S is for Stratigraphy

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"Stratigraphy" is the the layering of the soil deposits that, over time, form a chronology of the site. In other words, artifacts found in lower soil layers are usually older than those found in upper layers. Some soil layers form naturally, being deposited by wind or water or by the decaying vegetation. Others are formed by human activity from the accumulation of everyday living debris, digging holes for refuse pits, storage pits, or house basins, or intentionally putting down a layer of special soil. Also, when building a mound, soil is removed from one area (a borrow pit) and deposited in another to form a mound. This can mix the soils from several different layers, as well as the artiifacts that were in the borrowed soil. Also, mounds were often enlarged periodically by adding new layers of soil that may come from different borrow pits. This will create a stratigraphy within the mound itself.

Archaeologists use stratigraphy to understand the context in which they find artifacts. Again, artifacts in deeper layers are usually older, but not always. Sometimes a pit may be dug for storage or refuse and it may be deep enough to cut through seveal different soil layers. When the pit is filled in, some of the younger artifacts will end up at the bottom of the pit and thus will be younger than the older artifacts already in the soil layers next to the pit. This is why it is important to know the context of the artifacts that archaeologists find--whether the artifacts are from the pit or from the adjacent soil layers.