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