City of Rocks

Located in southern Idaho near the Utah border, the City of Rocks National Reserve is known for its massive peaks, spires, pinnalces, and rocks of granite.

The unusual landscape is a scenic landmark along the historic California Trail and is littered with unique chunks of granite—most of which is about 2.5 billion years old. The rocks shownhere are a part of the Green River Complex, an extensive belt of ancient rocks that are some of the oldest in the United States.

The rounded outcroppings form by slow rising (due to tectonic activity) and erosion, which strips away softer surface rock to expose the tough granite below. The granite domes were originally plutons: Chambers of magma that solidified before ever reaching the Earth's surface.

—LiveScience Staff

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Credit: USGS