Stratigraphy is a type of relative dating. It tells how old something is in relation to other objects, but cannot provide a year or specific date of use. The main principle behind stratigraphy is that of superposition. This says that older things are found below younger things. When archaeologists excavate sites, we find layers of soil, each marking a period of use of the site. Artifacts in the upper layers were laid down after those in lower levels. Stratigraphy is the record of these different layers or strata. By excavating sites and separating the artifacts from each layer, it is possible to see changes through time.