Stratigraphy

Stratigraphy is the study of layered rock, using the order, composition and fossil content of successive layers to read a sequence of events and assign relative ages to them. Its founding principle, the law of superposition, holds that in an undisturbed sequence each layer is younger than the one beneath it and older than the one above; the Danish anatomist and naturalist Nicolas Steno set this out in 1669, alongside the companion principle that layers are originally deposited close to horizontal, so a tilted or folded layer has been disturbed after it formed. More than a century later, the English surveyor William Smith extended the method into biostratigraphy, showing around 1815 that distinct fossil assemblages recur in the same order across widely separated outcrops and could be used to correlate and map strata a surveyor could not otherwise connect; his 1815 geological map of England and Wales, the first of its kind, is built on that insight. Both men are credited as founders of the field, Steno for its founding principle and Smith for making it a practical tool of mapping and correlation.