Devil's Tower, deemed the first national monument by Theodore Roosevelt in 1906, is an igneous intrusion that rises 1,267 feet above the surrounding area. Approximately 50 to 60 million years ago, during Tertiary time, pressures within western North America climaxed, uplifting the Rocky Mountains and the Black Hills. At this time or shortly after, magma (molten rock) welled up toward the surface of the earth, intruding into the already existing sedimentary rock layers. Devil's Tower is not a volcano. It is a laccolith, an intrusion of molten rock that pushed into surrounding sedimentary rocks before solidifying underground. The characteristic furrowed columns are the result of contraction which occurred during the cooling of the magma.
Most of the landscape surrounding Devils Tower is composed of sedimentary rocks. These rocks are formed from broken or dissolved fragments of other rocks and are usually deposited by water or wind.
The oldest rocks visible in Devils Tower National Monument were laid down in a shallow inland sea. This sea covered much of the central and western United States during Triassic time, 225 to 195 million years ago. This dark red sandstone and maroon siltstone, interbedded with shale, can be seen along the Belle Fourche River. Oxidation of iron rich minerals causes the redness of the rocks. This rock layer is known as the Spearfish Formation.
Above the Spearfish formation is a thin band of white gypsum, called the Gypsum Springs formation. Gypsum is an important mineral resource commonly used in making drywall. This layer of gypsum was deposited during the Jurassic time, 195 to 136 million years ago.
Seas retreated and returned. Gray-green shales deposited offshore in deep marine environments were interbedded with fine-grained sandstones, limestones, and sometimes thin beds of red mudstone. These rock layers, called the Stockade Beaver member, are part of the Sundance Formation—also of Jurassic age.
The Hulett Sandstone member, also part of the Sundance formation, is a yellow, fine-grained sandstone deposited on an ancient beach. Resistant to weathering, it forms the nearly vertical cliffs that encircle the Tower itself. Seas retreated and advanced; landforms developed and eroded. 100-200 million years ago, this area was once under a shallow sea. Over a period of millions of years, sediment was deposited on the floor of this sea. This sediment eventually turned in to sedimentary rock such as sandstone, shale and siltstone. Look to the north east and you will see the remnants of the shallow sea bed.
65 million years ago, pressures from within the earth forced the land upward. These pressures created the nearby Black Hills and Rocky Mountains. The pressure also forced molten rock toward the surface at the location where the tower now stands. Geologists believe that erosion uncovered the rock formations only one or two million years ago.
More information can be found at the National Park site. In order to log a find for this Earth Cache, please do the following:
1) Send a message to me with "GC3T8QC, Tower Geology" in the first line.
2) Look at the geologic formation to the north east. Compare it to the geologic formation to the north west. Describe the colors of the layers from the highest elevation down to the lowest elevation.
3) What types of rocks comprise each of the layers you listed in #2 above (igneous, metamorphic, or sedementary)?