Principle of Faunal Succession

The principle of faunal succession holds that sedimentary rock strata contain fossil plants and animals in a specific, reliable vertical order, and that this order can be recognized across wide horizontal distances even where the strata themselves are not continuous. William Smith documented the pattern in England during the first decade of the nineteenth century, drawn from years spent inspecting coal mines and canal cuttings for the Somersetshire Coal Canal Company, and used it to identify and correlate rock layers on his 1815 geological map; the French naturalists Georges Cuvier and Alexandre Brongniart documented similar patterns around the same time. Combined with the law of superposition, the observation that undisturbed strata are laid down with the oldest at the bottom, the principle lets geologists determine the relative order in which rock layers formed, and it remains the foundation of biostratigraphy, the dating of rock by the fossils it contains.