Double Arch

Coordinates: 38°41′29″N 109°32′25″W / 38.69139°N 109.54028°W / 38.69139; -109.54028

Double Arch is a close-set pair of natural arches, one of the more known features of Arches National Park in Utah, United States. From the Double Arch parking area it's a 0.5 mile (0.8 km) round trip to the arches. There are no guardrails or fences to prevent visitors from exploring directly beneath and through the arches.

The area was used as a backdrop for the opening scene of Indiana Jones and the Last Crusade, in which the arches are briefly visible. However, the cave shown in the movie does not exist.

Double Arch was formed differently from most of the arches in Arches National Park. It is what is known as a pothole arch; it formed by water erosion from above rather than more typical erosion from the side. The larger opening has a span of 148 feet (45 m) and a height of 104 feet (32 m).

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National park

A national park is a park in use for conservation purposes. Often it is a reserve of natural, semi-natural, or developed land that a sovereign state declares or owns. Although individual nations designate their own national parks differently, there is a common idea: the conservation of 'wild nature' for posterity and as a symbol of national pride. An international organization, the International Union for Conservation of Nature (IUCN), and its World Commission on Protected Areas, has defined "National Park" as its Category II type of protected areas.

While this type of national park had been proposed previously, the United States established the first "public park or pleasuring-ground for the benefit and enjoyment of the people", Yellowstone National Park, in 1872. Although Yellowstone was not officially termed a "national park" in its establishing law, it was always termed such in practice and is widely held to be the first and oldest national park in the world. The first area to use "national park" in its creation legislation was the US's Mackinac Island, in 1875. Australia's Royal National Park, established in 1879, was the world's third official national park. In 1895 ownership of Mackinac Island was transferred to the State of Michigan as a state park and national park status was consequently lost. As a result, Australia's Royal National Park is by some considerations the second oldest national park now in existence.

Arches National Park

Arches National Park is a US National Park in eastern Utah. The park is located on the Colorado River 4 miles (6 km) north of Moab, Utah. It is known for containing over 2,000 natural sandstone arches, including the world-famous Delicate Arch, in addition to a variety of unique geological resources and formations.

The park is located just outside Moab, Utah, and is 76,679 acres (119.811 sq mi; 31,031 ha; 310.31 km²) in area. Its highest elevation is 5,653 feet (1,723 m) at Elephant Butte, and its lowest elevation is 4,085 feet (1,245 m) at the visitor center. Forty-three arches are known to have collapsed since 1977. The park receives 10 inches (250 mm) of rain a year on average.

Administered by the National Park Service, the area was originally named a National Monument on April 12, 1929. It was redesignated as a National Park on November 12, 1971.

Geology

The national park lies atop an underground evaporite layer or salt bed, which is the main cause of the formation of the arches, spires, balanced rocks, sandstone fins, and eroded monoliths in the area. This salt bed is thousands of feet thick in places, and was deposited in the Paradox Basin of the Colorado Plateau some 300 million years ago when a sea flowed into the region and eventually evaporated. Over millions of years, the salt bed was covered with debris eroded from the Uncompahgre Uplift to the northeast. During the Early Jurassic (about 210 Ma) desert conditions prevailed in the region and the vast Navajo Sandstone was deposited. An additional sequence of stream laid and windblown sediments, the Entrada Sandstone (about 140 Ma), was deposited on top of the Navajo. Over 5000 feet (1500 m) of younger sediments were deposited and have been mostly eroded away. Remnants of the cover exist in the area including exposures of the Cretaceous Mancos Shale. The arches of the area are developed mostly within the Entrada formation.

Arch

An arch is a curved structure that spans a space and may or may not support weight above it. Arch may be synonymous with vault, but a vault may be distinguished as a continuous arch forming a roof. Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture, and their systematic use started with the Ancient Romans who were the first to apply the technique to a wide range of structures.

Basic concepts

An arch is a pure compression form. It can span a large area by resolving forces into compressive stresses and, in turn eliminating tensile stresses. This is sometimes referred to as arch action. As the forces in the arch are carried to the ground, the arch will push outward at the base, called thrust. As the rise, or height of the arch decreases, the outward thrust increases. In order to maintain arch action and prevent the arch from collapsing, the thrust needs to be restrained, either with internal ties or external bracing, such as abutments.

Fixed vs hinged arch

Double

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