The national park lies atop an underground salt bed, which is the main cause of the formation of the arches and spires, balanced rocks, sandstone fins, and eroded monoliths in the area. Thousands of feet thick in places, this salt bed was deposited over 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 residue from floods and winds and the oceans that came in intervals. Much of this debris was compressed into rock. At one time this overlying earth may have been one mile thick.

Salt under pressure is unstable, and thesat bed below Arches was no match for the weight of this thick cover of rock. Under such pressure it shifted, buckled, liquefied, and repositioned itself, thrusting the Earth layers upward into domes. Whole sections dropped into cavities. In places they turned almost on edge. Faults occurred. The result of one such 2,500-foot displacement, the Moab Fault, is seen from the visitor center.

As this subsurface movement of salt shaped the Earth, surface erosion stripped away the younger rock layers. Except for isolated remnants, the major formations visible in the park today are the salmon-colored Entrada Sandstone, in which most of the arches form, and the buff-colored Navajo Sandstone. These are visible in layer cake fashion throughout most of the park. Over time water seeped into the superficial cracks, joints, and folds of these layers. Ice formed in the fissures, expanding and putting pressure on surrounding rock, breaking off bits and pieces. Winds later cleaned out the loose particles. A series of free-standing fins remained. Wind and water attacked these fins until, in some, the cementing material gave way and chunks of rock tumbled out. Many damaged fins collapsed. Others, with the right degree of hardness and balance, survived despite their missing sections. These became the famous arches.

History

Humans have occupied the region since the last ice age 10,000 years ago. Fremont people and Ancient Pueblo People lived in the area up until about 700 years ago. Spanish missionaries encountered Ute and Paiute tribes in the area when they first came through in 1775, but the first European-Americans to attempt settlement in the area were the Mormon Elk Mountain Mission in 1855, who soon abandoned the area. Ranchers, farmers, and prospectors later settled Moab in the neighboring riverine valley in the 1880s. Word of the beauty in the surrounding rock formations spread beyond the settlement as a possible tourist destination.

The Arches area was first brought to the attention of the National Park Service by Matthias Hauber -Franken, passenger traffic manager of the Denver and Rio Grande Western Railroad. Wadleigh, accompanied by railroad photographer George L. Beam, visited the area in September 1923 at the invitation of Alexander Ringhoffer, a Hungarian-born prospector living in Salt Valley. Ringhoffer had written to the railroad in an effort to interest them in the tourist potential of a scenic area he had discovered the previous year with his two sons and a son-in-law, which he called the "Devil's Garden" (known today as the "Klondike Bluffs"). Wadleigh was impressed by what Ringhoffer showed him, and suggested to Park Service director Stephen T. Mather that the area be made a national monument.

The following year additional support for the monument idea came from Laurence M. Gould, a University of Michigan graduate student studying the geology of the nearby La Sal mountains, who was shown the scenic area by retired local physician Dr. J.W. "Doc" Williams.