ABSTRACT

Evidence for deep erosion of sediments in the Williston Basin region can be found throughout western North Dakota, eastern Montana, southern Saskatchewan, southwestern Manitoba, southeastern Alberta, and northwestern South Dakota. Evidence includes an erosion surface cutting across numerous stratigraphic units, a system of major escarpments, erosional remnants of previous topographic surfaces, and high-level drainage divides covered with alluvial sediments. Glacial erosion, aided by tremendous volumes of meltwater, probably scoured out systems of extremely wide and shallow glacial troughs on the northern Great Plains. These valleys are cut in the easily-eroded claystone bedrock of the region. The Missouri Escarpment, the Manitoba Escarpment, and escarpments bounding upland regions such as the Prairie Coteau, the Turtle Mountains, Moose Mountain, Riding Mountain, Duck Mountain, Porcupine Mountain and the Pasquia Hills are remnants of the walls of some of these valleys.

Late Cenozoic climatic models indicate as many as 25 major glacial episodes during the past 3.2 million years. Continental icesheets probably advanced into the Williston Basin region during most, if not all, of those glacial episodes. Alpine icecaps probably covered the Rocky Mountain uplands during each of the glacial episodes. Meltwater from these alpine icecaps repeatedly crossed the Williston Basin region only to be diverted along the margins of continental ice sheets. Glacial meltwater rivers, emanating from alpine icecaps in the Rocky Mountain region, carried distinctive glacio-fluvial alluvium. Gravels, capping upland regions in eastern Montana and the western Dakotas and elsewhere were probably deposited when rivers, carrying such glacio-fluvial material, were diverted along continental ice margins. Evidence for early diversions of one such river, the Yellowstone River, can be found along the asymmetric drainage divide east of the Little Missouri River. Buttes, composed of sediments usually mapped as (Oligocene) White River Group, probably represent topographic inversions of early Yellowstone River valleys. The distinctive sediments record both the diversions of the rivers and the progressive lowering of the Williston Basin region landscape.