stratigraphy

Branch of geology that deals with sedimentary rock layers (strata) and their sequence of formation. Its basis was developed by English geologist William Smith. The basic principle of superposition establishes that upper layers or deposits accumulated later in time than the lower ones.

Stratigraphy involves both the investigation of sedimentary structures to determine past environments represented by rocks, and the study of fossils for identifying and dating particular beds of rock. A body of rock strata with a set of unifying characteristics indicative of an environment is called a facies.

Stratigraphic units can be grouped in terms of time or lithology (rock type). Strata that were deposited at the same time belong to a single chronostratigraphic unit but need not be the same lithology. Strata of a specific lithology can be grouped into a lithostratigraphic unit but are not necessarily the same age.

Stratigraphy in the interpretation of archaeological excavations provides a relative chronology for the levels and the artefacts within rock beds. It is the principal means by which the context of archaeological deposits is evaluated.