Ancient Arches

Landscape Arch (over 300 ft long!) in the Devil's
Garden region of the park.

At Arches National Park, one can’t help but be preoccupied with geology (even if you’re a biologist). The blooming Ephedra, desert lizards sunning themselves, and twisted Juniper are impressive, but for a first time visitor to Arches like me, it is the sheer scale of rock and erosion that first hits the senses (more on the biota later).

Arches is near the heart of the Colorado plateau, a region roughly centered on the four corners of the US. Across the area, rivers like the Colorado and Green cut deeply into the ancient rock, eroding meandering canyons. Water is responsible in part for the formation of rock arches too. The process begins with cracks that form in a solid block of sandstone (a type of rock called Entrada sandstone is particularly conducive to arch formation). These cracks form in parallel bands. Water erodes along these cracks, eventually creating a series of parallel ridges of rocks known as fins. The final step in arch formation occurs when water (with a mild acid) pools above a denser layer of rock within an individual fin and slowly dissolves a cavity in the sandstone by loosening the matrix holding the sandstone particles together.

Two views of "Double O" Arch, also in the Devil's Garden region.

A sandstone spire in the foreground with the La Sal
Mountains in the background. I really liked the
juxtaposition of erosion-dominated sandstone with
the (probably) more recently uplifted mountains.

In addition to the arches for which the park is famous, the rust-colored sandstone takes on a variety of shapes, from sheer rock walls to spires. Flat-topped mesas, like ancient ruins crumbling over eons are present too, both inside and outside Arches. These formations were among my favorite, perhaps in part because the buttresses of rubble around their bases emphasized the process of decay and suggested their great age.

During the last two days in eastern Utah, the forces of erosion dominate in this region in my mind, manifest in every major shape on the landscape. Erosion is everywhere, whether orderly like grooves in the soft earth or chaotic like a jumble of boulders that remain from a cataclysmic collapse of a rock formation.

References

Arches National Park Visitors Guide, published by Canyonlands Natural History Association, and National Park Service interpretive signs.

Morris TH, Ritter SM, Laycock DP. 2012. Geology Unfolded. An Illustrated Guide to the Geology of Utah’s National Parks. BYU Press.

Sandstone fins (left) and decorative features in sandstone (right).

This region of the park is known as the Courthouse Towers and was one of my favorite areas.

Another view of sandstone formationa and the La Sal
Mountains in the background.

This is Delicate Arch, the iconic arch of the National Park.

I wonder how this precariously perched rock (top center)
got in that position. Earthquake? Slow action of
(frozen) water?