Abstract
The development of the conceptual ground model (CGM) is a critical component of any desk study or ground engineering project planning process. A key task of the engineering geologist is to develop the CGM in order to predict the occurrence of known terrain units, elements and facets within a given landsystem, and to communicate the lateral and vertical variability of engineering rocks and soils found within that system. This chapter details the significant ground components of glacial and periglacial landsystems within a geomorphological framework describing the sediments, structures and landforms that could reasonably be expected to be encountered in these terrains. Examples are provided of both modern and relict glacial and periglacial landforms, their mode of formation and their field recognition. Glaciogenic and periglacial sediments are described both in terms of their sedimentological and formal engineering description. The chapter provides a suggested naming nomenclature for these sediments that can be used within a BS 5930 description. An extensive photoglossary is presented as a field aide memoir, enabling the engineering geologist to identify these features once on site.
3.1 Introduction
This chapter aims to bring together the knowledge of glacial and periglacial conditions acquired by the scientific disciplines of engineering geology and geomorphology, which have followed separate paths of enquiry in the British Isles. Geomorphologists are primarily interested in understanding how landscapes develop over time by investigating Earth-surface processes and landforms. This is predominantly a scientific endeavour but many aspects of geomorphology have industrial applications, not least in civil engineering (Fookes et al. 2005, 2015). Engineering geologists are specifically responsible for the investigations needed to establish the ground conditions, geohazards and georesources of potential or existing development sites. The techniques for carrying out this work are well established (Chapter 7; Hencher 2012; Site Investigation Steering Group 2013) and a key component is the creation of a ground model (Fookes 1997). The ground model must be built around a framework of knowledge of the ground conditions, incorporating a broad understanding of the general geomorphological and geological conditions but becoming increasingly site-specific as new data are acquired. In the early stages of this process, engineering geology utilizes methods of terrain evaluation (Section 3.2) and classification (Section 3.3) to establish the general nature of the site and provide the basis for subsequent more expensive on-site ground investigations. These techniques are also widely used by geomorphologists in their investigations of process-form regimes in landscape evolution (e.g. Brunsden 1990; Cooke & Doornkamp 1990; Maund & Eddleston 1998; Griffiths 2001; Phipps 2001a, b, 2002; Evans 2003; Sharpe et al. 2007; Griffiths & Stokes 2008).
As part of the terrain evaluation process, engineering geologists draw upon conceptual ideas about the way landforms and geoenvironments in the area have evolved in order to create ground models. Examples of engineering geological ground models for glacial and periglacial conditions are presented in Section 3.4. This approach was articulated by Griffiths et al. (2012) in relation to the ground conditions in hot deserts, and we adopt the same approach in this chapter (Section 3.5) in relation to glacial and periglacial landscapes. Many of the models for glacial and periglacial landscapes have been developed by geomorphologists under the concept of ‘landsystems’ (Evans 2003, 2013a, b, c). An important distinction must be made between those models that represent present-day and active glacial and periglacial landsystems and those that display the relict landforms and materials associated with former glacial and periglacial environments, although active landsystems are employed wherever possible as the modern analogues for ancient settings.
In the British Isles it is mostly the relict landforms that are of relevance to present-day engineering, but these can only be understood by reference to active glacial and periglacial environments located in very different geographic regions at higher latitudes and/or altitudes. Because these models have primarily been created as part of scientific investigations they are hypothetical constructs that are still evolving and being discussed by the geomorphological community; they cannot therefore be regarded as definitive. However, they do provide engineering geologists with a geomorphological framework and a range of models that can be useful in understanding ground conditions at different sites. Nonetheless, some ambiguity in the terminology used by both parties remains and this chapter attempts to address this. The ambiguity is most notable once the investigations move from the generalized ground models in Section 3.5 to the detailed description of glacial and periglacial sediments and landforms in Sections 3.6, 3.7, 3.8. However, these last two sections are the core of this chapter and provide a suite of images that illustrate glacial and periglacial sediments, structures and landforms with modern examples, along with their relict equivalents as they might be found in the UK. The principal engineering significance of the phenomena are noted along with any specific investigative requirements.
3.2 Terrain evaluation
Terrain evaluation encompasses a range of geomorphological techniques and was identified by Lawrance et al. (1993) as a method for summarizing the physical aspects of a landscape. This was achieved initially through classification and then by assessing ground conditions expressed in terms of engineering requirements. Griffiths & Edwards (2001) took a similar definition. Although they preferred the expression ‘land surface’ rather than ‘terrain’ evaluation, the terms can be regarded as synonymous. Griffiths & Edwards (2001) stated that it is the evaluation and interpretation of land-surface and near-surface features using techniques that do not involve ground exploration by excavation (except small hand-dug pits or hand auger holes) or geophysics. Based on this definition terrain evaluation is integral to the development of the ground model (Fookes 1997), central to all successful civil engineering construction.
The techniques employed in terrain evaluation include: geomorphological mapping; geological mapping; engineering geological mapping; remote sensing interpretation; analytical photogrammetry; land systems mapping; and natural hazard and risk assessment. The output from a terrain evaluation is usually a suite of maps and block diagrams either held as hardcopy or as a series of overlays in a Geographical Information System (GIS). The map data can be classified into three categories (Griffiths 2004) as follows.
Element maps record the actual measurable and/or mapped ground conditions (i.e. the factual basis such as topography, bedrock geology, soils, morphology, land use, and location of previous site investigations).
Derivative maps and diagrams are obtained by either interpreting or combining various element maps (e.g. landforms based on an interpretation of the morphology; slope steepness derived from the topographic data; depth to bedrock compiled by combining ground investigation data, superficial deposit and bedrock maps; and block diagrams showing the landscape and subsurface geology in three dimensions).
Summary maps and diagrams integrate a range of derivative and element maps to identify combinations of hazards, resources or land-use issues that act either as constraints to any development or indicate the potential of the land for exploitation and development.
The compilations of such maps and diagrams have two requirements. First, an extended legend should be attached to each map and diagram or linked to GIS layers. Second, terrain evaluation studies must include an interpretative report that explains the basis for developing the derivative and summary maps.
This methodology is appropriate for use in all engineering geology investigations. The techniques were regarded as essential for engineering geological investigations in hot deserts (Shilston et al. 2012) and mountain roads (Hearn 2011a, b). These two publications describe the application of various terrain evaluation techniques in a number of case studies. Examples that apply the terrain evaluation approach to engineering projects associated with relict glacial and periglacial conditions in Great Britain are more limited, perhaps because the process would be deemed to be part of normal ground model development during the planning and interpretation of a site investigation. Waller & Phipps’ (1996) study of relict periglacial conditions along the alignment of the Channel Tunnel Link is one of the few that specifically refers to terrain systems mapping (see below). Notwithstanding, it is clear that the terrain evaluation approach has widespread acceptance in engineering geology as a technique for reconnaissance investigations. We explore below the important ways in which terrain evaluation techniques have led to the creation of various forms of ground model for engineering projects (Fookes 1997) through terrain classification and the creation of land systems.
3.3 Terrain classification
Terrain evaluation is an approach to investigation in civil engineering that covers a range of techniques, while terrain classification is an explicit process. Lawrance et al. (1993) indicated that terrain classification normally refers to the process of subdividing a landscape into land systems, land facets and land elements. Christian & Stewart (1953, p. 53), from their work with the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia, defined a land system as:
The topography and soils are dependent on the nature of the underlying rocks (i.e. geology), the erosional and depositional processes that have produced the present topography (i.e. geomorphology), and the climate under which these processes have operated. Thus the land system is a scientific classification of country based on topography, soils and vegetation correlated with geology, geomorphology and climate.
Finlayson (1984) used slightly different terminology to describe the PUCE system of terrain analysis further developed by CSIRO in Australia. The PUCE system comprises a very broad-scale landscape province (identified on maps at 1:250 000 scale) that is subdivided into terrain patterns, terrain units and terrain components (Fig. 3.1). Terrain patterns are equivalent to land systems, units to facets, and components to elements. In the present chapter we adopt the landsystems terminology, with definitions as follows.
Landsystem: a large area (e.g. 100 km2) with a recurring pattern of landforms, soils, vegetation, geology and hydrological regimes (Christian & Stewart 1953; Mitchell 1973; Lawrance et al. 1993). Normally land systems are mapped at small scales (e.g. >1:100 000). The geomorphological designation of units such as ‘active temperate glacial landscape’ is at the landsystem scale.
Land facet: a terrain unit of uniform slope, parent material, soils and hydrological conditions. Lawrance et al. (1993, p. 57) described it as ‘sufficiently homogenous to be considered uniform for most practical purposes’. Land facets combine to make up a landsystem and can be mapped at scales ranging between 1:10 000 and 1:100 000. Land facets can often be given simple morphographic descriptors such as pyramidal peak, outwash plain and drumlin.
Land element: the smallest unit of the classification, combined to make a land facet. These could include simple, more specific morphological descriptors such as concave slope, cliff and convex slope.
Fig. 3.1.
(a) Ground model (process-form model) of an active temperate glacial environment (Chapter 4). (b) Real-world setting being conceptualized. Skaftafellsjökull proglacial lake and moraines, Iceland (A. Russell).
Because engineering geology and geomorphology in Great Britain have developed independently, inconsistencies exist between their terminologies. One difference is the use of the term ‘landsystems’. For engineering geologists this is a component of terrain classification and usually undertaken at a small scale. However, geomorphologists (e.g. Eyles 1983; Evans 2003, 2013a, b, c) use the word ‘landsystems’ to describe geomorphological processes and landforms in both specific and generic situations. The concept of ‘process-form models’ or ‘landsystems’ has a long and distinguished history in geomorphology, and was first expounded by Chorley & Kennedy (1971). Chorley et al. (1984) used the expression ‘cascading process systems’ to cover landscape denudation and sediment cascades. However, Brunsden (2002, p. 103) proposed that, in the context of the geomorphological applications to engineering, the Earth's surface comprises
process-response [form] systems that are specified by the stress fabrics given by tectonic history and setting, the properties of the rocks, superficial deposits, landform geometries, climates and biomes arranged in patterns across the Earth's surface in a hierarchy of landscape types and scales.
It is this definition that allows us to recognize the link between geomorphological landsystems and the creation of ground models that is now a fundamental part of engineering geological practice (Section 3.4).
Lawrance et al. (1993) did suggest that in practice there is very little difference between a geomorphological map (see Brunsden et al. 1975; Smith et al. 2011), a geotechnical map (see Clark & Johnson 1975; Griffiths et al. 1994) and a landsystem/land facet map. However, geomorphological maps normally record actual conditions at specific sites, including information on landform genesis; maps of landsystems establish recurrent landscape themes that are presented as a generic three-dimensional model rather than representing one particular locality; and geotechnical maps will be mostly focused on the engineering parameters of rocks and soils.
3.4 Engineering geological, glacial and periglacial ground models
An understanding of the ground conditions is one of the key requirements of terrain classification for engineering purposes, and the principal issues of engineering geological significance in glacial and periglacial materials are presented in Table 3.1. In the engineering geological literature this understanding of ground conditions is increasingly expressed by creating ground models (Fookes 1991, 1997; Fookes et al. 2015), not least in the emphasis placed in BS5930 (Anon 2015) on forming and evaluating a ground model throughout the investigation process. The ground model attempts to provide a synopsis of the nature of the ground conditions, as demonstrated in Figures 3.1a and 3.2a for the glacial and near-glacial environments that are shown in the images in Figure 3.1b and 3.2b. There are differences in the manner in which engineering geologists and geomorphologists have utilized ground models but their end purpose, to understand the nature and development of the landscape, is the same. In Section 3.4.1 two examples of typical models are presented that were developed to provide general guidance to engineering geologists on the nature of present-day glacial and periglacial processes and landforms; they therefore provide analogues for glacial and periglacial processes that prevailed in Great Britain throughout significant periods of the Quaternary. However, as identified in Chapter 1, it has become apparent that engineering geology needs to incorporate the developments in geomorphological research in glacial and periglacial environments since these models were first proposed by Fookes (1997). These highly generalized engineering geological models do provide an introduction into the nature of the different landforms, sediments and structures that are found in both modern and relict glacial and periglacial environments. However, the full complexity of the ground conditions, processes and sediments requires a more comprehensive treatment and lexicon and this is provided in Sections 3.5, 3.6, 3.7, 3.8. This updated lexicon forms the basis for the more site-specific glacial and periglacial landsystems that identify individual processes, landforms and sediments presented in Chapters 4 and 5. These landsystems should be the basis for the development of all future engineering geological ground models, and some of the background to their development is presented in Sections 3.4.2.1 (glacial) and 3.4.2.2 (periglacial).
Fig. 3.2.
(a) Ground model (process-form model) of a debris-charged polythermal glacier margin typical of sub-polar settings (after Boulton 1967). (b) Real-world setting being conceptualized. Ellesmere Island sub-polar glacier snout, Arctic Canada (D.J.A. Evans).
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Table 3.1.
Principal engineering significance of glacial and periglacial materials
3.4.1 Engineering geological ground models
The International Association for Engineering Geology and the Environment (IAEG) Commission 25 has identified three types of engineering geological model (Table 3.2; Parry 2010; Parry et al. 2014), namely conceptual, observational and analytical. This terminology is used as a basis for this Working Party Report.
Conceptual models. Figures 3.3 and 3.4 provide examples of generalized conceptual models for active valley glacier and periglacial environments in the northern hemisphere from Fookes et al. (2007). These models do not represent any specific location, but constitute an amalgamation comprising the features and associated geomorphological processes found in active valley glacier and permafrost conditions. Within the models there is information on both the ground conditions and comments on issues concerning foundations to structures. This model represents a visual checklist of the features and processes that the engineering geologist and geomorphologist working on projects in such terrain might encounter, and is relevant to engineering design decisions.
Observational or physical models. An example that demonstrates that observational models have been employed for many decades is presented in Figure 3.5 (Croot & Griffiths 2001, based on Griffiths & Marsh 1986). This 3D model was created to identify the hazards to the A35 Axminster Bypass in Devon, UK. The model drew on data from 1:1250 scale field mapping of the geomorphology and a preliminary ground investigation. The model identified the processes and features that would have a direct effect on engineering design decisions, including: two phases of landsliding (active failures and relict active-layer detachment slides with the possibility of relict shear surfaces being present); extensive solifluction deposits with low shear strength horizons; the location of low-bearing-capacity alluvium; and active spring-head sapping.
Analytical models. In slope instability studies kinematic analysis, limit equilibrium, continuum (i.e. finite element and finite difference) and discontinuum (i.e. discrete element and brittle fracture) modelling would all fall under this category (for a discussion on these various types of models, see Stead & Coggan 2012). Such models fall readily within the purview of the geotechnical engineer although as always the accuracy of the input parameters, including the ground geometry and subsurface lithology and structure, will be dependent on the input from engineering geologists.
Observational and analytical engineering geological models can only really be compiled with accuracy once site-specific field data have been collected, although preliminary versions of these models may be produced during the desk studies. However, for detailed design work input from a ground investigation and detailed field mapping is necessary. These models must be updated regularly as new data are obtained during investigations and remain as live documents (Anon 2015), unlike the more ‘static’ conceptual models used during the planning and feasibility stages of a project.
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Table 3.2.
Types of ground model (after Parry et al. 2014)
Fig. 3.3.
Engineering geological conceptual model of a valley glacier (Fookes et al. 2007).
Fig. 3.4.
Engineering geological conceptual ground model of an active periglacial environment (Fookes et al. 2007).
Fig. 3.5.
A site-specific engineering geological conceptual ground model in an area of relict periglacial processes in SW England (after Croot & Griffiths 2001; reproduced with permission of the Geological Society of London).
3.4.2 Geomorphological landsystem models
As discussed in Section 3.3 geomorphologists use the landsystems concept to describe geomorphological processes and landforms in both generic and specific situations. Recently developed geomorphological landsystems models are therefore ideal for use in engineering geology and have the potential to provide significantly more site-specific characterizations of glaciated and periglaciated terrain than those presented in the generalized models in Figures 3.2 and 3.3. As an introduction to the more detailed work in Chapters 4 and 5, the current state of the art in modern exemplars for geomorphological landsystems for glacial and periglacial environments is presented in the following sections.
3.4.2.1 Glacial landsystems
A useful place to start in studying glacial landsystems is with the occurrence of glacial ice as this is the medium, in conjunction with its associated meltwater, that deforms and erodes the landscape and deposits the various materials that characterize the superficial geology of glaciated terrains. Identifying the modern analogue of the type of glacier that might have affected a landscape can be the first stage in establishing the nature of the glacial features and deposits that can be anticipated in an area. A number of different classifications of glaciers are available, and Benn & Evans (2010) identify a scheme comprising first- and second-order classifications. This scheme recognizes a form continuum that is dictated by a combination of climate and topography, so that the morphology of any one glacier is unique to its location on the Earth's surface.
Ice sheets and ice caps: these large ice masses submerge the landscape, at least in their central portions, and the major patterns of ice flow are largely independent of bed topography, with the exception of faster-moving ice streams and outlet glaciers located over troughs. Ice sheets are exemplified by the Antarctic and Greenland ice sheets, whereas ice caps are best represented by the largest ice masses in Svalbard, Ellesmere Island, Baffin Island and Iceland.
Ice domes are high areas of relatively slow-moving ice in an ice sheet or ice cap and may be located over either a topographic high or a vast depression. These domes act as dispersal centres from which ice flows radially. Domes are normally connected by ice divides, and ice sheets are commonly composed of numerous ice domes and ice divides, which may change position over time and hence initiate complex flow changes.
Ice streams and outlet glaciers are more rapidly flowing linear zones of ice through which most of the ice sheet or ice cap is discharged towards the periphery of the system, often along troughs or valleys, but in some cases between areas of slowly moving ice (pure ice streams).
Glaciers constrained by topography: these include ice masses that do not overwhelm the underlying terrain but rather are confined and directed by it, so that flow characteristics can be predicted from bedrock topography.
Ice fields do not possess a simple, dome-like surface but instead display flow patterns influenced by the underlying topography. They develop in upland topography at an altitude sufficient for ice accumulation, and examples include Columbia Ice Fields in the Canadian Rocky Mountains and the Patagonian Ice Fields. Ice may be drained from larger ice fields by valley glaciers.
Valley glaciers are formed by ice discharged from ice fields or cirques into deep bedrock valleys. They may have a simple, single-branched planform or dendritic network forms.
Transection glaciers form in deeply dissected glacierized mountain landscapes and constitute interconnected systems of valley glaciers in which ice overspills the pre-existing drainage divides.
Cirque glaciers are shaped by the armchair-shaped bedrock hollows (cirques) which act as their accumulation basin. The various sizes and forms of cirque glaciers lie on a continuum that is dictated by the evolutionary stage of the hosting cirque.
Piedmont glaciers are produced wherever valley glaciers debouch onto lowland areas after travelling through bedrock troughs. Excellent examples include the Malaspina Glacier, Alaska and Skeiðarárjökull, Iceland.
Niche glaciers, glacierets, ice aprons and ice fringes are the smallest glacier ice masses. Aprons are thin snow and ice accumulations adhering to mountain sides, and fringes occur along coasts where the glaciation level is at sea level. Glacierets are thin ice patches occupying depressions on mountain slopes and are produced by snow drifting and avalanching; ice avalanching from ice falls can form fall glaciers or reconstituted glaciers at lower elevations and below the equilibrium line altitude. Small ice masses located in niches or rock benches on mountain or valley sides are called niche glaciers.
Ice shelves: these are low-gradient, floating glacier tongues, produced where glaciers flow offshore or where sea ice thickens by surface accumulation and bottom accretion. Geographically, ice shelves are located in high polar settings. The thickening of an ice shelf so that it grounds on an offshore shoal or the margins of islands or coastal shelves results in the production of an ice rise.
As discussed in Chapter 1, engineering geology in the British Isles and the rest of Western Europe is mainly concerned with relict glacial landforms. Modern landsystems research has focused on present-day glaciers and their forelands in order to identify a range of landform-sediment suites that can be employed as exemplars for specific styles of glaciation in palaeoglaciological reconstructions (see Evans 2013a, b, c for overview). Additionally, there has been an increasing recognition that the landform-sediment suites from a single period of glacial activity may contain more complex overprinted signatures than previously widely acknowledged (e.g. Wilson & Evans 2000; Evans & Twigg 2002; Evans 2011). A reasonably comprehensive range of modern terrestrial glacial landsystem case studies/exemplars are now available (cf. Evans 2003, 2013a, b, c; Benn & Evans 2010) and are summarized in Table 3.3 with the glacial landsystems presented in Figure 3.6.
Fig. 3.6.
Example selection of major glacial landsystems, including active temperate (Evans & Twigg 2002), surging (Evans & Rea 2003), ice stream (Clark & Stokes 2003), sub-polar (Ó Cofaigh et al. 2003), plateau (Rea & Evans 2003) and mountain valley (Benn et al. 2003).
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Table 3.3.
Modern glacial landsystem exemplars
Based on this evaluation of modern analogues, the terminology that has been developed for glacial landsystems and which has been adopted for application in engineering geology is presented in Table 3.4. This terminology is used as the basis for the illustrated lexicon in Sections 3.5, 3.6, 3.7, 3.8 and explained in detail in Chapter 4.
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Table 3.4.
Adopted glacial landsystem terminology (Chapter 4)
3.4.2.2 Periglacial landsystems
The regions of active periglacial landsystems are those where ground is perennially or seasonally frozen and, while air temperatures are often below freezing, snow rarely lasts all summer. Such periglacial regions occur both at high altitudes and high latitudes and were formerly much more extensive during the Pleistocene. For engineering geologists working in western Europe and North America, understanding the effects of periglacial processes on the development of the present-day landscape is fundamental. Indeed, even in areas where relict glacial landforms and deposits now dominate the landscape, periglacial processes have often modified them during and after deglaciation.
The definitive work on the periglacial geomorphology of Great Britain remains the book by Ballantyne & Harris (1994), which contains details of relict periglacial landforms and deposits along with examples of the active equivalents. Ballantyne & Harris (1994) identified five types of periglacial climate, which we modify slightly below.
Polar desert: found in glacier-ice-free areas of the High Arctic and the Antarctic continent, where total plant cover is less than about 10%. Polar desert is characterized by a strong seasonal pattern of temperature fluctuation giving cool summers and very cold winters. Precipitation is usually <250 mm a−1, and so the ground surface tends to be dry and organic matter limited. Perennially frozen ground (permafrost) underlies a very thin (c. 0–50 cm) active layer that thaws in summer. The permafrost tends to be hundreds of metres thick and occurs within the continuous permafrost zone.
Arctic tundra: located south of the polar deserts in the Northern Hemisphere and still dominated by large seasonal temperature fluctuations and low amounts of precipitation. Tundra areas tend to contain more organic matter in their active layer than polar deserts, resulting in an active-layer thickness that is often similar to that of polar deserts, despite the warmer summers on tundra. Ground conditions may be wet in summer as the permafrost layer prevents the downward infiltration of meltwater. As with polar deserts, the permafrost is commonly hundreds of metres thick and occurs within the continuous permafrost zone.
Continental (boreal) climates: regions covered by boreal forest that lie to the south of the Arctic tundra (e.g. interior Alaska, boreal Canada and Siberia). Such regions have a greater temperature range between winter and summer than areas of Arctic tundra. Precipitation is higher than the tundra but with greater summer air temperatures the evaporation rates are also higher, often leading to drier ground surface conditions. Permafrost tends to be thick, cold and continuous in the northern part of these regions, but becomes thinner, warmer and discontinuous in their southern part. As with regions of Arctic tundra, vegetation and soil characteristics exert a strong control on ground temperatures, although active-layer thicknesses tend to exceed those of tundra regions and may reach 1–2 m.
Alpine: warmer than any of the above periglacial climates, with pronounced seasonal and diurnal temperature fluctuations. The higher altitudes are associated with higher rates of precipitation, much of which falls as snow. Permafrost may be continuous at higher elevations and/or beneath north-facing slopes (in the Northern Hemisphere) or discontinuous or absent at lower elevations and/or beneath south-facing slopes. The permafrost distribution is often highly complex over distances of tens to hundreds of metres due to the 3D nature of the topography and spatial variations in factors such as snow cover, slope angle, aspect and void size between debris particles.
Low-temperature range: these are associated with maritime areas (e.g. Iceland) and low-latitude mountains. In maritime areas the modifying affects of the sea result in a limited seasonal temperature range, but rainfall is usually high. In low-latitude mountains, there is little seasonal variation in temperature but a high diurnal range occurs throughout the year.
An attempt to distil this complexity of climate types and relate them to associated landforms was provided by Karte (1979), modified in Figure 3.7a.
Fig. 3.7.
(a) Conceptual model showing the typical landforms in active periglacial regions (modified from Karte 1979). (b) Schematic representation of the Late Devensian periglacial features of the Scottish Highlands that were probably active during the Loch Lomond Stadial (Ballantyne 1984).
Figure 3.7a fits within the framework of a conceptual model (Table 3.2) and, while useful for general guidance about active periglacial environments, it lacks sufficient detail to be of value in engineering geology. Of more interest is the schematic representation of the periglacial features found in the Highlands of Scotland that were probably active during the last period of major ice advance in the British Isles, the Loch Lomond Stadial (see Chapter 2), produced by Ballantyne (1984) and reproduced in Figure 3.7b. This conceptual model is a fine example of the value of producing schematic conceptual models to illustrate landforms. However, these models do not bring together the combination of landforms and deposits that allow the identification of interrelated landsystems that reflect the reality of the ground conditions that will be encountered in engineering geological investigations. Unlike glacial landsystems, discussed above, periglacial landsystems have not been identified before, either in Great Britain or elsewhere. In line with the work on glacial landsystems, based on an evaluation of modern analogues, the terminology that has been newly developed in this volume for periglacial landsystems and adopted for application in engineering geology is presented in Table 3.5. This terminology is used as the basis for the illustrated lexicon in Sections , 3.6, 3.7, 3.8 and explained in detail in Chapter 5. Based on the studies carried by the Engineering Group of the Geological Society, geomorphological landsystems were recognized as the core element linking geomorphological research and engineering geological requirements. As a result, the lexicon presented in Table 3.6 was compiled and this is illustrated in Sections 3.5, 3.6, 3.7, 3.8, 3.9, 3.10, 3.11, 3.12.
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Table 3.5.
Adopted periglacial landsystem terminology (Chapter 5)
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Table 3.6.
Glaciogenic and periglacial sediments, structures and landforms for Quaternary ground models
3.5 Glaciogenic sediment descriptors
This section outlines the developed naming convention, in accordance with BS 5930 (Anon 2015), for glaciogenic deposits for the geological origin and geological qualifier for origin terms (Table 3.7). Detailed table entries provide examples of sedimentological and engineering descriptions for each soil type, along with their diagnostic characteristics for field recognition and identification. Photographs show well-developed examples of the sediments. Table 3.8 details the descriptive sedimentological facie codes used in the typical sedimentological descriptions provided for each glaciogenic sediment.
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Table 3.7.
Geological origin name and qualifiers with examples in accordance with BS5930 (Anon 2015)
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Table 3.8.
