Devils Tower National Monument, in northeastern Wyoming, on the Belle Fourche River, about 110 miles east of Sheridan. The monument is the site of a tower of volcanic rock, known as Devils Tower, which rises 865 feet from a hill beside the Belle Fourche River. Visible from some directions for nearly 100 miles, the tower once served as a landmark for Sioux Indians and pioneers. The monument covers about 1,347 acres and is administered by the National Park Service. It was established in 1906 as the first national monument in the United States.