Devils Tower National Monument

Devils Tower (Lakota: Mato Tipila, which means “Bear Lodge”) is a monolithic igneous intrusion or volcanic neck located in the Black Hills near Hulett and Sundance in Crook County, northeastern Wyoming, above the Belle Fourche River. It rises dramatically 1,267 feet (386 m) above the surrounding terrain and the summit is 5,112 feet (1,558 m) above sea level.

Devils Tower was the first declared United States National Monument, established on September 24, 1906, by President Theodore Roosevelt. The Monument's boundary encloses an area of 1,347 acres (5.45 km2). (Marquis 54-55)

In recent years about 1% of the Monument's 400,000 annual visitors climb Devils Tower, mostly through traditional climbing techniques.

The landscape surrounding Devils Tower is composed mostly of sedimentary rocks. The oldest rocks visible in Devils Tower National Monument were laid down in a shallow sea during the Triassic period, 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 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. This layer of gypsum was deposited during the Jurassic period, 195 to 136 million years ago.

Created as sea levels and climates repeatedly changed, gray-green shales (deposited in low-oxygen environments such as marshes) were interbedded with fine-grained sandstones, limestones, and sometimes thin beds of red mudstone. This composition, called the Stockade Beaver member, is part of the Sundance Formation. The Hulett Sandstone member, also part of the Sundance formation, is composed of yellow fine-grained sandstone. Resistant to weathering, it forms the nearly vertical cliffs which encircle the Tower itself. (Marquis 54-55)

About 65 million years ago, during the Tertiary period, the Rocky Mountains and the Black Hills were uplifted. Magma rose through the crust, intruding into the existing sedimentary rock layers.

Geologists agree that Devils Tower was formed by the intrusion of igneous material. What they cannot agree upon is how, exactly, that process took place. Geologists Carpenter and Russell studied Devils Tower in the late 1800s and came to the conclusion that the Tower was indeed formed by an igneous intrusion. Later geologists searched for further explanations. Several geologists believe the molten rock comprising the Tower might not have surfaced; other researchers are convinced the tower is all that remains of what once was a large explosive volcano.

In 1907, scientists Darton and O'Hara decided that Devils Tower must be an eroded remnant of a laccolith. A laccolith is a large mass of igneous rock which is intruded through sedimentary rock beds but does not actually reach the surface, producing a rounded bulge in the sedimentary layers above. This theory was quite popular in the early 1900s since numerous studies had earlier been done on a number of laccoliths in the Southwest.

Other theories have suggested that Devils Tower is a volcanic plug or that it is the neck of an extinct volcano. Presumably, if Devils Tower was a volcanic plug, any ...