Stratigraphy at Harpers Ferry National Historic Park. (Paul A. Shackel, University of Maryland)
Relative chronology is based on a classic geological principle, known as the Law of Superposition. This law holds that, under normal circumstances, deeper layers of soil, sediment, or rock are older than those above them. Thus, relative chronology comes from stratigraphy—the sequences of layered, or stratified, deposits. Like geological exposures, archeological sites usually contain stratified layers. Some of them are the result of human activity, like house building. Others result from natural phenomena like rain and wind (Orser and Fagan 1995:97).
Relative chronologies in archeology derive from the close study of human occupation layers. In an undisturbed site, artifacts found together in the same strata will most likely date from the same occupation period. Further, artifacts found in a deep strata will be older than those found in a strata closer to the ground surface. While this allows archeologists to establish a relative chronology of the site's occupation and use, it does not tell the age of artifacts found within the stratum. (See Site formation in What are Archeological Resources?)
Archeologists may encounter stratigraphy that has been disrupted. Distortions can occur during or after material deposition that may cause strata to disappear in one area of the site and reappear farther along at a different distance from the surface. A stratum may not cover the entire site. Landfills, dumps, and landslides or other earth movements may distort a site's stratigraphy.
Interruptions or intrusions such as stones, tree roots, walls, wells and postholes may make interpreting the relationship of the strata on either side of the interruption difficult. Disturbances, such as storage or burial pits, trenches for postholes and building foundations, and natural occurrences like stream cuts, tree roots, and animal burrows, do not completely break a stratum's continuity. Because disturbances tend to mix soils from different stratum, later material could be interspersed with earlier material, thereby moving artifacts from their original strata to strata reflecting a different period (McMillon 1991:77-83).
Archeologists frequently encounter distortions, interruptions, and disturbances during excavations. All workers must be aware of them and of their importance to the final interpretations of the excavation. In one sense, though, disturbances are what excavation is about. Theoretically, the purpose of an excavation is to find human disturbances—generally referred to as artifacts and features (McMillon 1991:83)—and to investigate and interpret them as remnants of past human activity.
Try it yourself
View a movie that illustrates stratigraphy and the Law of Superposition.