Geologic Formations

A Story in Stone

The story of Arches begins roughly 65 million years ago. At that time, the area was a dry seabed spreading from horizon to horizon. If you stood in Devils Garden then, the striking red rock features we see today would have been buried thousands of feet below you, raw material as yet uncarved. Then the landscape slowly began to change.

First, geologic forces wrinkled and folded the buried sandstone, as if it were a giant rug and someone gathered two edges towards each other, making lumps across the middle called Anticlines. As the sandstone warped, fractures tore through it, establishing the patterns for rock sculptures of the future.

Next, the entire region began to rise, climbing from sea level to thousands of feet in elevation. What goes up must come down, and the forces of erosion carved layer after layer of rock away. Once exposed, deeply buried sandstone layers rebounded and expanded, like a sponge expands after it's squeezed (though not quite so quickly). This created even more fractures, each one a pathway for water to seep into the rock and further break it down.

Today, water shapes this environment more than any other force. Rain erodes the rock and carries sediment down washes and canyons to the Colorado River. Desert varnish appears where water cascades off cliffs. In winter, snowmelt pools in fractures and other cavities, then freezes and expands, breaking off chunks of sandstone. Small recesses develop and grow bigger with each storm. Little by little, this process turns fractured rock layers into fins, and fins into arches. Arches also emerge when potholes near cliff edges grow deeper and deeper until they wear through the cliff wall below them. In addition to grand arches, water dissolves small honeycomb formations called tafoni.

Over time, the same forces that created these arches will continue to widen them until they collapse. Standing next to a monolith like Delicate Arch, it's easy to forget that arches are impermanent. Yet the fall of Wall Arch in 2008 reminded us that this landscape continues to change. While some may fall, most of these arches will stand well beyond our lifetime: a lifetime blessed with an improbable landscape 65 million years in the making.

Why So Many Arches?

Arches National Park has the densest concentration of natural stone arches in the world. There are over 2,000 documented arches in the park, ranging from sliver-thin cracks to spans greater than 300 feet (97 m). Why are there so many arches in this place? How do they form? And what is an arch, anyway?

First, you need the right kinds of rock.
Sandstone is made of grains of sand cemented together by minerals, but not all sandstone is the same. The Entrada Sandstone was once a massive desert, full of shifting dunes of fine-grained sand. They formed a hard rock that is very porous (full of tiny spaces), while The Carmel Formation, made of sand and clays, is softer and resists water.

Crack it into parallel lines.
Deep beneath the surface lies a thick layer of salts. Squeezed by rock above and below, the salt bulged upward, creating long domes. The rock layers covering these domes were forced to crack, like the surface of freshly-baked bread, into a series of more-or-less parallel lines.

Next, add the right amount of rain.
On average, the park receives 8-10 inches (18-23 cm) of precipitation a year. That might not sound like much, but it’s enough to keep the engines of erosion working 24 hours a day, 365 days a year.

Rainwater soaks into the porous Entrada Sandstone easily, but gets trapped by the Carmel. It can slowly dissolve the calcite bonding the sand together—in other words, rotting rock from the inside out. In winter, water trapped in cracks expands when it freezes, then contracts, prying the rock apart.

If the park received too much precipitation, the sandstone could erode so quickly that arches might not have time to form. If it never rained here, the engines of erosion would stop.

Let the water do its work.
As erosion happens, a variety of shapes begin to appear. Rock walls erode into fins, then holes form. To be one of the park’s official stone arches, a hole must have an opening of at least three feet (1 m) in any one direction. There is no requirement for width; many arches in the park are so skinny you have to place your cheek against the rock to see any light through them.

Is a window a special kind of arch? Not really. “Windows” are arches that are particularly large, are located on a high wall or fin, or “frame” a particularly scenic view beyond.

What about bridges? A natural bridge spans a waterway—or somewhere water once ran. Very few bridges exist at Arches, but Natural Bridges National Monument, just two hours south of here, has three tremendous examples of this feature. All of them are visible from a paved road or hiking trail.

Make sure your rocks don’t rock and roll.
Luckily, earthquakes are rare in this area, otherwise these massive outdoor rock sculptures would splinter and collapse. The fact that over 2,000 still stand tells us this area has been rather geologically stable for at least 50,000 years.

Lastly, pick the right time to visit. (You did.)
The rock layers visible in the park today were once buried by over a mile of other rock that had to erode first to expose what lay beneath. Visitors millions of years ago might have seen a wide flat plain dotted with vegetation. Imagine a visit far into the future, when these layers have fully worn away. What new rock shapes might you discover then