Spatial Analysis Tools for Archaeologists

2011 Fieldwork
Project Fieldwork and Analysis: April 29 - August 22, 2011. University of California
Hopland Research and Extension Center (HREC), Hopland, CA
Bryan Gordon, Canadian Museum of Civilization

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Theory behind our petroglyph dating

We look for chipped rock art and hammerstone chips that fell to the artist’s feet when the art was made. These chips differ from soil and weathered rock art particles by their larger size and their sharp edges that resulted from hammering. Hammerstone chips are fewer, harder, more resilient and have a different color than rock art chips. To isolate hammerstone chips we first remove disturbed soil under the art, often down to 10 cm, then scrape the soil contents of each successive ¼ inch (6 mm) level over a 8x12 inch (20x30 mm) area into a dustpan. After transferring the contents of each level to nested sieves and shaking them, the few large hammerstone chips and any rootlets are removed from the topmost or ¼ inch (6 mm) mesh sieve. This is done with tweezers to avoid contaminating datable cultural or natural material.

Small hammerstone chips and soil particles pass into a 1.5 mm standard geological sieve or a ca. 10x20 inch building air vent filter of similar mesh available at hardware stores. Tiny chips and bits of organic material for AMS dating are tweezered into separately labeled aluminum foil pouches. Most dry soil passing this sieve is discarded as backdirt. As it may mask tiny hammerstone chips, a half cup (125 ml) sample is reserved for later washing and separation into its components. This is done by gravity in a 5 ft. (1.5 m) long water-filled rigid plastic pipe with a test tube attached via a reducing coupler to its base. Hammerstone chips fall ahead of the soil particles and can be removed and confirmed under a lens. Rock art and hammerstone chips in multiple levels represent palimpsests or multiple stages of art production. Dating organic material of natural or cultural origin in the same 6 mm level to these chips should date the art.

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