NORTH DAKOTA STATE UNIVERSITY - FARGO, N D ICE THRUST MASS IN BURLEIGH COUNTY, N.D. Click on the photo for an enlarged image. (54K) At Sibley Buttes, north of Sterling, N.D., a system of ice thrust masses consists of Cretaceous-aged sandstones (Fox Hills Formation) shoved a short distance southwest of their bedrock source area. (Photo by D.P. Schwert, North Dakota State University). [ Glacial Landforms of North Dakota ]