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.