09-13-19 – City Of Rocks National Preserve



The City of Rocks National Reserve, also known as the Silent City of Rocks, is a United States National Reserve and state park lying 2 miles (3.2 km) north of the south central Idaho border with Utah. It is widely known for its excellent rock climbing and rock formations.

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.

California Trail wagon trains of the 1840s and 1850s left the Raft River valley and traveled through the area and over Granite Pass into Nevada. Names or initials of emigrants written in axle grease are still visible on Register Rock. Ruts from wagon wheels also can be seen in some of the rocks.

The landscape of City of Rocks has been sculpted from granite that was intruded into the crust during two widely spaced times. The granite that composes most of the spires is part of the 28-million-year-old Almo pluton. However, some of the spires are made of granite that is part of the 2.5 billion-year-old Green Creek Complex that contains some of the oldest rocks in the western United States. The granite has eroded into a fascinating assortment of shapes.

City of Rocks was designated a National Reserve, a unit of the National Park Service, in recognition of the nationally significant geological and scenic values of its rock formations and the historical significance of the California Trail. 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, resulted from the contraction of the granite as it cooled, from an upward expansion of the granite as overlying materials were eroded away, and from regional tectonic stresses. The granite has eroded into a fascinating assortment of shapes as high as 600 feet (180 m). The upper surfaces of many of the rocks are covered with flat-floored weathering pits known as panholes. The most notable panhole is located on top of Bath Rock and is continuously filled with water from rain or snowmelt.

Source: Wikipedia

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