Descriptive sedimentological facies codes for diamicton, melange and predominantly non-fluvial coarse-clastic deposits
| Sediment name | 3.5.1.1 Subglacial traction till |
| Previous terms | Lodgement till, deformation till, melt-out till (also boulder clay, comminution till, lee-side cavity fills/ice-bed separation deposits, endiamict glaciotectonite, tectomict, soft-bed till, deforming bed till). |
| Diagnostic characteristics | Sediment deposited by a glacier sole either sliding over and/or deforming its bed, the sediment having been released directly from the ice by pressure melting and/or liberated from the substrate and then disaggregated and completely or largely homogenized by shearing. |
| Environment of formation | Subglacial traction zone/ice-bed interface. |
| Common structures present | Macroscopically massive to pseudo-laminated and/or various degrees of fissility. Individual till units often subdivided into upper and lower components or A and B horizons respectively, with the A horizon appearing massive and loose textured and the B horizon appearing more compact and fissile. Pseudo-laminated diamictons may have originated as Type A glaciotectonites; indeed, there is a process continuum that includes Type A glaciotectonite grading into subglacial traction till with increasing cumulative shear. Can contain discontinuous lenses, stingers or wisps of stratified sediment (intrabeds and/or interbeds; canal fills) which have been glaciotectonically disturbed to varying degrees. Clast macrofabrics can be relatively strong (clustered) with S1 eigenvalues >0.7 but are invariably girdle-like (S1 = 0.45–0.7). Stronger fabrics are usually displayed by larger, lodged cobbles and boulders, lodgement being indicated by striated and facetted upper surfaces. Microscopically they can display augens, lineations and symmetrical pressure shadows (planar deformation or discrete shear zone development) or galaxy structures, soft intraclasts and asymmetrical pressure shadows (rotational or pervasive deformation). Additionally, fractured quartz grains can indicate crushing under intense shear and water-escape structures and hydrofracture fills record the overpressurization of porewater. See van der Meer (1993), Menzies (2000) and Carr (2004) for a full catalogue of microscale features. |
| Principal engineering significance | Variable particle sizes, anisotropic permeability, anisotropic strength, anisotropic stiffness, perched water tables, coarse horizons. |
Typical images Fig. 3.8. (a) Breiðamerkurjökull till, Iceland, showing A and B horizons above and below knife, respectively (D.I. Benn). | |
 Fig. 3.8. (b) Stratified sand intrabeds/lenses in Skipsea Till, East Yorkshire, England (D.J.A. Evans). | |
 Fig. 3.8. (c) Clast pavement at boundary between gravelly and clast-poor subglacial traction till (Dmm), Whitburn, County Durham, England (D.J.A. Evans). | |
 Fig. 3.8. (d) Pseudo-laminated till (Dml), Alberta, Canada (D.J.A. Evans). | |
| Typical sedimentological description Massive, matrix-supported diamicton (Dmm) or pseudo-laminated diamicton (Dml) or highly fissile/sheared diamicton (Dmm(s)). Individual till units can contain or be separated by discontinuous stratified lenses, stringers or wisps. These can originate as either ice-bed separation deposits or deformed rafts of cannibalized substrate. Tills may contain or be separated by clast pavements. | Engineering description Figure 3.8c and d: Faintly laminated slightly gravelly sandy CLAY can appear thinly laminated with polished apparent bedding planes due to shearing. Individual units can be separated by discontinuous laminated lenses, stringers or wisps of sand or gravel. Tills may be separated by very thin beds of GRAVEL with occasional cobbles. |
| Principal references Boulton (1987), Boulton & Hindmarsh (1987), van der Meer (1993), Benn & Evans (1996, 2010), Evans (2000, 2017), Evans & Twigg (2002), van der Meer et al. (2003), Evans et al. (2006) | |
| Engineering geology case studies Kazi & Knill (1969), McGown (1971), Boulton & Paul (1976), McGown & Derbyshire (1977), Eyles & Sladen (1981), Little & Atkinson (1988), Bell (1991, 2000), Bell & Forster (1991), Culshaw et al. (1991), Gregory & Bell (1991), Hossain & McKinlay (1991), Marsland & Powell (1991), Hossain (1992), Clarke et al. (1998), Trenter (1999), McMillan et al. (2000), Winter (2004), Skipper et al. (2005), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.1.2 Glaciotectonite |
| Previous terms | Exodiamict glaciotectonite, comminution till, deformation till, tectomict. |
| Diagnostic characteristics | Rock or sediment that has been deformed by subglacial shearing/deformation but retains some of the structural characteristics of the parent material. Glaciotectonite is the most common but also the most variable glacial deposit. |
| Environment of formation | Proglacial, glacier submarginal and glacier basal traction zone, developed in deformable materials, both bedrock and soft sediments. |
| Common structures present | Glaciotectonites are subdivided into Type A and Type B varieties.
Type A glaciotectonites show evidence of penetrative deformation due to brittle deformation or intense localized ductile deformation. They may also form by pervasive deformation to high cumulative strains, thereby producing a tectonic foliation due to primary structures being attenuated and rotated towards parallelism with the direction of shear or transposed due to the imposition of a new foliation. This results in a weakly laminated appearance but may be massive at outcrop scale, with evidence of deformation apparent only in thin-sections. Lamination is often deformed or folded, indicative of pre-deformation structure/stratification. Type B glaciotectonites have undergone non-penetrative deformation, involving pervasive ductile deformation to small total cumulative strains; pre-deformational sedimentary structures are therefore still recognizable in the material. This results in weakly to highly deformed (folded, faulted, attenuated) pre-existing sediments, most easily recognized where these were stratified and characterized by a range of grain sizes and bedforms. |
| Principal engineering significance | Variable particle sizes, anisotropic permeability, anisotropic strength, anisotropic stiffness, perched water tables, coarse horizons, shear surfaces, sheared bedrock. |
Typical images Fig. 3.9. (a) Type B glaciotectonite comprising heavily folded and faulted stratified sands, silts and clays and a diamicton lens capped by subglacial traction till at Nelson River, Banks Island, Canada (D.J.A. Evans). | |
 Fig. 3.9. (b) Complexly folded Cretaceous strata in the Mud Buttes, Alberta, Canada. An example of a glaciotectonic cupola hill and hence Type B glaciotectonite (D.J.A. Evans). | |
 Fig. 3.9. (c) Highly attenuated, crudely to well-stratified and locally pseudo-laminated diamictons and associated clays, silts, sands and gravels comprising the ‘Irish Sea Till’ at Kilmore Quay, Ireland and representative of Types A and B glaciotectonite (D.J.A. Evans). | |
Fig. 3.9. (d) Sand and gravel boudin and discontinuous lenses in pseudo-laminated diamicton, indicative of deformation in a subglacial shear zone. Two possible depositional scenarios are possible here: firstly, the stratified sediments were deposited prior to glacier overriding and hence the sequence represents an example of intermediate Type A/B glaciotectonite; secondly, the stratified sediments originated as subglacial meltwater deposits or canal fills in a complex sequence of subglacial traction till emplacement events, Island of Funen, Denmark (D.J.A. Evans). | |
| Typical sedimentological description Highly variable in appearance ranging from weakly laminated, matrix-rich diamictons (Type A, penetrative) to deformed (folded, faulted, attenuated) stratified sediments (Type B, non-penetrative). Compressional features, such as open folds, thrust overfolds and nappes, are typical of proglacial to glacier submarginal deformation, whereas simple shear structures, such as isoclinal folds, boudins, tectonic laminae and conjugate shears, are produced in glacier submarginal to subglacial shear zones. | Engineering description Figure 3.9a, 3.9b, 3.9c, 3.9d: Variable materials including indistinctly laminated slightly gravelly sandy CLAYS to deformed (folded, faulted, attenuated) inter-stratified sediments. |
| Principal references Banham (1977), Pedersen (1989), Benn & Evans (1996), Evans et al. (1998, 2006), McCarroll & Rijsdijk (2003), van der Meer et al. (2003), Hiemstra et al. (2007), Ó Cofaigh et al. (2011), Evans (2017) | |
| Engineering geology case studies Kazi & Knill (1969), McGown (1971), Boulton & Paul (1976), McGown & Derbyshire (1977), Eyles & Sladen (1981), Little & Atkinson (1988), Bell (1991, 2000), Bell & Forster (1991), Culshaw et al. (1991), Gregory & Bell (1991), Hossain & McKinlay (1991), Marsland & Powell (1991), Hossain (1992), Clarke et al. (1998), Trenter (1999), McMillan et al. (2000), Winter (2004), Skipper et al. (2005), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.1.3 Supraglacial mass-flow diamicton/glaciogenic debris-flow deposit |
| Previous terms | Supraglacial morainic till, flow till, melt-out till, ablation till. |
| Diagnostic characteristics | Predominantly clast-supported, massive to crudely stratified or graded diamictons but the sedimentology of supraglacial depocentres is complex due to multiple cycles of redeposition during formation. Typical facies associations comprise interbedded diamictons and discontinuous bodies of laminated lacustrine sediments, and glaciofluvial sands and gravels. Internal disturbance is common and characterized by normal faulting, flow folding and soft sediment deformation. |
| Environment of formation | Glacier surfaces or on ice-cored moraines. |
| Common structures present | Although they appear largely massive, individual debris flows can form tabular or lens-shaped units, often with erosional, channelized bases and flat tops. Successive flows can therefore be distinguished by their upper and/or lower boundaries, which are commonly marked by basal concentrations of clasts, upper washed horizons/interbeds of silt, sand or gravel, or very subtle partings. Each of these characteristics, as well as any internal structures, reflect the nature of the mass-flow type, specifically related to the moisture content and coherence of the matrix. Supraglacial mass-flow deposits are most confidently identified when juxtaposed with typical supraglacial facies associations. At microscale, structures are particularly well developed at the basal boundaries of individual flows. Here the substrate can be characterized as small folds, thrusts and shears, associated with rotated to slightly attenuated diamicton pebbles derived from the overlying debris flow. The debris-flow-substrate interface can be marked by elongate ‘flames’ of the substrate material separating lobate or pendant structures of the debris-flow diamicton, which are progressively tilted downflow. The base of the debris flow can contain detached ‘flames’ or ribbons of the substrate material as well as indicators of rotational deformation such as circular, arcuate and galaxy-like grain arrangements. Importantly, none of these features is singularly diagnostic of debris-flow deposits and can also be found in subglacial tills. |
| Principal engineering significance | Variable particle sizes, anisotropic permeability, anisotropic strength, anisotropic stiffness, perched water tables, coarse horizons, shear surfaces, sheared bedrock. |
Typical images
Fig. 3.10. (a) Crudely stratified gravelly mass-flow deposits comprising a stacked sequence of discontinuous layers of predominantly clast-supported but locally matrix-supported diamictons separated in places by gravelly lags, Kvíárjökull, Iceland (D.J.A. Evans). | |
Fig. 3.10. (b) Thinly bedded, matrix-supported mass-flow diamictons with localized clast concentrations and separated by discontinuous partings, Morsarjökull, Iceland (D.J.A. Evans). | |
Fig. 3.10. (c) Very crudely stratified boulder-rich and predominantly clast-supported diamictons with contorted bedding structures and localized pockets of stratified sand and gravel, Gillespie's Beach, New Zealand (D.J.A. Evans). | |
| Typical sedimentological description Crudely stratified diamictons with a range of clast contents and matrix properties. Often interbedded with or separated by discontinuous layers of silt, sand and gravel. Can display crude grading, often with basal concentrations of clasts. Flow structures or soft sediment deformation features are visible wherever the deposits possess any stratification. | Engineering description Figure 3.10a, 3.10b, 3.10c: Often indistinctly bedded gravelly sandy CLAYS with medium to high cobble and low to medium boulder content. Occasionally materials fine upwards. Bedding affected by flow and soft structure deformation features. |
| Principal references Eyles (1979), Lawson (1979a, b), Johnson & Rodine (1984), Owen (1994), Johnson & Gillam (1995), Phillips (2006), Evans et al. (2010a, b), Evans (2017) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), McMillan et al. (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.1.4 Melt-out till |
| Previous terms | Melt-out till; subglacial melt-out till; supraglacial melt-out till; sublimation till |
| Diagnostic characteristics | Displays ‘pseudo-stratification’, which includes discontinuous and contorted and sheet-like lamina, layers and lenses, textural and compositional banding, with strong clast macrofabrics. |
| Environment of formation | Buried and isolated parts of debris-rich basal ice of former glacier snouts. Preservation potential low due to the build-up of high porewater pressures, which leads to remobilization and dewatering. Therefore likely to be rare and patchy pockets within mass-flow diamictons and crudely stratified sands and gravels. |
| Common structures present | Displays ‘pseudo-stratification’, which includes discontinuous and contorted and sheet-like lamina, layers and lenses, textural and compositional banding with indistinct contacts and internal flow structures and undeformed, intra-till soft sediment or soft bedrock rafts. Supercooled ice will create a deposit that displays silt or sandy-silt lamina, en echelon silt lamina, irregular lenses of coarse to fine sand that offset or cross bedding planes, short lenses of gravel, sand and silt, thin lenses of silt and clay-silt aggregates, dispersed subrounded, but non-striated pebbles and cobbles around which lamina bend. At the microscale, features include porewater pathways, compressed and lightly deformed clay-silt clasts and an absence of shear indicators. Strong macrofabrics that mimic those of sheared basal ice facies. |
| Principal engineering significance | Variable particle sizes, anisotropic permeability, anisotropic strength, anisotropic stiffness, perched water tables, coarse horizons, shear surfaces, sheared bedrock |
Typical images
Fig. 3.11. (a) Debris-rich (stratified) basal ice transitioning upwards into pseudo-stratified/laminated diamicton with strong macrofabrics and microscale porewater pathways. Linkage between stratified ice and overlying melt-out till is evidenced by transition zone 5–10 cm thick, Matanuska Glacier, Alaska (from Larson et al. 2016). | |
Fig. 3.11. (b) Interpretations annotated on the image in (a) (from Larson et al. 2016). | |
Fig. 3.11. (c) ‘Pseudo-stratification’ in melt-out till on glacial foreland showing discontinuous, contorted and sheet-like lamina, layers and lenses, textural and compositional banding with indistinct contacts and internal flow structures and undeformed, short lenses of gravel, sand and silt, including pebbles around which lamina bend, Matanuska Glacier, Alaska (from Larson et al. 2016). | |
| Typical sedimentological description Sediment released by the melting or sublimation of stagnant or slowly moving debris-rich glacier ice, and directly deposited without subsequent transport or deformation (Benn & Evans 1998; Evans et al. 2006). | Engineering description Figure 3.11a: Dark grey indistinctly very thinly bedded slightly sandy slightly gravelly CLAY. Overlying 50–100 mm thick transition zone of dark grey thinly laminated slightly sandy varying from slightly gravelly to gravelly; laminae inclined at 20°. Figure 3.11c: Grey thinly to thickly laminated slightly sandy slightly gravelly CLAY. Occasional lenses (up to 300 mm long and 20 mm thick) of sand or gravel. Laminae planar dipping 0–10° and bent around lenses and gravel clasts. |
| Principal references Lawson (1979a, b, 1981), Shaw (1988), Carlson (2004), Larson et al. (2016), Evans (2017) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), McMillan et al. (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.2.1 Rhythmites (non-genetic) |
| Previous terms | |
| Diagnostic characteristics | Stratified sediments that display rhythmic or cyclic repetition of beds, related specifically to alternations between dominant grain size distributions. The term rhythmite is a non-genetic term used to refer to a range of deposits that display cyclic alternations in bedding, but is often replaced when the exact origins of the rhythms are known (see varves, turbidites, cyclopels and cyclopsams). |
| Environment of formation | Subaqueous settings such as lakes and oceans but also developed rarely in glaciofluvial systems such as jökulhlaup-fed sandar and subglacial tunnels (see gravel rhythmites). The laminations are created in deeper water by overflows and interflows, below the influence of bioturbation, wind mixing, wave disturbance and wind-generated currents which tend to form massive muds and silts. Laminations are fining-up sequences of silt and clay, the laminae reflecting fluctuations in the grain size and quantity of incoming sediment, due to daily, meteorological or annual water and sediment discharge cycles. |
| Common structures present | Cyclic grain size variations that reflect alternating variations in sediment supply and depositional conditions. In finer grain sizes these are manifest as horizontal lamination, whereby relatively coarser sediments (sands) record periods of high sediment transport capacity and finer silts and clays represent quiescent conditions. In deep-water settings, the variation in ice-rafted debris can also reflect cyclic influx of coarser sediments, although as this is usually seasonal the term ‘varve’ is used (see glaciomarine varves). |
| Principal engineering significance | Anisotropic permeability, prone to liquefaction |
Typical images
Fig. 3.12. (a) Rhythmites in the bottomsets of a former ice-dammed lake. Also visible are areas of type B climbing ripple development, soft sediment deformation structures and pockets of granule gravel likely originating from iceberg rafting or localized sediment gravity flows from adjacent foreset beds, Heinabergsjökull, Iceland (D.J.A. Evans). | |
Fig. 3.12. (b) Sequence of variable cyclic changes in sands, silts and clays (rhythmites) in an ice-contact subaqueous fan, Berrygate Hill, East Yorkshire, England (D.J.A. Evans). | |
| Typical sedimentological description Predominantly horizontally laminated (fine grain sizes) or bedded (coarser than sand) sediments that record cyclic influx of material to a depocentre. Some small-scale bedforms may also develop in rhythmite sequences. | Engineering description Figure 3.12a: Horizontally thinly interlaminated CLAY and silty fine SAND. Figure 3.12b: Horizontally thinly to thickly interlaminated CLAY and silty fine SAND |
| Principal references Church & Gilbert (1975), Smith & Ashley (1985), Ó Cofaigh & Dowdeswell (2001) | |
| Engineering geology case studies Bell & Coultard (1991, 1997), Culshaw et al. (1991), Giraud et al. (1991), Bell (1998, 2000), McMillan et al. (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.2.2 Varves (seasonal rhythmites) |
| Previous terms | |
| Diagnostic characteristics | Varves are rhythmites produced on the bottoms of deep-water bodies by interflows and overflows on a seasonal or annual basis in glaciated catchments. Each season produces a coarse- and fine-grained couplet in response to the high and low discharges of the summer and winter seasons, respectively. |
| Environment of formation | Deep-water settings of glaciolacustrine depocentres. The production of glaciomarine varves has also been proposed, specifically for more proximal or fjord settings (see dropstone diamicton and glaciomarine varves (ice-rafted debris)). Deposition in both environments is regulated by the seasonal flux of meltwater. However, non-glaciated or nival catchments have also been known to produce varve sequences. |
| Common structures present | Annual meltwater cycles produce distinct silt–clay couplets (varves), in which the coarse and fine components are separated by sharp contacts due to marked differences in sediment supply between summer and winter. The coarse component is a product of overflow and interflow plumes generated during the ablation season, and the fine component records gradual settling of the finest material during winter when there is little or no incoming water to the water body. Microscale investigations of rhythmites have revealed that many are indeed varves, even though they do not display obvious couplets at outcrop scale. This is significant for more distal settings where couplets are very thin. Features that are identifiable at microscale are similar to those used for varve recognition at outcrop and include regular alternations between silt-dominated and clay laminae, sharp contacts between clay lamina and overlying silt-dominated lamina, and the normal grading or massive appearance of sand/silt laminae in the coarser parts of sequences. |
| Principal engineering significance | Anisotropic permeability, prone to liquefaction |
Typical images
Fig. 3.13. (a) Silt and clay varves from former ice-dammed lake sediments, Iceland (D.J.A. Evans). | |
Fig. 3.13. (b) 2 m high sequence of silt/clay varves with isolated dropstone, Lake Tekapo, New Zealand (D.J.A. Evans). | |
| Typical sedimentological description Horizontally bedded (laminated) silt–clay couplets separated by sharp boundaries and displaying distinct cyclicity. The coarser laminae can include some fine sands, and where this occurs a fining-up trend towards the clay layer can be observed. Varves can vary in thickness from year to year. Varves may contain some iceberg-rafted material, especially dropstones, if they were deposited in an ice-contact setting. | Engineering description Figure 3.13a and b: Horizontally thinly interlaminated (1–2 mm) CLAY and slightly fine sandy clayey SILT. Occasional randomly scattered well-rounded sandstone gravel. |
| Principal references Ashley (1975), Church & Gilbert (1975), Smith & Ashley (1985), Ohlendorf et al. (1997), Lamoureux et al. (2001), Ó Cofaigh & Dowdeswell (2001), Lamoureux & Gilbert (2004), Palmer et al. (2008) | |
| Engineering geology case studies Bell & Coultard (1991, 1997), Culshaw et al. (1991), Giraud et al. (1991), Bell (1998, 2000), McMillan et al. (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.2.3 Cycopels and cyclopsams (tidally influenced rhythmites) |
| Previous terms | |
| Diagnostic characteristics | Rhythmites deposited as couplets from overflow and interflow plumes in glaciomarine environments. Silt and mud couplets are called cyclopels and sand and mud couplets are called cyclopsams. |
| Environment of formation | Glaciomarine settings influenced by tidal forcings. Cyclopsams are generally deposited within 1 km of the point where the meltwater plume enters the sea. Cyclopels may also be deposited in proximal environments during periods of low discharge, but are more common between one and several kilometres of the efflux point. Tides control the sedimentation rate and style by creating: (a) the largest amounts of sand within the 2 hours following each low tide; and (b) the least amount around 1 hour after high tide, when silt and mud deposition dominates. In essence, rising tides bring greater water velocities and hence coarser materials, which gradually sink towards the bottom during high and ebb tide and then settle out in the slack water around low tide, all against a background of continuous fine silt and clay particle settling. Consequently, two laminae will be produced for each day, one for each tidal cycle. |
| Common structures present | Deposition from turbid plumes typically produces couplets of silt and mud (cyclopels) and sand and mud (cyclopsams). Couplet thicknesses vary due to glacier proximity. Each couplet has a sharp lower contact, recording the sudden onset of deposition of the coarsest material, and is normally graded, recording the gradual settling of progressively finer material. The grading records variations in sediment supply and settling rates, which are controlled by fluctuating meltwater stream discharge, tides and wind shear. Cyclopsams and cyclopels can be interbedded with turbidites, other mass-flow deposits and IRD, particularly in proximal environments. |
| Principal engineering significance | Anisotropic permeability, prone to liquefaction |
Typical images
Fig. 3.14. (a) Cyclopels (middle) and cyclopsams (bottom and top) in a vertical sequence in glaciomarine sediments in Clements Markham Inlet, Ellesmere Island, Canada (Stewart 1991). | |
Fig. 3.14. (b) Tidal rhythmites sheared and folded after deposition. Probably the force driving this deformation was an ice floe grounding on the sediment surface, Turnagain Arm, near Anchorage, Alaska (B. Higman). | |
| Typical sedimentological description Couplets of varying thickness of silt and mud (cyclopels) and sand and mud (cyclopsams). Each couplet has a sharp lower contact and is normally graded. Can be interbedded with turbidites, other mass-flow deposits and IRD. | Engineering description Figure 3.14a and b: Thinly interlaminated (<1–3 mm) grey CLAY and silty CLAY or CLAY and fine SAND. Sharp base to pairs. |
| Principal references Mackiewicz et al. (1984), Cowan et al. (1988), Powell & Molnia (1989), Cowan & Powell (1990), Ó Cofaigh & Dowdeswell (2001) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), McMillan et al. (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.2.4 Turbidites |
| Previous terms | |
| Diagnostic characteristics | An individual turbidite can be identified as a graded vertical sequence of horizontally bedded/laminated sediments created by the passage and slowing of turbidity currents. They also display proximal-to-distal fining, reflecting the rapid deposition of coarse material and the transport of finer material into deeper parts of the basin. The characteristic vertical ‘Bouma’ sequence comprises: (a) massive or normally graded sand or gravel; (b) planar-laminated sand; (c) ripple cross-laminated sand and silt; (d) interlaminated silt and/or clay; and (e) massive clay or silt. |
| Environment of formation | Subaqueous environments prone to gravitational mass movements or turbulent underflows. Common in deep-water settings where sedimentation rates are very high and turbidity currents or fluidal mass flows keep particles aloft within the flow body by turbulent suspension. The suspended particles make the flow denser than the surrounding water, causing it to move downslope along the bed and thereby initiating sediment transport by suspended load and bedload. Flow deceleration causes particles to settle out of suspension to produce normal grading or upwards fining. |
| Common structures present | Erosional or scoured bases give way vertically to all or some of the following: (a) massive or normally graded sand or gravel produced by rapid deposition from suspension of the coarsest material with little or no subsequent reworking; (b) planar-laminated sand or parallel lamination produced by sand deposited as bedload in the upper flow regime; (c) ripple cross-laminated sand and silt reflects the downflow migration of ripples during the reworking of (b) and/or the influx of new sediment from suspension during the upper to lower flow regime transition; (d) interlaminated silt and/or clay forms by the settling of suspended fines during the last stages of the flow; and (e) massive clay or silt is created by the gradual settling of residual suspended load in quiet water conditions. ‘Floating’ or outsized clasts can occur in (a) and (b). |
| Principal engineering significance | Anisotropic permeability, prone to liquefaction |
Typical images
Fig. 3.15. (a) Stacked sequence of fining-upwards cycles of gravel-filled scours, planar laminated sands and laminated fine sand and silt, representing relatively coarse-grained turbidites on the distal margins of a former ice-contact delta, Loch Lomond, Scotland (D.J.A. Evans). | |
Fig. 3.15. (b) Coarse-grained, ice-proximal turbidites with basal scours in glaciolacustrine deposits, Lake Pukaki, New Zealand (D.J.A. Evans). | |
| Typical sedimentological description Erosional or scoured base overlain vertically by all or some of the following: (a) massive or normally graded sand or gravel; (b) planar-laminated sand or parallel lamination; (c) ripple cross-laminated sand and silt; (d) interlaminated silt and/or clay; and (e) massive clay or silt. Outsized clasts can occur in (a) and (b). Glacially generated turbidites tend to be coarser than the classi Bouma sequence. | Engineering description Figure 3.15a and b: Erosional basal boundary overlain by all or some of the following: (a) Grey sandy GRAVEL grading upwards to brown gravelly SAND with rare coarse gravel and cobbles; (b) laminated brown silty SAND with rare coarse gravel and cobbles; (c) interlaminated brown SAND and SILT showing cross bedding; (d) interlaminated (0.5–2 mm) brown CLAY and SILT; and (e) structureless brown and grey silty CLAY. |
| Principal references Bouma et al. (1962), Shanmugam (1997), Kneller & Buckee (2000), Mulder & Alexander (2001) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), McMillan et al. (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.2.5 Dropstone mud and plumites/silt and mud drapes (ice-rafted debris) |
| Previous terms | Dropped para-till, iceberg till, iceberg dump till |
| Diagnostic characteristics | Dropstone muds are matrix-supported diamictons with strongly bimodal particle-size distributions, reflecting dominant suspension sedimentation and minor quantities of dropstones. Dropstone muds and drapes form extensive, blanket-like sheets covering pre-existing topography, and can be massive or weakly stratified. Laminations within such deposits are interpreted as the product of the cyclicity of suspension sedimentation from turbid meltwater plumes and are therefore called plumites. Dropstone muds can be differentiated from dropstone diamictons by their clast content, which should be less than 10% clasts per unit area. |
| Environment of formation | Subaqueous depositional environments influenced by ice-contact processes, more specifically in marine settings where interflow and overflow plumes dominate. Dropstone muds and plumites/drapes tend to dominate in more distal settings at the outer reaches of the influence of ice rafting. Turbid meltwater plumes emerge from efflux points below grounded ice and then rise to the surface or to intermediate depths, to be transported for many tens of kilometres from the ice front. Laminated sediments such as dropstone muds and plumites are common in deglaciated fjords and occur sporadically on high-latitude continental shelves, wherever reworking by iceberg scouring is ineffective. |
| Common structures present | Laminations comprise coarse lamina of fine sand or coarse silt fining upwards into a silt or clay lamina. The number of dropstones varies according to the sea-ice conditions prevalent at the time of deposition; where present in laminated sediments they will be associated with deformed and draped bedding. Some laminations may contain no ice-rafted debris due to the dominance of extensive shorefast sea ice. |
| Principal engineering significance | Variable particle sizes, coarse horizons, anisotropic permeability |
Typical images
Fig. 3.16. (a) Laminated silts and clays (plumites) with dropstone in raised glaciomarine deposits, Ellesmere Island, Canada (D.J.A. Evans). | |
Fig. 3.16. (b) Weakly stratified to laminated dropstone muds (includes silts and fine sands) and minor diamicton beds, Duck Hawk Bluffs, Banks Island, Canada (D.J.A. Evans). | |
| Typical sedimentological description Massive to laminated fine sands to clays, which may contain dropstones. Laminations consist of fine sand or coarse silt fining upwards into a silt or clay lamina. | Engineering description Figure 3.16a and b: Generally thickly to thinly interlaminated brown silty fine SAND, SILT and CLAY with rare to occasional subrounded to rounded gravel and cobbles up to 80 mm. |
