With relative dating, dates are expressed in relation to one another, for instance, earlier, later, more recent, and so forth. Each object at an archeological site has a different time relationship with every other object at that site. Artifacts deposited in one stratum—a more or less homogeneous material, visually separable from other levels by a distinct change in color, texture, or other characteristic—have a distinct relationship with artifacts recovered from strata (plural of stratum) above or below them. These kinds of time relationships between stratified layers are what archeologists call relative time or relative chronology. Archeologists use several methods to establish relative chronology including geologic dating, stratigraphy, seriation, cross-dating, and horizon markers. Each method is explained in this section.
Geologists study the earth and the natural forces that are involved in changes that take place. One aspect of this research is the study of time, or geologic dating. Geologists develop dates for various geological stages by relating them to climactic and geologic events that have been documented, but these dates are relative, not absolute. Nevertheless, they do help archeologists confirm some of their relative chronologies, and both fields use stratigraphy—the natural and cultural layering of soil—as the basis of these dating techniques (McMillon 1991:115). It is possible for geologists to determine absolute dates for geological occurrences, but most of the methods they use are accurate only when they are dealing with millions of years rather than for smaller increments such as tens of thousands of years (McMillon 1991:115).
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Learn about the concepts of geologic time, the formation processes of landforms, and national parks with especially cool examples of geological formations.