The Geology
The park lies atop an underground salt bed, which is basically responsible for the arches and spires, balanced rocks, sandstone fins, and eroded monoliths that make the area a sightseer's mecca. Thousands of feet thick in places, this salt bed was deposited across 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 and went at intervals. Much of this debris was compressed into rock. At one time this overlying layer of rock may have been more than a mile thick.
Slat under pressure is unstable, and the salt bed below Arches was no match for the weight of this thick cover of rock. Under such pressure the salt layer shifted, buckled, liquified, and repositioned itself, thrusting the rock layers upward into domes. Whole sections dropped into the cavities.
Faults deep in the Earch contributed to the instability on the surface. This movement produced vertical cracks slicing the overlying rock much like one slices a loaf of bread. These cracks eroded, separating the slices, and wind and water erosion later contributed to the development of arches.
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