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Landscape Arch on July 29, 2013 with a Third Quarter Moon. Landscape Arch is the longest natural arch in the world, measuring 93.3 meters (306 feet) from base to base. Since 1991 several slabs of sandstone rock have fallen from the underside of the arch, so the trail under the arch is now closed off. This image is 28% size of the original.
Most of the arches in Arches National Park are made out of Entrada Sandstone which was formed from sediments laid down on tidal flats, beaches, and coastal dunes of a sea sometime between 150 million to 200 million years ago in the Jurassic period. The Entrada beds were later uplifted by several hundred meters (several thousand feet) about 20 million years ago. This huge uplift created long, deep, vertical cracks in the sandstone, approximately 3 to 6 meters (10 to 20 feet) apart from each other. Slightly acidic rain water (water + carbon dioxide makes carbonic acid) broke down the calcium carbonate that cemented the sand grains together, releasing them to form fin structures. Exfoliation working on both sides of the fins peeled off slabs of the rock to eventually perforate the rock, forming windows. Water and frost enlarged the window holes as well as the abrasion by sand carrried by strong winds. The beds at the base at the base weathered more rapidly, undercutting the fins, causing the loosened pieces of the sandstone to fall, further enlarging the window openings. Once a window has been enlarged to be over 1 meter (three feet), it is considered an arch. Eventually, an arch becomes too thin to support itself and it collapses.
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last updated: August 13, 2019