Arches National Park Geology: Salt Walls, Sandstone Fins and Over 2,000 Arches

Arches National Park in Utah preserves more than two thousand natural sandstone arches — the greatest concentration of rock arches in the world. It also has balanced rocks, structures left by dissolving salt, folds pushed up by moving salt, petrified dune fields, and a maze of deep, narrow canyons.

This page is part of the National Park Service's Geodiversity Atlas, a product of the Geologic Resources Inventory. Geodiversity means the whole range of a park's geologic and soil resources and processes — rocks, minerals, sediments, fossils, landforms and the physical processes that shape them — and the atlas supports education, geoconservation, and management that treats the living and non-living parts of an ecosystem together.

The park's Geologic Resources Inventory report. Image from the NPS Geodiversity Atlas

Why so many arches here

The arches come out of a long geologic history:

At Arches, every condition an arch needs comes together:

1. Massive, hard, brittle sandstone, cracked by faulting into joints;
2. Soft layers or partings beneath or within it;
3. Salt-cored anticlines nearby, dissolving away;
4. A dry climate.

Most arches are in three rock units: the Dewey Bridge Member of the Carmel Formation, the Slick Rock Member of the Entrada Sandstone, and the Moab Member of the Curtis Formation.

The setting

The park sits in the middle of the "fault and fold belt" of the Paradox Basin, which dates from the Pennsylvanian (about 318 to 299 million years ago). Its most important feature is the Salt Valley anticline — the Salt Valley–Cache Valley salt wall, or diapir — where salt tectonics and salt dissolution stand out against colorful Mesozoic rock (about 251 to 65 million years old): the Moenkopi, Chinle, Wingate, Kayenta, Navajo, Carmel, Entrada, Curtis, Morrison, Dakota, Cedar Mountain and Mancos formations.

Classic sites

How an arch forms

Most arches and fins are in the Entrada Sandstone, where it lies over an anticline that developed closely spaced parallel fractures. Good examples of dissolution-made anticlines with Entrada arches are at Klondike Bluffs, Devils Garden, Fiery Furnace and Herdina Park. Elephant Butte, in the Windows section, is a remnant of Entrada that arched over an anticline probably not made by dissolution (Doelling, 1985).

1. The rock weathers. The Entrada is massive and crossbedded, which makes it prone to exfoliation — slabs peeling off the bedrock in concentric layers (Harris and others, 1997). Its Slick Rock Member is mostly quartz grains cemented by calcium carbonate. Rainwater picks up carbon dioxide from the air and becomes a weak carbonic acid, which dissolves calcium carbonate well; as the cement goes, loose sand collects on the rock.

2. Fins are carved. Wind and water erosion work along the big joints, widening them and carving the slabs into fins. The fins are thought to have formed in response to compression during the Laramide Orogeny (Doelling, 2000). Jointing in the park is discussed by Dyer (1983), Cruikshank (1993), and Cruikshank and Aydin (1993).

3. Openings form. Water seeps into fractures and along the contacts where the Entrada meets the Moab Member above or the Dewey Bridge Member below, opening thin gaps along these partings. Tension fractures climb into the rock above; groundwater dissolves the cement along them until the roof falls — leaving the stable arch shape, which relieves the stress and stops further collapse.

Watch: Geology of Arches video.

The Moab Fault

The Moab Fault, a major fault, runs alongside the Moab anticline just south of the visitor center. Drilling shows it cuts lengthwise through the middle of the Moab Valley salt wall, which is 3.2 km (2 miles) wide. The northeast block has dropped relative to the southwest block: at the visitor center, the Pennsylvanian Honaker Trail Formation sits against the Jurassic Entrada Sandstone, which takes at least 732 m (2,400 ft) of displacement (Doelling, 2000). Two branches of the fault can be seen from the visitor center.

• The swarm of normal faults on the Moab anticline also moved down to the northeast, and a normal fault on the crest of the Elephant Butte anticline dropped its northern block.
• Along Salt Valley and Cache Valley, surface faults dropped their blocks toward the valleys, and major faults, down to the northeast, are interpreted beneath the crests of the salt-cored anticlines in Moab and Salt valleys (Doelling, 1985, 2000).
• Where the Salt Valley fault reaches the surface, isolated knobs of Paradox Formation caprock, covered by the upper Chinle, form its southwest side, against collapsed Mesozoic rock on the northeast. Because salt dissolution has also deformed the Triassic-to-Cretaceous rock there, how much of the movement is the fault's alone can't be measured.

Dunes, living and petrified

Sand weathers out of many of the bedrock units and piles into local dune fields, as in the Devils Garden area. Ancient petrified dunes survive in the Mesozoic Wingate, Navajo and Entrada sandstones.

Biological soil crusts

The park's biological soil crusts live mostly in the top 1 to 4 mm of soil, so they shape what happens at the surface. They:

• bind soil and help it resist wind and water erosion;
• fix nitrogen from the air and feed nutrients to plants;
• improve how soil, plants and water interact, and let more water soak in;
• help the soil withstand and recover from many stresses.

Disturbing plants and crusts — naturally or by people — can mean more bare soil, faster erosion, more invasive and exotic plants, and weaker nutrient cycling, badly damaging how the soil works.

Fossils

The park lies in one of North America's richest fossil areas, with a thick sequence of land-deposited Mesozoic rock from the "Age of the Dinosaurs" (251 to 65 million years ago). Hundreds of fossil localities have been documented around its boundaries, and fieldwork inside the park since 2000 has found new and significant ones: vertebrates, invertebrates, plants and trace fossils.

All fossils in national parks are protected by the Paleontological Resources Preservation Act of 2009 (Public Law 111-11, Title VI, Subtitle D; 16 U.S.C. §§ 470aaa–470aaa-11).

The wider region

Arches belongs to the Colorado Plateaus physiographic province, and shares its geologic history and some of its formations with a region far larger than the park.

Maps, reports and data

Geologic Resources Inventory map of Arches National Park. Image from the NPS Geodiversity Atlas

The Geologic Resources Inventory produces digital geologic maps and reports for more than 270 natural resource parks, and many products are still in progress:

• The park's Geologic Resources Inventory report covers the features above in depth.
• Products are listed in the NPS DataStore.
• A Soil Resources Inventory for the park is complete and also in the DataStore.

Balanced Rock. Image from the NPS Geodiversity Atlas

More from the park

Geology · Arches · Rock strata · Biological soil crust · Desert varnish · Photo gallery · Park home · Utah Geological Survey

Sources

Based on "NPS Geodiversity Atlas—Arches National Park, Utah," National Park Service; a work of the United States government in the public domain. The atlas dates the Honaker Trail Formation at the visitor center to "about 295 million years ago (Pennsylvanian)," while giving the Pennsylvanian as about 318 to 299 million years ago, so the formation's age in years is left out here.