One important task of the archaeologist is determining how old the different layers at the site are. These layers, or strata, reflect different periods of occupation over the years. The strata accumulate over time, with the remnants of the most ancient inhabitants constituting the bottom layer and the most recent ones appearing on top. This layering is known as stratigraphy. Relative dates can be ascertained through careful examination of a site’s stratigraphy. Objects found in lower strata can generally be assigned to earlier time periods. When objects found at a site are similar to dated objects at another site, the latter helps establish the former’s relative date.
Another technique for relative dating is seriation. Like modern fashion, the styles of ancient pottery and other items change over time. By mapping these changes, archaeologists can develop a powerful tool for establishing the relative dates of groups of objects and the strata in which they are found.
In many cases, absolute dates can also be established. Sometimes evidence of an independently datable historical event (such as an earthquake, war, or king’s reign) is found within a particular stratum. In certain environments, dendrochronology (tree-ring dating) can also be used. In an increasing range of contexts, newer high-tech methods—including radiocarbon dating, electron spin resonance, and potassium-argon dating—can provide an absolute date range.