Arch Park

This park contains more than 2, 000 natural arches—the greatest concentration in the country. But numbers have no significance beside the grandeur of the landscape—the arches, the giant balanced rocks, spires, pinnacles, and slickrock domes against the enormous sky.

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.

The land has a timeless, indestructible look that is misleading. More than 700, 000 visitors each year threaten the fragile high desert ecosystem. One concern is a dark scale called biological soil crust composed of cyanobacteria, algae, fungi, and lichens that grow in sandy areas in the park. Footprints tracked across this living community may remain visible for years. In fact, the aridity helps preserve traces of past activity for centuries. Visitors are asked to walk only on designated trails or stay on slickrock or wash bottoms.

Did You Know?

There are more than 2, 000 arches in the park; to be classified as an arch, the opening must measure at least three feet across. The largest arch in the park, Landscape Arch, spans 306 feet (longer than a football field) base to base. New arches are constantly forming, while old ones occasionally collapse—most recently Wall Arch, which fell in 2008.

Arches National Park contains ephemeral pools, from a few inches to several feet in depth, that are essentially mini-ecosystems, home to tadpoles, fairy shrimp, and insects. The pools form among the sandstone basins, within potholes that collect the rare rainwater and sediment.

About 300 million years ago an inland sea covered what is now Arches National Park. The sea evaporated and re-formed more than 29 times, leaving behind salt beds thousands of feet thick.

Another unique aspect of the park is its knobby black ground cover, which is actually alive. A biological soil crust, it is composed of algae, lichens, and cyanobacteria (one of Earth’s earliest life forms), and provides a secure foundation for the desert plants.

Edward Abbey served as a seasonal ranger at Arches in the late 1950s, an experience that inspired his 1968 memoir, Desert Solitaire.

Copy for this series includes excerpts from the National Geographic Guide to the National Parks of the United States, Seventh Edition, 2012, and the National Parks articles featured in "Cutting Loose" in National Geographic Traveler.