In archaeology, especially in the course of excavation, stratification is of major interest and significance. Where archaeological finds are below the surface of the ground (as is most commonly the case), the identification of the context of each find is vital to enable the archaeologist to draw conclusions about the site and the nature and date of its occupation. In most cases, "features" can be identified, and these relate to the laying down of soil and other material over a period. For example, the contents of a ditch will constitute a separate feature, or context, from the layer into which the ditch was cut. It is the archaeologist's role to attempt to discover what layers exist and how they came to be created. (See: E. C. Harris, Principles of Archaeological Stratigraphy, 2 nd Edition. Academic Press, London and San Diego 1989)