The Cambrian Explosion

At the end of the eighteenth century, William Smith, an engineer who was working on the canals of Britain, discovered an empirical rule for identifying the sedimentary rocks of different regions. Smith noticed that any sedimentary stratum contains fossils that are never found in higher or in lower strata. Hence the idea that if two rocks have the same fossils, they also have the same geological age, even if one is at the bottom of a valley and the other on the top of a mountain. The fossils became in this way the key for reconstructing the past movements of the Earth, and Smith used them to draw the first geological map of the United Kingdom.