Devils Tower rises steeply for about 600 feet from a broad talus slope
at its base. The top of the Tower, at an altitude of 5,117 feet, is
about 1,270 feet above the Belle Fourche River. The Tower is about 800
feet in diameter at the base. The sides rise almost vertically from the
base for a distance of from 40 to 100 feet and then slope in more gently
to form a narrow bench. Above this bench, the sides again rise steeply,
at angles of 75° to over 85°, to within about 100 feet of the top where
the angle becomes less steep and the top edge of the Tower is somewhat
rounded. The top of the Tower is almost flat and measures about 180 feet
from east to west and about 300 feet from north to south.
One of the most striking features of the Tower is its polygonal columns
(fig. 53). Most of the columns are 5 sided, but some are 4 and 6 sided.
The larger columns measure 6 to 8 feet in diameter at their base and
taper gradually upward to about 4 feet at the top. The columns are
bounded by well-developed smooth joints in the middle part of the Tower,
but as the columns taper upward, the joints between them, rather than
being smooth, may be wavy and some of the columns may unite. Numerous
cross-fractures in the upper part of the Tower divide the column into
many small irregularly shaped blocks (fig. 53_A_).
[Illustration: Figure 53.—A. Northwest side of Devils Tower showing
how the columns taper or converge and in places unite near the top
and are cut by numerous cross-fractures.]
[Illustration: Figure 53.—B. Southwest corner of Devils Tower
showing the columns flaring out and merging to form the massive
base.]
The columns in the central and upper parts of the Tower are almost
vertical but flare out at the bench about 100 feet above the base (fig.
53_B_). On the southwest side the columns are nearly horizontal. Where
the columns flare out, several columns may join to form a larger, less
distinct column that merges with the massive base.
At the base of the tower, below the bench, the rock is massive and
jointing, poorly developed. Here the joints form large irregularly
shaped blocks rather than columns.
Columnar joints form as the result of contraction within a rock mass. In
igneous rock the contraction is the result of cooling; that is, the cold
solidified rock requires less volume than the same rock when molten. As
a rock cools it contracts, and the resulting tension is in a plane
parallel to the cooling surface. When rupture takes place, three
fractures radiate from numerous centers in the plane parallel to the
cooling surface. Ideally, the fractures are at 120° to each other. If
the centers were evenly distributed, the fractures from different
centers would join forming hexagonal (6 sided) columns. These fractures
will go deeper and deeper into the rock as cooling progresses. This
condition because of many factors, is seldom attained in nature, so the
columns may have 4, 5, 6, or even more sides.
The rock making up Devils Tower is classified as phonolite porphyry
(Darton and O’Harra, 1907, p. 6) and is of Tertiary age. The fresh
specimens have a light- to dark-gray or greenish-gray very fine-grained
groundmass with conspicuous crystals of white feldspar—commonly about
one-fourth to one-half inch in diameter—and smaller very dark-green
crystals of pyroxene. On the weathered surfaces the phonolite porphyry
is a light gray or brownish gray. Lichens growing on the rock may give
it a green, yellowish-green, or brown color.
Using a microscope, Albert Johannsen (Darton and O’Harra, 1907, p. 6)
identified the feldspar crystals as a soda-rich orthoclase and the
pyroxene crystals as augite with an outer zone of aegirite. In addition,
phenocrysts of apatite and magnetite, were identified. The groundmass,
according to Johannsen, consists of orthoclase laths in subparallel
arrangement, needles of aegirite, possibly some nephelite, small cubes
of magnetite, and secondary minerals of calcite, kaolin, chlorite,
analcite, and a anisotropic zeolite.
[Illustration: Figure 54.—Generalized section of the sedimentary
rocks of the Devils Tower National Monument.]
System
Series
Formation and member; Thickness, in feet; Columnar section;
Description
JURASSIC
Upper Jurassic
Sundance formation
Redwater shale member; 100+; Gray and gray-green shale. Thin
fine-grained sandstones in lower part; thin fossiliferous
limestones in upper part
Lak member; 40-65; Yellow soft fine-grained calcareous sandstone
Hulett sandstone member; 60-70; Yellow massive fine-grained
calcareous sandstone
Stockade Beaver shale member; 85-100; Gray and gray-green shales
with thin calcareous sandstones
UNCONFORMITY
Middle Jurassic
Gypsum Spring formation; 15-35; White massive gypsum interbedded
with thin red mudstone
UNCONFORMITY
TRIASSIC
Spearfish formation; 100; Red to maroon siltstone and sandstone
interbedded with some thin shale