Devils Tower, a dramatic igneous monolith in northeastern Wyoming, rises above the Belle Fourche River as a laccolithic butte of phonolite porphyry formed about 40.5 million years ago, its cooling and erosion creating the tall polygonal columns and surrounding scree that give it its distinctive form. Long sacred to Plains tribes including the Lakota, Cheyenne, and Kiowa, it is bound to traditions about bears and the Pleiades and is reflected in names such as Matȟó Thípila. After visits by U.S. expeditions in the nineteenth century, the area became a forest reserve, and in 1906 Theodore Roosevelt made it the first United States national monument. It later became a major climbing site.