City of Rocks National Reserve in south-central Idaho is nationally significant for both striking geology and overland trail history. Its granite spires, domes, walls, and towers were shaped by erosion from rock formed in two very different eras, including some of the oldest rock in the western United States. Long before formal protection, Shoshone and Bannock people hunted and gathered here. The area later became an important landmark on the California Trail and Salt Lake Alternate Trail, with emigrant signatures on Register Rock and wagon ruts still visible. Protected in stages after the 1950s, it also became a major climbing destination with more than 1,000 routes.