Devils Tower National Monument, 1,347 acres (546 hectares), overlooking the Belle Fourche River, NE Wyo.; it was the first designated U.S. national monument (1906). Devils Tower, 865 ft (264 m) high and narrowing in width from 1,000 ft (305 m) at its base to 250 ft (76 m) at its summit, is a cluster of rock columns formed by the cooling and crystallization of molten matter. Geologists assume it to be a remnant of an ancient lava intrusion into sedimentary strata which were later removed by wind and water action, leaving the more resistant "tower" to dominate the skyline. Sacred to many Lakota, Cheyenne, and members of other Plains tribes, who call it Bear Lodge, it is also a popular rock-climbing and tourist site, which has caused some controversy. See National Parks and Monuments, table.