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2.7.10 The geological timescaleIn assessing the relative age of rock formations, superposition of uninterrupted and undisturbed strata establishes a stratigraphic sequence (i.e. older below younger), while the thickness of deposition is a guide to the time period of sedimentary accretion. In palaeontology, the presence of fossils from the Cambrian period onwards promotes a correlation between strata in different areas. Geological Periods, collected into palaeontological Era, have been named after the districts in which the rock horizons were classically identified. A variety of geochronological techniques are available. The most recent past is indicated from dendrochronology (annular growth rings in very old trees), or the examination of a sequence of glacial meltwater varves (upper Pleistocene lacustrine deposits); both methods are related to changes in terrestrial climate and solar excursions. For anthropological specimens less than 1.5 million years old, fluorine dating (Oakley, 1980) may provide relative estimates of contemporaneous age of associated specimens, at least one of which has been dated by independent stratigraphic, cultural or absolute means. Their inter-relation is inferred from chemical measurement of fluorine/phosphate ratios (Glover and Phillips, 1965) of apatite mineral in bone, teeth, antlers, etc. from contiguous horizons. Absolute age may be computed from the decay of various radioisotopes which occur in specific relationships ‘starting’ from some geomorphological or biological event. Radiocarbon dating relies on a living organism exchanging carbon dioxide at an equilibrium level of 14C/12C until it dies; the level of 14C then decays according to its 5730 year half-life. Dates are reliable to 5% or better but this scale is only useful for events in the last 70 000 years. For earlier events, e.g. in the Tertiary Era, the measurement of 40K/40A ratios in micaceous and volcanic rocks has been notable in assessing the antiquity of fossil remains of very early hominids and anthrapoids found in East Africa. Dating of older (e.g. Palaeozoic) rock formations relies on determining the ratios of longer lived isotopes, e.g. 87Rb/87Sr or various uranium/lead ratios. Estimates of the age of the earth’s crust have been derived from ratios of 206Pb and 207Pb to (nonradiogenic) 204Pb. (See section 4.6.2 for an exposition of the uranium and thorium radioactive disintegration series.) Harland and colleagues (1990) developed a chronostratic scale of rock sequences (with data through 1988) which assigns ages with standardised stratigraphic reference points. In the following table, the geological periods are mostly given classic (West) European stage names but some broadly equivalent East European and Asian strata (marked ‘*’) have been included. For more detailed reference there is a useful wall chart by his co-author Smith (1990) based on their book but with some minor revisions. In the table, the age point for the base bed of most systems has been abstracted from data in Harland et al. (1990). References M. J. Glover and G. F. Phillips (1965) J. Appl. Chem.,
15, 570–576. G.F.Phillips The geological timescale 1989 Age points are expressed in million years; uncertainties are generally around 5 in the last digit
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