The City of Rocks , also known as the Silent City of Rocks, is a U.S. National Reserve and Idaho state park lying two miles (3 km) north of the south central border with Utah. The rock spires in the City of Rocks and adjacent Castle Rocks State Park are largely composed of granitic rock of the Oligocene Almo pluton and Archean Green Creek Complex.... Found on http://en.wikipedia.org/wiki/Mcdowall