City of Rocks

City of Rocks, also called the Silent City of Rocks, is a granite rock formation in south-central Idaho, about 2 miles north of the Utah border in Cassia County, protected as a United States National Reserve and Idaho state park. Most of its spires come from the 28-million-year-old Almo pluton, while some formations are cut from granite of the 2.5-billion-year-old Green Creek Complex, among the oldest exposed rock in the western United States. The landscape developed through exfoliation, a weathering process in which thin rock plates and scales sloughed off along joints formed by the granite's contraction as it cooled, by upward expansion as overlying material eroded away, and by regional tectonic stress. Rock spires reach as high as 600 feet, and the reserve spans 14,407 acres. Emigrants travelling the California Trail between 1843 and the 1850s left inscriptions written in axle grease on rock faces here, signatures still visible today at sites such as Register Rock and Camp Rock. This entity documents City of Rocks specifically as a geological formation, a granite spire landscape shaped by exfoliation from two plutons of very different ages.