The Geology of City of Rocks National Reserve, Idaho

Adapted from the National Park Service Geodiversity Atlas, drawing on the Geologic Resources Inventory.

The reserve

City of Rocks National Reserve lies in the southern Albion Mountains of Cassia County, in south-central Idaho. Established on November 18, 1988, it covers about 5,830 hectares (14,407 acres) of naturally sculpted granite spires and domes, a landscape significant to the Northern Shoshone people. It centers on three upland basins, Circle Creek, Twin Sisters and Emigrant, floored by broad plains eroded into the granite and flanked by high mountains cut by narrow canyons. The reserve was created to protect the scenery of the California Trail landscape, its historic rural setting and its granite features, while interpreting them and managing recreation.

World-renowned rocks

The reserve's geology is famous among rock climbers and researchers alike. Its rare, densely packed granite spires and domes earned it designation as a national natural landmark. Its rocks and landforms let scientists study the ancient and recent tectonic events that raised the mountainous interior of the western United States, and the weathering processes that shape the land and expose buried rocks and structures.

The bedrock is mainly igneous and metamorphic. It includes some of the oldest rocks in North America: metamorphosed igneous rocks of the Archean Green Creek Complex, more than 2.5 billion years old. Right against them lies the Oligocene Almo pluton, about 28 million years old, so the contact between them spans a gap of billions of years.

The view east from East Ridge over City of Rocks National Reserve. Hollows called panholes form by weathering on top of some granite outcrops and fill with water after rain. NPS photo by Wallace Keck.

Regional setting and maps

The reserve belongs to the Columbia Plateau physiographic province, as the Park Service classifies it, and shares its geologic history with a much wider region. The Park Service's Geologic Resources Inventory, which produces digital geologic maps and reports for more than 270 natural resource parks, has mapped the reserve.

Part of the Geologic Resources Inventory map of the reserve. National Park Service.

Sources

• National Park Service, "NPS Geodiversity Atlas—City Of Rocks National Reserve, Idaho." https://www.nps.gov/articles/nps-geodiversity-atlas-city-of-rocks-national-reserve-idaho.htm
• Thornberry-Ehrlich (2010); National Park Service (2018).