LANDSCAPE ARCH, DELICATE ARCH, AND DOUBLE ARCH IN ARCHES NATIONAL PARK, SOUTHEASTERN UTAH

Arches National Park in southeastern Utah has the greatest concentration of natural sandstone arches in the world. The park, located in the Paradox fold and fault belt in the northern Paradox Basin, showcases spectacular Colorado Plateau geology with colorful sedimentary rocks, ancient sand dunes, cliffs, domes, fins, and pinnacles—as well as the arches. The sandstone arches in the park and surrounding region formed by unique circumstances involving Middle Pennsylvanian to present movement of subsurface salt; Middle Pennsylvanian to Paleogene deposition; and Neogene to Quaternary folding, faulting, erosion, and salt dissolution. Most arches in the park are formed in two geologic units, the Slick Rock Member of the Entrada Sandstone and the Moab Member of the Curtis Formation, both deposited in Middle Jurassic eolian dune environments. The intervening contact is the J-3 unconformity.

Five primary factors are required to form and preserve exceptionally large sandstone arches: 1 – massive, moderately hard, isotropic sandstone beds, 2 – weak basal layers, 3 – parallel joints (here formed by salt-cored anticlines), 4 – rapid erosion, and 5 – a dry climate. Rarely do all these phenomena occur in one place, but they do in Arches National Park.

Over 2000 (approximately 60 “large”) natural arches have been catalogued in the park, including three that may be the most famous in the world, Landscape, Delicate, and Double Arches. Landscape and Double Arches are in the Slick Rock Member whereas Delicate Arch includes the Slick Rock and Moab Members. Landscape Arch has an incredible span of 290 feet (88 m) (77 feet [23 m] high), the largest known sandstone arch in the world and the fifth largest of any type (a few limestone, karst-formed arches are larger) (measurements by the Natural Arch and Bridge Society). Its gravity-defying ribbon of rock narrows to only 6 feet (2 m) in thickness (it is very old and not expected to stand much longer). Landscape is classified as a freestanding arch but originally began as a cliff-wall arch. The iconic Delicate Arch is also freestanding and has a horizontal span of about 32 feet (9.7 m) and a vertical span of 46 feet (14 m). Double Arch is the third largest in the park, consisting of two giant pothole arch spans joined at one end. Its largest opening has a span of 148 feet (45 m) and a height of 104 feet (32 m).