| Principal references Powell (1984), Syvitski et al. (1987), Hesse et al. (1997), Cowan et al. (1999), Ó Cofaigh & Dowdeswell (2001) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), McMillan et al. (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.2.6 Dropstone diamicton and glaciomarine varves (ice-rafted debris) |
| Previous terms | Dropped para-till, waterlain till |
| Diagnostic characteristics | Dropstone diamictons are matrix-supported diamictons with relatively large concentrations of clasts derived from iceberg rafting and fine-grained matrixes produced by background suspension sedimentation from plumes. They can form extensive, blanket-like sheets covering pre-existing topography, and can be massive or weakly stratified. Dropstone diamictons can be differentiated from dropstone muds by their clast content, which should be greater than 10% clasts per unit area. Where stratified and displaying rhythmic sequences, dropstone diamictons have been termed glaciomarine varves. This reflects the alternations between IRD-poor, fine-grained muds and stratified, matrix-supported diamictons. The laminated muds in such sequences are interpreted as the products of suspension sedimentation from turbid meltwater plumes (see plumites), whereas the stratified diamictons reflect rain-out of IRD combined with suspension settling when iceberg melting is dominant. The diamictons are regarded as winter layers, introduced by iceberg sedimentation, when plumes are suppressed due to reduced meltwater production. |
| Environment of formation | Subaqueous depositional environments influenced by ice-contact processes, more specifically in marine settings in the case of varves, where interflow and overflow plumes dominate and in relatively proximal locations where icebergs release significant quantities of debris. |
| Common structures present | Massive to stratified diamictons with variable quantities of IRD, ranging from individual clasts with associated deformed bedding to clast clusters/lenses created by iceberg dumping. Cyclicity (varves) can be identified in vertical sequences of alternating laminated muds and stratified diamictons. |
| Principal engineering significance | Variable particle sizes, coarse horizons, anisotropic permeability |
Typical images
Fig. 3.17. (a) Massive to crudely stratified diamicton, interpreted as dropstone diamicton, Lake Pukaki, New Zealand (D.J.A. Evans). | |
Fig. 3.17. (b) X-radiographs of laminated glaciomarine sedimentary units from a core in east Scoresby Sund, Greenland showing interstratified mud with sand and silt layers and dropstones, overlain by fine, laminated mud (MacKensen & Marienfeld 2000). | |
| Typical sedimentological description Massive to stratified diamictons with variable quantities of IRD, ranging from individual clasts with associated deformed bedding to clast clusters/lenses created by iceberg dumping. Cyclicity (varves) can be identified in vertical sequences of alternating laminated muds and stratified diamictons. | Engineering description Figure 3.17a: Indistinctly very thinly bedded greyish brown slightly gravelly sandy CLAY. Gravel is sub-angular to rounded of various lithologies; occasionally occurring in clusters. |
| Principal references Powell (1984), Dowdeswell & Dowdeswell (1989), Dowdeswell & Murray (1990), Cai et al. (1997), Cowan et al. (1997), Jaeger & Nittrouer (1999), Smith & Andrews (2000), Ó Cofaigh & Dowdeswell (2001) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), McMillan et al. (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.2.7 Undermelt diamicton |
| Previous terms | Waterlain till, undermelt till, subaquatic melt-out till, subaqueous basal till, grounding-line till, dropped para-till. |
| Diagnostic characteristics | Stratified diamictons containing numerous dropstones, winnowed lags and poorly sorted gravelly lenses. |
| Environment of formation | Subaqueous sedimentation beneath or immediately proximal to floating ice (ice shelves) by sustained rain-out from melting debris-rich ice. Constant reworking by bottom/traction currents, turbidity currents and sediment gravity flows. |
| Common structures present | Significant stratification but discontinuous stratified sediment lenses and large quantities of dropstones. Not always possible to distinguish undermelt diamicton from dropstone diamicton deposited from multiple icebergs using internal characteristics alone. |
| Principal engineering significance | Variable particle sizes, coarse horizons, perched water tables, anisotropic permeability |
Typical image
Fig. 3.18. Stratified diamicton from Duck Hawk Bluffs, Banks Island, Canada (D.J.A. Evans). | |
| Typical sedimentological description Stratified diamicton with large numbers of dropstones. Discontinuous pockets of poorly sorted gravels and sands as well as palimpsest lags. | Engineering description Figure 3.18: Very thinly interbedded brown gravelly SAND and grey slightly sandy CLAY. Frequent sub-angular to rounded fine and medium gravel of various lithologies throughout. |
| Principal references Gibbard (1980), Gravenor et al. (1984), Powell (1984) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), McMillan et al. (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.2.8 Iceberg-contact deposits (ice keel turbate, iceberg dump structures and mounds) |
| Previous terms | Iceberg till, iceberg dump till |
| Diagnostic characteristics | Diamictons and clusters of clasts deposited directly from icebergs without intervening gravitational transport where bergs lodge on the sea or lake bed and debris melts out in situ. Iceberg scouring can also modify pre-existing depositional structures, creating a massive, structureless diamicton called ice-keel turbate. |
| Environment of formation | Ice-contact, largely proximal subaqueous environments heavily influenced by icebergs. Deposition is directly from grounded icebergs as debris melts out in situ or from icebergs rolling over and rapidly dumping debris from their surfaces. Iceberg scouring can also modify pre-existing deposits and initiate re-suspension of fine-grained sediment. Icebergs can also initiate turbid underflows to produce turbidites (see turbidites). |
| Common structures present | Iceberg dump structures/mounds are planar-based mounds or symmetrical cones of coarse materials, which can occur in isolation, offlap each other or be stacked vertically and inter-stratified with finer-grained sediments. Multiple dumping events due to repeated overturning of, or basal melt-out from, the same iceberg, will produce more complex features. Some relatively coarse-grained infills within finer-grained rhythmite sequences, where boundaries are subvertical, often fault delineated and sharply defined, can represent the locations of iceberg melt-out often grounding and partial burial in rapidly aggrading lake or ocean floor deposits. Ice keel turbate can be very difficult to differentiate from non-stratified dropstone diamictons, some glaciotectonites and subglacial tills on internal characteristics alone; stratigraphic context needs to be considered. |
| Principal engineering significance | Variable particle sizes, anisotropic permeability |
Typical images
Fig. 3.19. (a) Poorly sorted gravel within a fault-defined infill of silt/sand rhythmites in proximal glaciolacustrine deposits, created by debris-rich iceberg melt-out, Lake Pukaki, New Zealand (D.J.A. Evans). | |
Fig. 3.19. (b) Iceberg dump mound in the proximal bottomsets of a former ice-contact delta, Loch Lomond, Scotland (D.J.A. Evans). | |
| Typical sedimentological description Mounds consist of poorly sorted gravels with bedding dipping outwards from the cone centre, or comprise a central core of clast-rich diamicton grading upwards and outwards to poorly sorted gravel. Melt-out traces of debris-charged icebergs are typically relatively coarse-grained infills within finer-grained rhythmite sequences. They are defined by subvertical, often fault-delineated boundaries with surrounding sediments. Iceberg grounding and ploughing produce small-scale folds and low-angle thrusts and thrust slices of pre-existing sediment. Fault surfaces can be polished and slickensided. Floating lake or sea ice can produce ‘soft sediment striations’, which are grooves and intervening flutes with a relief of about 1 cm and superimposed with millimetre-scale striae. | Engineering description Figure 3.19b: Conical mounds of silty sandy GRAVEL with bedding dipping out from centre of cone. Cone core comprises sandy clayey GRAVEL grading upwards to sandy silty GRAVEL with low to medium cobble content. |
| Principal references Vorren et al. (1983), Thomas & Connell (1985), Woodworth-Lynas & Guigné (1990), Marienfeld (1992), Dowdeswell et al. (1994), Eden & Eyles (2001), Winsemann et al. (2003), Eyles et al. (2005) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), McMillan et al. (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.2.9 Glaciogenic debris flow (debrites)/subaqueous slide and slump deposits (including cohesive and cohesionless) |
| Previous terms | Subaquatic flow till, submarine flow till |
| Diagnostic characteristics | Debris-flow deposits are created by sediment gravity flows, which result from the flowage of concentrated sediment-water mixtures. Subaqueous slides and slumps occur whenever slopes fail along internal shear planes and undergo downslope transport. Debris flows, slides and slumps can undergo transformations during transport depending on whether or not they lose or gain fluid, resulting in lateral and vertical changes in sedimentary characteristics. For example, a cohesive debris flow can become liquefied if it enters a water body and hence becomes disaggregated or fluidal flows (see turbidite). Indeed, sediment gravity flows lie on a process-form continuum, with cohesive debris flows at the relatively better-drained end (see supraglacial mass-flow diamicton) and turbidites at the saturated end of the continuum. Subaqueous debris flows are traditionally subdivided into cohesive and cohesionless varieties based upon the presence or absence of matrix cohesion. Cohesive flows contain some clay matrix so that they act as a fluid with cohesive strength within which large clasts, boulders and blocks of pre-existing sediment can be rafted. The upper part of the flow commonly consists of a semi-rigid ‘plug’ riding passively on a thin basal shear zone, but the flow top can become diluted due to turbulent mixing with the overlying water column, thereby causing a transformation into a turbulent underflow. Cohesionless debris flows are high-concentration, non-turbulent flows which tend to occur in sandy and gravelly materials, with grains being supported during transport by a variety of dispersive pressure, buoyancy effects, liquefaction and fluidization. At the liquefied end of the cohesionless debris-flow continuum are grain flows, fluidized flows and liquefied flows, overlapping with fall deposits created on steeper slopes (see subaqueous debris-fall deposits). Deposition from a cohesionless flow will occur when grain suspension ceases and grains begin to settle out and regain contact with each other, displacing porewater upwards. This causes flows to shut down or ‘freeze’ from the bottom upwards and to develop water-escape and -loading structures. |
| Environment of formation | Subaqueous sediment gravity flows, slides and slumps are created in glaciogenic settings wherever sedimentation rates are high, specifically at the ice-water interface around meltwater efflux points (i.e. ice-contact fans, grounding line fans, morainal banks) and on rapidly aggrading depocentres (i.e. deltas, trough-mouth fans and other offshore glaciomarine sediment piles, supraglacial lake basins). Deposition in glaciolacustrine and glaciomarine settings results in the rapid, strongly episodic transfer of large amounts of debris, which can build up thick sediment successions. Cohesionless flows can be triggered by sudden elevations in porewater pressure associated with transient shocks, such as iceberg calving events, earthquakes and sudden loading by overpassing sediment. Liquefaction and hydroplaning of cohesionless mass flows on submarine continental margins can result in long run-out distances and thereby create ‘contourites’. |
| Common structures present | Subaqueous debris flows produce diamictons that are often difficult to differentiate from subglacial tills and some glaciotectonites, due to the mixing of heterogeneous sediment sources to create a continuum from stratified to massive diamictons. Numerous flow deposits can be stacked in thick complex sequences, the base of each flow often marked by an erosional scour in pre-existing sediment or previous flow deposits. Cohesive debris-flow deposits are typically composed of two tiers comprising the basal plug zone of massive, matrix-supported diamicton or muddy sands and gravels, often containing large clasts and/or soft-sediment rafts, passing upwards into normally graded and stratified gravels, sands and silts, deposited from turbulent flows. The base of the plug zone can also be marked by a sheared horizon, identified by the presence of attenuated inclusions/wisps of underlying sediment and shear planes. Normal or inverse grading can also be present in this zone; normal grading reflects the sinking of large particles through weak, shearing sediment, whereas inverse grading is due to a combination of upwards clast migration due to shearing and a sieving effect. Cohesionless debris-flow deposits are typically dipping, sheet-like masses often with erosional/channel scoured bases or gradational (welded) contacts where the flow has incorporated underlying material. The resulting debrites typically range from well to poorly sorted gravel, sand or pebbly sand. Internally, each flow units may be massive or exhibit inverse or inverse-to-normal grading. Particle fabrics in gravelly cohesionless debris flows commonly show preferred a-axis orientations parallel to flow, and up-flow a-axis imbrication. Dewatering can produce water-escape structures such as pipes and ball and pillow structures. Slides and slump deposits form sheet-like masses from decimetres up to 100 m thick. In slides, the internal beds are mainly undisturbed, but may exhibit compressional or tensional deformation near the toe and head zones, respectively. Slumps display more extensive evidence of internal deformation, consisting of overfolds, box folds, imbrication of soft clasts and branching thrust faults. Disruption followed by only short-distance transport results in gravity-induced soft sediment deformation (i.e. sediment melange, flow structures and attenuation features). |
| Principal engineering significance | Shear surfaces, anisotropic strength, anisotropic permeability |
Typical images
Fig. 3.20. (a) Stacked subaqueous mass-flow deposits and intervening discontinuous beds of gravelly sands and clast lags (cohesive debris flows), Loch Quoich, Scotland (D.J.A. Evans). | |
Fig. 3.20. (b) Lensate bodies of clast-rich, matrix-supported diamictons separated by discontinuous partings, clast lags and gravel pockets, indicative of subaqueous cohesive debris flows, Coosderrig, Ireland (D.J.A. Evans). | |
Fig. 3.20. (c) Gravelly mass-flow diamictons separated by discontinuous lenses of rhythmically bedded sands and silts indicative of cohesionless subaqueous debris flows and their late-stage turbulent underflow deposits, Connemara, Ireland (D.J.A. Evans). | |
| Typical sedimentological description Stratified to massive diamictons that are often difficult to differentiate from subglacial tills and some glaciotectonites. Stratification and sorting improves towards the cohesionless end of the debris-flow continuum. Tend to be arranged in stacked sequences, with each flow unit possessing an erosional (scoured) base and/or separated by winnowed lags or gravelly sand lenses. Cohesive debris-flow deposits are typically two-tiered: (a) The basal zone = massive (often sheared), matrix-supported diamicton or muddy sands and gravels, often containing large clasts and/or soft-sediment rafts and sometimes characterized by normal or reverse grading; and (b) the upper zone = normally graded and stratified gravels, sands and silts. Cohesionless debris-flow deposits are typically dipping, sheet-like masses, often with erosional/channel scoured bases or gradational (welded) contacts, and typically range from well to poorly sorted gravel, sand or pebbly sand. Each flow unit may be internally massive or exhibit inverse or inverse-to-normal grading. Particle fabrics commonly show preferred a-axis orientations parallel to flow, and up-flow a-axis imbrication. Dewatering can produce water-escape structures such as pipes and ball and pillow structures. Slides and slump deposits are sheet-like masses from decimetres up to 100 m thick. Internal structures are typically either mainly undisturbed or deformed, displaying compressional or tensional structures depending on head or tail zone location as well as gravity-induced soft sediment deformation features. | Engineering description Figure 3.20a and b: Structureless grey or brown slightly sandy gravelly CLAY. Gravel is sub-angular to rounded of various lithologies. Occasional discontinuous lenses (50 × 250 mm) of indistinctly thinly laminated silty SAND. Figure 3.20c: Structureless grey or brown slightly sandy gravelly to very gravelly CLAY with frequent cobbles. Gravel and cobbles are sub-angular to rounded of various lithologies. Occasional discontinuous lenses (50 × 250 mm) or very thin beds of indistinctly thinly laminated silty SAND. |
| Principal references Lowe (1976), Johnson & Rodine (1984), Nemec & Steel (1984), Postma (1986), Prior & Bornhold (1988), Nemec (1990), Ghibaudo (1992), Nemec et al. (1999), Eyles & Eyles (1992), Laberg & Vorren (2000), Mulder & Alexander (2001), Hiemstra et al. (2004) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), McMillan et al. (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.2.10 Subaqueous debris-fall deposits (including olistostromes) |
| Previous terms | |
| Diagnostic characteristics | In debris falls (often also referred to as ‘grain falls’, although this term also implies suspension sedimentation), clasts fall individually or as masses of strongly dispersed particles. Each particle is driven in a downslope direction by its own momentum, and will bounce, slide or roll when impacting the bottom. This gives rise to downslope coarsening and a fining-upwards or normally graded sequence due to the tendency for larger particles to be deposited first and then to be overrun by the finer-grained tail of the avalanche. The term ‘olistostrome’ is used for very-large-scale debris falls in the geological record in which a range of materials are liberated from pre-existing sediment piles to produce mélanges, a scale of failure that is possible in large glacially influenced marine basins for example. |
| Environment of formation | On steeper subaqueous slopes that are aggrading rapidly due to the high sedimentation rates associated with ice-proximal or ice-contact settings. The extreme relief in such locations causes cohesionless flows to operate as debris falls. Gravel foreset beds, typical of ice-contact and glacier-fed deltas as well as grounding-line fans, may also operate in this way. |
| Common structures present | Debris-fall deposits exhibit downslope coarsening or fall sorting due to the tendency for the larger particles with most momentum to travel furthest. Normal grading is also common in relation to this fall sorting. Often difficult to identify in ancient deposits due to the debris-fall material becoming enveloped by later fine-grained sediments, thereby causing them to resemble diamictic debris flows. Diagnostic criteria in this instance might be isolated clasts overlain by layered drapes of fine sediment, indicating that the fine drape was deposited after the clasts. Load structures or plough marks might indicate that the fall material has come to an abrupt rest in pre-existing sediments. |
| Principal engineering significance | Shear surfaces, anisotropic strength, anisotropic permeability |
Typical image
Fig. 3.21. Sequence of fining-upwards (normally graded) gravelly clinoforms from glacial lake sediments. Infilling of void spaces with sand could be indicative of post grain fall sedimentation or operation as a cohesionless mass flow, Lake Pukaki, New Zealand (D.J.A. Evans). | |
| Typical sedimentological description Downslope (down clinoform) coarsening or fall sorting as well as normal grading of individual falls. Difficult to identify in ancient deposits due to the debris-fall material becoming enveloped by later fine-grained sediments, thereby causing them to resemble diamictic debris flows. Load structures or plough marks can indicate that the fall material has come to an abrupt rest in pre-existing sediments. | Engineering description Figure 3.21: Medium beds of slightly sandy GRAVEL fining upwards to sandy fine GRAVEL. Gravel at base is sub-angular to well-rounded closely packed. Gravel at top is sub-angular. |
| Principal references Nemec (1990), Bornhold & Prior (1990) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), McMillan et al. (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.2.11 Palimpsest lags |
| Previous terms | Boulder pavements |
| Diagnostic characteristics | Concentrations of clasts occurring in discrete horizons in stratified sediment sequences. |
| Environment of formation | Shallow glaciolacustrine and glaciomarine environments where currents can rework bottom materials by transporting away fine-grained sediments, leaving coarse lags. This is especially effective during periods of non-deposition or reduced sediment influx and hence lags can represent significant hiatuses in the stratigraphic record. |
| Common structures present | Discrete horizons/concentrations of clasts either within sediments units or most commonly between them. |
| Principal engineering significance | Variable particle sizes, coarse horizons, anisotropic permeability |
Typical image
Fig. 3.22. Cobble to boulder palimpsest lag within stacked sequence of gravelly subaqueous debris-flow deposits and dropstone diamictons, Lake Coleridge, New Zealand (D.J.A. Evans). | |
| Typical sedimentological description Horizon of clasts lying between, or less commonly within, fine-grained laminated sediments, dropstone muds or massive mud drapes. Can be differentiated from clast pavements in tills by their lack of bevelled/facetted tops and unidirectional striae. | Engineering description Figure 3.22: Grey sandy generally rounded GRAVEL of various lithologies, overlying 100 mm layer of rounded COBBLES. Variable amount of matrix in cobble layer, probably washed in from above, and not always present. Underlain by brown slightly sandy slightly gravelly CLAY with low cobble content. |
| Principal references Powell (1984), Eyles & Lagoe (1990) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), McMillan et al. (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.3.1 Hyperconcentrated flow deposits (jökulhlaup-type flood deposits) |
| Previous terms | |
| Diagnostic characteristics | Hyperconcentrated flows are transitional between debris flows and normal stream flows and particles are maintained in suspension by fluid turbulence. The flows can be homogeneous or be composed of an upper, low-concentration component and a lower, coarse-grained, high-concentration ‘carpet’. Deposition of this material occurs when shear stresses and flow velocities fall in association with a reduction in stream gradient and/or flow depth. This creates partial or wholesale stalling (‘freezing’) or locking of the suspension, reflecting mass transport and rapid deposition. |
| Environment of formation | Glaciofluvial settings where meltwater discharges are very high (i.e. jökulhlaup situations) and loose sediment is readily available. This creates very high suspended sediment concentrations or hyperconcentrated flows of mixed grain size. |
| Common structures present | Hyperconcentrated flow deposits occur in sheets or channelized lenses and comprise matrix-supported gravels or diamictons often containing rip-up clasts. The latter can be composed of a variety of materials derived from pre-existing deposits eroded by the flood waters, but are commonly represented by fragments of local tills. Individual flow units may be entirely massive or display internal structures, such as crude stratification and basal inverse- or inverse-to-normal grading. The stratification is usually crude horizontal or cross-bedding, likely reflecting pulsed deposition from surges in the flow. There is also commonly an upwards increase in evidence of fluvial processes (e.g. better sorting, well-developed bedding and cross-bedding, winnowed gravels) resulting from the transition from hyperconcentrated to normal stream flow during falling discharges. Clast fabrics in the massive parts of hyperconcentrated flow deposits tend to show preferred a-axis orientations parallel to flow and up-flow a-axis imbrication. |
| Principal engineering significance | Variable particle sizes, anisotropic permeability |
Typical image
Fig. 3.23. Matrix-supported gravels in a vertical sequence of outwash deposits, recording a hyperconcentrated flow event, Daviot, Scotland (D.J.A. Evans). | |
| Typical sedimentological description Sheets or channelized lenses containing matrix-supported gravels or diamictons, often with rip-up clasts. Individual flow units may be entirely massive or display internal structures, such as crude horizontal stratification or cross-bedding, and basal inverse- or inverse-to-normal grading. Upwards increase in evidence of fluvial processes (e.g. better sorting, well-developed bedding and cross-bedding, winnowed gravels) is common. Clast fabrics in the massive units tend to show preferred a-axis orientations parallel to flow and up-flow a-axis imbrication. | Engineering description Figure 3.23: Brownish grey very gravelly slightly sandy CLAY with medium cobble and low boulder content. Gravel, cobbles and boulders are subrounded to well-rounded of various lithologies. Occasional interbeds of indistinctly laminated brown clayey SAND. |
| Principal references Maizels (1989a, b), Todd (1989), Russell & Marren (1999), Mulder & Alexander (2001), Benvenuti & Martini (2002), Russell & Knudsen (2002), Marren (2005) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), McMillan et al. (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.3.2 Gravel rhythmites (flood facies) |
| Previous terms | |
| Diagnostic characteristics | Coarse-grained stratified sediments that display rhythmic or cyclic repetition of beds, related specifically to alternations between dominant grain size distributions. The term rhythmite is a non-genetic term used to refer to a range of deposits that display cyclic alternations in bedding, but is often replaced when the exact origins of the rhythms are known (see rhythmites, varves, turbidites, cyclopels and cyclopsams). Gravel rhythmites are the coarse-grained equivalents of these deposits and have also been called ‘coarse-grained turbidites’ in order to convey their pulsed delivery by hyperconcentrated flows. |
| Environment of formation | Proglacial and glacier submarginal environments affected by outburst floods or jökulhlaups. They are relatively rare but have been reported from glaciofluvial systems such as jökulhlaup-fed sandar and subglacial tunnels where waves of hyperconcentrated flows deliver coarse-grained outwash sediments to slack-water areas. |
| Common structures present | Cyclic grain size variations that reflect alternating variations in sediment supply and depositional conditions. Coarser-grained gravel rhythmites have been observed in jökulhlaup-fed glaciofluvial deposits where they comprise alternating coarser and fine couplets (e.g. proximal pebbles and granules or distal cross-bedded sands and granules). They have also been termed coarse-grained rhythmites (turbidites) whey they infill slack water areas to form granule to cobble gravels with outsized clasts interspersed with granule to sandy gravels. |
| Principal engineering significance | Coarse horizons, anisotropic permeability |
Typical images
Fig. 3.24. (a) Coarse-grained rhythmites (turbidites) deposited during the 1996 Skeiðarárjökull jökulhlaup, Iceland (Russell & Knudsen 1999). | |
Fig. 3.24. (b) Giant gravel rhythmite produced by glacial lake outburst floods, Altai Mountains, Siberia (P.A. Carling). | |
Fig. 3.24. (c) Gravel rhythmites (flood facies), former Bonne Glacier, Pont de Ponsonnas, France (D. Giles). | |
Fig. 3.24. (d) Gravel rhythmites (flood facies), former Bonne Glacier, Pont de Ponsonnas, France (D. Giles). | |
| Typical sedimentological description Rhythmic sequence of coarse-grained stratified sediments defined by cyclic alternations between dominant grain size distributions. Gravel rhythmites typically comprise alternating coarser and fine couplets, the grain sizes of which will vary but generally can be related to either proximal or distal sedimentation. Proximal settings produce coarse couplets of boulder- to pebble-sized gravels and fine couplets of cobble- to granule-sized gravels. Distal settings will tend to record the same cyclicity but in couplets typically dominated by granule gravels and cross-bedded sands. A common attribute of proximal gravel rhythmites is the occurrence of outsized clasts produced where the hyperconcentrated flow immobilizes. | Engineering description Figure 3.24a: Dark grey to black sandy GRAVEL with low cobble content. Gravel and cobbles are rounded to well-rounded of various lithologies. With medium-spaced very thin beds of dark brownish grey silty SAND. Figure 3.24c and d: Dark grey to black sandy GRAVEL with low cobble content. Gravel and cobbles are rounded to well-rounded of various lithologies. Crude interbedding with medium spaced very thin beds of dark brownish grey silty SAND. |
| Principal references Smith (1993), Russell & Knudsen (1999), Carling et al. (2009) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), McMillan et al. (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.3.3 Plane bed deposits |
| Previous terms | |
| Diagnostic characteristics | One of the four basic fluvial bedform types (also includes ripples, dunes and antidunes), which are patterns in the bedload representative of a dynamic equilibrium response of the bed to prevailing flow conditions. They are repetitive, mobile structures that migrate down-current. |
| Environment of formation | Any fluvial system where the sandy bedload is modified by the flow velocity. Plane beds are created by net deposition of sediment in beds of horizontally bedded or laminated sand, with the laminae recording minor fluctuations in flow velocity or sediment supply. |
| Common structures present | Horizontally bedded or laminated very fine to coarse sand. Upper flow regime conditions create ‘parting lineation’ or bedding planes that display thin, linear grooves and ridges aligned parallel to the former flow direction. Lower flow regime conditions also produce horizontally bedded and laminated sands, but the sand tends to be coarse to very coarse and parting lineation is absent. |
| Principal engineering significance | Anisotropic permeability |
Typical image
Fig. 3.25. Plane bedded sand with dropstone in glaciomarine sediments, Ellesmere Island, Canada (D.J.A. Evans). | |
| Typical sedimentological description Horizontally bedded or laminated very fine to very coarse sand. | Engineering description Figure 3.25: Thinly to thickly laminated greyish brown and black fine to coarse SAND. With rare (1 No) well-rounded coarse gravel. |
| Principal references Allen (1982, 1983, 1985), Ashley (1990), Miall (1996) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), McMillan et al. (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.3.4 Cross-bedded facies (dunes and antidunes) |
| Previous terms | |
| Diagnostic characteristics | Two of the four basic fluvial bedform types (also includes ripples and plane beds), which are patterns in the bedload representative of a dynamic equilibrium response of the bed to prevailing flow conditions. They are repetitive, mobile structures that migrate down-current in response to the erosion of sediment from their upstream faces (high shear stress) and the deposition of sediment on their lee faces (low shear stress). |
| Environment of formation | Any fluvial system where the bedload is modified by the flow velocity. Although they are commonly developed in sandy bedload, gravel dunes and antidunes are also common in glaciofluvial systems where the high discharges are capable of mobilizing larger grain sizes as bedload, especially during jökulhlaup conditions. |
