FW Geoarchaeologists Help Map San Francisco Bay Area Prehistory

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Cores in the Far Western Geoarchaeology Lab.

Cores upon cores fill the shelves of the Far Western Geoarchaeology Lab in Davis. These long tubes of “dirt” tell us a lot about prehistoric landscape evolution, and thus can help determine whether or not a location might have potential for archaeological findings. In most situations, backhoe trenching is the most effective way to identify sites. When backhoe trenching is not possible, in urban areas for example, or when the potential depth for a site exceeds the range of mechanical excavation, we conduct hydraulic continuous-core sampling to identify sites.

Coring in San Francisco.

Coring in San Francisco.

 

 

When archaeologists dig through the layers of earth carefully, the different soils and buried surfaces can be visually seen. Cores do the same thing, like inserting a straw into a layer cake, sometimes reaching 65 feet below surface. Each four-foot section of the core is pulled up in two-inch-diameter plastic liners, brought to the lab, sliced down the center, and splayed open to reveal the stratigraphic layers.

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OSL samples from cores.

Dating the layers can be done in a couple of ways. Most often, radiocarbon dating is used to get close estimates of how old plant, bone, or shell is in a certain layer, or when now buried surfaces were exposed at the surface. Other times, however, there is not enough organic matter to be sampled. In those cases, Optically Stimulated Luminescence (OSL) dating can be used. OSL samples must be removed from their original location in complete darkness, and kept in the dark until tested to provide accurate dates. The dates for each layer let geoarchaeologists map similar types of strata throughout a particular location. This helps archaeologists figure out where sites might be buried.

Schematic Cross Section of Study Area in San Francisco.

Schematic Cross Section of Study Area in San Francisco.

 

Recently opened core exposing artificial fill at the surface (~1.5–2.4 meters below surface), underlain by recent alluvium (~2.5–3.5 meters below surface), and followed in turn by a dense prehistoric shell midden (~3.5–5.5 meters below surface) formed on a Pleistocene-age sand dune (starting ~5.5 meters below surface).

Recently opened core exposing artificial fill at the surface (~1.5–2.4 meters below surface), underlain by recent alluvium (~2.5–3.5 meters below surface), and followed in turn by a dense prehistoric shell midden (~3.5–5.5 meters below surface) formed on a Pleistocene-age sand dune (starting ~5.5 meters below surface).

 

 

 

Once buried land surfaces are identified in the cores, they can be sampled to not only determine their age and whether they contain archaeological materials, but also tested to see what types of small seeds, pollen, or other diagnostic material. This can be used to reconstruct the type of landscape that was present when that layer was at or near the surface.

Currently, Far Western Geoarchaeologists are using cores to map the potential for intact buried land surfaces below the historic-era extent of the San Francisco Bay. As ocean levels have risen, they have covered up landforms where people once lived. Recent findings indicate that we may be able to quite accurately map where some people lived for long periods of time, and perhaps returned over and over again generations later.

For interested archaeologists and non-archaeologists alike, we suggest reading: Waters, Michael R.,1992, Principles of Geoarchaeology: A North American Perspective. The University of Arizona Press.

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