Explore Geology
geology fieldnotes title

City of Rocks

National Reserve


cover of park brochure

park geology subheading
Photo of a meadow at City of Rocks
City of Rocks National Reserve, Idaho

"We encamped at the city of rocks, a noted place from the granite rocks rising abruptly out of the ground," wrote James Wilkins in 1849. "They are in a romantic valley clustered together which gives them the appearance of a city." Wilkins was among the first wagon travelers to fix the name City of Rocks to what looked like a "dismantled rock-built city of the Stone Age." California Trail pioneers were leaving civilization as they knew it in the East for new lives in the West. Some wrote their names in axle grease on rock faces, and their signatures can still be seen today. No doubt thirsty on this northern edge of the Great Basin Desert, one emigrant saw the distant rocks in August like "water thrown up into the air from numerous artificial hydrants." Beginning in 1843, City of Rocks was a landmark for emigrants on the California Trail and the Salt Lake ALternate Trail and later on freight routes and the Kelton, Utah to Boise, Idaho, stage route. The area's historical and geological values, scenery, and opportunities for recreation led to its designation as City of Rocks National Reserve in 1988.

More like mother and daughter than siblings, the Twin Sisters of City of Rocks are made up of two different rocks. The difference helps explain how the City of Rocks landscape came to be. The darker sister is made of rock in a formation that geologists refer to as the Green River complex. It is 2.5 billion years old and is some of the oldest rock in the lower 48 states. The lighter sister (granite) is made of rock in a far younger formation called the Almo Pluton. It is 25 million years old. Both formations began as molten matter deep in the Earth's crust. Eventually the Almo Pluton was thrust up through the Green River Complex, while both formations still lay beneath the surface and other layers of rock. As time passed, the overlying rocks and the formations beneath them cracked. Along the cracks and fissures erosion took place more rapidly and exposed the rocks of the Almo Pluton and the Green River Complex. The exposed rocks were then shaped by the forces of erosion. In weathering, the tops of rocks were dissolved by rainwater; and minerals such as iron oxide, are redeposited to form crust-like caps. These caps are more resistant to weathering than the underlying rock, and this causes the formation of spires and pinnacles. When the caps erode, the innder rock can be molded by erosion into many caves, arches, bath tubs, and hollow boulders seen at the City of Rocks. On the sides of spires water seeps into cracks and frost wedging occurs. When water freezes it expands and has the ability to crack of large slabs of rock. This process has already removed some of the layers of rock bearing 150-year-old signatures left by the pioneers. Today, many people see animals, faces, or other shapes in the rock formations.

park maps subheading

The General park map handed out at the visitor center is available on the park's map webpage.

For information about topographic maps, geologic maps, and geologic data sets, please see the geologic maps page.

photo album subheading

A geology photo album has not been prepared for this park.

For information on other photo collections featuring National Park geology, please see the Image Sources page.

books, videos, cds subheading

Currently, we do not have a listing for a park-specific geoscience book. The park's geology may be described in regional or state geology texts.

Please visit the Geology Books and Media webpage for additional sources such as text books, theme books, CD ROMs, and technical reports.

Parks and Plates: The Geology of Our National Parks, Monuments & Seashores.
Lillie, Robert J., 2005.
W.W. Norton and Company.
ISBN 0-393-92407-6
9" x 10.75", paperback, 550 pages, full color throughout

The spectacular geology in our national parks provides the answers to many questions about the Earth. The answers can be appreciated through plate tectonics, an exciting way to understand the ongoing natural processes that sculpt our landscape. Parks and Plates is a visual and scientific voyage of discovery!

Ordering from your National Park Cooperative Associations' bookstores helps to support programs in the parks. Please visit the bookstore locator for park books and much more.

geologic research subheading

Information about the park's research program is available on the park's research webpage.

For information about permits that are required for conducting geologic research activities in National Parks, see the Permits Information page.

The NPS maintains a searchable data base of research needs that have been identified by parks.

A bibliography of geologic references is being prepared for each park through the Geologic Resources Evaluation Program (GRE). Please see the GRE website for more information and contacts.

selected links subheading

NPS Geology and Soils Partners

NRCS logoAssociation of American State Geologists
NRCS logoGeological Society of America
NRCS logoNatural Resource Conservation Service - Soils
USGS logo U.S. Geological Survey

teacher feature subheading

Currently, we do not have a listing for any park-specific geology education programs or activities.

For resources and information on teaching geology using National Park examples, see the Students & Teachers pages.
updated on 01/04/2005  I   http://www.nature.nps.gov/geology/parks/ciro/index.cfm   I  Email: Webmaster
This site is best viewed in Internet Explorer 6.0 or Netscape 7.0