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Such time determinations are made and the record of past geologic events is deciphered by studying the distribution and succession of rock strata, as well as the character of the fossil organisms preserved within the strata.
Earth’s surface is a complex mosaic of exposures of different rock types that are assembled in an astonishing array of geometries and sequences.
Fossils help geologists establish the ages of layers of rock.
Johann Gottlob Lehmann of Germany reported on the succession of rocks in the southern part of his country and the Alps, measuring and describing their compositional and spatial variation.He recognized that the marine organisms now found as fossils in rocks exposed in the Tuscan Hills were simply ancient animals that lived in the region when it had been covered by the sea and were eventually buried by muds along the seafloor.He also recognized that the rivers of northern Italy, flowing south from the Alps and emptying into the sea, had done so for a very long time.and the ensuing widespread dissemination of Steno’s ideas, other natural scientists of the latter part of the 17th and early 18th centuries applied them to their own work.The early English geologist John Strachey, for example, produced in 1725 what may well have been the first modern geologic maps of rock strata.