Stratigraphy (Archeology)

Stratigraphy is the study of layered materials (strata) that were deposited over time—their lateral and vertical relations, as well as their composition. The basic law of stratigraphy, the law of superposition, states that lower layers are older than upper layers, unless the sequence has been disturbed. Stratified deposits may include soils, sediments, and rocks, as well as man-made structures such as pits and postholes. The adoption of this principle by archeologists greatly improved excavation and archeological dating methods.

By digging from the top downward, the archeologist can trace the buildings and objects on a site back through time using techniques of typology (i.e., the study of how types change in time). Object types, particularly types of pottery, can be compared with those found at other sites in order to reconstruct patterns of trade and communication between ancient cultures. When combined with stratification analysis, an analysis of the stylistic changes in objects found at a site can provide a basis for recognizing sequences in stratigraphic layers.

Archeological stratigraphy, which focuses on stratifications produced by man, was derived largely from the observations of stratigraphic geologists, or geomorphologists. A geomorphologist studies stratigraphy in order to determine the natural processes, such as floods, that altered and formed local terrain. By comparing natural strata and man-made strata, archaeologists are often able to determine a depositional history, or stratigraphic sequence—a chronological order of various layers, interfaces, and stratigraphic disturbances. Stratigraphic data may be translated into abstract diagrams, with each deposit's diagram positioned relative to the deposits above and below it. By this method, archeologists can illustrate the stratigraphic sequence of a given site with a single diagram. Such a diagram, showing the different layers with the oldest at the bottom and the youngest at the top, may cover 3,000 years. The diagram also records finds such as pits, post holes, and burials that may have belonged to a single period. The archeologist may also document the site with notes about the relationships of stratigraphic units and soil composition.