Law of Superposition

The law of superposition is a foundational principle of stratigraphy and geology stating that, in an undisturbed sequence of sedimentary rock layers, the oldest layer lies at the bottom and each successive layer above it is progressively younger, since a layer of sediment can only be deposited on top of a surface that already exists. Danish scientist Nicolas Steno first articulated the principle in his 1669 work De solido intra solidum naturaliter contento dissertationis prodromus, reasoning from observations of rock strata in Tuscany, alongside his own principles of original horizontality and lateral continuity, which together established a rational basis for interpreting the order of geological events from the rock record. The principle was later extended into a practical dating tool by English geologist and engineer William Smith, who in the early nineteenth century recognized that particular layers could also be identified and correlated across different locations by their characteristic fossil content, an approach that became the principle of faunal succession. The law of superposition remains a basic working assumption of relative dating in geology and archaeology, letting a researcher establish the chronological order of undisturbed layers before any absolute, numerical date is available for any of them.