Geological Survey Bulletin 1021-I Geology of Devils Tower National Monument, Wyoming

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 conglomuerate, 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 depositiomi 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 withdrawl, 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, amid 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.