The Sundance formation consists of an alternating sequence of
greenish-gray shale, light-gray to yellowish-brown sandstone and
siltstone, and gray limestone. The formation crops out above the gypsum
and red shale of the Gypsum Spring formation on the bluffs and low
rolling hills that surround the Tower. The formation consists of four
members that are, in order of age from oldest to youngest, the Stockade
Beaver shale member, the Hulett sandstone member, the Lak member, and
the Redwater shale member (fig. 54) (Imlay, 1947, p. 227-273).
_Stockade Beaver shale member._—In general, this member, because it is
composed mostly of shale, is poorly exposed. The best exposures of the
lower part are on the hill at the east boundary of the Monument and
along the steep slope south of the Tower. The upper part is fairly well
exposed on the south side of the ridge north of the Tower, near the
north boundary of the Monument. The member has a thickness of 85 to 100
feet.
The composition differs considerably in detail from one exposure to
another, but in general it consists of gray-green shale with interbedded
fine-grained calcareous sandstone. At the base of the member, at nearly
all exposures, is a thin sandstone, 1 to 24 inches thick, containing
black or dark-gray water-worn chert pebbles that have a maximum
dimension of about 2 inches. Above the basal sand, the lower half of the
member is composed mostly of gray-green shale, which locally contains
some interbedded fine-grained calcareous sandstone, thin sandy and shaly
limestone or dolomitic limestone, and rarely thin beds of red mudstone.
The upper half of the member consists of dark-gray to gray-green shale
with interbedded fine-grained calcareous sandstone that range from less
than 1 foot to 6 feet in thickness.
The contact of the Stockade Beaver shale member with the overlying
Hulett sandstone member is gradational. The sandstone becomes more
abundant in the upper part of the Stockade Beaver shale, and the contact
between those two members is placed at that point where the sandstone
makes up more than 50 percent of the rocks.
_Hulett sandstone member._—The Hulett sandstone member is resistant to
weathering and forms a conspicuous, almost vertical, cliff that nearly
encircles the Tower. This member ranges in thickness from about 60 to 70
feet.
The Hulett sandstone member consists, in general, of massive
fine-grained glauconitic calcareous sandstone. It is typically yellow or
brownish yellow but locally may be pink or red. In the lower 5 to 10
feet the sandstone is in beds from less than 1 inch to 2 feet thick
separated by gray or greenish-gray shale partings of from less than 1
inch to 6 inches thick. Many of the sandstone beds at the base of the
member are ripple marked.
The 50 to 60 feet in the middle of the member consists of massive beds
that range in thickness from 5 to 20 feet. This portion is well cemented
and forms the conspicuous cliff seen throughout the area. The upper 5 to
10 feet is thin bedded (beds from less than 1 inch to 6 inches in
thickness) locally shaly, and poorly cemented. This grades upward into
the overlying sandstone and siltstone of the Lak member.
_Lak member._—The Lak member crops out above the cliff of Hulett
sandstone that almost encircles the Tower, and it underlies a broad
rolling area in the northwestern part of the Monument. The member is
rarely exposed because it is composed of soft sandstone and siltstone
that usually weather to gentle slopes and become covered with
vegetation. The best exposure is on the steep hill east of the Tower and
northwest of the bridge across the Belle Fourche river.
This member is 65 feet thick a few hundred feet east of the Tower, but
mapping within the Monument and measured sections within a few miles of
the Monument indicate that the average thickness is about 45 feet.
The Lak member is typically poorly bedded soft, very fine-grained
calcareous sandstone and siltstone with a few thin gray-green sandy
shale partings. At the base and near the top of the member may be a few
thin (less than 1 inch to 6 inches thick) well-cemented sandstone beds
that form small ridges. The sandstone and siltstone grade almost
imperceptibly from one to the other. The color ranges from light yellow
brown and yellow to red. In the Devils Tower area, shades of yellow and
yellowish brown are most common.
The contact of the Lak with the overlying Redwater shale member can be
observed only in the exposure east of the Tower. Here, the upper 3 feet
of the Lak is a yellowish-brown calcareous silty sandstone with a few
discontinuous sandy shale partings (less than 1 inch thick), and the
lower 3 feet of the overlying Redwater shale consists of dark-gray-green
shale with interbedded, thin silty sandstone.
_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).