PETROGLYPHS DATING THE OTHER 95% OF ROCK ART

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Hammerstones are mainly elongated cobbles of shock-resistant quartz, quartzite or basalt, that are pocked at one or both ends. Petroglyph production is much faster with a hammerstone than a line grinder because shock penetrates rock faster than grinding. Line grinders smooth and align the pockmarks, producing rock flour. Rock flour is whitened from size reduction, even when its parent rock is either dark sandstone or basalt. Very fine, rounded sand grains and flatter rock flour may be separable but need washing for lens differentiation.

Hammerstones rarely occur on the surface because they are used for other tasks. Their chips were given little past attention, but our tests show their size and sharp edges correspond to hammerstone pockmarks (Slide 6). Large chips are caught in the basket; smaller chips in the sieve (Slide 7). Large sandstone chips also fall from the art (Slide 8), while sandstone flour (Slide 9) from the art is caught in the sieve.

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Slide 6
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Slide 7
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Slide 8
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Slide 9

(Slide 10) Our first petroglyph tests were on Gabriola Island, opposite Nanaimo on south Vancouver Island. (Slide 11) We began at the famous Thunderbird sandstone petroglyph, choosing a 12 mm soil thickness rather than 5 mm because art tools or their chips may exceed 5 mm. (Slide 12) We found sandstone chunks and sandstone, basalt and quartzite hammerstones or their chips in most levels to 26, with rock flour to 31. Sandstone grinders were in four levels, their flour in 14 levels. (Slide 13) Hammerstones and their chips and line grinders were mainly quartzite, then quartz and basalt.

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Slide 10
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Slide 11
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Slide 12
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Slide 13
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