to join U. S. Highway 14 at a point 29 miles northwest of Sundance, Wyo., and 33 miles northeast of Moorcroft, Wyo. The entrance to the National Monument may also be reached by a road (paved in Wyoming) that goes northeastward from the entrance, via Hulett and Aladdin, Wyo., to Belle Fourche, S. Dak., a distance of about 54 miles, where it joins U. S. Highways 212 and 85.

Figure 52.—Index map showing location of Devils Tower National Monument.

Public campgrounds and a natural history museum are maintained by the National Park Service at the base of the Tower about 3 miles by paved road from the Monument entrance.

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. 53A).

Figure 53.—A. Northwest side of Devils Tower showing how the columns taper or converge and in places unite near the top and are cut by numerous cross-fractures.

Figure 53.—B. Southwest corner of Devils Tower showing the columns flaring out and merging to form the massive base.

The columns in the central and upper parts of the Tower are almost vertical but flare out at the bench about 100 feet above the base (fig. 53B). On the southwest side the columns are nearly horizontal. Where the columns flare out, several columns may join to form a larger, less distinct column that merges with the massive base.

At the base of the tower, below the bench, the rock is massive and jointing, poorly developed. Here the joints form large irregularly shaped blocks rather than columns.

Columnar joints form as the result of contraction within a rock mass. In igneous rock the contraction is the result of cooling; that is, the cold solidified rock requires less volume than the same rock when molten. As a rock cools it contracts, and the resulting tension is in a plane parallel to the cooling surface. When rupture takes place, three fractures radiate from numerous centers in the plane parallel to the cooling surface. Ideally, the fractures are at 120° to each other. If the centers were evenly distributed, the fractures from different centers would join forming hexagonal (6 sided) columns. These fractures will go deeper and deeper into the rock as cooling progresses. This condition because of many factors, is seldom attained in nature, so the columns may have 4, 5, 6, or even more sides.

The rock making up Devils Tower is classified as phonolite porphyry (Darton and O’Harra, 1907, p. 6) and is of Tertiary age. The fresh specimens have a light- to dark-gray or greenish-gray very fine-grained groundmass with conspicuous crystals of white feldspar—commonly about one-fourth to one-half inch in diameter—and smaller very dark-green crystals of pyroxene. On the weathered surfaces the phonolite porphyry is a light gray or brownish gray. Lichens growing on the rock may give it a green, yellowish-green, or brown color.

Using a microscope, Albert Johannsen (Darton and O’Harra, 1907, p. 6) identified the feldspar crystals as a soda-rich orthoclase and the pyroxene crystals as augite with an outer zone of aegirite. In addition, phenocrysts of apatite and magnetite, were identified. The groundmass, according to Johannsen, consists of orthoclase laths in subparallel arrangement, needles of aegirite, possibly some nephelite, small cubes of magnetite, and secondary minerals of calcite, kaolin, chlorite, analcite, and a anisotropic zeolite.

Figure 54.—Generalized section of the sedimentary rocks of the Devils Tower National Monument.

SEDIMENTARY ROCKS

The sedimentary rocks that surround Devils Tower have a total exposed thickness of about 400 feet. They are divided, from oldest to youngest, into the Spearfish formation of Triassic age, the Gypsum Spring formation of Middle Jurassic age, and