[Illustration: _Figure 15.—Looking east toward Cheyenne at “the Gangplank.” Interstate Highway 80 and the Union Pacific Railroad follow the Gangplank from the High Plains in the distance onto the Precambrian rocks (older than 570 m.y.) of the Laramie Mountains in the foreground. Photograph by R. D. Miller, U.S. Geological Survey._] [Illustration: _Figure 16.—Aerial view of Scotts Bluff National Monument, Nebr. Buttes on the south side of the valley of the North Platte River isolated by erosion from High Plains in the background. Highest butte stands about 800 feet above valley floor._] The High Plains extends southward from the Pine Ridge escarpment, near the South Dakota-Nebraska border (fig. 3), to the Edwards Plateau in Texas. The Platte, the Arkansas, and the Canadian Rivers have cut through the High Plains. That part of the High Plains south of the Canadian River is called the Southern High Plains, or the Llano Estacado (staked plain). The origin of this name is uncertain, but it has been suggested that the term Llano Estacado was applied by early travelers because this part of the High Plains is so nearly flat and devoid of landmarks that it was necessary for those pioneers to set lines of stakes to permit them to retrace their routes. The Llano Estacado is bounded on the west by the Mescalero escarpment (fig. 4) and on the east by the Caprock escarpment. The southern margin with the Edwards Plateau is less well defined, but King Mountain, north of McCamey, Tex., is a scarp-bounded southern promontory of the High Plains. The remarkably flat surface of the Llano Estacado is abundantly pitted by sinks and depressions in the surface of the Ogallala Formation; these were formed by solution of the limestone by rainwater and blowing away or deflation by wind of the remaining insoluble particles. Many of these solution-deflation depressions are aligned like strings of beads, suggesting that their location is controlled by some kind of underlying structure, such as intersections of joints in the Ogallala Formation. The solution-deflation depressions are less abundant north of the Canadian River, but occur on the High Plains surface northward to the Arkansas River and along the eastern part of the High Plains north of the Arkansas to the South Fork of the Republican River.