The geology of the Devils Tower National Monument was mapped during the summer of 1954 by the U. S. Geological Survey in collaboration with the National Park Service. The work was part of a study of the geology of the northern and western parts of the Black Hills region conducted by the Survey on behalf of the Division of Raw Materials of the U. S. Atomic Energy Commission. The author wishes to acknowledge the assistance of the National Park Service and, in particular, Mr. Raymond McIntyre, Superintendent of Devils Tower National Monument. GEOLOGY The rocks exposed in the Devils Tower National Monument may be divided on the basis of their origin into two general types; igneous and sedimentary. The Tower itself is composed of igneous rock; that is, rock formed directly by cooling and crystallization of once molten materials. The rocks exposed in the remainder of the Monument are sedimentary; that is, they were formed by the consolidation of fragmental materials derived from other rocks or accumulations of chemical precipitates that were deposited either on the floors of prehistoric seas or near the shores of such seas. These rocks, which crop out around the igneous mass, are layers of shale, sandstone, siltstone, mudstone, gypsum, and limestone. Devils Tower owes its impressiveness to the differing rates of erosion of these rock types—the soft sedimentary rocks erode more easily than the hard igneous rock—and to the contrast of the somber color of the igneous column to the brightly colored bands of sedimentary rock that surround its base. DEVILS TOWER Devils Tower rises steeply for about 600 feet from a broad talus slope at its base. The top of the Tower, at an altitude of 5,117 feet, is about 1,270 feet above the Belle Fourche River. The Tower is about 800 feet in diameter at the base. The sides rise almost vertically from the base for a distance of from 40 to 100 feet and then slope in more gently to form a narrow bench. Above this bench, the sides again rise steeply, at angles of 75° to over 85°, to within about 100 feet of the top where the angle becomes less steep and the top edge of the Tower is somewhat rounded. The top of the Tower is almost flat and measures about 180 feet from east to west and about 300 feet from north to south. One of the most striking features of the Tower is its polygonal columns (fig. 53). Most of the columns are 5 sided, but some are 4 and 6 sided. The larger columns measure 6 to 8 feet in diameter at their base and taper gradually upward to about 4 feet at the top. The columns are bounded by well-developed smooth joints in the middle part of the Tower, but as the columns taper upward, the joints between them, rather than being smooth, may be wavy and some of the columns may unite. Numerous cross-fractures in the upper part of the Tower divide the column into many small irregularly shaped blocks (fig. 53_A_).