The Spearfish formation consists of red to maroon siltstone and sandstone interbedded with mudstone or shale. Locally, greenish-blue shale partings are found in the siltstone and sandstone. The formation is poorly cemented and weathers very easily forming, for the most part, gentle slopes, as on the northeast and southwest sides of the monument. Where it does form cliffs, as south of the Tower, the cliffs are cut by many sharp gullies.
No fossils have been found in the Spearfish formation in the Devils Tower region, but elsewhere in Wyoming, stratigraphically equivalent rocks contain land vertebrates of Triassic age.
GYPSUM SPRING FORMATION
The Gypsum Spring formation is exposed in a thin but almost continuous band around the Tower on the southwest to northeast sides. It also crops out near the top of the small hill at the eastern boundary of the National Monument, a few hundred feet north of the Registration Building. This formation is composed mostly of white gypsum, which stands out conspicuously between the red beds of the underlying Spearfish formation and beds of gray-green shale at the base of the overlying Sundance formation.
The Gypsum Spring formation ranges in thickness from about 15 to about 35 feet. It is thickest on the hill at the eastern boundary of the Monument. Here the formation is made up of a lower unit consisting of a bed of white massive gypsum 20 feet thick overlain by 14 feet of interbedded white gypsum and dark-maroon mudstone. The formation is 15 feet thick along the cliff directly south of Devils Tower. At this place, the formation consists of 12 feet of white massive gypsum interbedded with 1-6 inch thick beds of dark-maroon mudstone overlain by 3 feet of dark-brownish-red mudstone. The differences in thickness are primarily the result of erosion of the Gypsum Spring formation prior to the deposition of the Stockade Beaver shale member of the Sundance formation (Imlay, 1947, p. 243).
SUNDANCE FORMATION
The Sundance formation consists of an alternating sequence of greenish-gray shale, light-gray to yellowish-brown sandstone and siltstone, and gray limestone. The formation crops out above the gypsum and red shale of the Gypsum Spring formation on the bluffs and low rolling hills that surround the Tower. The formation consists of four members that are, in order of age from oldest to youngest, the Stockade Beaver shale member, the Hulett sandstone member, the Lak member, and the Redwater shale member ([fig. 54]) (Imlay, 1947, p. 227-273).
Stockade Beaver shale member.—In general, this member, because it is composed mostly of shale, is poorly exposed. The best exposures of the lower part are on the hill at the east boundary of the Monument and along the steep slope south of the Tower. The upper part is fairly well exposed on the south side of the ridge north of the Tower, near the north boundary of the Monument. The member has a thickness of 85 to 100 feet.
The composition differs considerably in detail from one exposure to another, but in general it consists of gray-green shale with interbedded fine-grained calcareous sandstone. At the base of the member, at nearly all exposures, is a thin sandstone, 1 to 24 inches thick, containing black or dark-gray water-worn chert pebbles that have a maximum dimension of about 2 inches. Above the basal sand, the lower half of the member is composed mostly of gray-green shale, which locally contains some interbedded fine-grained calcareous sandstone, thin sandy and shaly limestone or dolomitic limestone, and rarely thin beds of red mudstone. The upper half of the member consists of dark-gray to gray-green shale with interbedded fine-grained calcareous sandstone that range from less than 1 foot to 6 feet in thickness.
The contact of the Stockade Beaver shale member with the overlying Hulett sandstone member is gradational. The sandstone becomes more abundant in the upper part of the Stockade Beaver shale, and the contact between those two members is placed at that point where the sandstone makes up more than 50 percent of the rocks.