![]() Other ideas have suggested that Devils Tower is a volcanic plug or that it is the neck of an extinct volcano (Figure 3). This idea was quite popular in the early 1900s when numerous studies were done on a number of laccoliths in the American southwest. This produces a rounded bulge in the sedimentary layers above the intrusion (Figure 2). A laccolith is a large, mushroom-shaped mass of igneous rock which intrudes between the layers of sedimentary rocks but does not reach the surface. In 1907, scientists Darton and O’Hara decided that Devils Tower must be an eroded remnant of a laccolith. The simplest explanation is that Devils Tower is a stock-a small intrusive body formed by magma which cooled underground and was later exposed by erosion (Figure 1). Later geologists searched for more detailed explanations. Geologists Carpenter and Russell studied Devils Tower in the late 1800s and concluded that the Tower was formed by an igneous intrusion (the forcible entry of magma through other rock layers). Numerous theories have been suggested to explain how Devils Tower formed. What they cannot agree upon are the processes by which the magma cooled to form the Tower, or its relationship to the surrounding geology of the area. Geologists agree that Devils Tower began as magma, or molten rock buried beneath the Earth’s surface. 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. Approximately 50 to 60 million years ago, during Tertiary time, tectonic pressures within western North America climaxed, uplifting the Rocky Mountains and the Black Hills. Seas retreated and advanced landforms developed and eroded. Resistant to weathering, these form the nearly vertical cliffs that encircle the Tower itself. Their origins trace back to sand deposited on an ancient beach, with many outcrops exhibiting preserved symmetrical ripples. The Hulett Sandstone and Lak Members, also part of the Sundance Formation, are yellow, fine-grained sandstones. ![]() These rock layers, called the Stockade Beaver Member, are part of the Sundance Formation-also of Jurassic age. Off-shore clay deposits in deep marine environments became gray-green shales interbedded with sandstones, limestones, and thin beds of red mudstone. During the Jurassic period (195 to 136 million years ago) seas periodically retreated and returned. As mineral rich water evaporated deposits of gypsum were left behind. This rock layer is the Spearfish Formation.Ībove the Spearfish Formation is the Gypsum Springs Formation. ![]() Oxidation of iron-rich minerals causes the red color of the rocks. Dark red sandstone and maroon siltstone, interbedded with shale, can be seen along the Belle Fourche River. This sea covered much of the central and western United States during the Triassic period, 225 to 195 million years ago. The oldest rocks visible in Devils Tower National Monument were deposited in a shallow inland sea. ![]() Many fossils are found in sedimentary rocks, providing us with clues about ancient ecosystems. This process, known as deposition, is common in river deltas and coastal areas. These rocks are formed from solidification of minerals or organic material, and are usually deposited by water or wind. Most of the landscape surrounding Devils Tower is composed of sedimentary rocks. Fossilized remains of marine plants like crinoids and creatures such as oysters and belemnites are found in these rocks. The visible layers of sedimentary rocks today give us clues to the ancient environments under which they formed. ![]()
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