Hulett sandstone member.—The Hulett sandstone member is resistant to weathering and forms a conspicuous, almost vertical, cliff that nearly encircles the Tower. This member ranges in thickness from about 60 to 70 feet.
The Hulett sandstone member consists, in general, of massive fine-grained glauconitic calcareous sandstone. It is typically yellow or brownish yellow but locally may be pink or red. In the lower 5 to 10 feet the sandstone is in beds from less than 1 inch to 2 feet thick separated by gray or greenish-gray shale partings of from less than 1 inch to 6 inches thick. Many of the sandstone beds at the base of the member are ripple marked.
The 50 to 60 feet in the middle of the member consists of massive beds that range in thickness from 5 to 20 feet. This portion is well cemented and forms the conspicuous cliff seen throughout the area. The upper 5 to 10 feet is thin bedded (beds from less than 1 inch to 6 inches in thickness) locally shaly, and poorly cemented. This grades upward into the overlying sandstone and siltstone of the Lak member.
Lak member.—The Lak member crops out above the cliff of Hulett sandstone that almost encircles the Tower, and it underlies a broad rolling area in the northwestern part of the Monument. The member is rarely exposed because it is composed of soft sandstone and siltstone that usually weather to gentle slopes and become covered with vegetation. The best exposure is on the steep hill east of the Tower and northwest of the bridge across the Belle Fourche river.
This member is 65 feet thick a few hundred feet east of the Tower, but mapping within the Monument and measured sections within a few miles of the Monument indicate that the average thickness is about 45 feet.
The Lak member is typically poorly bedded soft, very fine-grained calcareous sandstone and siltstone with a few thin gray-green sandy shale partings. At the base and near the top of the member may be a few thin (less than 1 inch to 6 inches thick) well-cemented sandstone beds that form small ridges. The sandstone and siltstone grade almost imperceptibly from one to the other. The color ranges from light yellow brown and yellow to red. In the Devils Tower area, shades of yellow and yellowish brown are most common.
The contact of the Lak with the overlying Redwater shale member can be observed only in the exposure east of the Tower. Here, the upper 3 feet of the Lak is a yellowish-brown calcareous silty sandstone with a few discontinuous sandy shale partings (less than 1 inch thick), and the lower 3 feet of the overlying Redwater shale consists of dark-gray-green shale with interbedded, thin silty sandstone.
Redwater shale member.—This member encircles Devils Tower, but at most places it is covered by talus from the Tower. Even where it is not covered by talus, it is poorly exposed. It consists mostly of shale that weathers into gentle slopes, which are usually covered by vegetation. The Redwater shale is partly exposed on Fossil Hill, northwest of Devils Tower, and on the hill in the northwest corner of the Monument. The best exposures are on Fossil Hill.
The top of the Redwater shale member is not exposed within the limits of the Monument; consequently, the thickness could not be determined. In surrounding areas the Redwater shale ranges in thickness from 150 to 190 feet. It is at least 100 feet thick on the hill in the northwest corner of the Monument.
The Redwater shale consists mostly of light-gray to dark gray-green soft shale. In the lower 20 or 30 feet are beds of yellow soft sandstone, 3 inches to 2 feet thick. In the upper part, ranging from 50 feet above the base to the top, are lenticular beds of fossiliferous limestone 1 inch to 4 feet thick. Two such beds of fossiliferous limestone are exposed on Fossil Hill.