A Bedrock Source of Swan River Chert

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REPORTS

ABSTRACT

Swan River chert is a lithic material that was commonly utilized by precontact people on the southern part of the Canadian prairie provinces and the adjoining states since at least Clovis times. The chert occurs in glacial till and other Quaternary deposits within the area of use. It occurs in highest density west of lakes Winnipegosis and Manitoba. Archaeologists have speculated about its bedrock source at least since 1980. This paper describes one primary geological source of Swan River chert located in west-central Manitoba. Rather than a continuous bedded layer the chert occurs in randomly distributed solution chimneys in Devonian carbonates of the Souris River Formation exposed in the Mafeking quarry, west-central Manitoba.

Keywords: Swan River chert; primary bedrock source, solution chimneys; Devonian carbonates, Souris River Formation.

Usage of the label "Swan River chert" dates back to the Glacial Lake Agassiz surveys headed by M. Hill and M. Tamplin for the Department of Anthropology, University of Manitoba in the mid1960s. The label was applied to a lithic material that occurred in abundance at archaeological sites in the Swan River valley and adjacent areas. In its natural state, the chert ranges from white through gray to brown and even black, and is normally very tough to work. The bright colors and luster commonly seen on archaeological specimens are the result of the chert having been heat-treated by precontact artisans to improve the flaking quality of the stone, a process that can easily be duplicated by experimentation (see Low 1996). Heat-treated samples range from red, orange, pink, purple, black, gray, blue, white, and even transparent. The material also displays a high variability in texture. The macroscopic texture of Swan River chert is typically vuggy, with abundant irregular shaped small cavities, cracks and joints. Some varieties however, may be homogeneous with few vugs. Vugs typically are lined with small (< 1 mm) quartz crystals. Individual samples display a high degree of variation, having a mottled texture of clear, translucent, and opaque areas. Samples typically have small (< 1 mm) irregular-shaped 'spots' that form feathery or acicular patterns. Campling (1980) describes a complex microscopic texture as being characterized by: 1) granoblastic subhedral to euhedral quartz, often in radial aggregates, and pore filling, 2) cryptocrystalline chalcedony forming spherical aggregates and filling small vugs, and 3) massive anhedral cryptocrystalline quartz forming the matrix.

DISTRIBUTION OF SWAN RIVER CHERT

Material that has been referred to as "Swan River chert" occurs in glacial till throughout southwestern Manitoba, southern Saskatchewan and southeastern Alberta and contiguous areas of Minnesota, North Dakota and Montana, with the highest frequency being in southwestern Manitoba (Low 1996; Figure 1). Chert nodules are readily found eroding out of glacial till at salt flats in the Dawson Bay area, and stream and river cuts along the Porcupine Hills (Manitoba Escarpment) to the west. In Alberta, one of us (Gryba) has recognized nodules of Swan River chert in gravel pits as far west as Lethbridge and Stettler, and among the boulder-gravel deposited into Glacial Lake McConnel (north of Ft. McMurray) by a discharge of glacial lake Agassiz water down the Clearwater River (Smith and Fisher 1993). Further west, between Alberta Highway 2 (the Calgary-Edmonton highway) and the Cordilleran till situated east of the Rocky Mountains, the Laurentide till is of a more northerly origin (Klassen 1989:Figure 2:20; Prest et al. 1967) and lacks Swan River chert. The occurrence of Swan River chert in low proportions in archaeological sites within the Rocky Mountain Foothills (e. g., Gryba 1983:46) is undoubtedly due to the material having been transported there by precontact groups moving in off the Northern Plains.

Until now, archaeologists have questioned the bedrock source for Swan River chert. …