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 the Sundance formation of Late Jurassic age ([fig. 54]).
SPEARFISH FORMATION
The Spearfish formation crops out in the southern and northeastern parts of the Devils Tower National Monument along the valley of the Belle Fourche River and its tributaries and forms conspicuous brownish-red to maroon cliffs that border the Belle Fourche valley for several miles in the Devils Tower region. The formation is 450 to 600 feet thick in the northern Black Hills area (Darton, 1909, p. 28); however, only the uppermost 100 feet are exposed within the National Monument.