Photo by Wallace Keck

City of Rocks, an extraordinary encirclement of granite rising out of the gently rolling sagebrush country in south-central Idaho, has attracted and intrigued people since they first entered this region. The Shoshone camped here as did the emigrants traveling along the California Trail. One of the reserve's most notable qualities is its large degree of biological diversity concentrated in a relatively small area. The great variety of textures, colors and shapes in the natural landscape contributes considerably to the reserve's scenic quality.

City of Rocks was designated a national natural landmark in recognition of the nationally significant geological and scenic values of its rock formations. Rock formations in the reserve developed through an erosion process called exfoliation, during which thin rock plates and scales sloughed off along joints in the rocks. The joints, or fractures, probably resulted from contractions when the rock cooled or from expansions when overlying materials eroded away and eliminated confining pressure. The granite has eroded into a fascinating assortment of shapes as high as 200 feet. There are more than 500 panholes in one small area in the reserve. The most notable panhole is located on top of Bath Rock and frequently fills with water from rain or snow melt.