_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 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.