Devils Tower

 

Devils Tower (also known as Bear Lodge Butte) is a butte, possibly laccolithic, composed of igneous rock in the Bear Lodge Ranger District of the Black Hills, near Hulett and Sundance in Crook County, northeastern Wyoming, above the Belle Fourche River. It rises 386 m above the Belle Fourche River, standing 265 m from summit to base. The summit is 1,559 m above sea level.

Devils Tower was the first United States national monument, established on September 24, 1906 by President Theodore Roosevelt. The monument's boundary encloses an area of 545 ha.

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. 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, Jurassic in age. Overlying this formation is the Sundance Formation During the Paleocene Epoch, 56 to 66 million years ago, the Rocky Mountains and the Black Hills were uplifted.[citation needed] Magma rose through the crust, intruding into the existing sedimentary rock layers.

Geologists Carpenter and Russell studied Devils Tower in the late 19th century and came to the conclusion that it was formed by an igneous intrusion. In 1907, geologists Nelson Horatio Darton and C. C. O'Harra (of the South Dakota School of Mines) theorized that Devils Tower must be an eroded remnant of a laccolith.

The igneous material that forms the Tower is a phonolite porphyry intruded about 40.5 million years ago, a light to dark-gray or greenish-gray igneous rock with conspicuous crystals of white feldspar. As the magma cooled, hexagonal columns formed (though sometimes 4-, 5-, and 7-sided columns were possible), up to 6.1 m wide and 180 m tall.

As rain and snow continue to erode the sedimentary rocks surrounding the Tower's base, more of Devils Tower will be exposed. Nonetheless, the exposed portions of the Tower still experience certain amounts of erosion. Cracks along the columns are subject to water and ice erosion. Portions, or even entire columns, of rock at Devils Tower are continually breaking off and falling. Piles of broken columns, boulders, small rocks, and stones, called scree, lie at the base of the tower, indicating that it was once wider than it is today.

The geologically-related Missouri Buttes are located 5.6 km northwest of Devils Tower.