Devils Tower National Monument, Wyoming Poster

"Devils Tower National Monument, Wyoming" by Catherine Sherman. Devils Tower rises from the hills of northern Wyoming under a cloudy sky. 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 1,347 acres (545 ha). The igneous material that forms the Tower is a phonolite porphyry intruded about 40.5 million years ago, a light to dark-grey or greenish-grey igneous rock with conspicuous crystals of white feldspar. As the magma cooled, hexagonal (and sometimes 4-, 5-, and 7-sided) columns formed, each about six feet in diameter. As the rock continued to cool, the vertical columns shrank in width and cracks began to occur at 120-degree angles, generally forming compact 6-sided columns. The Sioux who lived in this area explain the grooved sides with a story of girls playing nearby. Suddenly giant bears began to chase them. The girls climbed up on a rock and prayed for the Great Spirit to protect them. The Great Spirit lifted the rock, creating a tower into the sky. The bears could not reach the girls, but tried, leaving deep claw marks on the side of the tower. These claw marks explain the column-like shapes on the outside of Devils Tower.