| Common structures present | The migration and vertical accretion of gravel and sands sheets to produce dunes gives rise to cross-bedded sands and gravels. Two principal types of structure are apparent. (1) ‘Planar cross-bedded gravels and sands’ are produced by the migration of transverse, 2D dunes with approximately planar lee surfaces. They can also be created by the growth of small deltas from bars into deep channels and gravel foresets are common in the ‘megadunes’ produced during jökulhlaup-scale discharges. (2) ‘Trough cross-bedded gravels and sands’ are formed by the migration of 3D dunes or 2D dunes with scalloped lee faces. Cross-beds have curved surfaces, concave-up and concave downstream, reflecting deposition in lee-side hollows. Simple trough cross-bedding occurs as solitary scoops eroded into other facies, whereas more complex ‘festoon cross-bedding’ forms co-sets or multiple sets of cross-cutting units. Planar and trough cross-beds can be cross-cut by low-angle erosion surfaces called reactivation surfaces, which may be planar, curved or irregular. Where water flow reaches supercritical, specific bedforms called antidunes are created by low-amplitude sinusoidal waves which are in phase with water surface waves. The migration of antidunes forms foresets or backsets, depending on the direction of bedform migration. |
| Principal engineering significance | Groundwater, high-permeability zones |
Typical images
Fig. 3.26. (a) Stacked sequence of planar bedded gravels (clinoforms) recording bar migration in ice-contact outwash. Gravel sets are separated by cross-bedded to laminated sands which record low-flow stages, Stranraer, Scotland (D.J.A. Evans). | |
Fig. 3.26. (b) Cross-bedded gravels and sands, Villeneuve, Alberta, Canada (D.J.A. Evans). | |
Fig. 3.26. (c) Cross-bedded gravels and sands with scour and fill sequences, Kells, Ireland (D.J.A. Evans). | |
| Typical sedimentological description Dunes are recognizable as cross-bedded sands and gravels, either as planar cross-bedded gravels and sands or as trough cross-bedded gravels and sands. The former can also include gravel foresets in flood megadunes. The latter can be manifest as simple trough cross-bedding or complex, festoon cross-bedding. Cross-beds can be cross-cut by low-angle erosional, planar, curved or irregular, reactivation surfaces. Antidunes are wave-like hummocks, recognizable by their internal foresets or backsets. | Engineering description Figure 3.26a, 3.26b, 3.26c: Cross-laminated to very thinly cross-bedded fine SAND and sandy GRAVEL. |
| Principal references Boothroyd & Ashley (1975), Miall (1977a, b), Allen (1982, 1985), Ashley (1990), Rust & Gibling (1990), Carling (1996), Alexander et al. (2001), Carling et al. (2009) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), McMillan et al. (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.3.5 Ripple cross-laminations (including climbing ripple drift) |
| Previous terms | |
| Diagnostic characteristics | One of the four basic fluvial bedform types (also includes plane beds, dunes and antidunes), which are patterns in the bedload representative of a dynamic equilibrium response of the bed to prevailing flow conditions. They are repetitive, mobile structures that migrate down-current in response to the erosion of sediment from their upstream faces (high shear stress) and the deposition of sediment on their lee faces (low shear stress). |
| Environment of formation | Any fluvial system or shallow subaqueous (glaciolacustrine or glaciomarine) setting where the sandy bedload is modified by the flow velocity. Ripples are created by the deposition of sand by the migration and vertical accretion of current ripples in the lower flow regime. Vertical sequences of ripples display distinctive internal structures called ‘ripple cross-lamination’, the form of which depends on the balance between down-current migration and suspension sedimentation during ripple formation. |
| Common structures present | There are three principal types of cross-lamination. Type A are created when more sediment is eroded from the stoss sides than is added from suspension. This results in successive lee-side positions of the ripples being preserved as cross-laminae that dip down-current, but the stoss positions being removed by net erosion. As a result, individual sets of lee-side laminae are separated by diachronous erosion surfaces that dip up-current. The angle of climb of these surfaces varies according to the net amount of deposition from suspension averaged over the entire bedform and also creates specific structures. If the angle of climb is zero ripples record migration only, giving rise to ‘starved ripples’. If the angle of climb is identical to the angle of the original stoss faces, the ripples record an exact balance between deposition from suspension and erosion on the stoss face of the bedform, giving rise to ‘critical cross lamination’. Type B records successive stoss face as well as lee face positions, and records net deposition from suspension over the whole bedform, although net deposition is still highest on the lee faces. Type S, also known as ‘sinusoidal cross-lamination’ or ‘draped lamination’, records only small variations in thickness between stoss-side and lee-side laminae and a very steep angle of climb; ripple profiles are therefore only weakly asymmetric or symmetrical. This represents small or no ripple migration due to weak or zero current and a dominance of suspension sedimentation. Vertical gradations between these ripple types record temporal changes in current flow and suspended sedimentation rates. |
| Principal engineering significance | Groundwater, high-permeability zones |
Typical images
Fig. 3.27. (a) The production of ripple cross-lamination. Ripples climb at angle θ whose tangent is the mean aggradation rate Vy divided by the downstream migration rate Vx (from Ashley et al. 1982). | |
Fig. 3.27. (b) Climbing ripple drift, showing predominantly Type A cross-lamination in glaciolacustrine deposits, Drumheller, Alberta, Canada (D.J.A. Evans). | |
Fig. 3.27. (c) Type B cross-lamination in esker deposits, Teesdale, England (D.J.A. Evans). | |
| Typical sedimentological description Type A: Cross-lamination shows successive lee-side positions of ripples preserved as cross-laminae that dip down-current, with no stoss-side deposits. Sets of lee-side laminae are separated by diachronous erosion surfaces that dip up-current. Zero angles of climb result in starved ripples. Identical angles of climb and angle of stoss face result in critical cross-lamination. Type B: Cross-lamination shows successive stoss face as well as lee face positions. Type S: Sinusoidal cross-lamination or draped lamination, where ripples are only weakly asymmetric or symmetrical. | Engineering description Figure 3.27b: Light brown ripple bedded silty fine SAND. Figure 3.27c: Light grey wavy bedded silty fine SAND. |
| Principal references Jopling & Walker (1968), Ashley et al. (1982), Allen (1985) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), McMillan et al. (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.5.3.6 Gravel sheets |
| Previous terms | |
| Diagnostic characteristics | Gravel sheets accumulate through clast-by-clast accretion on low-relief parts of river beds, normally in longitudinal and bank-attached bars. Sediment transport and deposition are strongly episodic and take place during peak flows and subsequent waning flows. At other times gravel sheets are usually exposed above water level and can be superimposed by overbank fines or sandy bedforms. Steeper dipping bedding (clinoforms) can develop during jökulhlaups, when gravel megabars are deposited. |
| Environment of formation | Gravel-bed rivers where water current flow is episodically sufficient to move gravel as bedload. Particularly well developed in proximal to intermediate reaches of glaciofluvial systems. |
| Common structures present | Gravel sheets comprise massive to crudely bedded clast-supported gravels, with the clast framework being determined by grain-by-grain deposition from bedload. Fine gravel particles can become trapped in the spaces between larger clasts, creating a poorly sorted deposit, although crude horizontal stratification can develop during fluctuating flows. Clast clusters can develop where they are trapped upstream of obstructions. Fining-up units can record accumulation to shallower water levels, whereas coarsening-up units record downstream bar migration. Openwork structures are observed but more commonly a fine matrix will occupy the spaces between clasts as a result of filtering of sand or silt following gravel deposition. Clast fabrics typically show upstream imbrication of a–b planes, and a preferred orientation of a-axes transverse to flow. Jökulhlaup conditions will give rise to the development of giant bars within which large-scale gravel cross-bedding or even foresets will develop on slipfaces. |
| Principal engineering significance | Anisotropic permeability, groundwater, perched water tables |
Typical images
Fig. 3.28. (a) Stacked gravel sheets, Villeneuve, Alberta, Canada (D.J.A. Evans). | |
Fig. 3.28. (b) Fine to medium gravel sheets and intervening planar bedded sands, Duck Hawk Bluff, Banks Island, Canada (D.J.A. Evans). | |
| Typical sedimentological description Massive to crudely bedded, commonly poorly sorted, clast-supported gravels, often displaying crude horizontal stratification. Can include clast clusters and fining-up units (shallowing water depths) or coarsening-up units (downstream bar migration). Can display openwork structure or more commonly a fine matrix support. Clast fabrics typically show upstream imbrication of a–b planes, and a preferred orientation of a-axes transverse to flow. Jökulhlaup can create giant bars containing large-scale gravel cross-bedding or foresets. | Engineering description Figure 3.28a: Very thinly interbedded laminated brown fine SAND and grey sandy fine and medium sub-angular to subrounded GRAVEL of various lithologies. Figure 3.28b: Medium to thinly interbedded laminated brown fine SAND and grey sandy fine and medium sub-angular to subrounded GRAVEL of various lithologies. |
| Principal references Boothroyd & Ashley (1975), Rust (1975), Miall (1977a, b, 1985), Brayshaw (1984), Collinson (1996) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), McMillan et al. (2000), Reeves et al. (2006a, b) | |
3.6 Periglacial sediment descriptors
Periglacial sediments are described and illustrated in this section in a similar way to glaciogenic sediments in Section 3.5. Periglacial sediments are widespread in regions such as the UK and NW Europe, particularly on frost-susceptible soils or rocks, even in glaciated terrains. As the examples below show, the thickness of periglacial sediment sequences may vary from <1 to 20 m or more, particularly where granular head deposits or slopewash deposits accumulate near the foot of hillslopes. Photographs show well-developed examples of the sediments. Table 3.7 outlines the developed naming conventions in accordance with BS 5930 (anon) for periglacial deposits for the geological origin and geological qualifier for origin.
| Sediment name | 3.6.1.1 Granular head deposits |
| Previous terms | Head |
| Diagnostic characteristics | Clasts (pebbles to boulders) set within a sandy to silty matrix. Completely remoulded. |
| Environment of deposition | Periglacial hillslopes and valley bottoms. Debris derives typically from local bedrock and/or Quaternary deposits. In chalk downlands, granular head deposits are termed ‘coombe deposits’ or ‘coombe rock’. |
| Common structures present | Strong downslope alignment of elongate clasts, commonly imbricated (depositional clast fabric). Vertical to steeply dipping elongate clasts in places (post-depositional clast fabric caused by frost heave). |
| Principal engineering significance | Variable particle sizes, coarse horizons, anisotropic permeability, anisotropic strength, anisotropic stiffness, high-permeability zones, perched water tables, shear surfaces, low shear strength |
Typical images
Fig. 3.29. (a) Granular head deposits exposed in coastal cliffs. Elongate clasts are strongly aligned, with their long axes protruding from the cliff face. Section between Lannacombe Bay and Start Point, Devon, England (J. Murton). | |
Fig. 3.29. (b) Granular head deposits in coastal cliffs. Head deposits are stratified and overlie a shore platform. Wembury Bay, Devon, England (J. Murton). | |
| Typical sedimentological description Figure 3.29a: Massive, matrix-supported to clast-supported diamicton; angular to subrounded granules to boulders of mica schist, elongate ones oriented downslope. Figure 3.29b: Stratified matrix-supported diamicton and pebbly silt; angular to subrounded granules to cobbles of Dartmouth Group, elongate ones oriented downslope; subhorizontal strata few centimetres to few tens of centimetres thick, parallel to land surface (head apron). | Engineering description Figure 3.29a: Brown slightly sandy very gravelly SILT with medium cobble and low boulder content. Gravel, cobbles and boulders (up to 500 mm) are elongate to tabular, sub-angular strong of mica schist. Elongate particles dip 10° downslope. Figure 3.29b: Reddish brown becoming light reddish brown at base, thinly to thickly bedded slightly sandy gravelly SILT. Occasionally with low cobble content, cobbles are tabular flat-lying angular to subrounded of Dartmouth Group. |
| Principal references Harris (1987, 2013), Scourse (1987), Hutchinson (1991), Ballantyne & Harris (1994), Bates et al. (2003), Knight (2005), Murton et al. (2015) | |
| Engineering geology case studies Higginbottom & Fookes (1970), Grainger & Harris (1986), Culshaw et al. (1991), Hutchinson (1991), Bell (2000), Croot & Griffiths (2001), Reeves et al. (2006a, b) | |
| Sediment name | 3.6.1.2 Clay-rich head deposits |
| Previous terms | Head |
| Diagnostic characteristics | Reworked (partially remoulded) clayey debris. |
| Environment of deposition | Hillslopes underlain by clay-rich bedrocks (e.g. London Clay Formation, Lias Group clays, Atherfield Clay Formation, Gault Formation clays). |
| Common structures present | Lithorelicts of original clay fabric are incorporated within a matrix of soft clay. Fragments of more resistant rocks from upslope and sometimes non-local clasts derived from adjacent sediments. Shear surfaces occur within or beneath clay-rich head deposits. |
| Principal engineering significance) | Ground movement, shear surfaces, swell/shrink potential, low shear strength, perched water tables |
Typical images
Fig. 3.30. (a) Lithorelicts in a soft clay matrix, clayey mudslide debris. Sediment core is c. 100 mm from top to bottom. Sevenoaks, Kent, England (J.N. Hutchinson). | |
Fig. 3.30. (b) Basal shear associated with clay-rich head deposits, scale in inches. Sevenoaks, Kent, England (J.N. Hutchinson). | |
| Typical sedimentological description Figure 3.30a: Massive matrix-supported diamicton, with locally derived clay lithorelicts within a soft clay matrix. Figure 3.30b: Polished, striated shear surface in clay. | Engineering description Figure 3.30a: Soft grey very gravelly CLAY. Gravel is generally angular fine to coarse of extremely weak mudstone. Figure 3.30b: Shear surface is planar smooth along striations and highly polished. |
| Principal references Harris (1987, 2013) | |
| Engineering geology case studies Weeks (1969), Chandler (1970a, b, 1972a), Higginbottom & Fookes (1970), Harris (1977), Culshaw et al. (1991), Hutchinson (1991, 2010), Skempton & Weeks (1976), Spink (1991), Skempton et al. (1991), Bell (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.6.1.3 Slopewash deposits |
| Previous terms | Chalk meltwater deposits (Evans 1968) (Overland flow may derive from a number of sources beside meltwater, e.g. summer rain, return flow). |
| Diagnostic characteristics | Thinly stratified and sorted deposits of silt, sand or pebbles. |
| Environment of deposition | Usually located on or adjacent to lower hillslopes, commonly interbedded with head deposits. May form discrete fans. |
| Common structures present | Thin stratification (few millimetres to several centimetres), more or less parallel and gently dipping downslope. |
| Principal engineering significance | Variable particle sizes, low shear strength, shear surfaces, coarse horizons, groundwater |
Typical images
Fig. 3.31. (a) Stratified silty and chalky slopewash deposits overlying coombe rock (chalky granular head deposits), Black Rock, Brighton, East Sussex, England (J. Murton). | |
Fig. 3.31. (b) Stratified slopewash deposits. Image is c. 1 m wide. Black Rock, Brighton, East Sussex, England (J. Murton). | |
| Typical sedimentological description Figure 3.31b: Stratified pebbly silt and gravel; chalk granules and pebbles, flint pebbles and cobbles, rounded to sub-angular; subhorizontal to undulating strata, few millimetres to c. 150 mm thick; small channel-fill structure below sheet of paper; contacts sharp to gradational. | Engineering description Figure 3.31b: Very thinly to thinly interbedded brown SILT, white sandy SILT and brown gravel. Brown SILT is sandy slightly gravelly. White SILT is slightly sandy, slightly gravelly of sub-angular flint. Brown sandy GRAVEL is fine to coarse of flint. Bedding is wavy, essentially horizontal. |
| Principal references Kerney (1963), Evans (1968), Preece et al. (1995), Murton et al. (2001) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.6.1.4 Fluvio-colluvial deposits |
| Previous terms | Dry Valley and Nailbourne deposits |
| Diagnostic characteristics | Gravel that tends to be better sorted and sometimes better stratified than granular head deposits (e.g. coombe rock), but less sorted and less stratified than river terrace deposits (i.e. intermediate between solifluction and fluvial deposits). |
| Environment of deposition | Commonly found beneath the axes of dry valleys (e.g. in the chalk downlands) and extending up the sides of their lower slopes, where fluvio-colluvial deposits may interdigitate with finer-grained slopewash deposits. |
| Common structures present | Crudely stratified or massive. Varies between openwork gravel (i.e. lacks matrix of <2 mm particles) to silty-sandy gravel (silt or sand in interstices between gravel particles). In areas of sandy bedrock, fluvio-colluvial deposits may be sandy rather than gravelly. |
| Principal engineering significance) | Variable particle sizes, coarse horizons, anisotropic permeability, groundwater |
Typical images
Fig. 3.32. (a) Vertical cross-section through infilled dry chalkland valley. Fluvio-colluvial deposits represented by sheet of flint gravel near the ground surface, Birling Gap, East Sussex, England (J. Murton). | |
Fig. 3.32. (b) Flint gravel fluvio-colluvial deposits intercalated with stratified silty slopewash deposits, Birling Gap, East Sussex, England (J. Murton). | |
| Typical sedimentological description Figure 3.32b: Flint gravel, massive, with sharp lower contact (involuted to erosive) intercalated with thinly stratified silt; overlies brecciated chalk (type 2 brecciation) with gently folded flint bands. | Engineering description Figure 3.32b: Light brown sandy flint GRAVEL intercalated with brown laminated slightly fine sandy SILT |
| Principal references Evans (1968), Lewin (1969), Williams (1971), Shakesby (1975), Shephard-Thorn (1988), Murton et al. (2003), West & Williams (2012) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.6.1.5 Talus deposits |
| Previous terms | Scree |
| Diagnostic characteristics | Clast- or matrix-supported diamicton, often openwork at surface, resting at angles >33° at the foot of a cliff. |
| Environment of deposition | Foot of a rock outcrop or rock wall. |
| Common structures present | Fall sorting: irregular downslope increase in clast size. Tightly wedged, poorly sorted clasts. Stacked clast- or matrix-supported diamicton units representing reworking by debris flows, sometimes intercalated with buried soils or slopewash deposits. |
| Principal engineering significance | Coarse horizons, anisotropic permeability, groundwater |
Typical images
Fig. 3.33. (a) Section through near-surface relict (vegetated) talus deposits. Tightly interlocking clasts with an infill of fine sediment at the base of the pit is overlain by two debris-flow units in which an immature organic palaeosol (dark grey bands) has developed, and a slopewash layer of fine gravel caps the section, Baosbheinn, NW Scotland (S. Hinchliffe). | |
Fig. 3.33. (b) Partly relict talus accumulation below Torridon Sandstone cliff, Scorr Ruadh, Strathcarron, NW Scotland (C.K. Ballantyne). | |
Fig. 3.33. (c) Near-surface stratigraphy of relict talus accumulations on Quinag (Q) and Stac Pollaidh (SP), NW Highlands, Scotland (Hinchliffe & Ballantyne 2009). | |
| Typical sedimentological description Figure 3.33a: Poorly sorted clast- or matrix-supported diamictons (debris-flow deposits) intercalated with organic-rich soil horizons and underlain by tightly wedged boulders (rockfall debris). Fines (<2 mm) content 25–30% by weight, comprising mainly sand with negligible clay (<2 µm). Figure 3.33b: Relict talus accumulation below Torridon sandstone cliff. Surface is partly vegetated and partly boulder-covered, and extensively reworked by intermittent debris flows. | Engineering description Figure 3.33a: Brown and brownish grey silty very gravelly SAND with medium cobble and low boulder content. Gravel, cobbles and boulders are angular to sub angular of sandstone. Indistinct bedding. Occasional very thin beds of black organic clayey silt. |
| Principal references Salt & Ballantyne (1997), Hinchliffe et al. (1998), Curry & Black (2002), Hinchliffe & Ballantyne (2009) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.6.1.6 Avalanche deposits |
| Previous terms | |
| Diagnostic characteristics | Poorly sorted angular openwork debris with perched clasts and occasional soil drapes. |
| Environment of deposition | Valley-side, slope-foot and valley-floor locations in mountain environments. |
| Common structures present | Loosely packed angular coarse debris with perched boulders and soil drapes deposited by ablating snow. Boulder tongues, splay deposits and (rarely in Britain) impact ramparts at basal break of slope. |
| Principal engineering significance | Coarse horizons, anisotropic permeability, groundwater |
Typical images
Fig. 3.34. (a) Dirty snow avalanche over snow cover, Meall Buidhe, southern Grampian Highlands, Scotland (C.K. Ballantyne). | |
Fig. 3.34. (b) Active avalanche boulder tongues, Pools of Dee, Lairig Ghru, Cairngorm Mountains, Scotland (C.K. Ballantyne). | |
Fig. 3.34. (c) Typically angular snow avalanche debris overlying snow cover, Longyeardalen, Svalbard, Norway (H. Christiansen). | |
| Typical sedimentological description Figure 3.34c: Poorly sorted accumulation of angular (sometimes very angular) clasts up to 600 mm long that accumulates on ablating snow pack and is draped over the substrate as the snow melts, forming a loosely packed clast-supported deposit with ‘perched’ surface clasts and sometimes drapes of fine soil. | Engineering description Figure 3.34c: Surface layer of very angular to angular generally flat BOULDERS (up to 600 mm long) with some cobbles. |
| Principal references Ballantyne (1989), Luckman (1992), Ballantyne & Harris (1994) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.6.1.7 Blockslope deposits |
| Previous terms | Scree, debris-mantled slope deposits. |
| Diagnostic characteristics | Mantle of mobile or formerly mobile regolith derived from weathering of bedrock on upper slopes of mountains with gradients of c. 5–35°. Depending on lithology, this may range from openwork to matrix-supported diamictons. |
| Environment of deposition | Formed by predominantly mechanical (frost) weathering of underlying bedrock and subsequent downslope movement of debris over the upper slopes of mountains. Depending on lithology, may range from openwork to matrix-supported diamicton. |
| Common structures present | Sheets and lobes terminating downslope at pronounced steps or risers, representing former movement. Vertical sorting; imbrication of coarse deposits; downslope alignment of clasts. Ploughing boulders on frost-susceptible diamictons. Depth to bedrock highly variable. |
| Principal engineering significance | Coarse horizons, anisotropic permeability, groundwater |
Typical images
Fig. 3.35. (a) Granite blockslope on Creag an Leth-choin, Cairngorm Mountains, Scotland. Boulder lobes have developed on the blockslope as a result of Lateglacial solifluction (C. K. Ballantyne). | |
Fig. 3.35. (b) Section through a schist blockslope deposit, Ben Wyvis, Highlands, Scotland (C.K. Ballantyne). | |
| Typical sedimentological description Figure 3.35a: Granite blockslope in the Cairngorm Mountains. Boulder lobes with risers up to 2.0 m high provide evidence of former (Lateglacial) downslope movement of debris by solifluction. These are now inactive. Figure 3.35b: Vertically sorted blockslope deposit comprising openwork boulders overlying a stony matrix-supported diamicton developed in schist, Ben Wyvis, Scotland. | Engineering description Figure 3.35b: Brown gravelly slightly sandy CLAY. Surface layer of angular BOULDERS with some cobbles. |
| Principal references Innes (1986), Ballantyne & Harris (1994), Ballantyne (2013a) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.6.1.8 Debris-flow deposits |
| Previous terms | Mudflow deposits; sediment gravity flow deposits |
| Diagnostic characteristics | Crudely bedded clast or matrix-supported diamicton produced by rapid downslope flow of poorly sorted debris mixed with water. |
| Environment of deposition | Lower valley sides and valley floors below steep (>28°) sediment-mantled slopes. Many flows terminate downslope on a debris cone, produced by recurrent debris-flow accumulation. |
| Common structures present | Stacked clast- or matrix-supported diamicton units. Inverse grading of clasts in individual flow units, imparting crude bedding; slope-parallel clast stringers; dominantly downslope alignment of clast long axes. Intercalated with organic soils and/or thin beds of sand or silt representing post-flow surface wash |
| Principal engineering significance | Low shear strength, shear surfaces, compressible soils, ground movement |
Typical images
Fig. 3.36. (a) Stacked debris-flow units (matrix-supported diamicton) near the foot of a basalt talus slope. Top of each unit is marked by an organic horizon representing soil formation prior to emplacement of the overlying unit, Isle of Skye, Scotland (C.K. Ballantyne). | |
Fig. 3.36. (b) Section through prograding debris-flow deposits on a relict debris cone at the foot of a limestone scarp. Section is 4 m high, Split, Croatia (C.K. Ballantyne). | |
| Sedimentological description Matrix-supported diamicton interbedded with organic layer. In diamicton, fines (<2 mm) content 25–30% by weight, comprising mainly sand with negligible clay (<2 µm). Diamicton layers massive, sharp lower contact, gradational upper contacts. | Engineering description Figure 3.36a: Thickly colour banded brown, grey and black, slightly sandy gravelly CLAY; long axes of gravel oriented parallel to slope. Top of each bed is thin TOPSOIL. Figure 3.36b: Brown and brownish grey silty very gravelly SAND with medium cobble content. Gravel and cobbles are angular to sub-angular of sandstone. Indistinct bedding. Occasional very thin beds of black organic clayey silt. |
| Principal references Wells & Harvey (1987), Brazier & Ballantyne (1989), Ballantyne (2002a) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), Winter et al. (2006), Reeves et al. (2006a, b) | |
| Sediment name | 3.6.2.1 Sorted sand and gravel |
| Previous terms | |
| Diagnostic characteristics | Stratified and sorted sheets or lenses of sand, pebbly sand or gravel, often observed in pre-Holocene river terraces in Great Britain. |
| Environment of deposition | Low-amplitude bars, shallow ephemeral channels and smaller sandy bedforms in floodplains of braided or wandering rivers. |
| Common structures present | Massive or horizontally crudely bedded gravel with units typically ≤90–125 cm thick, often extend laterally >30 m, frequently superimposed, form stacked multistorey units. Gravel commonly matrix-supported, interstices filled with silt, sand and gravels, coarser gravels commonly clast-supported. Sand facies tend to fill scours or form horizontal strata of massive sand. Ice-wedge pseudomorphs and/or involutions common. |
| Principal engineering significance | Anisotropic permeability, groundwater, aggregate resource |
Typical image
Fig. 3.37. Sorted sand and gravel within the Balderton Sand and Gravel Member Whisby Quarry, Lincoln, England. Note ice-wedge pseudomorph in centre (J. Murton). | |
| Typical sedimentological description Stratified sand, pebbly sand and sandy gravel; sand medium to coarse-grained; pebbles to cobbles, rounded to sub-angular, clast-supported to matrix-supported; strata horizontal to moderately inclined, few centimetres to several tens of centimetres thick; lower contacts sharp to gradational and planar to undulating or concave-upwards; iron staining common, especially towards top of exposure; downturned strata in downwards-tapering wedge structure. | Engineering description Figure 3.37: Medium interbedded SAND, gravelly SAND and sandy GRAVEL. SAND is reddish brown to dark brown. Sandy GRAVEL is light brown fine to coarse flint. Gravelly SAND is brown; gravel is fine and medium of flint. Colours darker near top of exposure due to iron staining. Bedding horizontal to 10°, downturned into wedge structure which contains mélange of surrounding materials, 1.5 m wide tapering to 3 m depth. |
| Principal references Bryant (1983a, b), Seddon & Holyoak (1985), Brandon & Sumbler (1991), Collins et al. (1996), Maddy et al. (1998), Briant et al. (2004a, b, 2005), Lewin & Gibbard (2010), van Huissteden et al. (2013) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.6.2.2 Channel scours |
| Previous terms | |
| Diagnostic characteristics | Broad, shallow concave-up erosion surfaces (≤15 m wide, ≤1.5 m deep) incised into sorted gravel and sand, and filled with cross-stratified sand and gravel. |
| Environment of deposition | Unstable, shallow, ephemeral channels in floodplains of braided or wandering rivers. |
| Common structures present | Concave-up base, usually truncating strata in underlying sand and gravel. Cross-stratified fill of sand and gravel. |
| Principal engineering significance | Variable particle sizes, groundwater, aggregate resource |
Typical image
Fig. 3.38. Infilled channel scour in the Balderton Sand and Gravel Member, Norton Bottoms Quarry, Lincolnshire, England (J. Murton). | |
| Typical sedimentological description Stratified sand, pebbly sand and sandy gravel (as per description of sorted sand and gravel in Fig. 3.37) containing sharp, concave-up surface (erosive scour base) overlain by trough cross-stratified sand (scour fill); top of scour fill truncated by planar gently dipping lower contact (erosion surface) of sheet of stratified pebbly sand. | Engineering description Figure 3.38: Brown sandy flint GRAVEL over light brown laminated SAND in turn overlying channel infill of cross-laminated sandy flint GRAVEL grading down to silty fine SAND; channel eroded into brown sandy flint GRAVEL. |
| Principal references Bryant (1983a, b), Maddy et al. (1998), Briant et al. (2004a, b, 2005), Lewin & Gibbard (2010), van Huissteden et al. (2013) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.6.2.3 Fine-grained sediments and organic beds in channel fills and on floodplains |
| Previous terms | |
| Diagnostic characteristics | Dark-coloured to blackish organic-rich silt or sand forming sheet or infilling channel in sorted sand and gravel. |
| Environment of deposition | Channel fills and on floodplains of braided, wandering or meandering rivers. |
| Common structures present | Sharp, planar to concave-up lower contact. Stratification sometimes present in organic-rich silt or sand. |
| Principal engineering significance | Variable particle sizes, compressible soils, perched water tables |
Typical image
Fig. 3.39. Organic-rich silt and sand infilling a shallow channel and forming thin sheet within sorted gravel in Baston Manor Pit, Lincolnshire, England (P. Worsley). | |
| Typical sedimentological description Dark grey to black organic silt and sand; massive to horizontally stratified; sharp planar to concave-up lower contact with underlying stratified gravel. | Engineering description Figure 3.39: Brown cross-laminated sandy GRAVEL and gravelly SAND over thickly bedded dark grey organic SAND and black organic SILT overlying grey indistinctly bedded sandy GRAVEL. |
| Principal references Bryant (1983a, b), Maddy et al. (1998), Briant et al. (2004a, b, 2005), Lewin & Gibbard (2010), van Huissteden et al. (2013) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.6.3.1 Fluvio-aeolian sand |
| Previous terms | |
