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).