Comparative analysis of the Souris gravel and sand deposit, a precontact lithic resource in southwestern Manitoba

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Thomson, Sharon L.
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Gravel deposits in the vicinity of Souris, Manitoba, contain various types of siliceous rock, including chert, agate, quartzite, silicified wood, silicified lignite, and fused shale or porcellanite. These gravels are derived from early Tertiary erosion of the Rocky Mountains. In the absence of better-quality outcrops of material suitable for knapping, the Souris gravel exposure was likely utilized by local Native groups as a precontact source of stone for tool manufacture. Visually-similar silicates are found both on archaeological sites throughout southwestern Manitoba and in deposits in Wyoming, Montana, South Dakota and North Dakota. In order to both compare the Souris material with similar material to the south and to test the reliability of macroscopic visual identification, samples from Souris and the United States were tested and compared with archaeological material using thin sections, x-ray diffraction, instrumental neutron activation analysis, and oxygen isotopic analysis. Results indicate that visually-similar silicates can be differentiated from each other by petrographic analysis. Examination reveals that some flakes from the Treleaven and Snyder II North sites identified as "petrified wood" are actually silicified lignite. Similarly, based upon very high U levels, two very small translucent flakes previously identified as agate have been re-identified with some confidence as silicified wood/silicified lignite. Four flakes of silicified lignite from the archaeological sources are virtually indistinguishable microscopically from North Dakota Knife River Flint. Usage of the term "Knife River Flint", which implies a specific source area, should be tempered by the knowledge that silicified lignite is found in secondary gravel deposits over a wide geographic area. Outcrops of such gravel at Souris, Manitoba contain silicified lignite that is generally of poorer quality than silicified lignite from aboriginal quarries in North Dakota.