| Diagnostic characteristics | Evidence of both fluvial activity (e.g. small palaeochannels; irregular scour surfaces overlain by massive to faintly stratified granule-bearing sand; and locally derived rip-up clasts) and aeolian activity (e.g. horizontal to subhorizontal stratified sand; see coversand). |
| Environment of deposition | Sand sheets and alluvial fans subject to intermittent fluvial activity. |
| Common structures present | Horizontal to gently dipping sandy strata 1 mm to several centimetres thick, parallel to sub-parallel. Small, infilled palaeochannels. Irregular scours infilled with sediment. |
| Principal engineering significance) | Variable particle sizes, perched water tables |
Typical image
Fig. 3.40. Stratified fluvio-aeolian sand at base of Girton coversand sequence. Red = 0.5 m. Nottinghamshire, England (Trent and Peak Archaeology). | |
| Typical sedimentological description Horizontal to subhorizontally stratified sand and clay laminae; strata few millimetres to several centimetres thick, undulating; low-angle erosion surfaces, some concave-upwards; locally pebbly sand. | Engineering description Figure 3.40: White and grey laminated silty fine SAND with closely to very closely spaced laminae of grey clay. Bedding horizontal and wavy. |
| Principal references Straw (1963), George (1992), Bateman (1998), Bateman et al. (2000), Baker et al. (2013) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.6.4.1 Loess |
| Previous terms | Brickearth, head brickearth |
| Diagnostic characteristics | Homogeneous and porous structure, absence of bedding (typical in primary ‘airfall’ loess), and a sigmoidal cumulative grain-size distribution curve with abundant coarse silt (20–50 µm), and a median particle size between 25 and 35 µm; faint stratification (few millimetres to few centimetres thick), locally present, may represent primary loess or loess reworked (e.g. by sheetwash). In Great Britain, loess may contain small components of sand (1–2% maximum), clay (<2 µm; 15–18%), primary carbonates (≤15–18%), iron (1.5–2% total iron) and organic carbon (<0.2% by weight). Calcareous tubes (‘rhizoliths’, which mark sites of roots) between c. 0.5 mm and a few millimetres in diameter and vertically to horizontally oriented may extend through the loess. |
| Environment of deposition | Cold steppe-tundra environments, partially vegetated, outside the margins of the British–Irish Ice Sheet. |
| Common structures present | Usually massive, but sometimes faint horizontal strata few centimetres to several centimetres thick are present. Vertical prismatic structure common in vertical sections through loess. Commonly found within involutions (e.g. on chalklands) |
| Principal engineering significance | Collapsible soil |
Typical image
Fig. 3.41. Loess (brown) of late Devensian age above Reculver Sand Member (light grey) of Thanet Formation of Paleocene age. Note faint horizontal colour bands near base of loess and vertical prismatic structures, Pegwell Bay, Kent, England (J. Murton). | |
| Typical sedimentological description Massive to faintly stratified silt; strata colour bands horizontal-subhorizontal, parallel, few centimetres to tens of centimetres thick; vertical prismatic structure spaced few tens of centimetres apart; sheet-like geometry of silt; pebbly silt in basal few tens of centimetres. | Engineering description Figure 3.41: Top: Reddish-brown fissured indistinctly bedded SILT. Fissures are medium to widely spaced vertical. Base: Light-brown laminated gravelly silty calcareous SAND. Gravel is of sandstone, shells and shell fragments. |
| Principal references Pitcher et al. (1954), Catt (1985, 2008), Derbyshire & Mellors (1988), Rose et al. (2000), Antoine et al. (2003), Clarke et al. (2007) | |
| Engineering geology case studies Culshaw et al. (1991), Northmore et al. (1996), Bell (2000), Reeves et al. (2006a, b), Milodowski et al. (2015) | |
| Sediment name | 3.6.4.2 Coversand (sand-sheet deposits) |
| Previous terms | Blown sand |
| Diagnostic characteristics | Occurs as sheet of horizontally to subhorizontally stratified sand, usually <5 m thick, which forms a low-relief cover across a landscape. |
| Environment of deposition | Cold steppe-tundra environments outside the margins of the British–Irish Ice Sheet and adjacent to a sand source (e.g. sandy floodplain). |
| Common structures present | Strata 1 mm to several centimetres thick, parallel to sub-parallel, horizontal to subhorizontal. Well-sorted sand with a mean grain size of about 150–200 µm. Grains moderately to well-rounded. |
| Principal engineering significance | Aggregate resource, groundwater |
Typical images
Fig. 3.42. (a) Coversand, Leziate, Norfolk, England (P. Hoare). | |
Fig. 3.42. (b) Coversand, Girton, Nottinghamshire, England (M. Bateman). | |
| Typical sedimentological description Horizontally to subhorizontally stratified silty to medium-grained sand; strata few millimetres to few centimetres thick, parallel to sub-parallel, planar to undulating, of alternating sand and silty sand. | Engineering description Figure 3.42a and b: Interlaminated brown silty fine SAND and light brown fine to medium SAND. |
| Principal references Straw (1963), Catt (1977), Buckland (1982), Schwan (1988, 1990), George (1992), Bateman (1995, 1998, 2008), Hoare et al. (2002), Antoine et al. (2003) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.6.4.3 Dune sand |
| Previous terms | |
| Diagnostic characteristics | Large-scale and high-angle cross-stratification (dune foresets) in sand that is well sorted to moderately sorted. |
| Environment of deposition | Cold-climate sand dunes adjacent to a sand source (e.g. sandy floodplain). |
| Common structures present | Gravelly coarse sand within some dune deposits may contain gravel particles up to c. 10 mm diameter, transported by saltation and/or creep during strong winds. Dune sand is commonly associated with coversand. |
| Principal engineering significance | Aggregate resource, groundwater |
Typical images
Fig. 3.43. (a) Periglacial sand dune deposits. Dunes developed on surface of coversand, Girton, Nottinghamshire, England (Trent and Peak Archaeology). | |
Fig. 3.43. (b) High-angle stratification interpreted as slipfaces of former dunes, Girton, Nottinghamshire, England (M. Bateman). | |
| Typical sedimentological description Cross-stratified sand with tangential toesets; high-angle foresets (up to 20°); strata few millimetres to several centimetres thick, parallel to sub-parallel; sand medium-grained and well-sorted. | Engineering description Figure 3.43a and b: Light greyish-brown cross-laminated fine and medium SAND. |
| Principal references Bateman (1998), Baker et al. (2013) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.6.4.4 Niveo-aeolian sand deposits |
| Previous terms | Sand sheets, dunes |
| Diagnostic characteristics | Poorly or moderately sorted, predominantly structureless sand sheets. Dominated by medium to coarse sand (0.2–2.0 mm), sometimes containing granules and small pebbles. |
| Environment of deposition | Formed by simultaneous deposition of mixed aeolian sediment and wind-blown snow. The coarseness of the sand reflects mainly tractional deposition of saltating and creeping grains. In Britain, niveo-aeolian sands have been described only on mountains, but some lowland coversands may contain a niveo-aeolian component. |
| Common structures present | Sand sheets are often structureless, and lack dune deposits. Poor to moderate sorting reflecting short travel distances and admixture of different grain sizes during ablation of sand-rich snow cover. Organic-rich horizons (in Holocene deposits) represent immature soils. |
| Principal engineering significance | Aggregate resource, groundwater |
Typical images
Fig. 3.44. (a) Niveo-aeolian sand sheets, An Teallach, NW Scotland (C.K. Ballantyne). | |
Fig. 3.44. (b) Section through niveo-aeolian sand deposits, An Teallach, NW Scotland (C.K. Ballantyne). | |
Fig. 3.44. (c) Section through niveo-aeolian sand deposits, An Teallach, NW Scotland (C.K. Ballantyne). | |
| Typical sedimentological description Figure 3.44a: Holocene niveo-aeolian sand sheets up to 4 m thick on An Teallach, NW Scotland. Now largely restricted to lee slopes; these were much more extensive until 300 years ago. Figure 3.44b and c: Sections cut through the An Teallach sand deposits. Banding reflects staining by iron and manganese oxide, not grain-size variations or structural discontinuities. Unconformities are sometimes present where accumulation was interrupted by erosion. | Engineering description Figure 3.44a and b: Brown grey and black colour-laminated SAND. |
| Principal references Ballantyne & Whittington (1987), Ballantyne & Morrocco (2006), Morrocco et al. (2007) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.6.5.1 Blockfield deposits |
| Previous terms | Felsenmeer, mountain-top detritus. |
| Diagnostic characteristics | Shallow mantle of in situ bouldery regolith ranging from openwork to clast- or matrix-supported diamictons. |
| Environment of deposition | Formed by predominantly mechanical (frost) weathering of underlying jointed bedrock. Mainly restricted to high summits and plateaux that escaped glacial erosion under a former cover of cold-based glacier ice. |
| Common structures present | Variable composition of fine (<2 mm) sediment from zero to c. 40% by weight. Crude inverse sorting, with largest clasts at or near the surface; downwards increase in clast angularity. Downwards transition from regolith via detached angular clasts to intact jointed bedrock. Depth commonly <1.0 m. |
| Principal engineering significance | Coarse horizons, groundwater |
Typical images
Fig. 3.45. (a) Section through openwork quartzite summit blockfield, Glas Mheall Liath, An Teallach, NW Scotland (C.K. Ballantyne). | |
Fig. 3.45. (b) Pit in quartzite plateau blockfield. A clast-supported diamicton underlies a thin surface layer of openwork boulders, Sàil Liath, An Teallach, NW Scotland (C. Hopkinson). | |
Typical images
Fig. 3.45. (c) Pit in shallow sandstone blockfield, northern plateau of An Teallach, NW Scotland (C. Hopkinson). | |
| Typical sedimentological description Figure 3.45a: Openwork quartzite blockfield deposit with inverse grading and infill of silty-sand fine sediment below 0.7 m depth. Figure 3.45b: Clast-supported diamictic quartzite blockfield deposit overlying bedrock at c. 1.0 m depth. In the fine fraction, clay averages 2.9% by weight silt 15.6% and sand 81%. Figure 3.45c: Shallow non-sorted clast- and matrix-supported blockfield regolith c. 0.4 m thick overlying intact Torridon sandstone bedrock. In the fine fraction, clay averages 4.7% by weight, silt 31.1% and sand 64.2%. | Engineering description Figure 3.45a: BOULDERS with some cobbles of sub-angular to subrounded grey quartzite. Below 0.7 m with some silty and infill. Figure 3.45b: Boulders with some cobbles and a little gravelly sand underlain by brown slightly sandy clayey GRAVEL. Figure 3.45c: Greyish brown slightly silty sandy GRAVEL with high cobble and boulder content. Gravel, cobbles and boulders are angular to sub-angular of grey sandstone. |
| Principal references Ballantyne (1998), Hopkinson & Ballantyne (2014) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), Reeves et al. (2006a, b) | |
| Sediment name | 3.6.5.2 Brecciated bedrock |
| Previous terms | Frost-weathered mantle |
| Diagnostic characteristics | Layer typically c. 1.5 m to a few metres thick beneath flat to gently sloping ground. Fractures predominantly horizontal to subhorizontal, increase in spacing as depth increases, and define rock blocks that are commonly tabular and parallel to the surface. (This represents the type 1 brecciation distinguished in Chapter 5.) |
| Environment of formation | Beneath plateaux and valleys typically underlain by frost-susceptible bedrocks (e.g. chalk and other fine-grained limestones, silty sandstones, clays, slates, schists) but may include hard low-porosity rocks. |
| Common structures present | Two distinct layers sometimes present in brecciated chalk. Upper layer deformed into small open folds (involutions), commonly a few to several centimetres in amplitude and a few decimetres in wavelength; axial planes more or less vertical and some folds have a catenary shape; blocks commonly sub-angular to subrounded, and open millimetre-scale joints between them filled with chalk powder and small fragments; sharp to gradational lower contact. Lower layer undeformed, comprising angular to sub-angular chalk blocks with matched sides. |
| Principal engineering significance | Shattered bedrock, coarse horizons, anisotropic permeability, anisotropic strength, anisotropic stiffness, groundwater |
Typical images
Fig. 3.46. (a) Brecciated chalk (Margate Chalk Member, Newhaven Chalk Formation). Section c. 7 m high. Beresford Gap, Kent, England (J. Murton). | |
Fig. 3.46. (b) Brecciated chalk (Margate Chalk Member, Newhaven Chalk Formation), Grenham Bay, Margate, Kent, England (D. Giles). | |
Fig. 3.46. (c) Brecciated near-surface Dalradian quartzite. Pole is about 1.3 m long. Sound but well-jointed rock (not shown) at 2.0–2.5 m depth (i.e. beneath the brecciated layer) is inferred from a separate exposure. An alternative interpretation is that the brecciated quartzite represents bedrock glaciotectonized by overriding ice. Given that all of Scotland, including Jura, was under the last and previous ice sheets, glaciotectonic brecciation of bedrock might explain (1) the features that appear to be silty void fills; and (2) rotation of clasts towards the top and right of the photograph, which could be due to movement of the ice over this area. Isle of Jura, Scotland (C. K. Ballantyne). | |
Fig. 3.46. (d) Brecciated slates of the Polzeath Slate Formation, Camel Estuary, Daymer Bay, Cornwall, England (D. Giles). | |
| Typical sedimentological description Figure 3.46a and b: Predominantly horizontal fractures, parallel to ground surface that define tabular blocks of chalk with matched sides; fracture density declines with depth beneath ground surface (typically 10–20 mm vertical spacing towards top of section v. 100–200 mm towards base). Figure 3.46c: Horizontal to subhorizontal and vertical to subvertical fractures define angular blocks of quartzite of tabular, equant or irregular shape; fracture density declines with distance beneath surface. Figure 3.46d Predominantly horizontal to subhorizontal fractures defining angular platy fragments in lower half of image grades upwards into clast-supported diamicton with angular to subrounded platy fragments of slate and matrix of clay to silt; upper part of diamicton contains poorly defined involutions. | Engineering description Figure 3.46a and b: Surface layer of structureless CHALK composed of cream slightly gravelly sandy silt. Occasional rinded flints. Most of face comprises very weak low density white CHALK. Fractures are extremely closely spaced (10–20 mm) at top becoming very closely to closely spaced (100– 200 mm) towards base, open and infilled (1/3/10) with cream comminuted chalk (Newhaven Chalk Formation, Margate Chalk Member, low density, Grade C4 becoming B3 with depth). Figure 3.46c: Strong light grey medium grained QUARTZITE. Fractures are predominantly steeply dipping but also randomly oriented very closely to closely spaced (25/100/250) curved, undulating, rough clean. (Weathered DALRADIAN QUARTZITE.) Figure 3.46d: Uppermost 25% of image: measured from top: brown clayey GRAVEL of flat angular slate. Gravel is randomly orientated. 25–50% of image: clay content decreases and gravel orientation becomes more ordered with depth, so now is a brown slightly clayey GRAVEL of flat angular slate, particles dipping about 25° into face. Lowermost half of image: medium strong brown SLATE. Cleavage dips about 25° into face and at the time of logging cleavage fractures were extremely closely spaced (5/10/20). Occasional inclined fractures moderately stained grey. |
| Principal references Bradshaw & Ingle Smith (1963), Chandler (1972b), Horswill & Horton (1976), Williams (1987), Murton (1996a), Murton et al. (2006), Ealey (2012), Murton & Giles (2016) | |
| Engineering geology case studies Culshaw et al. (1991), Bell (2000), Reeves et al. (2006a, b) | |
3.7 Macrostructural, erosional and sediment architectural element descriptors
Macroscale features are not only easier to observe than microstructures (Section 3.8) but also provide a more holistic impression of former process, especially in complex sedimentary sequences. Macrostructures in particular are often developed in large sedimentary packages and can record significant post- and syndepositional disturbance. Primary sedimentary structures can also be analysed in large and even complex packages using architectural elements (Miall 1977b, 1985, 1996), which have been employed to represent large-scale components of the fluvial landscape and can be combined to form depositional systems representative of different fluvial styles. The architectural element technique has also been applied to complex glacial stratigraphies, where it constitutes a systematic classification scheme to aid in the interpretation of heterogeneous sediment sequences.
Macrostructures in soil or rock can be observed in different ways. Natural exposures in bluffs fringing rivers, lakes and coasts, for example, often show macrostructures within their geomorphic or stratigraphic context, which assists their identification and interpretation. Likewise, artificial excavations, particularly in trial pits and along cuttings for roads or pipelines, often reveal a variety of glacial or periglacial macrostructures in the upper metres of the ground. Imaging of borehole sides or logging of borehole cores can provide useful information, although in just one dimension and at small scale; as such, these should be seen as complementing, rather than replacing, observations from large two- or three-dimensional exposures.
Observation of macrostructures depends strongly on the experience of the observer and on the condition of the exposure. This chapter is designed to help the former, but exposure condition can be harder to assess. Some exposures readily show macrostructures, for example where wind has etched out fine lamination in sand or where rain has washed debris from rock outcrops to reveal striae. Other exposures, however, require careful preparation and patience. This may involve clearing away slumped sediment for several hours with a spade or mechanical excavator, and then using a trowel to reveal the fine detail of cross-lamination or involutions. Subtle but important discontinuities in soil or rock may only become apparent as the exposure dries, and so monitoring for some days may be necessary to identify features such as relict periglacial shears.
| Structure name | 3.7.1.1 Clastic dykes, intrusions and hydrofracture fills |
| Diagnostic characteristics | High artesian pressures within the glacial system can lead to the hydrofracturing of pre-existing glacial sediments and the upwards emplacement of clastic dykes and intrusions. The dykes can extend vertically into sediments, often ending in plumes or burst-out structures with fan-shaped gravel clusters. They are aligned subvertically in the sediment and may show evidence of infilled branches where the pressurized dyke fill has exploited pre-existing fractures. The sediment infill will comprise poorly sorted gravels with clasts orientated parallel to the dyke walls. Downwards injection of material is also possible. |
| Principal engineering significance | Variable particle sizes, anisotropic permeability, anisotropic strength, anisotropic stiffness, high-permeability zones, perched water tables |
Typical images
Fig. 3.47. (a) Clastic dyke in pumice gravels and beneath till, Slettjökull, Iceland (D.J.A. Evans). | |
Fig. 3.47. (b) Till and clastic dykes over glaciofluvial outwash, Sandfellsjökull, Iceland (D.J.A. Evans). | |
Fig. 3.47. (c) Hydrofracture fill cutting through till, Skeidararsandur, Iceland (A. Russell). | |
Fig. 3.47. (d) Hydrofracture fill developed in glaciolacustrine clays and silts, Fjallsjökull foreland, Iceland (D.J.A. Evans). | |
| Principal references Le Heron & Etienne (2005), van der Meer et al. (2009) | |
| Structure name | 3.7.1.2 Soft sediment deformation and slump and loading structures |
| Diagnostic characteristics | Soft sediment deformation structures: Involves any deformation structure created by failure or loading of soft sediments. Subdivided into slump structures and loading structures. Slump structures: Created by mass failure on depositional slopes. Includes folds and thrusts and are often intraformational (e.g. interbedded sequences in deltas on a sloping lake floor). Loading structures: Caused by inverted density stratification where a material of higher density (e.g. sand) rests on a less dense material (e.g. liquefied clay) causing instability within the mass. Sand tends to sink and the finer particles inject upwards producing ball-and-pillow structures (hemispherical or kidney-shaped masses of sand that show internal laminations) and flame structures (wavy or flame-shaped tongues of silt and clay that project upwards into an overlying layer). |
| Principal engineering significance | Anisotropic permeability, anisotropic stiffness, anisotropic strength |
Typical images
Fig. 3.48. (a) Ball and pillow (loading) structures, Upgang, Whitby, East Yorkshire, England (D.J.A. Evans). | |
Fig. 3.48. (b) Slump structures (folds) in glaciolacustrine rhythmites, New Zealand (D.J.A. Evans). | |
| Principal references Allen (1982), Prior & Bornhold (1988), Nemec (1990), Nichols et al. (1994), Jones et al. (1999), Hindmarsh & Rijsdijk (2000), Evans & Benn (2004), Hiemstra et al. (2004) | |
| Structure name | 3.7.1.3 Dewatering structures |
| Diagnostic characteristics | Dewatering structures are caused by fluidization processes within the sediment as porewaters are expelled from a sediment bed. Water escapes vertically and laterally within the sediment body, displacing and entraining particles along its path, producing dish (thin, dark-coloured, subhorizontal, flat to concave-upwards, clayey laminations that occur principally in sand and silts units) and pillar structures (vertical to near-vertical, cross-cutting columns and sheets of structureless or swirled sand that cut through either massive or laminated sands). Pillar structures generally occur in association with dish structures. Larger, coarser grained structures are often called clastic dykes or burst-out structures. |
| Principal engineering significance | Anisotropic permeability, anisotropic stiffness, anisotropic strength |
Typical images
Fig. 3.49. (a) Dewatering dykes in glaciolacustrine deposits, New Zealand (D.J.A. Evans). | |
Fig. 3.49. (b) Dewatering or burst-out structure in glaciolacustrine deposits, Lake Tekapo, New Zealand (D.J.A. Evans). | |
Fig. 3.49. (c) Pillar structures, Red Cliff, North Ferriby, East Yorkshire, England (D.K. Murton). | |
Fig. 3.49. (d) Dish structures, Finningley, South Yorkshire, England (J. Murton). | |
| Principal references Lowe (1976), Jones et al. (1999), Rijsdijk et al. (1999), van der Meer et al. (1999), Evans & Benn (2004) | |
| Structure name | 3.7.1.4 Microfluting |
| Diagnostic characteristics | Typically <100 mm wide, aligned parallel to direction of ice flow. Uniform in cross-section and are usually propagated from a large boulder, a collection of boulders or a bedrock obstacle. Typically composed of subglacial traction till but may also contain glaciotectonite, glaciofluvial sands and gravels. Often termed sole casts. |
| Principal engineering significance | Variable particle sizes, coarse horizons, anisotropic permeability, high-permeability zones |
Typical images
Fig. 3.50. (a) Microfluting at base of subglacial traction till, Lethbridge, Canada (D.J.A. Evans). | |
Fig. 3.50. (b) Microfluting at base of the Svinafellsjökull glacier, Iceland (D.J.A. Evans). | |
| Principal references Westgate (1968), Ehlers & Stephan (1979), Clark & Hansel (1989), Eyles & Boyce (1998), Krüger & Kjær (1999), Piotrowski et al. (2001), Jørgensen & Piotrowski (2003) | |
| Structure name | 3.7.1.5 Boulder pavements |
| Diagnostic characteristics | Horizontal layers of coarse clasts, one or more cobbles or boulders thick, commonly with planar, striated upper surfaces. Lag deposits marking former positions of ice–till interface. |
| Principal engineering significance | Coarse horizons, anisotropic permeability, groundwater |
Typical images
Fig. 3.51. (a) Boulder pavement, Whitburn, Tyneside, England (D.J.A. Evans). | |
Fig. 3.51. (b) Boulder pavement on recently deglaciated foreland, Fjallsjökull, Iceland (D.J.A. Evans). | |
| Principal references Eyles (1988), Clark (1991), Hicock (1991) | |
| Structure name | 3.7.1.6 Canal fills |
| Diagnostic characteristics | Infills of former braided canal systems that develop at the ice–bed interface during periods of decoupling. Recognizable as stratified lenses with flat upper surfaces and concave-up bases. Contain primary depositional structures or bedforms in addition to thin sand layers and stringers. |
| Principal engineering significance | Anisotropic permeability, groundwater, variable particle sizes |
Typical images
Fig. 3.52. (a) Canal fills between subglacial traction tills, Patagonia, Argentina (D. Benn). | |
Fig. 3.52. (b) Canal fill between mudstone bedrock and subglacial traction till, Alberta, Canada (D.J.A. Evans). | |
| Principal references Eyles et al. (1982), Clark & Walder (1994), Evans et al. (1995) | |
| Structure name | 3.7.2.1 Glaciotectonic foliations |
| Diagnostic characteristics | Planar surfaces that develop in glacial sediments as a result of deformation. Foliation may be defined by a spatial variation in composition leading to banding or layering or by a preferred orientation of clasts. Sediments may exhibit several generations of foliations indicative of different deformation phases. |
| Principal engineering significance | Low shear strength, anisotropic strength, shear surfaces |
Typical images
Fig. 3.53. (a) Glaciotectonic foliation induced in underlying mudstone bedrock, Alberta, Canada (D.J.A. Evans). | |
Fig. 3.53. (b) Glaciotectonic foliation developed in stratified glaciofluvial sands and gravels, Germany (D.J.A. Evans). | |
| Principal references van der Wateren (1995), Phillips et al. (2008, 2011a, b), Waller et al. (2011), Evans & Benn (2014), Evans (2017) | |
| Structure name | 3.7.2.2 Glaciotectonic lineations |
| Diagnostic characteristics | Linear features that occur persistently within the body of the glaciogenic sediment. Can be further categorized into intersection, crenulation, stretching and mineral lineations together with slickenside striations. |
| Principal engineering significance | Low shear strength, anisotropic strength, shear surfaces |
Typical images
Fig. 3.54. (a) Stretching lineation developed upon foliation surfaces within a glaciotectonite, West Runton, Norfolk, England (E. Phillips). | |
Fig. 3.54. (b) Glaciotectonic lineations or fissile partings in subgacial traction till, Skalafellsjökull, Iceland (D.J.A. Evans). | |
| Principal references van der Wateren (1995), Phillips et al. (2008, 2011a, b), Waller et al. (2011), Evans & Benn (2014), Evans (2017) | |
| Structure name | 3.7.2.3 Glaciotectonic boudinage |
| Diagnostic characteristics | Structure where relative competent layers of glaciogenic deposits become stretched and elongated during glaciotectonic deformation. Layers may become separated into blocks or lens-shaped structures or boudins. Where the process is incomplete the layers show a narrowing or pinching; these are referred to as pinch and swell structures that can be used to determine the extension direction within the deformed sediments. |
| Principal engineering significance | Low shear strength, anisotropic strength, shear surfaces |
Typical images
Fig. 3.55. (a) Disrupted blocks of chalk bedrock contained within a glaciotectonite, East Runton, Norfolk, England (E. Phillips). | |
Fig. 3.55. (b) Glaciotectonic boudinage with chalk augen, West Runton, Norfolk, England (D. Giles). | |
Fig. 3.55. (c) Sand intraclasts with well-preserved bedding in highly foliated glaciotectonite, West Runton, Norfolk, England (E. Phillips). | |
Fig. 3.55. (d) Boudinage, extensional faults and crenulation cleavage within glaciotectonite, West Runton, Norfolk, England (E. Phillips). | |
| Principal references van der Wateren (1995), Phillips et al. (2008, 2011a, b), Waller et al. (2011), Evans & Benn (2014), Evans (2017) | |
| Structure name | 3.7.2.4 Glaciotectonic shear zones |
| Diagnostic characteristics | Planar zones of relatively intense shearing deformation generated by predominantly ductile subglacial deformation of the glaciogenic sediments. Shear zones can occur at all scales within the deforming beds. |
| Principal engineering significance | Low shear strength, anisotropic strength, shear surfaces |
Typical images
Fig. 3.56. (a) Glaciotectonized bedrock (shear zone) beneath Bannow Till, County Wexford, Ireland (D.J.A. Evans). | |
Fig. 3.56. (b) Shear zone with detail enlarged in the rhythmites of the canal fills in Horden Till, Durham, England (D.J.A. Evans). Sr, Reidel shear zone; Sb, boudinage zone; Sh, homogenized zone. | |
| Principal references van der Wateren (1995), Phillips et al. (2008, 2011a, b), Waller et al. (2011), Evans & Benn (2014), Evans (2017) | |
| Structure name | 3.7.2.5 Glaciotectonic folds |
| Diagnostic characteristics | Curved or wave-like structures that develop in glacial sediments as a result of ductile deformation of the sediment beds or other pre-existing planar structures present within the deposits. Can occur on a variety of scales and at a variety of wavelengths. Folds can be synclinal, anticlinal and can be upright, inclined or recumbent. Can be further classified on the basis of their profile into parallel, concentric, similar, chevron and kink folds. |
| Principal engineering significance | Anisotropic strength, anisotropic stiffness |
Typical images
Fig. 3.57. (a) Kink fold detail, Pukaki, New Zealand (D.J.A. Evans). | |
Fig. 3.57. (b) Deformed and folded Skipsea Till, Barmston, East Yorkshire, England (D. Giles). | |
Fig. 3.57. (c) Fold and thrust fault in glaciolacustrine deposits, Lake Pukaki, New Zealand (D.J.A. Evans). | |
Fig. 3.57. (d) Recumbent isoclinal folds and glaciotectonic foliation in glaciotectonite, West Runton, Norfolk, England (E. Phillips). | |
| Principal references van der Wateren (1995), Phillips et al. (2008, 2011a, b), Waller et al. (2011), Evans & Benn (2014), Evans (2017) | |
| Structure name | 3.7.2.6 Glaciotectonic faults and thrusts |
| Diagnostic characteristics | Fault: Planar fracture within glaciogenic sediments that has accommodated significant displacement/movement parallel to the fracture plane. Subdivided into normal faults where the hanging wall (sediment mass resting on the fault plane) has moved down, reverse faults where the hanging wall has moved up and strike-slip or wrench faults where movement occurred parallel to the strike of the fault plane. Lag faults or lags are a type of normal fault where the fault plane dips at an angle of 30° or less. Thrusts: Type of reverse fault within glaciogenic sediments where the fault plane dips at an angle of 30° or less. Thrusts can be single or conjugate. Formed through horizontal compression. |
| Principal engineering significance | Shear surfaces, anisotropic strength, anisotropic stiffness |
Typical images
Fig. 3.58. (a) Reverse-faulted glaciolacustrine rhythmites, Happisburgh, Norfolk, England (D. Giles). | |
Fig. 3.58. (b) Faulting in glaciofluvial sands and gravels, Glaven Pit, near Holt, Norfolk, England (D. Giles). | |
Fig. 3.58. (c) Faulted glaciofluvial sands, Sandsend, Whitby, North Yorkshire, England (D. Giles). | |
Fig. 3.58. (d) Faulted laminated sand elements within glaciofluvial sands, Glaven Pit, near Holt, Norfolk, England (D. Giles). | |
Fig. 3.58. (e) Glaciotectonic thrust fault, cryoturbation above Lemont Formation, Waubonsee, Chicago, Illinois, USA (D.H. Voorhees). | |
Fig. 3.58. (f) Liquefied silts and sands along glaciotectonic thrusts, Iceland (D.J.A. Evans). | |
| Principal references van der Wateren (1995), Phillips et al. (2008, 2011a, b), Waller et al. (2011), Evans & Benn (2014), Evans (2017) | |
| Structure name | 3.7.2.7 Glaciotectonic grabens and half-grabens |
| Diagnostic characteristics | Sediment block that has dropped down between two sub-parallel normal faults that dip towards each other (graben) or a sediment block bounded by a normal fault on one side only (half-graben). |
| Principal engineering significance | Shear surfaces, anisotropic strength, anisotropic stiffness |
Typical images
Fig. 3.59. (a) Complex graben structure in slackwater deposits of the jökulhlaup outwash, Skeiðarárjökull, Iceland (D.J.A. Evans). | |
