In North Dakota, a high area on the east side of the Williston basin acted as a barrier to the advance of the ice, most of which was diverted southeastward. The margin of the ice sheet, however, lapped onto the bedrock high, where it stagnated. Earlier advances moved farthest south; the later advances stopped north of the present course of the Missouri River—their maximum position marked by ridges of unsorted, glacially transported rock debris (till) called terminal moraines. North of the terminal moraines is a distinctive landscape characterized by a rolling, hummocky, or hilly surface with thousands of closed depressions between the hills and hummocks, most of them occupied by lakes. This is the deposit left by the stagnant or dead ice, and it is called dead-ice moraine. The rolling upland in North Dakota that is covered by dead-ice moraine and ridges of terminal moraines from the last glacial advances is called the Coteau du Missouri (fig. 20). A gently sloping scarp, several hundred feet high and mostly covered by glacial deposits (referred to collectively as drift), separates the Coteau du Missouri from the lower, nearly flat, drift-covered plains of the Central Lowland to the east. This escarpment, which is called the Missouri escarpment, is virtually continuous across the State of North Dakota southward into South Dakota. The base of the Missouri escarpment is the eastern boundary of the Great Plains in these northern states. [Illustration: _Figure 20.—Ground moraine on the Coteau du Missouri, northwestern North Dakota. Photograph by R. M. Lindvall, U. S. Geological Survey._] The advancing ice front blocked one after another of the northward-flowing streams of the region, diverting them eastward along the ice front. Shonkin Sag, north of the Highwood Mountains near Great Falls, Mont., is an abandoned diversion channel of the Missouri River, occupied when the ice front stood close to the north slopes of the Highwoods. Much of the present course of the Missouri River from Great Falls, Mont., to Kansas City, Mo., was established as an ice-marginal channel, and the east-flowing part of the Little Missouri River in North Dakota was formed in the same way. These valleys were cut during the last 2 million years. The north-flowing part of the Little Missouri River and the east-flowing courses of the Knife, Heart, and Cannonball Rivers in North Dakota are for the most part older, preglacial courses. The Little Missouri was dammed by the ice, and its waters impounded to form a huge lake during the maximum stand of the ice, but the deposits of this glacial lake are few and make no imprint on the landscape.