A natural arch or natural bridge is a natural rock formation where a rockarch forms, with an opening underneath. Most natural arches form as a narrow bridge, walled by cliffs, become narrower from erosion, with a softer rock stratum under the cliff-forming stratum gradually eroding out until the rock shelters thus formed meet underneath the ridge, thus forming the arch. Natural arches commonly form where cliffs are subject to erosion from the sea, rivers or weathering (subaerial processes); the processes "find" weaknesses in rocks and work on them, making them larger until they break through.
The choice between bridge and arch is somewhat arbitrary. The Natural Arch and Bridge Society identifies a bridge as a subtype of arch that is primarily water-formed. By contrast, the Dictionary of Geological Terms defines a natural bridge as a "natural arch that spans a valley of erosion."
The park is located just outside of Moab, Utah, and is 76,679 acres (31,031 ha) 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 have collapsed due to erosion since 1970. The park receives 10 inches (250 mm) of rain a year on average.