Fig. 3.59. (b) Half-graben in recent glaciolacustrine deposits, Hoffellsjökull, Iceland (D.J.A. Evans). | |
| Principal references van der Wateren (1995), Phillips et al. (2008, 2011a, b), Waller et al. (2011), Evans & Benn (2014), Evans (2017) | |
| Structure name | 3.7.3.1 P-forms |
| Diagnostic characteristics | Plastically moulded forms are smoothed depressions eroded into bedrock. Forms can be transverse (aligned at right angles to ice-flow), longitudinal (aligned parallel to ice flow) or non-directional. Can take on a variety of shapes such as mussel-shaped, sickle-shaped, commas, troughs, spindle-shaped flutes and furrows. Can vary widely in size but are mostly small scale. |
| Principal engineering significance | |
Typical images
Fig. 3.60. (a) P Forms, Nordenskiold Glacier foreland, Svalbard, Norway (D.J.A. Evans). | |
Fig. 3.60. (b) P Forms looking west in direction of former ice flow, Camas Mor beach, near Keanchulish, Sutherland, Scotland (B. Davies). | |
| Principal references Dahl (1965), Gjessing (1965), Kor et al. (1991), Benn & Evans (2010) | |
| Structure name | 3.7.3.2 Lunate fractures |
| Diagnostic characteristics | Larger arcuate fractures that may measure a few centimetres to more than a metre across. Similar to crescentic gouges except that the horns of lunate fractures point down-ice-flow direction. Both may be present on the same rock outcrop. |
| Principal engineering significance | Sheared bedrock |
Typical images
Fig. 3.61. (a) Lunate fractures in gabbro bedrock indicating ice flow from left to right, Coire Lagan, Skye, Scotland (D. Giles). | |
Fig. 3.61. (b) Lunate fractures on striated bedrock indicating ice flow from left to right, Coire Lagan, Skye, Scotland (D. Giles). | |
| Principal references Laverdière et al. (1979, 1985) | |
| Structure name | 3.7.3.3 Crescentic gouges |
| Diagnostic characteristics | Larger arcuate gouges that may measure a few centimetres to more than a metre across. Similar to lunate fractures except that the horns of crescentic gouges point up-ice-flow direction. Both may be present on the same rock outcrop. |
| Principal engineering significance | Sheared bedrock |
Typical images
Fig. 3.62. (a) Crescentic gouges reflecting ice flow direction from right, Nigardsbreen, Norway (D.J.A. Evans). | |
Fig. 3.62. (b) Crescentic gouges on smoothed and striated bedrock, ice flow bottom to top, Greenland (D. Roberts). | |
| Principal references Laverdière et al. (1979, 1985) | |
| Structure name | 3.7.3.4 Crescentic fractures |
| Diagnostic characteristics | Arcuate fracture planes which may dip either up-ice or down-ice. May have their horns similarly pointing up-ice- or down-ice-flow direction. Fractures with up-ice horns commonly occur in isolation, whereas those with down-ice-pointing horns generally occur in a series. Width of individual fractures decreases in down-ice direction. Series of crescentic fractures provide a clear sense of former ice-flow direction. |
| Principal engineering significance | Sheared bedrock. |
Typical images
Fig. 3.63. (a) Crescentic fractures, Peggy's Cove, Nova Scotia, Canada (D.J.A. Evans). | |
Fig. 3.63. (b) Crescentic fractures with ice movement away from viewer, Isfallsglaciaren, Sweden (D.J.A. Evans). | |
| Principal references Laverdière et al. (1979, 1985) | |
| Structure name | 3.7.3.5 Chattermarks |
| Diagnostic characteristics | Crescent-shaped fractures, usually a few centimetres across, with their open or concave sides facing in down-ice-flow direction. Commonly occur as a series of closely spaced fractures nested one inside the other, resulting from repeated fracture events beneath a single overpassing clast. Spacing between chattermarks commonly very consistent. |
| Principal engineering significance | Sheared bedrock |
Typical images
Fig. 3.64. (a) Chattermarks and adjacent striae indicating ice movement towards the top of the image, Coire Lair, Scotland (C.K. Ballantyne). | |
Fig. 3.64. (b) Chattermarks within striae, Konowbreen, Svalbard, Norway (D.J.A. Evans). | |
| Principal references Laverdière et al. (1979, 1985) | |
| Structure name | 3.7.3.6 Rat tails |
| Diagnostic characteristics | Small residual longitudinal ridges extending down-ice from resistant rock knobs or nodules. Small-scale equivalent of crag and tails. |
| Principal engineering significance | Sheared bedrock |
Typical images
Fig. 3.65. (a) Rat tails on Möberg, Breidamerkurjökull, Iceland (D.J.A. Evans). | |
Fig. 3.65. (b) Rat tails on the lee side of feldspathic inclusions within basaltic rock, Hudson Bay, Canada (Laverdière et al. 1985). | |
| Principal references Laverdière et al. (1985), Shaw (1994), Benn & Evans (2010) | |
| Structure name | 3.7.3.7 Grooves, striations and polished surfaces |
| Diagnostic characteristics | Long scratches on a rock, a few millimetres across, usually in clusters. Often well developed on the stoss side of roches moutonnées. Grooves can be quite deep and often infilled. As the glacier or ice sheet moves forward, rocks imbedded in the ice scratch the underlying materials. If small, these linear features are termed striations. Grooves are larger features that may be regular or irregular and may show the direction of glacier movement. |
| Principal engineering significance | Sheared bedrock |
Typical images
Fig. 3.66. (a) Striae and polished bedrock, Isfallsglaciaren, Sweden (D.J.A. Evans). | |
Fig. 3.66. (b) Bedrock groove with striae and polished surface, Skalafellsjökull, Iceland (D.J.A. Evans). | |
| Principal references Iverson (1991), Rea (2013a) | |
| Structure name | 3.7.4.1 Channel elements |
| Diagnostic characteristics | Channel infills. Single or multistorey, finger, lens or sheet-like sand and gravel forms with concave-up erosional base. May include any combination of lithofacies as well as smaller-scale architectural elements. Facies include ripple cross-laminated sands and cross-bedded sand and gravels. Channels with high sinuosity may be subject to chute or neck cut-off, and be filled with fine-grained overbank flood deposits. Can be further divided into ribbon and sheet channels, crevasse channels and chute channels. |
| Principal engineering significance | Variable particle sizes, anisotropic permeability, groundwater |
Typical images
Fig. 3.67. (a) Multistorey channel infill, buried channel, Overstrand, Norfolk (D. Giles). | |
Fig. 3.67. (b) Channel elements, Skeiðarárjökull, Iceland (D.J.A. Evans). | |
| Principal references Miall (1985, 1996), Jones et al. (1999), Benn & Evans (2010) | |
| Structure name | 3.7.4.2 Downstream accretion elements |
| Diagnostic characteristics | Principal products of accretion within bar complexes of major sand-bed channels. Lenses of related sandy bedforms resting on a flat or channelled base. Typically related to downstream migration of simple or compound macroforms and bars. Bars are characteristically 1–15 m high, 10–1000 m long. Consist of several (possibly many) co-sets of downstream-dipping cross-beds. Particularly characteristic of braided streams. |
| Principal engineering significance | Variable particle sizes, anisotropic permeability, groundwater |
Typical images
Fig. 3.68. (a) Downstream accretion macroforms overlying bouldery flood gravels, Lake Pukaki, New Zealand (D.J.A. Evans). | |
Fig. 3.68. (b) Downstream accretionary elements, Alberta, Canada (D.J.A. Evans). | |
| Principal references Miall (1985, 1996), Jones et al. (1999), Benn & Evans (2010) | |
| Structure name | 3.7.4.3 Lateral accretion elements |
| Diagnostic characteristics | Wedge, sheet or lobes of sandy bedforms or gravel elements, characterized by internal lateral accretion surfaces. Includes heterolithic inclined strata and epsilon cross-stratification. Cross-beds dip transversely or obliquely to the main channel trend. Can also develop on the insides of channel bends where deposition occurs while erosion takes place on the opposite bank. Facies may be predominantly gravelly or sandy. |
| Principal engineering significance | Variable particle sizes, anisotropic permeability, groundwater |
Typical images
Fig. 3.69. (a) Lateral accretion elements, Skeiðarárjökull, Iceland (D.J.A. Evans). | |
Fig. 3.69. (b) Lateral accretion elements in glaciofluvial outwash, Lleiniog, Wales (D.J.A. Evans). | |
| Principal references Miall (1985, 1996), Jones et al. (1999), Benn & Evans (2010) | |
| Structure name | 3.7.4.4 Gravel bar and bedform elements |
| Diagnostic characteristics | Gravel bars and bedforms. Lens or blanket-shaped sediment units, usually tabular sets of trough and planar cross-bedded gravels. May occur as multistorey sheets, interbedded with sandy bedforms. Major element of many sandar. |
| Principal engineering significance | Variable particle sizes, anisotropic permeability, groundwater |
Typical images
Fig. 3.70. (a) Gravel bar and bedform elements, Glaven Pit, near Holt, Norfolk, England (D. Giles). | |
Fig. 3.70. (b) Gravel bar and bedform in glaciofluvial deposits, North Ferriby, Yorkshire, England (D.J.A. Evans). | |
| Principal references Miall (1985, 1996), Jones et al. (1999), Benn & Evans (2010) | |
| Structure name | 3.7.4.5 Sediment gravity-flow elements |
| Diagnostic characteristics | Narrow or elongate lobes or multistorey poorly sorted gravel sheets may be interbedded with gravel bars or sandy bedforms. Deposited by debris flow. Typically have irregular, non-erosional bases and are 0.5–3 m thick, with widths up to 20 m and downstream length of several kilometres. Flow units may be lobate in plan view. Amalgamated flows are common with cumulative thicknesses of several metres. Grading and inverse grading common. Common in the proximal reaches of sandar and along margins of valley trains. |
| Principal engineering significance | Variable particle sizes, anisotropic permeability, groundwater |
Typical images
Fig. 3.71. (a) Sediment gravity flow deposits, Skeidararjokull foreland, Iceland (D.J.A. Evans). | |
Fig. 3.71. (b) Sediment gravity flow facies in recent glaciofluvial outwash, Fjallsjökull, Iceland (D.J.A. Evans). | |
| Principal references Miall (1985, 1996), Jones et al. (1999), Benn & Evans (2010) | |
| Structure name | 3.7.4.6 Sandy bedform elements |
| Diagnostic characteristics | Lens, sheet, blanket or wedge-shaped sand bodies, formed by migration of ripples, dunes, sand waves, transverse and linguoid bedforms, as well as upper-flow-regime plane beds or washed-out humpback dunes. May be present as channel fill or minor bars, or as a component of simple or compound bars within downstream and lateral accretionary elements. Also present as a component of crevasse channels and crevasse-splay elements. Vertical stacking of different bedform types indicates long- or short-term changes in the flow regime. |
| Principal engineering significance | Variable particle sizes, anisotropic permeability, groundwater |
Typical images
Fig. 3.72. (a) Sandy bedform elements, Happisburgh, Norfolk, England (D. Giles). | |
Fig. 3.72. (b) Sandy bedform element architecture, near Sea Palling, Norfolk, England (D. Giles). | |
| Principal references Miall (1985, 1996), Jones et al. (1999), Benn & Evans (2010) | |
| Structure name | 3.7.4.7 Laminated sand-sheet elements |
| Diagnostic characteristics | Sheet or blanket laminated tabular sand bodies. May represent deposition from non-channelized flows, as bar-top, bar-flank sands, or as deposits of overbank sheet floods. Uncommon in proximal sandar but can occur in distal deposits. |
| Principal engineering significance | Variable particle sizes, anisotropic permeability, groundwater |
Typical images
Fig. 3.73. (a) Laminated sand-sheet elements (top of image) overlying sandy dune beds, Parana, Brazil (P. Ashworth). | |
Fig. 3.73. (b) Planar parallel laminations in laminated sand-sheet elements, Richards Island, western Arctic Canada (J. Murton). | |
| Principal references Miall (1985, 1996), Jones et al. (1999), Benn & Evans (2010) | |
| Structure name | 3.7.4.8 Overbank fines elements |
| Diagnostic characteristics | Thin to thick blankets or drapes of overbank material, massive or laminated silts and clays. May include pedogenic horizons. Can occur as fill of abandoned channels. Major component of floodplain deposits. Can occur locally within sandar deposits, particularly in distal environments. |
| Principal engineering significance | Variable particle sizes, perched water tables, compressible soils |
Typical images
Fig. 3.74. (a) Overbank fines interstratified with Gms facies on Robillaird Island, west Banks Island, Arctic Canada (D.J.A. Evans). | |
Fig. 3.74. (b) Overbank fines elements interbedded with imbricated glaciofluvial gravels and cobbles, Ponsonnas, Isère, France (D. Giles). | |
| Principal references Miall (1985, 1996), Jones et al. (1999), Benn & Evans (2010) | |
| Structure name | 3.7.5.1 Periglacial involutions |
| Diagnostic characteristics | Contorted strata comprising unconsolidated sediments, organic material or brecciated bedrock on a centimetre to metre scale. Structures include folds, flame structures, diapirs, ball-and-pillow structures, broken and dislocated strata and lenses. Beneath flat ground, involutions occur as small-scale patterned ground. Beneath sloping ground, involutions are commonly elongated or overturned in downslope direction. Formed by repeated frost-action processes, resulting in mixing of soil. Non-sorted soil deformation features characterized by fluidized structures with smooth boundaries and laminae that generally occur on low-angle surfaces with little or no surface morphological expression. Can greatly vary in form with six different morphological structures recognized according to their sedimentary architecture based on symmetry and amplitude-wavelength ratio of folds, and frequency and patterns of occurrence. Cryogenic origin with dominantly liquid or plastic deformation of the soil. Termed involutions when the genesis of deformation is uncertain. |
| Principal engineering significance | Anisotropic permeability, variable particle sizes, anisotropic strength, anisotropic stiffness |
Typical images
Fig. 3.75. (a, b) Involutions shown in (a) horizontal section as small-scale, non-sorted patterned ground. Vertical section (b) through chalky coombe rock and gravel, College Lake Wildlife Centre, Marsworth, Buckinghamshire, England (Murton et al. 2015). | |
Fig. 3.75. (c) Flat-topped diapiric involution of clay extending upwards through sand and gravel, Broomfield, Essex, England (C. Whiteman). | |
| Principal references Ballantyne & Harris (1994), Vandenberghe (1988, 1992, 2013), Harris et al. (2000), Murton et al. (2003), French (2007) | |
| Structure name | 3.7.5.2 Periglacial creep folds |
| Diagnostic characteristics | Large-scale folds developed in zones of disturbed mudstones up to 20 m thick beneath gentle hillslopes. Folds include tight and kink types, and are locally overturned. Upwards increase in degree of disturbance and weathering. Unlike the structures associated with landslides, creep folds are not underlain or overlain by an extensive shear surface. |
| Principal engineering significance | Anisotropic permeability, anisotropic strength, shear surfaces, low shear strength |
Typical images
Fig. 3.76. (a) Intensely folded and partially overturned mudstones of the Green Ammonite Member, Charmouth Mudstone Formation, Seatown, Dorset, England (R. Gallois). | |
Fig. 3.76. (b) Sketch of section above highlighting the periglacial creep folds within the Charmouth Mudstone Formation (modified from Gallois 2010). | |
| Principal reference Gallois (2010) | |
| Structure name | 3.7.5.3 Platy structures |
| Diagnostic characteristics | Fractures more or less parallel to the original ground surface, therefore commonly horizontal to gently dipping. Fractures tend to be undulating and extend laterally for typically 30–150 mm. Vertical spacing between fractures is commonly several millimetres to a few centimetres. Platy aggregates of frost-susceptible sediment (usually silty or silt–clay mixtures) between fractures. Occur to depths of a few to several metres beneath the ground surface. |
| Principal engineering significance | Anisotropic permeability, low shear strength |
Typical images
Fig. 3.77. (a) Platy structure developed within granitic weathering profile. Image c. 1 m wide, Dartmoor, Devon, England (J. Murton). | |
Fig. 3.77. (b) Platy structure developed in silt- and clay-rich soil at the base of the active layer, Summer Island, Canada (J. Murton). | |
| Principal references Harris (1985), van Vliet-Lanoë (1985, 2010), Huijzer (1993), Ballantyne & Harris (1994), van Vliet-Lanoë et al. (2004) | |
| Structure name | 3.7.5.4 Relict periglacial shears |
| Diagnostic characteristics | Underlie sloping ground developed on weathered clay. Shear surfaces may be polished, striated or gleyed. Shallow, low-angle basal shears are the most extensive form, at depths of c.1.5–3.0 m, at or near the base of the reworked clay or top of destructured clay, and sub-parallel to ground surface. Deeper, subhorizontal continuous shears occur near base of weathered clay at depths of c. 4–8 m. Smaller, discontinuous shears produced by internal deformation during mass movement are commonly associated with both shallow and deeper shears. High-angle shears sometimes present, occur to depths of c. 3 m beneath slopes of 5° or less. Shears often difficult to see in freshly dug sections when soil is at natural water content. Time is needed for drying to cause shrinkage so that clays pull apart along shear surfaces. |
| Principal engineering significance | Ground movement, shear surfaces, low shear strength, anisotropic permeability |
Typical images
Fig. 3.78. (a) Basal shear surface with striations and polish in clay-rich head deposits, Sevenoaks, Kent, England (J.N. Hutchinson). | |
Fig. 3.78. (b) Close-up of principal slip surface and Riedel shears in trial pit F2, Sevenoaks Bypass, Kent, England (A. Weeks). | |
| Principal references Chandler (1970a, b, 1972a), Skempton & Weeks (1976), Harris (1977, 2013), Hutchinson (1991), Spink (1991), Skempton et al. (1991) | |
| Structure name | 3.7.5.5 Thermal contraction cracks |
| Diagnostic characteristics | In vertical cross-section, thermal contraction cracks in old ice wedges often form narrow V-shapes (2–4 m deep, 2–20 mm wide at ground surface). Exceptionally wide cracks (up to 200 mm) probably result from unusually high coefficients of thermal contraction. Cracks tend to be vertical as contraction is constrained laterally, and they taper downwards as horizontal tensile stresses decline with depth. Thermal contraction cracks in bedrock tend to be more irregular in size and shape than those in soils due to geological influences (e.g. joints, stratification, faults). They may infill with comminuted rock debris and steeply to vertically oriented elongate clasts. |
| Principal engineering significance | Anisotropic permeability, perched water tables, anisotropic strength |
Typical images
Fig. 3.79. (a) Thermal contraction crack formed in the second winter after drainage of Lake Illisarvik, Arctic Canada (J. R. Mackay). | |
Fig. 3.79. (b) Vein-like structure marking location of thermal contraction cracking, brecciated chalk, Grenham, Kent, England (J. Murton). | |
| Principal references Mackay (1974, 1975, 1990, 1992, 1993, 2000), Allen (1984), Huijzer (1993), Ballantyne & Harris (1994), Mackay & Burn (2002), Murton & Kolstrup (2003), French (2007), Murton (2013a), Murton & Giles (2016) | |
| Structure name | 3.7.5.6 Ice-wedge pseudomorphs |
| Diagnostic characteristics | Ice-wedge pseudomorphs or casts are common in terrestrial cold-climate sediment sequences. Ice wedges form by repeated infilling of thermal contraction cracks in permafrost and comprise steeply to vertically laminated bodies of ice. The size and shape of wedges, the relative age of wedge infill and host material depend on the interactions between thermal contraction cracking and ground-surface stability. Thaw of ice wedges may result in secondary infilling of the space left by melting ice, sometimes producing a pseudomorph of the original ice wedge. The infill usually comprises material similar to that beside or above it. Secondary infill is commonly massive, often poorly sorted and lacks systematic vertical trends in particle size. Vertical to steeply dipping layers, often indistinct, within some pseudomorphs. Concave-up sediment layers, stacked within gravelly pseudomorphs. Large clasts or faulted blocks of sediment within some pseudomorphs. Elongate clasts may be vertically to steeply aligned within pseudomorphs. Lining (a layer of fine sediment) borders the sides of some pseudomorphs. Tunnels or irregular voids infilled to various degrees. Downturned strata in adjacent host material. Normal faults, commonly stepped, adjacent to pseudomorphs. Intraformational structures develop wholly within a geological unit, whereas interformational structures extend down from the top of a geological unit. Where the host material is bedrock, the rock tends to be frost-susceptible and brecciated adjacent to vein- or wedge-shaped structures. Ice-wedge pseudomorphs often have a polygonal configuration in plan. |
| Principal engineering significance | Anisotropic permeability, anisotropic strength |
Typical images
Fig. 3.80. (a) Ice-wedge pseudomorph within periglacial fluvial sand and gravel, Whisby Quarry, Lincoln, England (J. Murton). | |
Fig. 3.80. (b) Ice-wedge pseudomorph extending down through involuted layer into brecciated chalk, Marsworth, Bucks, England (J. Murton). | |
Fig. 3.80. (c) Ice-wedge pseudomorph, Barmston, North Yorkshire, England (D. Giles). | |
| Principal references Paterson (1940), Higginbottom & Fookes (1970), Morgan (1971), Seddon & Holyoak (1985), Worsley (1987, 2014), Harris (1989), Hutchinson (1991), West (1993), Ballantyne & Harris (1994), Croot & Griffiths (2001), Waltham & Fookes (2001), Murton & Kolstrup (2003), Murton (2013a) | |
| Structure name | 3.7.5.7 Relict sand wedges |
| Diagnostic characteristics | Form by infilling of thermal contraction cracks with sand, usually aeolian. Infill comprises vertical to steeply dipping lamination, sometimes arranged in bundles. Sand veins may extend out from the sides or toes of wedges in sand (apophyses). Sand veins are commonly 1–10 mm thick, 100 mm–4 m deep, sharp planar to curved sides, internally massive, truncating stratification in host sand and, commonly, other sand veins. Veins may bifurcate and rejoin and are commonly laterally and vertically discontinuous. Inclusions of host material are sometimes cross-cut by sand veins. Pebbles or granules, where present, are generally concentrated at or near top of sand wedges and any gravel content very distinctly decreases with depth. Upturned strata in host material adjacent to sand wedges. Ventifact lags commonly extend laterally from tops of sand wedges. Large-scale polygonal network of wedges. |
| Principal engineering significance | Anisotropic permeability, anisotropic strength, variable particle sizes, low shear strength |
Typical images
Fig. 3.81. (a) Sand wedge with aeolian sand infill displaying vertical laminae, Great Waltham, Essex, England (C. Whiteman). | |
Fig. 3.81. (b) Buried, inactive sand wedge and adjacent sand veins, Richards Island, western Arctic coast, Canada (J. Murton). | |
| Principal references Worsley (1966, 2016), Rose et al. (1985), Ballantyne & Harris (1994), Murton et al. (2000), Hutchinson (2010), Murton (2013a) | |
| Structure name | 3.7.5.8 Composite-wedge pseudomorphs |
| Diagnostic characteristics | Composite wedges form by infilling of thermal contraction cracks with a mixture of ice and clastic sediment (usually sand). If the permafrost thaws, the resulting structure shows a combination of characteristics of ice-wedge pseudomorphs and relict sand wedges. |
| Principal engineering significance | Anisotropic permeability, anisotropic strength, variable particle sizes, low shear strength |
Typical images
Fig. 3.82. (a) Composite-wedge pseudomorph, with both primary and secondary infilling, Somersham, Cambridgeshire, England (R. West). | |
Fig. 3.82. (b) Partially thawed composite wedge. Dark, organic-rich infill above ice-rich part of wedge, Richards Island, Canada (J. Murton). | |
| Principal references West (1993), Fish et al. (1996), Briant et al. (1999), Murton & Kolstrup (2003), Murton (2013a) | |
| Structure name | 3.7.5.9 Vertical to steeply dipping elongate clasts |
| Diagnostic characteristics | Beneath flattish ground or within almost-horizontal strata the long (‘a’) axes of elongate pebbles and cobbles are dominantly vertical to steeply dipping (i.e. ‘on end’); such clasts are often found within or adjacent to involutions. Within some stony periglacial slope deposits, the intermediate (‘b’) axes are dominantly vertical or steeply dipping (i.e. ‘on edge’) and the long (‘a’) axes are aligned downslope. (Measurement of the orientation of the long, intermediate and short axes of each clast is required to distinguish those that are ‘on end’ from those ‘on edge’.) |
| Principal engineering significance | Anisotropic permeability |
Typical images
Fig. 3.83. (a) Vertical to steeply dipping elongate pebbles and cobbles within fluvial gravel, Llanon, Wales (J. Murton). | |
Fig. 3.83. (b) Vertical to steeply dipping clasts (flint) within involuted layer above brecciated chalk, Grenham Bay, Kent, England (J. Murton). | |
| Principal references Potts (1971), Watson & Watson (1971), Ballantyne & Harris (1994) | |
| Structure name | 3.7.5.10 Downslope-deflected strata |
| Diagnostic characteristics | Rockhead deformation which occurs within a deformed layer up to c.1 m thick between in situ bedrock and overlying granular head deposits; the deformed layer becomes increasingly fractured (brecciated) towards the top. Downslope movement of soil resulting from repeated cycles of volumetric expansion (frost heave) and contraction (thaw consolidation). Net downslope movement occurs because expansion occurs normal to the slope, but contraction has a gravity-imposed vertical component. Deflection may also result from creep of ice-rich permafrost. |
| Principal engineering significance | Anisotropic permeability |
Typical images
Fig. 3.84. (a) Downslope deflected strata in slate with overlying granular head deposits, Polurrian Cove, Cornwall, England (P. Ealey). | |
Fig. 3.84. (b) Downslope deflected slaty strata beneath granular head deposits at White Strand, Cahersiveen, Ireland (D. Moss). | |
| Principal references Cresswell (1983), Ballantyne & Harris (1994) | |
| Structure name | 3.7.5.11 Gulls |
| Diagnostic characteristics | Widened steeply inclined to vertical fissures or joints in the strata that may have been wholly or partially infilled with material from above or from the sides. Gulls can be closed at the top yet open out at depth to apertures of several metres. Commonly a vertical displacement of the strata across the gull. Formed as part of the cambering process as blocks of strata move downslope under permafrost conditions, most likely during thaw of ice-rich permafrost in incompetent mudrock beneath competent caprock in which the gulls develop. |
| Principal engineering significance | Ground movement, shear surfaces, voids, low shear strength, anisotropic permeability, anisotropic stiffness, high-permeability zones |
Typical images
Fig. 3.85. (a) Gulls in Jurassic Oolitic Limestone, Quarry Road, University of Bath, NE Somerset, England (D. Giles). | |
Fig. 3.85. (b) Gulls enhanced by dissolution in Hythe Formation calcareous sandstones, Hermitage Quarry, Kent, England (D. Giles). | |
| Principal references Hollingworth et al. (1944), Higginbottom & Fookes (1970), Chandler et al. (1976), Horswill & Horton (1976), Hawkins & Privett (1981), Worssam (1981), Hutchinson (1991, 1992), Parks (1991), Ballantyne & Harris (1994), Hutchinson & Coope (2002), Hawkins (2013), Self & Farrant (2013), Farrant et al. (2015) | |
| Structure name | 3.7.5.12 Dip and fault structures |
| Diagnostic characteristics | Series of discrete blocks of capping strata, separated by parallel gulls, which have rotated to give a downhill dip greater than the general dip of the slope camber. Formed under permafrost conditions with a competent capping rock overlying a more ductile clay layer. Formed as part of the cambering process. None of these structures have been shown to be moving under present-day environmental conditions. |
| Principal engineering significance | Ground movement, shear surfaces, voids, low shear strength |
Typical images
Fig. 3.86. (a) Dip and fault structures, Bath University, Bath, North East Somerset, England (D. Giles). | |
Fig. 3.86. (b) Dip and fault structures showing rotation and displacement, Bath University, Bath, NE Somerset, England (D. Giles). | |
| Principal references Hollingworth et al. (1944), Higginbottom & Fookes (1970), Chandler et al. (1976), Horswill & Horton (1976), Hawkins & Privett (1981), Hutchinson (1991, 1992), Parks (1991), Ballantyne & Harris (1994), Hutchinson & Coope (2002), Hawkins (2013) | |
| Structure name | 3.7.5.13 Superficial valley disturbances and valley bulges |
| Diagnostic characteristics | In alternations of competent rocks and weaker shales evidence of dilation, faulting or crumpling may underlie valley bottoms, with joint opening but with little flexure of valley sides. Where a competent rock caps stiff clays and shales, argillaceous strata from beneath valley sides may extrude upwards beneath valley bottoms, producing a valley bulge and sympathetic flexures and subsidence of the capping rock. Deformation can be intensive. With predominantly well-bedded ductile stiff clays with little capping rock, multiple folds and wrinkles can occur. Displacements resulting from stress relief accompany the formation of all valleys, enhanced during periods of greater flow and rapid valley down-cutting (e.g. during postglacial melt). Generates an upwards movement at the base of the valley and an inwards movement at its sides. Frost heave and thaw consolidation through multiple freeze–thaw cycles can add to these valley-floor disturbances. Mode of disturbance depends on geological structure and lithology present controlling brittleness and ductility, together with the degree of periglacial influence. Possible arching, voids and décollement planes present within the disturbed ground. |
| Principal engineering significance | Ground movement, shear surfaces, voids, low shear strength, anisotropic permeability |
Typical images
Fig. 3.87. (a) Near isoclinal folding in Jurassic mudstones and clays, A46 Batheaston Bypass, NE Somerset, England (D. Giles). | |
Fig. 3.87. (b) Superficial valley disturbances, Newbridge Ball Clay Quarry, Devon, England (K. Privett). | |
| Principal references Hollingworth et al. (1944), Higginbottom & Fookes (1970), Chandler et al. (1976), Horswill & Horton (1976), Hawkins & Privett (1981), Hutchinson (1991, 1992), Parks (1991), Ballantyne & Harris (1994), Hutchinson & Coope (2002) | |
| Structure name | 3.7.5.14 Buried hollows |
| Diagnostic characteristics | Drift-filled hollows penetrating deeply into the underlying strata. Encountered beneath Quaternary terrace gravels, 90–470 m top width, with the base of the depression up to 30 m below ground level. In some cases the underlying strata have risen as diapirs into the bottom of the depression. Several hypotheses exist on mode of formation, including scour at river channel confluences, bottom-up subsidence features or remnants of former pingos, thermokarst features, formed through complex local subsidence or possible ice-wedge or frost-heaved diapiric structures. |
| Principal engineering significance | Variable particle sizes, anisotropic permeability, compressible soils, anisotropic stiffness, anisotropic strength, groundwater, perched water tables, voids |
Typical images
Fig. 3.88. (a) Conceptualized buried hollow geometry, Battersea Power Station, London, England (Clayton 2009). | |
Fig. 3.88. (b) Buried hollow at site of Blackwell Tunnel, London, England (modified from Hutchinson 1991). | |
| Principal references Wakeling & Jennings (1976), Berry (1979), Hutchinson (1980, 1991), Griffin (1988), Newman (2009), Banks et al. (2015), Collins et al. (2015) | |
3.7.6 Superimposed or overprinted sedimentary and structural signatures
Glacial and periglacial processes often operate on existing materials deposited as a product of previous process-form regimes, and may therefore only modify the characteristics of the original deposit. Glaciotectonite is an excellent example of this inheritance of sedimentological signatures, whereby Type B glaciotectonite may display a very strong pre-deformation signature. Similarly, periglacial processes will commonly modify and re-distribute component materials of pre-existing glacial deposits. The combinations of end-member products are infinite, but the following figures give some indication of overprinting signatures that might be observed in the field.
