by Matthew Law

Stratigraphy is the key concept that powers the science of archaeology. In essence it is the study of the layers, cuts and other contexts on an archaeological site and the order in which the events that created them took place. This chronological ordering of the site is called a stratigraphic sequence. The concept of stratigraphy derives originally from geology, where it was realised that the uppermost rock strata in an unmodified sequence were younger than those underneath them (Barker 1993: 19). By recording the stratigraphic sequence, archaeologists are able to demonstrate how a site developed through time. It also allows a sequence for dating the finds, such as pottery or metalwork, retrieved from the different layers. Even though the archaeologist may not know an exact date for the artefacts, they will be able to tell which types are earlier relative to other types from the same site by the layer's position in the stratigraphic sequence. 

The stratigraphic process starts with the C horizon at the bottom of the sequence. This is the natural geological bedrock. Organic matter  and windblown sediments on the surface of the bedrock combine with elements of the bedrock that have changed form through weathering to form the topsoil, or A horizon. An interface layer between the A and C horizon, the B horizon, also develops (Hunter 1996: 40). This structure of horizons is very stable, and so will not alter except as a result of external action (Hunter 1996: 40). When such action takes place, for example digging and later filling in a pit, the disturbance to the stratigraphic layers in irreversable (Hunter 1996: 40) and can be seen by archaeologists.

References:

Barker, P., 1993: Techniques of Archaeological Excavation. Third Edition (London: Routledge) 

Hunter, J., 1996: Recovering Buried Remains, in Hunter, J., Roberts, C., and Martin, A. Studies in Crime: An Introduction to Forensic Archaeology (London: Routledge)

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