Abstract

The Dirt Hills structure is expressed at the surface as a broad, arcuate moraine, 7.5 km wide, 120 m high, and 40 km long. The structure is composed mainly of bedrock and drift consisting of three slabs stacked to form a single block 215 m thick. The shear zones that bound the slabs are mainly in clays of the Bearpaw Formation. The Dirt Hills structure originated during the last deglaciation when the glacier, under compressive flow, readvanced up the Missouri Coteau escarpment and stacked the subglacial slabs of bedrock and drift. The subparallel, arcuate ice-thrust ridges are formed presumably by imbricate thrust faults originating from the uppermost shear zone.

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