| 3.7.6.1 Interbedded diamictons and thin stratified lenses |
Fig. 3.89. Interbedded diamictons and thin stratified lenses representative of paraglacially reworked till (debris-flow diamictons), Leirdalen, Norway (A. Curry). |
| Principal references Curry (2000), Curry & Ballantyne (1999), Harris (1998) |
3.7.6.2 Glaciotectonite
Fig. 3.90. Glaciotectonite showing the amalgamation zone between Type A glaciotectonite (top), Type B glaciotectonite (middle) and stratified periglacial slope deposits (base), Ireland (D.J.A. Evans). |
| Principal reference Ó Cofaigh & Evans (2001) |
| 3.7.6.3 Ice-wedge pseudomorph devloped in glaciolacustrine sediments |
Fig. 3.91. Ice-wedge pseudomorph developed in glaciolacustrine sediments, Banks Island, Canada (Evans et al. 2014b) (D.J.A. Evans). |
| Principal reference Evans et al. (2014b) |
| 3.7.6.4 Ground ice developed in refrozen mass-flow deposits |
Fig. 3.92. Ground ice (dark grey) developed in refrozen mass-flow deposits (medium grey). Pencil marks a thaw unconformity between the base of the refrozen mass-flow deposits (base of early Holocene active layer) and underlying buried basal ice of the Laurentide Ice Sheet, Summer Island, Canada (J. Murton). |
| Principal references Murton & French (1994), Murton et al. (1995, 2005) |
| 3.7.6.5 Periglacial involutions within refrozen mass-flow deposits |
Fig. 3.93. Periglacial involutions (load casts of brown sand) within refrozen mass-flow deposits (dark grey). Base of spade marks base of a thaw unconformity between the base of the refrozen mass-flow deposits (base of early Holocene active layer) and underlying buried basal ice of the Laurentide Ice Sheet, Summer Island, Canada (J. Murton). |
| Principal references Murton & French (1994), Murton et al. (1995, 2005) |
| 3.7.6.6 Banded massive ground ice beneath glacial diamicton |
Fig. 3.94. Banded massive ground ice about 5 m thick beneath several metres of glacial diamicton (till and colluvium derived from till), near Tuktoyaktuk, Canada. The ice formed by ice segregation and/or injection of water under high pressure after deposition of till. Anticline marks recent upwards bulging of massive ice beneath a small thaw slump (depression at surface), somewhat similar to valley bulging in clay in the UK. (J. Murton). |
| Principal references Mackay & Dallimore (1992), Waller et al. (2011, 2012) |
3.8 Microstructures in glacial and periglacial sediments
Readers and users of this book will be increasingly familiar with some of the dramatic effects that glacial and periglacial processes have on our current and relict landscapes. There are many photographs in this chapter and throughout this book that show outcrops and manifestations of these features. However, all practitioners, and not least the engineering geologists, should bear in mind that the periglacial and glacial processes can also have an equally dramatic effect on the soils and rocks at a microscopic scale.
The effects of ice formation and thaw causes a wide range of small-scale features which have significant impacts on the key engineering parameters of strength, deformability and permeability of many soils. For example:
The previous presence of ice in a soil, sometimes through repeated freeze–thaw cycles, will de-structure many rocks and soils.
Repeated freeze–thaw cycles also introduce great heterogeneity at a microscopic scale; these features are present and described as part of the weathering profile across the Northern Hemisphere.
The heterogeneity can include particle realignment at a variety of scales, and this is now well understood to have very great impacts on the shear strength of soils. Where this has not been fully understood is evinced by the back catalogue of major soil and embankment failures in periglacially affected ground.
Cryoturbation can change the anisotropy of the soil mass.
Ice can introduce voids in the soil which can remain present after the ice has gone, possibly affecting the mass permeability, or in which there has been subsequent deposition or growth of material or crystal linings.
The analyses of thin-sections of glacial deposits allows the identification of a range of features that not only replicate at microscale what is observed at macroscale, but also provide observations and data on diagnostic structures not visible at outcrop. For example:
Deformation structures in particular are now widely identified in tills with their relatively high-strain signatures and can be differentiated from the lower-strain indicators of gravity mass flow.
Contemporaneous liquefaction and water-escape conduits that trace the passage of pressurized water (hydrofractures) during shear can also be identified.
Styles of deformation and material origins can be assessed through the differentiation of glaciotectonic (shear) lamination v. glaciotectonite derived from stratified sediments.
More challenging but still possible is the distinguishing of subaqueous massive diamictons from subglacial matrix-supported tills, through the recognition of a range of deformation structures in subglacial materials such as crushed quartz grains, grain lineations, pressure shadows, augen or boudin-shaped instraclasts, unistrial and masepic fabric domains, rotational features and unidirectional fabrics. These can be contrasted with those in glaciomarine sediments, such as winnowed plasma texture, dropstones, in situ microfossils, a lack of shear deformation structures, graded bedding, a lack of plasmic fabric and high microfabric dip angles similar to those recognized in clast macrofabrics, together with vertical lineations created by the passage of dropstones through soft-bottom sediments.
Some depositional signatures, however, remain difficult to differentiate; larger-scale stratigraphic context is therefore critical to genetic interpretation. For example, the deformation structures observed in iceberg-turbated deposits are predominantly impossible to differentiate from those of subglacial deformation based upon micromorphology alone.
Many of these small-scale features are described and illustrated in the following sections. For more examples of glaciogenic microstructures, see Menzies & Maltman (1992), van der Meer (1993, 1996), Hiemstra & Rijsdijk (2003) and Phillips et al. (2007, 2011a, b, 2013a, b).
3.8.1 Periglacial microstructures in engineering soils
Periglacial microstructures in engineering soils provide valuable information about processes and geohazards. Where the soil is frost-susceptible (i.e. silt or silt-clay mixtures), such microstructures are particularly abundant. Under active periglacial conditions, moist frost-susceptible soils typically develop ice lenses in the upper centimetres to metres of frozen ground. When the ice lenses melt, they often leave a distinctive ‘platy’ or ‘lenticular’ microstructure. In fact, these structures are often visible to the naked eye as ‘platy structures’ (Section 3.7). Both micro- and macrostructures indicate the former occurrence of the ice-segregation process and result in anisotropic soil strength and permeability, with water able to percolate more freely laterally than vertically.
Another key periglacial microstructure to watch out for in ground investigations on argillaceous substrates, especially in clay-rich head deposits, is relict periglacial shears. As with platy structures, shears can be observed both micro- and macroscopically. They can be easier to record in thin-section than in the field however, because sharp discontinuities seen through a microscope are independent of soil moisture whereas macroscopic shears often only become apparent in the faces of trial pits or natural sections as the soil dries and the shears open a little. If ground investigations do not allow time for repeated observation of drying faces, then it may therefore be prudent to collect several oriented blocks of undisturbed soil, impregnate them with resin and identify any periglacial microstructures within them. Where relict periglacial shears are identified, there is a risk of their reactivation and so measures can be taken to limit this risk.
| 3.8.1.1 Relict periglacial shear microstructure Shear surfaces within cohesive bedrock or clay-rich head deposits that are no longer active under present-day environmental conditions, but which may be reactivated (e.g. by excavation or loading). |
 Fig. 3.95. (a) Plane-polarized light (PPL) and (b) cross-polarized light (XPL). Petrographic thin-section through the main pre-existing shear surface at Walton's Wood, Staffordshire, England. The preparation of a thin-section of a clay to such resolution was a first in engineering geology. This pioneering work was achieved in the laboratory of Soil Mechanics Ltd in December 1962. Early & Skempton (1972) recorded that ‘After several weeks experimenting, Mr Early and Mr Nenninger succeeded in producing a good thin section across the slip surface, using the Carbowax technique. Under the microscope, with crossed nicols, strong orientation of the clay particles along the slip surface is observed. The ambient clay shows almost random orientation’ (D. Norbury). |
| Principal references Early & Skempton (1972), Skempton & Weeks (1976), Hutchinson (1991), Spink (1991), Harris (2013) |
| 3.8.1.2 Platy or lenticular microstructure Comprises platy or lenticular peds separated by elongate voids that mark the sites of former ice lenses. The microstructure develops by ice segregation in silty and silt-clay soils. |
Fig. 3.96. Lenticular microstructure (‘lamellar fabric’) in an earth hummock, PPL. Peds are brown, voids are white. Vallonnet Alps, France (B. van Vliet-Lanoë). |
| Principal references Harris (1985), van Vliet-Lanoë (1985, 2010), van Vliet-Lanoë et al. (2004) |
| 3.8.1.3 Banded microstructure or ‘banded fabric’ Comprises alternating bands of coarser and finer soil. It develops where particles of silt or clay accumulate as cappings on the upper face of platy or lenticular peds. Banded microstructure occurs in soils subject to repetitive freeze–thaw cycles, where platy and lenticular microstructure is affected by micro-erosion, particle translocation and plastic deformation. |
Fig. 3.97. Banded fabric in an earth hummock, PPL, Vallonnet Alps, France (B. van Vliet-Lanoë). |
| Principal references Harris (1985), van Vliet-Lanoë (1985, 2010), van Vliet-Lanoë et al. (2004) |
| 3.8.1.4 Granular microstructure Consists of granular peds and develops where frequent freeze–thaw cycles and variable freezing directions, usually in the upper part of a very fine silt to clayey soil, cause rounding of originally angular blocky peds, or where infiltrating meltwater disrupts pre-existing clay coatings or infillings. Rotation of soil peds may occur as a result of differential frost heave and/or solifluction. |
Fig. 3.98. Granular microstructure in an earth hummock, PPL, Vallonnet Alps, France (B. van Vliet-Lanoë). |
| Principal references Harris (1985), van Vliet-Lanoë (1985, 2010), Murton & French (1994), van Vliet-Lanoë et al. (2004) |
| 3.8.1.5 Silt cappings microstructure Cappings of silt and coarse clay on the top of coarse grains or peds develop by downwards translocation of fine sediment, as ice melts within thawing soil. The cappings tend to resist collapse or dispersion because of desiccation, mechanical compaction and flocculation in soil subject to repetitive freezing and thawing. |
Fig. 3.99. Silty cappings (dark brown) on top of sediment aggregates in sorted patterned ground, PPL, Mount Jacques Cartier, Quebec, Canada (B. van Vliet-Lanoë). |
| Principal references Harris (1985), van Vliet-Lanoë (1985, 2010), van Vliet-Lanoë et al. (2004) |
| 3.8.1.6 Vesicular microstructure Voids that tend to be rounded in silty or sandy soils and mammilated in clayey or organic soils, and are often found in the upper centimetres of frost-affected ground. Vesicles have been attributed to expulsion of air during either freezing or thawing. Also termed ‘pinhole structures’. |
Fig. 3.100. Vesicles or pinholes (white) in silt-sand soil in upper horizon of a sorted circle, PPL, field of view (FOV) = 9 mm wide, Ny-Ålesund, Spitsbergen, Svalbard, Norway (B. van Vliet-Lanoë). |
| Principal references Harris (1985), van Vliet-Lanoë (1985, 2010), van Vliet-Lanoë et al. (2004) |
3.8.2 Periglacial microstructures superimposed on glaciogenic sediments
Periglacial microstructures are commonly superimposed on glaciogenic sediments because periglacial conditions typically follow deglaciation. In addition, many locations in southern England, for example, lay between the maximum ice limit in the Middle Pleistocene (c.430 ka, near London) and the maximum ice limit in the Late Pleistocene (c. 20 ka, in northern or eastern England). The extensive till sheets of midland and eastern England have therefore suffered widespread periglacial modification. Evidence for this can be observed as macrostructures (Section 3.7) and as periglacial microstructures similar to those described previously. Below are some examples of such microstructures that have developed on glaciogenic sediments.
| 3.8.2.1 Platy or lenticular microstructure Comprises platy or lenticular peds separated by elongate voids that mark the sites of former ice lenses. The microstructure develops by ice segregation in silty and silt–clay soils. |
 Fig. 3.101. (a) PPL and (b) XPL. Platy or lenticular microstructure (‘flattened pebbles’ in van der Meer et al. 1993) in diamicton from a crevasse squeeze ridge. Horizontal to subhorizontal voids (black in lower image) are thought to mark the sites of former ice lenses. FOV = 18 mm wide, Inexpressible Island, Nansen Ice Shelf, Terra Nova Bay, Victoria Land, Antarctica (J. van der Meer). |
| Principal reference van der Meer et al. (1993) |
| 3.8.2.2 Granular microstructure Consists of granular peds and develops where frequent freeze–thaw cycles and variable freezing directions, usually in the upper part of a very fine silt to clayey soil, cause rounding of originally angular blocky peds, or where infiltrating meltwater disrupts pre-existing clay coatings or infillings. Rotation of soil peds may occur as a result of differential frost heave and/or solifluction. Aggregates of similar size and shape can also occur within ice lenses, massive segregated-intrusive ice and debris-rich glacier ice; such aggregates may form during and/or prior to ice segregation, although the exact mechanism is uncertain. |
Fig. 3.102. (a) PPL and (b) XPL with gypsum wedge. Pebble structure in silty-clay diamicton (till). ‘Pebbles’ comprise sediment aggregates attributed to freeze–thaw cycles and gelifluction of till. Calcitans (see below) partially or completely surround many aggregates. FOV = 11.2 mm wide, Mount Provender, Shackleton Range, Antarctica (J. van der Meer). |
| Principal reference van der Meer et al. (1993) |
| 3.8.2.3 Silt cappings microstructure Cappings of silt and coarse clay on the top of coarse grains or peds develop by downwards translocation of fine sediment, as ice melts within thawing soil. The cappings tend to resist collapse or dispersion because of desiccation, mechanical compaction and flocculation in soil subject to repetitive freezing and thawing. |
 Fig. 3.103. (a) PPL and (b) XPL. Silt capping in diamicton from a crevasse squeeze ridge. FOV = 5.9 mm wide, Inexpressible Island, Nansen Ice Shelf, Terra Nova Bay, Victoria Land, Antarctica (J. van der Meer). |
| Principal reference van der Meer et al. (1993) |
| 3.8.2.4 Calcitans microstructure Calcitans are discontinuous coatings of secondary calcium carbonate that form by precipitation beneath particles. In this example, they are not found on the same side of the aggregates but in different orientations, which indicates that the aggregates have rotated after the calcitans started to form. Rotation probably accompanied gelifluction. |
Fig. 3.104. (a) PPL and (b) XPL with gypsum wedge. Calcitans (dark brown in (a)) surrounding sediment aggregates (‘pebble structure’; multicoloured in (b)) in silty-clay till, FOV = 6.4 mm wide, Mount Provender, Shackleton Range, Antarctica (J. van der Meer). |
| Principal reference van der Meer et al. (1993) |
| 3.8.2.5 Clay cutans or ferri-argillans microstructure Cutans are thought to have formed by illuviation from percolating water, even in the dry environment of Antarctica. |
Fig. 3.105. (a) PPL and (b) XPL. Cutans or ferri-argillans (brown in both images) infilling pores in diamicton from a crevasse squeeze ridge, FOV = 5.9 mm wide, Inexpressible Island, Nansen Ice Shelf, Terra Nova Bay, Victoria Land, Antarctica (J. van der Meer). |
| Principal reference van der Meer et al. (1993) |
| 3.8.2.6 Oriented clay domains microstructure Small clusters of clay particles (indicated by interference colours) are oriented parallel and orthogonal to the surface of the grains or aggregates that they encase; the encasing therefore forms a lattice-like pattern (termed ‘latti-skelsepic plasmic fabric’). The strong reorientation of clays is attributed by van der Meer et al. (1993) to freeze–thaw and/or wetting/drying processes, and the plasmic fabrics are interpreted to indicate that rotation of aggregates was more important than planar movement. Voids are black. |
Fig. 3.106. XPL. Oriented clay domains in silty-clay till, FOV = 7.0 mm wide, Mount Provender, Shackleton Range, Antarctica (J. van der Meer). |
| Principal reference van der Meer et al. (1993) |
| 3.8.2.7 Banded microstructure Comprises alternating bands of coarser and finer soil. It develops where particles of silt or clay accumulate as cappings on the upper face of platy or lenticular peds. Banded microstructure occurs in soils subject to repetitive freeze–thaw cycles, where platy and lenticular microstructure is affected by micro-erosion, particle translocation and plastic deformation. In the images below, the banding is indicated by subhorizontal curving bands that are clear in the upper image and brown in the lower. |
Fig. 3.107. (a) PPL and (b) XPL. Banded microstructure in sandy till, FOV = 18.0 mm wide. Ter Idzard, Netherlands (J. van der Meer). |
| Principal reference Rappol et al. (1989) |
3.8.3 Glaciogenic sediment microstructures
3.8.3.1 Base of subglacial traction till
Fig. 3.108. Laminated silt and very fine-grained sand overlain by gravelly diamicton containing sub-angular to rounded locally derived fragments of sandstone. The laminated fine-grained sediments represent the ‘high strain zone’, which forms the contact between the diamicton and underlying subglacially deformed sediments (Phillips et al. 2007). |
| Principal references Phillips (2006), Phillips et al. (2007, 2008, 2011a, b, 2013a, b), Phillips & Auton (2008), Phillips & Merritt (2008), Burke et al. (2009), Lee et al. (2013), Phillips & Lee (2013), Phillips & Hughes (2014) |
3.8.3.2 Base of subglacial mass-flow diamicton
Fig. 3.109. Base of subglacial mass-flow diamicton into meltwater canal. Shows localized shearing in diamicton, and disruption and fragmentation due to liquefaction of underlying stratified sediments, Whitburn, County Durham, England (E. Phillips, 2016, pers. comm.). |
| Principal references Phillips (2006), Phillips et al. (2007, 2008, 2011a, b, 2013a, b), Phillips & Auton (2008), Phillips & Merritt (2008), Burke et al. (2009), Lee et al. (2013), Phillips & Lee (2013), Phillips & Hughes (2014) |
3.8.3.3 Bedrock-rich subglacial traction till
Fig. 3.110. Diagram showing the orientation of the long and short axes of coarse sand to gravel-sized clasts within sample (Phillips & Auton 2008). |
| Principal references Phillips (2006), Phillips et al. (2007, 2008, 2011a, b, 2013a, b), Phillips & Auton (2008), Phillips & Merritt (2008), Burke et al. (2009), Lee et al. (2013), Phillips & Lee (2013), Phillips & Hughes (2014) |
3.8.3.4 Faulted lake sediments
Fig. 3.111. Representative thin-section of deformed laminated silt and clay overlain by sand. Deformation structures include sand-filled hydrofractures and water escape conduits, soft-sediment deformation structures (disharmonic folds and convolute lamination) and a prominent reverse fault marked by lenses of sheared clay (Phillips et al. 2007). |
| Principal references Phillips (2006), Phillips et al. (2007, 2008, 2011a, b, 2013a, b), Phillips & Auton (2008), Phillips & Merritt (2008), Burke et al. (2009), Lee et al. (2013), Phillips & Lee (2013), Phillips & Hughes (2014) |
3.8.3.5 Hydrofracture
Fig. 3.112. Annotated scan of thin-section showing main structures within the complex, cross-cutting sequence of sand, silt and clay fills present within the subvertical section of a laterally extensive hydrofracture system present (scale bar, 10 mm) (Phillips & Merritt 2008). |
| Principal references Phillips (2006), Phillips et al. (2007, 2008, 2011a, b, 2013a, b), Phillips & Auton (2008), Phillips & Merritt (2008), Burke et al. (2009), Lee et al. (2013), Phillips & Lee (2013), Phillips & Hughes (2014) |
3.8.3.6 Massive till
Fig. 3.113. (a, b) Microstructural map showing the relationships between the clast microfabrics and spatially related microstructures (arcuate grain alignment, etc.) developed within this late Devensian subglacial traction till. Basal grey-brown metasandstone and granite-rich till (Phillips et al. 2011a, b). |
| Principal references Phillips (2006), Phillips et al. (2007, 2008, 2011a, b, 2013a, b), Phillips & Auton (2008), Phillips & Merritt (2008), Burke et al. (2009), Lee et al. (2013), Phillips & Lee (2013), Phillips & Hughes (2014) |
3.8.3.7 Soft-sediment deformation
Fig. 3.114. Thin-section characterized by soft-sediment deformation (folding, faulting) of laminated fine sand, silt and clay. Deformation is most intense in the lower and central parts of the section, with these sediments also showing evidence of fluidization and remobilization of the silt to fine-sand laminae (scale bar = 10 mm) (Phillips 2006). |
| Principal references Phillips (2006), Phillips et al. (2007, 2008, 2011a, b, 2013a, b), Phillips & Auton (2008), Phillips & Merritt (2008), Burke et al. (2009), Lee et al. (2013), Phillips & Lee (2013), Phillips & Hughes (2014) |
| 3.8.3.8 Pseudo-stratified diamicton Created by liquefaction at the ice–bed interface, these are in fact subglacial traction tills in which tectonic lamination is created during shear of liquefaction features. 
