Geological Survey Bulletin 1021-1 Geology of Devils Tower National Monument, Wyoming

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, silt stone, 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 750 to over 850, 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.

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.

FIGURE 53.—B. South side of Devils Tower showing the columns flaring out and merging to form 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.

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).

FIGURE 54.—Generalized section of the sedimentary rocks of the Devils Tower National Monument. (click on image for an enlargement in a new window)

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 (Dalton, 1909, p. 28); however, only the uppermost 100 feet are exposed within the National Monument.

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 cemnented 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 sequense 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.

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 Monumnent. The member is rarely exposed because it is composed of soft sandstone and siltstone that usually weather to gentle slopes amid 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.

The Sundance formation contains clams, oysters, belemnites (squids), and other marine fossils that establish its age as Late Jurassic (Imlay, 1947, p. 244—264).

STREAM TERRACE DEPOSITS AND ALLUVIUM

Stream deposits (alluvium) are found in the valleys of the small streams around the Tower and, in particular, in the valley of the Belle Fourche River, that cuts across the southeast corner of the Monumuent. The deposits consist of unconsolidated gravel, sand, silt, and mud.

Along the Belle Fourche River, northwest of the river and between it and the main road, the river cut a terrace in the Spearfish formation. On the terrace were deposited gravel and sand.

TALUS AND LANDSLIDE MATERIAL

The talus and landslides are composed primarily of the material from the Tower and the Hulett sandstone.

Talus from the Tower forms a broad apron that completely surrounds the Tower. The talus extends from high on the shoulders of the Tower down to and across the sedimentary rock. Locally, landslides of this talus have extended through valleys in the sedimentary rock down almost to the level of the surrounding streams. The talus from the Tower is composed of fragments of the columns that range from a few inches in diameter to large sections of the columns as much as 8 feet in diameter and 25 feet long.

The cliff of Hulett sandstone that surrounds the Tower breaks off into rectangular blocks that form talus slopes at the base of the cliffs and locally large landslides down the hill below the cliffs. These blocks of Hulett sandstone range in size from a few inches to many feet in diameter. The talus material from the Tower has at several places overlapped the cliff of Hulett sandstone and become mixed with the material from the cliff.

About 1,400 feet north of the Tower are two patches of what is believed to be talus formed from sedimentary rocks that once surrounded the Tower. The talus consists of fragments of medium grained brownish-white sandstone and, what is apparently, a highly silicified gray or white fine-grained quartzite. The sandstone resembles that found in the Lakota (Darton and O'Hara, 1907, p. 3) that lies about 200 feet stratigraphically above the Redwater shale in the area west of the Monument.

The sandstone and quartzite occur in angular blocks that range from less than 1 inch to several feet in diameter. The spaces between the blocks are filled with a yellowish or brownish-white sand.

The Lakota sandstone at one time surrounded the Tower and it is believed that these blocks are residual blocks that have not been removed by erosion.

STRUCTURE

The sedimentary rocks in the National Monument, and in the surrounding area, are gently folded into many small rolls, basins and domes, which locally are cut by faults of small displacement. These small folds are superimposed on a large dome that is collapsed in the middle.

Devils Tower is near the middle of the collapsed dome. From one-half to about a mile from the Tower the sedimentary rocks dip gently from 2° to 5° away from the Tower to form a broad dome. Within a radius of about 2,000 to 3,000 feet of the Tower, the dips change, and the rocks dip, in general, from 3° to 5° towards the Tower to form a shallow structural basin. In the basin itself and on the dome are several small folds. As an example, Spring No. 1 southwest of the Tower is in the center of a comparatively sharp syncline or downfold at the edge of the basin. Fossil Hill northwest of the Tower is another small structural basin. The beds along the top and on the north side of Fossil Hill dip from 12° to 52° S. Those on the south side of the hill, north of the road, apparently dip very gently northward.

Three faults were observed in the area of the National Monument. Two of the faults are in the Hulett sandstone west of the main road and west of the Tower, and the third is in the northwestern side of the Tower near its base (pl. 30). The faults in the Hulett sandstone are probably vertical, and the displacement along them is believed to be less than 10 feet. The fault at the base of the Tower is a low-angle fault that trends northwesterly. The attitude of this fault at the point where it disappears below the talus is: strike, N. 41° W.; dip, 21° NE. The fault zone is 4 to 12 inches wide and is filled with a yellowish-green clay and sheared fragments of altered phonolite porphyry. The rock of the Tower below this fault in somewhat altered; the groundmass is a light greenish gray, and the normally shiny crystals of feldspar have a dull earthy luster.