Devils Tower, America's First National Monument

Devils Tower in the Black Hills of northeastern Wyoming was the first U.S. National Monument, designated by President Theodore Roosevelt in 1906. The tower is a massive igneous rock formation rising 1,267 feet above the Belle Fourche River, formed when molten magma intruded into surrounding rock, cooled slowly, and contracted into the distinctive hexagonal columns visible today. The surrounding rock eroded over millions of years, leaving the harder igneous column standing alone on the plains. Devils Tower is sacred to at least 20 Native American tribes, and a voluntary climbing closure during June is observed by most climbers out of respect.

Source: Wikipedia