Devils Lake formed through a long sequence of geologic changes that began about 65 million years ago and culminated during the Ice Age. During the Tertiary Period, erosion shaped eastern North Dakota’s gray shale into a gently rolling plain as the climate cooled from tropical conditions to slightly cooler than today. When the Pleistocene Epoch began about 2.6 million years ago, the region drained northward to Hudson Bay through extensive river systems, including two rivers in the Devils Lake region, one of them the large preglacial Knife/Cannonball River. Repeated glaciations blocked these northward drainage routes and forced water southeastward, while glacial meltwater filled old river valleys with thick layers of sand and gravel that were later buried beneath impermeable glacial sediment. One of these buried deposits became the Spiritwood Aquifer, which passes beneath the area now occupied by the bays of Devils Lake and was chiefly responsible for the lake’s origin. Before the last major southward ice advance, the land was covered by earlier glacial sediment over shale of the Pierre Formation, and the region was probably a tundra landscape with deep permafrost. When the Late Wisconsinan glacier advanced into the area about 12,000 years ago, it overrode the saturated, confined Spiritwood Aquifer, raising water pressure beneath and ahead of the ice until the overlying materials gave way. Glacial sediment and blocks of shale were thrust upward and pushed a short distance south, creating both a lowland depression and a range of hills made of stacked slabs of sediment and bedrock. The flooded depression became the chain of bays that make up Devils Lake and Stump Lake, while the rugged ice-thrust hills include Sully’s Hill. During this thrusting, sediment-laden groundwater also burst to the surface, in some places through cracks in the glacier, depositing cone-shaped hills of sand and gravel such as Devils Heart Butte near Tokio.