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.