The City of Rocks National Reserve, also known as the Silent City of Rocks, is a United States National Reserve and state park lying 2 miles (3.2 km) north of the south central Idaho border with Utah. It is widely known for its excellent rock climbing and rock formations. The rock spires in the City of Rocks and adjacent Castle Rocks State Park are largely composed of granitic rock of the Oligocene Almo pluton and Archean Green Creek Complex.
The City of Rocks is a popular rock climbing area, with numerous traditional and bolt-protected routes. In the 1980s, it was home to some of the most difficult routes in the USA, mostly developed by Idaho climber Tony Yaniro. Climbers in the region refer to the area as simply 'The City'. California Trail wagon trains of the 1840s and 1850s left the Raft River valley and traveled through the area and over Granite Pass into Nevada. Names or initials of emigrants written in axle grease are still visible on Register Rock. Ruts from wagon wheels also can be seen in some of the rocks.