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Geological periods

The first academic geologists, in the early 19th century, divide fossil-bearing rocks into three major periods. They name these periods in two different ways. One gives merely a time sequence: Primary for the oldest rocks, then Secondary, and Tertiary for the most recent. The other method describes them in terms of the life which they contain in fossil form: the Primary era is called Palaeozoic ('old life'); the Secondary is Mesozoic ('middle life'); the Tertiary is Cenozoic ('recent life').

Primary and Secondary soon prove inadequate and are replaced by many subdivisions; and Quaternary has been added to Tertiary. But the original broad structure is still used.

As the fossil-bearing rocks are studied in more detail, names are provided for a succession of more precise periods. In the case of the two earliest eras, Palaeozoic and Mesozoic, many of these names derive from the regions where the rocks are first described.

For example, the four earliest Palaeozoic eras relate to the southwest of Britain: Cambrian (Welsh), Ordovician and Silurian (from two Celtic tribes in the region) and Devonian (from Devon). By contrast the next Palaeozoic era, the Carboniferous (Latin for 'coal bearing'), acknowledges the formation of fossil fuels. The best-known Mesozoic era - the Jurassic, in which the dinosaurs thrive - is named from the Jura mountains of France and Switzerland.

The most recent era has been subdivided by geologists into much shorter epochs. These have been given names, deriving from Greek, which all share the cen of Cenozoic. So the Cenozoic ('recent life') era moves forward from the Palaeocene ('old recent') epoch through the Eocene ('dawn recent'), Oligocene ('little recent'), Miocene ('less recent'), Pliocene ('more recent'), Pleistocene ('most recent') and Holocene ('wholly recent', in which we live).

Modern techniques of radiometric dating have given dates to this geological timescale. Combined with the fossil evidence, these provide a full chronology of life on earth.

The approximate starting date of each successive period or epoch, in millions of years ago, is as follows:

Palaeozoic Cambrian 600; Ordovician 500; Silurian 450; Devonian 400; Carboniferous 345; Permian 280.

Mesozoic Triassic 225; Jurassic 190; Cretaceous 135.

Cenozoic Palaeocene 65; Eocene 54; Oligocene 38; Miocene 26; Pliocene 12; Pleistocene 1.7; and finally the present epoch, the Holocene, from 10,000 years ago.

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