Fig. 3.115. (a) Microstructural map and high-resolution scan taken from the thinly stratified diamicton. The orientation of the long axes of the fine-sand- to pebble-sized clasts included within the diamicton layers (labelled A–F) are shown in the series of rose diagrams. Inset is a false colour image of the thin-section highlighting the distribution of the clay-rich laminae within this stratified till (Phillips et al. 2013a, b). |
Fig. 3.115. (b) Stratified diamicton from 5 m thick multiple diamicton sequence. Shows diffuse stratification in a subglacial diamicton due to disruption and liquefaction of pre-existing stratified sediments, Nairn, Scotland (E. Phillips, 2016, pers. comm.). |
| Principal references Phillips (2006), Phillips et al. (2007, 2008, 2011a, b, 2013a, b), Phillips & Auton (2008), Phillips & Merritt (2008), Burke et al. (2009), Lee et al. (2013), Phillips & Lee (2013), Phillips & Hughes (2014) |
3.8.3.9 Glaciotectonite in lake sediments
Fig. 3.116. Deformed laminated clay, silt and fine-grained sand. (a) Well-developed low-angle reverse faults and thrusts in sample. Minor folding and faulting occurs in the hanging walls of these structures. (b) Layer-parallel thrusting in sample resulted in localized imbrication and repetition of the very fine-grained sediments. Minor injection of fluidized sand occurred along the reverse faults deforming the laminated sand, silt and clay in the upper part of the sample, Coire Mhic-sith, Drumochter, Scotland (Phillips et al. 2007). |
| Principal references Phillips (2006), Phillips et al. (2007, 2008, 2011a, b, 2013a, b), Phillips & Auton (2008), Phillips & Merritt (2008), Burke et al. (2009), Lee et al. (2013), Phillips & Lee (2013), Phillips & Hughes (2014) |
3.8.3.10 Till over laminated lake sediments
Fig. 3.117. Example of a completed microstructural map of polydeformed, thinly laminated sand silt and clay overlain by subglacial traction till, Strathspey, Scotland (Phillips et al. 2011a, b). |
| Principal references Phillips (2006), Phillips et al. (2007, 2008, 2011a, b, 2013a, b), Phillips & Auton (2008), Phillips & Merritt (2008), Burke et al. (2009), Lee et al. (2013), Phillips & Lee (2013), Phillips & Hughes (2014) |
3.8.3.11 Melt-out till
Fig. 3.118. Micro-characteristics of the bedded diamicton interpreted as melt-out till. Thin-section showing silt-rich laminated diamicton with laminations, black lines (black lines), deformed aggregates of clayey silt (yellow fill) and porewater pathways (magenta lines). Note that aggregates occur in bands and have their long axes generally sub-parallel to lamina, Matanuska Glacier, Alaska (Larson et al. 2016). |
| Principal references Phillips (2006), Phillips et al. (2007, 2008, 2011a, b, 2013a, b), Phillips & Auton (2008), Phillips & Merritt (2008), Burke et al. (2009), Lee et al. (2013), Phillips & Lee (2013), Phillips & Hughes (2014), Larson et al. (2016) |
3.9 Terrain unit descriptors
The tables in Sections 3.10–3.11 document the variety of landforms that can be expected in terrains that have previously experienced glacial and periglacial conditions. The entries should be used to develop ground models for these environments and to inform Risk Registers compiled for ground engineering projects being undertaken in these areas. Each entry has the structure of the following table.
| Terrain unit | Landform feature name to be used in the developed ground model or Risk Register. |
| Image | Image of relict form as aid to field recognition. Engineers need to understand the modern analogue, but more significantly need to recognize the relict form in site investigations. |
| Form/topography | Diagnostic characteristics for field recognition. |
| Landsystem | The landsystem in which the described landform could be expected to be encountered (Table 3.4 for glacial landsystems, and Table 3.5 for periglacial landsystems). |
| Process of formation | Details of the key glacial/periglacial processes involved in the landform formation. |
| Modern analogue | Image of modern landform analogue as an aid to identification of relict forms. Examples of where features can be observed today. |
| Associated features | Other landforms that could be expected to be present in the developed ground model or Risk Register. |
| Principal engineering significance | Significance of landform with respect to developed ground model or Risk Register (Table 3.1 detailing the engineering aspect and significance). |
| Principal references | List of key definitive references for landform, together with engineering geological case studies. |
3.10 Glacial landsystems
This section outlines landforms that could be expected to be encountered in environments that have been directly affected by glacial ice and should form part of ground models developed for these terrains. Photographs show well-developed examples of the landforms, both as relict examples together with modern analogues.
| Terrain unit | 3.10.1.1 Areal scouring |
| Image |
Fig. 3.119. (a) Landscape of areal scour, Inchnadamph, Scotland (D.J.A. Evans). |
| Form/topography | Irregular depressions with intervening bosses sometimes labelled ‘knock and lochan’ topography. |
| Landsystem | Ice-sheet-related landsystem: subglacial footprint |
| Process of formation | Large-scale ice-sheet erosion of bedrock in lowland areas. |
| Modern analogue |
Fig. 3.119. (b) Active glacial areal scouring, Hoffellsjökull, Iceland (D.J.A. Evans). |
| Associated features | Megascale glacial lineations, megaflutes and megagrooves, riegel, rock basins, whalebacks, roche moutonnées, crag and tails, glacial erratics |
| Principal engineering significance | Voids, compressible soils, shattered bedrock |
| Principal references | McManus (1970), Rea & Evans (1996), Glasser & Bennett (2004), Benn & Evans (2010), Evans (2013b) |
| Terrain unit | 3.10.1.2 Glaciated valleys and glacial troughs |
| Image |
Fig. 3.120. (a) Trough to north of Mount Edith Cavell, Canadian Rockies, Canada (D.J.A. Evans). |
| Form/topography | A relatively straight, steep-sided, U-shaped valley with a parabola cross-profile and irregular long profile with rock riegel and over-deepened rock basins being typical features. |
| Landsystem | Upland glacial landsystem (bedrock terrain): subglacial footprint |
| Process of formation | Eroded channels of present or former glaciers or ice streams. |
| Modern analogue |
Fig. 3.120. (b) Modern trough formed by debris-covered Hooker Glacier, New Zealand (D.J.A. Evans). |
| Associated features | Hanging valley, arêtes, trough heads, cirques, cols, horns, nunataks, roche moutonnées, crag and tails, glacial erratics |
| Principal engineering significance | Over-steepened zones, overstressed zones, shattered bedrock, sheared bedrock, variable particle sizes, anisotropic permeability, anisotropic strength |
| Principal references | Nichol & Reynolds (2002), Glasser & Bennett (2004), Benn et al. (2005), Waltham (2008), Benn & Evans (2010), Evans (2013b), Long et al. (2015) |
| Terrain unit | 3.10.1.3 Hanging valleys |
| Image |
Fig. 3.121. (a) Hanging valley, Labrador, Canada (D.J.A. Evans). |
| Form/ topography | Tributary valley whose floor is well above that of the adjacent main valley. |
| Landsystem | Upland glacial landsystem (bedrock terrain): subglacial footprint |
| Process of formation | Typical of glaciated uplands where it results from glacial widening and/or deepening of the main valley. |
| Modern analogue |
Fig. 3.121. (b) Modern hanging valleys, Hochstetter icefall, New Zealand (D.J.A. Evans). |
| Associated features | Glaciated valleys and glacial troughs, arêtes, trough heads, cirques, cols, horns, nunataks, roche moutonnées, crag and tails, glacial erratics |
| Principal engineering significance | Shattered bedrock, sheared bedrock, over-steepened zones |
| Principal references | Benn et al. (2005), Waltham (2008), Benn & Evans (2010), Evans (2013b) |
| Terrain unit | 3.10.1.4 Arêtes |
| Image |
Fig. 3.122. (a) Arêtes, Oksfjordjokelen, Norway (D.J.A. Evans). |
| Form/topography | A sharp, narrow, often pinnacled ridge separating two valleys. |
| Landsystem | Upland glacial landsystem (bedrock terrain): subglacial footprint |
| Process of formation | Typically formed when two glaciers erode parallel valleys. Arêtes can also form when two opposing cirques erode headwards towards one another, although frequently this results in a saddle-shaped col. The arête edge is sharpened by freeze–thaw weathering and the slope on either side of the arête steepened through mass-wasting events and the erosion of exposed unstable rock. |
| Modern analogue |
Fig. 3.122. (b) Arêtes and glaciers near Kunzom La, India (D.J.A. Evans). |
| Associated features | Glaciated valleys and glacial troughs, trough heads, cirques, cols, horns, nunataks |
| Principal engineering significance | Shattered bedrock |
| Principal references | Glasser & Bennett (2004), Benn et al. (2005), Waltham (2008), Benn & Evans (2010), Evans (2013b) |
| Terrain unit | 3.10.1.5 Glacial watershed breaches |
| Image |
Fig. 3.123. (a) Glacial watershed breach, Belledonne Massif, France (D. Giles). |
| Form/topography | Glacially eroded trough that cuts through a ridge and so breaches the original valley watershed. |
| Landsystem | Upland glacial landsystem (bedrock terrain): subglacial footprint |
| Process of formation | Formed when the outflow of a glacier is impeded, its thickness consequently increases, and ultimately a new escape route (the breach) is exploited. Process termed diffluence when a single glacier spills out of its valley and transfluence when several breaches are formed due to the accumulation of a large ice sheet. |
| Modern analogue |
Fig. 3.123. (b) Watershed breaching by Hoffellsjökull, Iceland (S. Gudmundsson). |
| Associated Features | Arêtes, glaciated valleys and glacial troughs, trough heads, cirques, cols, horns, nunataks |
| Principal engineering significance | Shattered bedrock |
| Principal references | Benn et al. (2005), Waltham (2008) |
| Terrain unit | 3.10.1.6 Whalebacks |
| Image |
Fig. 3.124. (a) Streamlined whalebacks, Coire Lagan, Skye, Scotland (D.J.A. Evans). |
| Form/topography | A streamlined smoothed or scratched bedrock knoll with symmetrical longitudinal profiles, several metres to a few hundred metres high, resembling a whale in profile. |
| Landsystem | Upland glacial landsystem (bedrock terrain): subglacial footprint |
| Process of formation | Formed by abrasion of both stoss and lee sides of a rock knoll. Small whalebacks can form under only a few hundred metres of ice, larger ones under deep ice streams. |
| Modern analogue |
Fig. 3.124. (b) Striated whaleback, Konowbreen, Svalbard (D.J.A. Evans). |
| Associated features | Rock drumlins, roch moutonnées, riegel, rock steps, cirques |
| Principal engineering significance | Sheared bedrock |
| Principal references | Evans (1996), Glasser & Bennett (2004), Rea (2013a) |
| Terrain unit | 3.10.1.7 Trough heads |
| Image |
Fig. 3.125. (a) Trough head, Deepdale, Lake District, England (D.J.A. Evans). |
| Form/topography | Valleys shaped by valley glaciers and ice streams within ice sheets that have a distinct trough form. The trough head is situated at the top of the valley enclosed by glacially moulded cliffs and may have been overlooked by extensive snow-gathering areas. |
| Landsystem | Upland glacial landsystem (bedrock terrain): subglacial footprint |
| Process of formation | Glacial erosion at head of valley. |
| Modern analogue |
Fig. 3.125. (b) Modern trough head, Oksfjordjokelen plateau icefield, Norway (D.J.A. Evans). |
| Associated features | Rock basins, riegel, cirques |
| Principal engineering significance | Sheared bedrock, over-steepened zones, overstressed zones |
| Principal references | Benn et al. (2005), Waltham (2008) |
| Terrain unit | 3.10.1.8 Rock basins |
| Image |
Fig. 3.126. (a) Rock basin in Kananaskis cirque, Canada (D.J.A. Evans). |
| Form/topography | A lake- or sea-filled bedrock depression typically over-deepened. |
| Landsystem | Upland glacial landsystem (bedrock terrain): subglacial footprint |
| Process of formation | Rockhead depression calved out by a glacier moving downslope. |
| Modern analogue |
Fig. 3.126. (b) Rock basins exposed by receding cirque glaciers, Torngat Mountains, Labrador, Canada (NAPL). |
| Associated features | Cirques, riegel, rock steps |
| Principal engineering significance | Shattered bedrock, voids, anisotropic strength, compressible soils |
| Principal references | Anderson (1970), Nichol & Reynolds (2002), Benn et al. (2005), Nichol et al. (2007), Evans (2013b) |
| Terrain unit | 3.10.1.9 Rock steps |
| Image |
Fig. 3.127. (a) Striated rock steps, Oksfjordjokelen, Norway (D.J.A. Evans). |
| Form/topography | Series of rock steps composed of more resistant rocks with small depression behind them where the weaker rock is exposed. Often these depressions are infilled with water to form staircase lakes which can be connected by small streams. These lakes are also termed paternoster lakes. |
| Landsystem | Upland glacial landsystem (bedrock terrain): subglacial footprint |
| Process of formation | As glaciers flow down-valley they encounter exposed bedrock of varying resistance to erosion. The glacier will erode down into weaker rock but have to flow over the stronger rock. |
| Modern analogue |
Fig. 3.127. (b) Bedrock step beneath Oksfjordjokelen, Norway (D.J.A. Evans). |
| Associated features | Glacial troughs, trough heads, riegel, whalebacks |
| Principal engineering significance | Shattered bedrock, sheared bedrock |
| Principal references | Benn et al. (2005), Evans (2013b) |
| Terrain unit | 3.10.1.10 Cirques (corrie or cwm) |
| Image |
Fig. 3.128. (a) Cirque and moraines, Jardalen, Norway (D.J.A. Evans). |
| Form/topography | Armchair-shaped hollow with steep sides and back wall often containing a rock basin with a tarn. Open downslope and bounded by a cliff or steep slope headwall which is arcuate in plan. |
| Landsystem | Upland glacial landsystem (bedrock terrain): subglacial footprint |
| Process of formation | Formed as a result of glacial erosion high on a mountainside. Periglacial freeze–thaw weathering and glacial quarrying facilitate erosion of the headwall. |
| Modern analogue |
Fig. 3.128. (b) Modern cirque glacier, Labrador, Canada (D.J.A. Evans). |
| Associated features | Rock basins, glaciated valleys, glacial troughs, hanging valleys, arêtes |
| Principal engineering significance | Shattered bedrock, over-steepened zones, overstressed zones |
| Principal references | Glasser & Bennett (2004), Waltham (2008), Evans (2013b) |
| Terrain unit | 3.10.1.11 Cols |
| Image |
Fig. 3.129. (a) Cols with arêtes, cirques, glacial troughs and valleys, Bakening, Kamchatka, Russia (T. Gernon). |
| Form/ topography | High pass or saddle in a ridge which lies on the line of the watershed between two mountain vallleys, often on a prominent ridge or arête. |
| Landsystem | Upland glacial landsystem (bedrock terrain): subglacial footprint |
| Process of formation | Formed by two headwards-eroding cirques that reduce an arête. |
| Modern analogue |
Fig. 3.129. (b) Cols at various stages of being exposed above Flaajökull, Iceland (S. Gudmundsson). |
| Associated features | Arêtes, glaciated valleys, glacial troughs, cirques |
| Principal engineering significance | Shattered bedrock |
| Principal references | Evans, I.S. (2008), Evans, D.J.A. (2013b), Waltham (2008) |
| Terrain unit | 3.10.1.12 Horns |
| Image |
Fig. 3.130. (a) Horns, Les Aiguilles d'Arves, French Alps, France (D. Giles). |
| Form/topography | A pyramidal, sharp-pointed peak. |
| Landsystem | Upland glacial landsystem (bedrock terrain): subglacial footprint |
| Process of formation | Formed as a result of the backwards erosion of cirque glaciers on three or more sides. |
| Modern analogue |
Fig. 3.130. (b) Horns being exposed by retreating niche glacier, Nufsfjord, Norway (D.J.A. Evans). |
| Associated features | Arêtes, cirques, cols, glaciated valleys, glacial troughs |
| Principal engineering significance | Shattered bedrock |
| Principal references | Evans, I.S. (2008) Evans, D.J.A. (2013b), Waltham (2008) |
| Terrain unit | 3.10.1.13 Nunataks |
| Image |
Fig. 3.131. (a) Nunatak above level of ice during the Loch Lomond Stadial, Binnein Mòr, Scotland (C.K. Ballantyne). |
| Form/ topography | An island of bedrock or mountain top which protruded above the surface of an ice sheet or highland icefield. |
| Landsystem | Upland glacial landsystem (bedrock terrain) |
| Process of formation | Rock mass left protruding above ice field, subjected to freeze–thaw weathering. |
| Modern analogue |
Fig. 3.131. (b) Nunataks of the British Empire Range, Ellesmere Island, Canada (D.J.A. Evans). |
| Associated features | Periglacial trimlines |
| Principal engineering significance | Shattered bedrock |
| Principal references | Waltham (2008), Ballantyne (2013a) |
| Terrain unit | 3.10.1.14 Roche moutonnées |
| Image |
Fig. 3.132. (a) Roche moutonnée, Hoffellsjökull, Iceland (D.J.A. Evans). |
| Form/topography | Rocky hillock with a gently inclined, smooth up-valley facing slope. |
| Landsystem | Upland glacial landsystem (bedrock terrain): subglacial footprint |
| Process of formation | Formed by glacial abrasion and a steep rough down-valley facing slope resulting from glacial plucking. |
| Modern analogue |
Fig. 3.132. (b) Modern roche moutonnée, Skalafellsjökull, Iceland (D.J.A. Evans). |
| Associated features | Whalebacks, crag and tails |
| Principal engineering significance | Shattered bedrock, sheared bedrock |
| Principal references | Glasser (2002), Rea (2013a) |
| Terrain unit | 3.10.1.15 Röthlisberger channels |
| Image |
Fig. 3.133. (a) Röthlisberger channel with esker emerging at the margin of Fjallsjökull, Iceland (D.J.A. Evans). |
| Form/topography | The ice channels leave no trace after the glacier melts unless the channel becomes choked with sediment and is left as a pseudomorph cast. |
| Landsystem | Ice-sheet-related landsystem: subglacial footprint |
| Process of formation | Semi-circular channels cut upwards into ice by meltwater. |
| Modern analogue |
Fig. 3.133. (b) Röthlisberger channel in Glacier Peak outlet, Washington, USA (M. Pelto). |
| Associated features | Eskers, tunnel valleys |
| Principal engineering significance | Variable particle sizes, anisotropic permeability |
| Principal references | Röthlisberger (1972), Boulton & Paul (1976), Eyles (2006) |
| Terrain unit | 3.10.1.16 Crag and tail ridges |
| Image |
Fig. 3.134. (a) Digital elevation model of crag and tails, Sweden (DEM processed by T. Dowling). |
| Form/topography | Teardrop-shaped rock outcrop. The crags are cliffs or buffs developed in near-cylindrical masses of strong rock. The tail is formed in weaker less-resitant rocks sheltered from erosion in its lee. |
| Landsystem | Ice sheet related landsystem: subglacial footprint |
| Process of formation | Developed by glacial erosion of rocks on unequal resistance. |
| Modern analogue |
Fig. 3.134. (b) Crag and tail recently emerged at margin of Nordenskiold Glacier, Svalbard, Norway (D.J.A. Evans). |
| Associated features | Roche moutonnées, whalebacks |
| Principal engineering significance | Shattered bedrock, sheared bedrock |
| Principal reference | Benn & Evans (2010) |
| Terrain unit | 3.10.1.17 Riegel |
| Image |
Fig. 3.135. (a) Rock bar (riegel) Kananaskis, Canada (D.J.A. Evans). |
| Form/topography | A rock barrier that extends across a glaciated valley, usually comprising stronger, more resistant, rock than above and below, and commonly having a smooth up-valley facing slope and a rough down-valley facing slope. Can be holding back a lake or, when breached, be present as an upstanding transverse ridge. Barrier may show limited signs of abrasion on the up-glacier side and much fracturing on the down-glacier side. |
| Landsystem | Upland glacial landsystem (bedrock terrain): subglacial footprint |
| Process of formation | Usually form where there is a band of stronger resistant rock crossing a valley. |
| Modern analogue |
Fig. 3.135. (b) Glacier outlet lobe moving over bedrock ridges or riegels, Sukkertoppen, Greenland (DGI). |
| Associated features | Rock basins, whalebacks, roch moutonnées |
| Principal engineering significance | Shattered bedrock, sheared bedrock |
| Principal reference | Hambrey (1994) |
| Terrain unit | 3.10.1.18 Push and squeeze moraines |
| Image |
Fig. 3.136. (a) Old and new push moraines, Skaftafellsjökull, Iceland (D.J.A. Evans). |
| Form/topography | Small moraine ridges, <10 m in height. Vary widely in composition with subglacial tills, mass movement deposits, water-sorted sediments or large boulders dependent on the nature of the sediment on the glacier (ice) foreland. Where glacier margins are debris-covered, consist mainly of supraglacial material dumped onto the foreland during the summer and pushed up during the winter. Commonly have asymmetric profiles with gentle proximal slopes, often covered with small flutings. Steep distal slopes. Broadly arcuate in form, but can be irregular and winding, reflecting the morphology of the glacier snout. Relict squeeze moraines rare. |
| Landsystem | Ice-sheet-related landsystem: ice-marginal complexes |
| Process of formation | Produced by minor, often annual ice advances in either subaqueous or terrestrial settings. Younger moraines pushed on top of older moraines reflecting advance and retreat cycles of the ice. Squeeze moraines formed by the squeezing of saturated sediments from beneath the ice margins. Tend to degrade rapidly. Subject to reworking by ice advance. |
| Modern analogue |
Fig. 3.136. (b) Push moraines, Flaajökull, Iceland (D.J.A. Evans). |
| Associated features | Other morainic landforms, flutes, sandar/outwash fans and plains, pitted sandar (kettled outwash plain), kettle holes/ponds |
| Principal engineering significance | Aggregate resource, variable particle sizes, anisotropic permeability, anisotropic strength, anisotropic stiffness, perched water tables |
| Principal references | Boulton & Paul (1976), Newbery & Subramaniam (1977), Newbery & Baker (1981), Boulton (1986), Lewis (1991), Van Overmeeren (1997), Brabham et al. (2005), Evans (2013c) |
| Terrain unit | 3.10.1.19 Dump moraines and ice-marginal aprons |
| Image |
Fig. 3.137. (a) Dump moraine at margin of sub-polar glacier, Ellesmere Island, Canada (D.J.A. Evans). |
| Form/topography | Material accumulating on the surface of a glacier. Released to give a chaotic mix of glacial debris. Dump moraine size related to supraglacial volume and length of ice stillstand period. Thin veneer of coarse-grained material deposited on retreat. |
| Landsystem | Ice-sheet-related landsystem: ice-marginal complexes |
| Process of formation | Form where the ice margin remains stationary during debris accumulation, material released through mass flowage, fall or glaciofluvial transport. Material dumped onto adjacent terrain. Can be bulldozed into push moraines if ice undergoes a subsequent readvance. Dumping of large quantities of supraglacial debris builds up a latero-frontal dump moraine. |
| Modern analogue |
Fig. 3.137. (b) Dump moraine and ice-marginal apron, Dobbin Bay, Ellesmere Island, Canada (D.J.A. Evans). |
| Associated features | Other morainic landforms, flutes, sandar/outwash fans and plains, pitted sandar (kettled outwash plain), kettle holes/ponds |
| Principal engineering significance | Aggregate resource, variable particle sizes, anisotropic permeability, anisotropic strength, anisotropic stiffness, perched water tables |
| Principal references | McGown (1971), Newbery & Subramaniam (1977), Boulton & Eyles (1979), Eyles (1979), Newbery & Baker (1981), Lewis (1991), Evans (2013c) |
| Terrain unit | 3.10.1.20 Latero-frontal fans and ramps |
| Image |
Fig. 3.138. (a) Stratified moraine ridge or ice contact fan, Lake Clearwater, New Zealand (D.J.A. Evans). |
| Form/topography | Coalescent debris fans. Identifiable by pronounced asymmetric cross-section. Comprising a distal slope much shallower in gradient than on latero-frontal dump moraines. Steep proximal or ice-contact face. Composition dominated by massive often bouldery debris-flow deposits up to tens of metres thick. Interbedded with water-sorted and possible glaciolacustrine sediments. Deposits dip away from glacier snout and, if predominantly glaciofluvial in origin, grade distally into outwash tracts or sandar. |
| Landsystem | Ice-sheet-related landsystem: ice-marginal complexes |
| Process of formation | Deposition by debris flows and glaciofluvial processes around stationary glacier or ice margins. Results in the construction of latero-frontal fans and ramps. Glaciolacustrine sediments present reflect the short-lived proglacial lakes ponded between the ice margin and the fans during ice retreat phases. |
| Modern analogue |
Fig. 3.138. (b) Cirque glaciers, moraines and debris-flow fans, Lahul, Himalaya, India (D.J.A. Evans). |
| Associated features | Other morainic landforms, flutes, sandar/outwash fans and plains, pitted sandar (kettled outwash plain), kettle holes/ponds |
| Principal engineering significance | Aggregate resource, variable particle sizes, anisotropic permeability, anisotropic strength, anisotropic stiffness, perched water tables |
| Principal references | McGown (1971), Newbery & Subramaniam (1977), Newbery & Baker (1981), Lewis (1991), Benn et al. (2003), Evans (2013c) |
| Terrain unit | 3.10.1.21 Medial moraines |
| Image |
Fig. 3.139. (a) Medial moraine debris on west Breiðamerkurjökull foreland, Iceland (D.J.A. Evans). |
| Form/topography | Usually form elevated ridges. Most medial moraines are straight but they can become folded due to compressive flow if the ice spread out laterally as a piedmont glacier. Most medial moraines tend not to be preserved as they contain relatively little debris and are subjected to intense reworking during ice melt. They can be difficult to recognize on the glacier foreground. Debris is angular and frost-shattered. |
| Landsystem | Ice-sheet-related landsystem: supraglacial debris complexes. |
| Process of formation | Ablation results on the concentration of debris on the surface of the glacier. May not appear on the surface until the snout is approached. Debris may be a shallow feature or extend to the base of the glacier. |
| Modern analogue |
Fig. 3.139. (b) Aerial photo mosaic (1945) showing medial moraines on surface of Breiðamerkurjökull, Iceland (USAAF). |
| Associated features | Other morainic landforms, flutes, sandar/outwash fans and plains, pitted sandar (kettled outwash plain), kettle holes/ponds |
| Principal engineering significance | Aggregate resource, variable particle sizes, anisotropic permeability, anisotropic strength, anisotropic stiffness, perched water tables |
| Principal references | McGown (1971), Newbery & Subramaniam (1977), Dawson (1979), Newbery & Baker (1981), Lewis (1991), Hambrey (1994), Evans (2013c), Benn & Evans (2010) |
| Terrain unit | 3.10.1.22 Hummocky moraine and controlled moraines |
| Image |
Fig. 3.140. (a) Hummocky moraine, Drumheller-Stettler, Alberta, Canada (D.J.A. Evans). |
| Form/topography | Moundy irregular morainal topography. Can appear chaotic or linear in pattern. Occurrences with pronounced transverse linear elements are termed controlled moraines. |
| Landsystem | Ice-sheet-related landsystem: supraglacial debris complexes |
| Process of formation | Moraine deposited during the melt-out of debris-mantled glaciers or ice sheets. Sedimentology of hummocky moraine can be complex reflecting multiple cycles of redeposition during its formation. |
| Modern analogue |
Fig. 3.140. (b) Surge hummocky moraine, Tungnaarjökull, Iceland (D.J.A. Evans). |
| Associated features |