What is radioactive dating
The earliest geological time scales simply used the order of rocks laid down in a sedimentary rock sequence (stratum) with the oldest at the bottom.
However, a more powerful tool was the fossilised remains of ancient animals and plants within the rock strata.
Divisions in the geological time scales still use fossil evidence and mark major changes in the dominance of particular life forms.
For example, the Devonian Period is known as the 'Age of Fishes', as fish began to flourish at this stage.
These sequences apply from the beginning of the Cambrian period, which contains the first evidence of macro-fossils.However, the end of the Devonian was marked by the predominance of a different life form, plants, which in turn denotes the beginning of the Carboniferous Period.The different periods can be further subdivided (e.g.After Charles Darwin's publication Origin of Species (Darwin himself was also a geologist) in 1859, geologists realised that particular fossils were restricted to particular layers of rock.This built up the first generalised geological time scale.