The geologic history of the Devils Tower area is part of that of the
Black Hills region. The uplift of the Black Hills and the subsequent
erosion have exposed the rocks, from which the geologic history of the
area may be interpreted.
Most of the rocks within the area around the Black Hills are composed of
sediments deposited by water. These sedimentary rocks, which overlie
much older rocks (Precambrian), were deposited in a series of successive
layers during time intervals from the Cambrian period to well into the
Tertiary period. Deposits in the ancient seas are represented by
limestone, shale and some sandstone; deposits on low lands adjacent to
seas, as flood plains and deltas, by sandstone, siltstone, and some
mudstone; and deposits along streams by conglomerate, sandstone and
siltstone. Between the periods of deposition were intervals when the
land was relatively high, and in certain areas all of the sediments of
an earlier period were eroded away.
The oldest formation exposed in the National Monument, the Spearfish
formation, was deposited during Triassic time along flat lands bordering
the sea. Arms of the sea locally invaded the area to leave deposits of
gypsum, which are found near the base of the Spearfish in areas outside
the National Monument. The Gypsum Spring formation was deposited in the
sea in Middle Jurassic time following a period of uplift and erosion
that occurred after the deposition of the Spearfish formation. After the
Gypsum Spring formation was deposited, the sea retreated, and another
period of erosion followed before Late Jurassic time when the sea
invaded the area again and the Sundance formation was deposited. The
depth and conditions for deposition in this sea changed from time to
time, as shown by the alternating beds of shale and sandstone in the
Sundance formation.
Following the deposition of the Sundance formation, there were several
periods when the area was above sea level and when the sea covered the
area. During the periods when it was above sea level, the higher land
was eroded, and the sediments deposited at a lower level. When the area
was covered by the sea, marine sediments, principally shales, were
deposited. Near the end of the Cretaceous period, the sea made its final
withdrawal, and the sediments from late Cretaceous time to the present
were deposited in fresh water.
The Black Hills uplift developed primarily during early Tertiary time,
although it may have started in very late Cretaceous time. At this time
the present general structural features of the Black Hills area were
developed, and, probably, the igneous rocks, such as Devils Tower, were
intruded (Jaggar, 1901, p. 266). Following this, the Black Hills area
was repeatedly uplifted, and erosion exposed the older sedimentary and
intrusive rocks. Even today streams continue to strip more and more rock
from the country, leaving only local deposits, such as alluvium and
terrace deposits, along the valleys.