Around 1800, William Smith in England, who was a canal surveyor, noticed that he could map out great tracts of rocks on the basis of their contained fossils. The sequences he saw in one part of the country could be correlated (matched) precisely with the sequences in another. He, and others at the time, had discovered the first principles of stratigraphy -- that older rocks lie below younger rocks and that fossils occur in a particular, predictable order.
Then, geologists began to build up the stratigraphic column, the familiar listing of divisions of geological time -- Jurassic, Cretaceous, Tertiary, and so on. Each time unit was characterized by particular fossils. The scheme worked all round the world, without fail.
From the 1830s onwards, geologists noted how fossils became more complex through time. The oldest rocks contained no fossils, then came simple sea creatures, then more complex ones like fishes, then came life on land, then reptiles, then mammals, and finally humans.
2007-01-02
01:59:38
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10 answers
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asked by
advgman52
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