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ABSTRACT

The Marias River canyon in north-central Montana, incised into Upper Cretaceous strata of the Great Plains during latest Pleistocene to Holocene time, served as a locus of human activity tied to the unique floral and faunal resources it provided ancient peoples. Erosion of the main canyon walls resulted in deposition of tributary junction alluvial fans characterized by debris-flow and hyperconcentrated flow sediment transport processes where side canyons emerged onto the alluvial valley floor. These alluvial-fan deposits preserve Late Precontact archaeological remains accessible due to their postburial exposure where partially eroded by the meandering channel of the Marias River (Goose Bill site complex). Archaeological materials are also preserved high on the dissected canyon walls where movement of hillslope sediment and colluvium by sheetwash led to their burial (Sparrowhawk site).

Deposition of sediment of contrasting physical attributes (grain size, sorting, clay mineral content) within the canyon influences soil substrate properties, favoring growth of specialized plant communities in both alluvial-fan and fluvial environments. These relations exemplify the strong geoecological connections among depositional environment and sediment characteristics, substrate properties, and vegetation community development. The plant communities, interacting in conjunction with the physical landscape, provided a range of habitats utilized by such large mammals as plains bison, Rocky Mountain elk, Audubon sheep, pronghorn, Rocky Mountain mule deer, and white-tailed deer.

Four major bison bone layers, some including bison hair and other soft tissues, with associated stone arrow points and stone flakes (artifacts) and evidence of bone processing are preserved in alluvial-fan deposits exposed along the bank of the Marias River at the Goose Bill site complex. These relations illustrate: (1) human activities dating to ~100-595 yr B.P. (falling within the Late Precontact Period of regional archaeological prehistory) that reflect subsistence reliance on bison as a food source, (2) the importance of tributary alluvial fans proximal to canyon walls in preserving archaeological remains, and (3) the role of the meandering Marias River channel in destroying these deposits over a time period of <100 yr. Sediment deposition by overland flow higher up the canyon walls at the Sparrowhawk site preserves a somewhat older (710-830 yr B.P.) archaeological record suggestive of a broader spectrum of resource (food) processing spatially decoupled from an area of bison kill/death.

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