Geology of Griggs and Steele Counties
Griggs and Steele Counties, located at the eastern edge of the Williston basin, are underlain by 400 to 2,600 feet of Paleozoic and Mesozoic rocks that dip gently to the west. The Cretaceous Greenhorn, Carlile, Niobrara, and Pierre Formations lie directly beneath the glacial drift, and shale of the Pierre Formation is exposed in several places along the Sheyenne River. The Pleistocene Coleharbor Formation, which covers most of the area, consists mainly of glacial, fluvial, and lake sediment. The Coleharbor Formation averages 200 to 300 feet thick, but it is as much as 550 feet thick in some of the buried valleys. The Holocene Walsh Formation occurs in parts of the area, chiefly sloughs and river bottomland. It consists mainly of alluvial and eolian sediment. Griggs County and the western two-thirds of Steele County are part of the Drift Prairie, which is characterized by flat to gently rolling topography that is rugged in areas of end moraines and intense ice thrusting, subdued on the ground moraine and outwash plains. Associated with these major landforms are numerous washboard moraines, drumlins, eskers, kames, meltwater trenches, and water-washed areas. The eastern third of Steele County is a nearly flat area covered by lake deposits of the glacial Lake Agassiz. As the Late Wisconsinan glacier in eastern North Dakota thinned and receded eastward, it was increasingly affected by the topography over which it was flowing. This resulted in lobation of the glacier. Locally intense areas of thrusting developed within the lobate glacier, and large blocks of subglacial material were moved short distances. Large areas of Griggs County were washed by water flowing from the glacier, and in some areas gravel and sand were deposited. Continued withdrawal of the glacier resulted in ponding of melt water in parts of the two counties. These and other ponds tended to coalesce at lower and lower elevations, eventually forming Lake Agassiz, which flooded part of eastern Steele County.