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This has led to some confusion. Current recommendations, led by the Geological Society (Zalasiewicz et al. 2004) are to adopt the geochronology scheme, which has been used in this chapter.
Part of the geological timescale, covering the last 545 million years of earth's history (Phanerozoic—from the greek meaning ‘visible life’), is shown in Table 6.1 (Gradstein & Ogg 1996) with the youngest at the top. This shows the broadest sub-divisions of era and period (the Neogene and Palaeogene are often referred to as the Tertiary, though this name may fall into disuse). The names for the eras derive from the greek meaning ‘ancient’, ‘middle’, and ‘new’ life. Periods further sub-divide into early, middle, and late (or in some cases simply early and late), and thence into epochs and ages (refer to Appendix C). Time before the Phanerozoic is usually referred to as the Pre-Cambrian.
Clays are found in three principal geological environments: as primary deposits in the sedimentary environment, as alteration products of other materials in the weathering and
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