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 Monument. 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 down-fold 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 is somewhat altered; the groundmass is a light greenish gray, and the normally shiny crystals of feldspar have a dull earthy luster.

GEOLOGIC HISTORY

The geologic history of the Devils Tower area is part of that of the Black Hills region. The uplift of the Black Hills and the subsequent erosion have exposed the rocks, from which the geologic history of the area may be interpreted.

Most of the rocks within the area around the Black Hills are composed of sediments deposited by water. These sedimentary rocks, which overlie much older rocks (Precambrian), were deposited in a series of successive layers during time intervals from the Cambrian period to well into the Tertiary period. Deposits in the ancient seas are represented by limestone, shale and some sandstone; deposits on low lands adjacent to seas, as flood plains and deltas, by sandstone, siltstone, and some mudstone; and deposits along streams by conglomerate, sandstone and siltstone. Between the periods of deposition were intervals when the land was relatively high, and in certain areas all of the sediments of an earlier period were eroded away.

The oldest formation exposed in the National Monument, the Spearfish formation, was deposited during Triassic time along flat lands bordering the sea. Arms of the sea locally invaded the area to leave deposits of gypsum, which are found near the base of the Spearfish in areas outside the National Monument. The Gypsum Spring formation was deposited in the sea in Middle Jurassic time following a period of uplift and erosion that occurred after the deposition of the Spearfish formation. After the Gypsum Spring formation was deposited, the sea retreated, and another period of erosion followed before Late Jurassic time when the sea invaded the area again and the Sundance formation was deposited. The depth and conditions for deposition in this sea changed from time to time, as shown by the alternating beds of shale and sandstone in the Sundance formation.

Following the deposition of the Sundance formation, there were several periods when the area was above sea level and when the sea covered the area. During the periods when it was above sea level, the higher land was eroded, and the sediments deposited at a lower level. When the area was covered by the sea, marine sediments, principally shales, were deposited. Near the end of the Cretaceous period, the sea made its final withdrawal, and the sediments from late Cretaceous