Perched high above the Colorado River, the park is part of southern Utah's extended canyon country, carved and shaped by eons of weathering and erosion. Some 300 million years ago, inland seas covered the large basin that formed this region. The seas refilled and evaporated—29 times in all—leaving behind salt beds thousands of feet thick. Later, sand and boulders carried down by streams from the uplands eventually buried the salt beds beneath thick layers of stone. Because the salt layer is less dense than the overlying blanket of rock, it rises up through it, forming it into domes and ridges, with valleys in between.

Most of the formations at Arches are made of soft red sandstone deposited 150 million years ago. Much later, groundwater began to dissolve the underlying salt deposits. The sandstone domes collapsed and weathered into a maze of vertical rock slabs called "fins." Sections of these slender walls eventually wore through, creating the spectacular rock sculptures that visitors to Arches see today.