West of the Black Hills, in Wyoming, the Tongue River and the Powder River have excavated the Powder River Basin and produced similar features (fig. 25). The east-flowing tributaries of the Missouri River—the Knife, Heart, and Cannonball Rivers in North Dakota and the Grand, Moreau, Belle Fourche, Cheyenne, Bad, and White Rivers in South Dakota—similarly have shaped the landscape. Most of these rivers flow in broad, old valleys, established more than 2 million years ago, before the first advance of the continental ice sheets. Some of these valleys have been widened by recession of the valley walls by badland development. Badlands are formed by the cutting action of rivulets and rills flowing down over a steeply sloping face of soft, fine-grained material composed mainly of clay and silt. The intricate carving by thousands of small streams of water produces the distinctive rounded and gullied terrain we call badlands. Badlands National Monument in South Dakota (fig. 26) has been established in the remarkable badlands terrain cut into the White River Group along the north valley wall of the White River, and the South Unit of Theodore Roosevelt National Memorial Park is in the colorful badlands of the Little Missouri River, formed on the Fort Union Formation (fig. 27). The White River also has cut a steep scarp along its southern wall that is called the Pine Ridge escarpment. This escarpment defines the boundary between the Missouri Plateau and the High Plains here. [Illustration: _Figure 25.—View northeast across the Deckers coal mine and the Tongue River in the Powder River Basin, southeastern Montana. Typical terrain of unglaciated Missouri Plateau. Small mesas with cliffed escarpments on capping layer of resistant sandstone, such as those in the foreground, are common. Coal mine is about 1 mile across. Photograph by R. B. Taylor, U. S. Geological Survey._] The landscape of the Unglaciated Missouri Plateau has been determined largely by the action of streams, but in some areas igneous intrusions and volcanoes have produced small mountain masses that interrupt the plain, and valley glaciers have modified the valleys in some of these mountains. [Illustration: _Figure 26.—Badlands in Badlands National Monument, S. Dak. Photograph by W. H. Raymond, III, U. S. Geological Survey._] [Illustration: _Figure 27.—Badlands of the Little Missouri River in South Unit of Theodore Roosevelt National Memorial Park, N. Dak. View looking northwest from Painted Canyon Overlook along Interstate Highway 94, west of Belfield._] THE COLORADO PIEDMONT