Sterile deposit

In archaeology, natural is a term to denote a layer (stratum) in the stratigraphic record where there is no evidence of anthropogenic activity. While there may be "natural" layers interbedded with archaeologically interesting layers, such as when a site was abandoned for long periods of time between occupations by man, the top (the horizon) of the natural layer below which there is no anthropogenic activity on site, and thus where the archaeological record chronologically begins, is the sought-for point to terminate digging. This final natural layer is often the underlying geological makeup of the site that was formed by geological processes. It is the goal of complete excavation to remove the entirety of the archaeological record all the way to the final "natural", thus leaving only the natural deposits of pre-human activity on site. If the excavation is related to development, the impact assessment may stipulate excavation will cease at a certain depth, because the nature of the development will not disturb remains below a certain level; such an excavation may not reach a natural or sterile layer. Thus one always has to overdig a site (dig past the top of the natural) in order to establish the natural.

Natural can be a relative term. On urban sites where research interests may make a detailed examination of the earliest part of the record impractical, prehistoric or rudimentary human activity may go unrecorded, as opposed to an equivalent horizon on a rural site for which the study team's agenda is to look for prehistoric evidence. Chemical and soil process over time often obscure and cause decomposition of cultural materials, and thus a human-occupied layer may look natural. Additionally, early prehistoric tools were manufactured from natural materials, such as bone, stone and fiber; they do not stand out as clearly as metal, glass and plastic. The effect of decompositional processes is that the older an archaeological deposit is, the more it will appear similar to the underlying geology. For some archaeologists, a basic rule of thumb is "the greater the contrast a context has with the natural, the younger it is." Similarly, United States prehistoric archaeologists often rely on significantly diminished counts of lithic flake debitage to assess the excavation unit's trend toward natural stratigraphy. While a trend may be recognized, a stratum is not called natural or sterile, unless it is void of cultural materials.

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