Year of Award

2012

Document Type

Professional Paper

Degree Type

Master of Arts (MA)

Degree Name

Anthropology (Cultural Heritage Option)

Department or School/College

Department of Anthropology

Committee Chair

Douglas MacDonald

Commitee Members

John Douglas, Marc Hendrix

Keywords

arid sediments, Besant culture, sandstone cave, stratigraphic analysis

Publisher

University of Montana

Abstract

Horseshoe Cave is located south of Ashland, Montana and at the northern end of the Powder River Basin. Its earliest documented human use and occupation dates to 2,000 years before the present. Evidence of that past use and occupation includes diagnostic bone and stone tools, hearth features and animal remains. The geoarchaeological investigation of this sandstone cave site reveals the cave host rocks to be sandstones of the Tongue River Member of the Early Tertiary Fort Union Formation. Results of the geoarchaeological inquiry demonstrate that a difference in the calcite cementation between the two host sandstone units is the leading geologic factor in the creation of the cave. The cap-rock material is found to contain twice the amount of calcite cement than does the underlying cave-forming wall-rock unit. This difference in hardness allows both gravity and water to remove the much softer sandstone to form a cave feature that today is some 7 m wide and 3 m high at its opening and that extends some 12 m to its back. The cave forming process appears to have been initiated tens of thousands of years ago when the gradient between the cave floor and the adjacent drainage bottom was steeper than at present. The pace of cave formation appears to have all but ceased and the majority of materials currently being deposited on the cave floor are from the grain-by-grain disintegration of the surrounding sandstone, a sedimentary process augmented by the occasional rock fall and from fine grained clastic materials being blown into the cave during arid times. The annual sedimentation rate ranges between 0.3 and 0.5 mm. A sieve analysis of a 1.35 m column of sediment, which spans some 2000+ years, indicates that the past climatic regime of the sediments found in the cave collection includes at least four periods of warm and dry conditions, periods delineated by an increase in fine-grained sediment. When the arid patterns are overlain on wall profiles containing cultural features it is apparent that none of these features coincide with the dry intervals. This is an association that strongly suggests the area immediately surrounding Horseshoe Cave was not as favorable an environment as might be found along the larger regional drainage bottoms or the foothills of the Big Horn Mountains of Wyoming and the Black Hills of South Dakota.

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© Copyright 2012 Norman Bernard Smyers