Friday, January 15, 2010

Grains of Sand

For archaeologists, soils brim with information. During our initial training, we learn how to glean some of that data and make sense of it. But lots of archaeologists lots of the time throw away useful information with each shovel full of soil that falls through the screen.

Grains of sand, for example, can be classified and the sediment of which they are a part can be interrogated. Where did the sediment come from? How was it modified? How did that sediment play a role in the human use--or avoidance--of a particular landform.

A very useful and remarkably inexpensive tool rarely seen on archaeological sites is the sand grain size and shape chart. This handy little tool folds up to the size of a wallet--and, unfortunately, as thin as many wallets-- and conveniently fits into the jacket pocket of a Munsell soil color book.

The chart illustrates sand grains of different sizes, allowing the field worker to describe the material as fine, medium, or coarse. Examine sediments under magnification of, say, X10 (ten diameters, or ten times the actual size), and compare the grain shapes to the Roundness chart. Large, angular grains probably haven't been transported very far, while small, spheroid grains may have been carried great distances. Large (coarse) grains probably were deposited in a high energy, high velocity environment (wind or water) while finer sediments settle out in lower energy environments.

As a case in point, there are many relict sand dunes in northern Anne Arundel County and on Maryland's Eastern Shore that formed several thousand years ago during a stretch of dry weather. The sand grains that dominate these deposits tend to be coarse and well rounded, a product of wind erosion in an increasingly arid environment. When those winds encountered any obstruction--an existing dune, a clump of trees--the obstruction sufficiently sapped energy from the wind to allow the heavier particles to precipitate, forming an elongate sandy rise that continued to accumulate additional material. These typically appear on aerial photographs and topographic maps as crescent-shaped rises aligned northeast to southwest. They were favored occupation sites for many Archaic period peoples, probably because they typically were associated with wetlands with rich resources.

Grain size and shape in the various deposits encountered at Port Tobacco also can reveal information about how they formed and, by extension, how the various landforms in and around Port Tobacco were created and modified over the millennia.


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