Stratigraphy is the best-known and most commonly used method of relative dating. Stratigraphy is based on the observation that soil is deposited in successive layers, or strata. The oldest layers of soil (and any artifacts or fossils within them) will appear beneath more recent layers of soil (and any artifacts or fossils within them). In addition to using the location of layers of soil to date fossils deposited within these layers, biological anthropologists also sometimes make use of other items consistently found in a specific layer of the soil. These items are referred to as indicator artifacts because they help indicate the relative age of fossils and other artifacts. The best indicator artifacts are those that have a wide geographic distribution, are presence for a short period of geological time, and/or are from a species that underwent rapid evolutionary change. Different indicator artifacts have been used to ascertain relative age in different areas of the world. In Africa, elephants, pigs, and horses have been used to establish relative dates of different geological strata. The stratigraphy at Olduvai Gorge in East Africa, for example, was established based on fossil pigs. The various species of pig in successive strata are different and distinct, allowing paleoanthropologists to distinguish the strata based on the pig species found within them. Once the stratigraphy of an area is established, the relative ages of two different fossils in different sites can be determined by the associated indicator artifacts. If a site has been disturbed, stratigraphy will not be a satisfactory way to determine relative age. In such a situation, it may be possible to use absolute dating methods to estimate the age of fossils found together in a disturbed site.

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