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Roger Weller, geology instructor                     

Mary Higgins
Physical Geology
Spring 2007

                                                   Arches National Park, Utah


Rocks have attracted visitors to Arches National Park for thousands of years. However, sightseeing has not been the main activity for very long. Hunter-gatherers migrated into the area about 10,000 years ago at the end of the Ice Age. As they explored Courthouse Wash and other areas in what are now Arches, they found pockets of chert and chalcedony, microcrystalline quartz perfect for making stone tools. Chipping or knapping these rocks into dart points, knives, and scrapers, they created debris piles that are still visible to the trained eye.


Photo by National Park Services

Water and ice, extreme temperatures and underground salt movement are responsible for the sculptured rock scenery of Arches National Park. On clear days with blue skies, it is hard to imagine such violent forces, or the 100 million years of erosion that created this land that boasts the greatest density of natural arches in the world. The more than 2,000 cataloged arches range in size from a three-foot opening; the minimum considered an arch, to the longest one, Landscape Arch, which measures 306 feet from base to base. New arches are being formed and old ones are being destroyed. Erosion and weathering are relatively slow but are relentlessly creating dynamic landforms that gradually change through time. Occasionally change occurs more dramatically. In 1991 a slab of rock about 60 feet long, 11 feet wide and 4 feet thick fell from the underside of Landscape Arch, leaving behind an even thinner ribbon of rock. Delicate Arch, an isolated remnant of a bygone fin, stands on the brink of a canyon, with the dramatic La Sal Mountains for a backdrop. Towering spires, pinnacles and balanced rocks perched atop seemingly inadequate bases vie with the arches as scenic spectacles.

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 in 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.

Salt 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, liquefied and repositioned itself, thrusting the rock layers upward into domes. Whole sections dropped into the cavities.

Faults deep in the Earth contributed to the instability on the surface. The result of one such 2,500-foot displacement, the Moab Fault, is seen from the visitor center. This movement also produced vertical cracks that later contributed to the development of arches. 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.

Photographs of Arches National Park

Photo by National Park Services

Winds later cleared 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. Pothole arches form by chemical weathering as water collects in natural depressions and eventually cuts through to the layer below. This is the geologic story of Arches - probably. The evidence is largely circumstantial.

Cryptobiotic Crust. Itís alive, so watch your step! But it won't bite you. Once called cryptogamic soil, this dark crust covers much of the untrampled desert. Composed of cyanobacteria as well as lichen, algae and fungi, this covering protects against erosion, absorbs moisture provided nitrogen and other nutrients for plant growth. Avoid crushing these life-giving organisms. Stay on trails. Without these crusts, many of the larger plants could not survive, and if the plants go, so do the animals. The desert could lose much of the life that makes it such a magical place.


Photo by National park services


Landscape Arch is the longest Arch in Arches National Park, measuring 306 feet from base to base. In 1991, a massive slab of rock fell from its underside, resulting in an even thinner ribbon of rock.