• Twin Sisters

    City Of Rocks

    National Reserve Idaho

Erosion

Fracture

Photo by wallace keck

Erosional processes transport the products of weathering and mass wasting away from their source, and in the Albion Mountains water is the most effective erosive agent. Accompanying the weathering that created the hollow balls, pinnacles, and other features in the park are slope failures, rockfall and topple, debris flows, slides, creep, soil deformation, and colluvium deposits. The most active areas of slope formation and erosion are in the low parts of the upland basins at City of Rocks.

Some of the granitic rock spires at City of Rocks National Reserve are among the oldest in North America. In their long history, these rocks were affected by multiple phases of deformation, resulting in folds, faults, joints, and other planes of weakness. These features compromise the strength of any rock unit and in many cases profoundly affect the geomorphologic processes that shape the landscape at City of Rocks. Younger rock units, such as the granite of the Almo pluton, are also traversed by cracks, known as joints. Joints are fractures in rock produced by stresses that cause the rock on either side of the fracture to be displaced perpendicular to the fracture plane. The orientation and spacing of joints strongly affects the size, shape, and distribution of spires.

 
panhole

photo by kris hawkins

As erosion removes more material from the overlying surface, a release of pressure causes the rocks to expand and crack. Earth materials are responding to pressures within the earth, and recent small-scale fractures and joints attest to this.

Wind is a major force that redistributes fine-grained materials, such as sand, soil, and soil resources (e.g., litter, organic matter, and nutrients) within and among ecosystems. Erosion and deposition by wind are important at City of Rocks National Reserve and can be affected by human activities that disturb stabilizing ground features.

Did You Know?

Creekside Trail adjacent to a large rock and winding through trees.

Over 22 miles of trails wind through the City of Rocks National Reserve. Enjoy hiking, horse back riding or mountain biking through granite spires surrounded by high desert flora and fauna.