Moab and the Arches

I’ve lived in the southwest for 14 years and finally got the opportunity to visit the area around Moab, Utah. We spent a few days camping in Moab and first visited Arches National Park. Arches has the world’s largest concentration of natural sandstone arches, over 2,000. The park also contains other geologic formations such as sandstone fins, balanced rocks, pinnacles, and spires. Millions of years of deposition, erosion, and other geologic events shaped the layers of rock. 

How Arches Form from Erosion

Arches lies near the heart of a high desert called the Colorado Plateau at an elevation an average of 3,000 feet above sea level with peaks over 12,000 feet above sea level. Roughly 65 million years ago the area was a dry seabed. The red rock formations seen currently in the park were buried beneath the seabed at that time. Geologic forces wrinkled and folded the buried sandstone to form anticlines (kind of like a carpet that has been pushed inward to form lumps across the middle). As the sandstone warped, fractures tore through it.

Skyline Arch

Next, the entire region began to rise, climbing from sea level to thousands of feet in elevation. The forces of erosion carved layer after layer of rock away and exposed deeply buried sandstone layers that expanded and fractured enough to allow water to seep into the rock and further break it down. Water continues to shape the environment through freezing and thawing cycles that break off chunks of sandstone and rain eroding the rock and carrying sediment down washes and canyons. Little by little fractured rock layers turn into fins and then the fins turn into arches.

Broken Arch

Most of the exposed arches are of the Navajo Sandstone, Carmel Formation, and the Entrada Sandstone Slick Rock Member that was laid down during the Triassic period, roughly 150 million years ago. The Carmel layer is a mix of sand and clay and form a rock more dense and less porous than the Entrada Sandstone, which was once a massive desert of fine-grained sand that turned into a very porous sandstone. Underneath these sandstones lie a thick layer of salts that flowed and bulged upward into long domes that forced the sandstones above to crack. As water soaked into the porous Entrada sandstone, it puddled above the more dense Carmel Formation where it eroded the sandstone into a cavity, and in time, an arch.

Tunnel Arch

Turret Arch

There are many types of arches: cliff wall arches such as Park Avenue Arch, Biceps Arch, and Visitor Center Arch; free standing arches such as North Window Arch, Delicate Arch, Landscape Arch, and Double Arch; natural bridges (which required a hike too far to see this time); and pothole arches such as Pothole Arches and Bean Pot Arch.

As we drove the scenic drive through the park, we stopped to hike easy trails to various arches and rock formations. We hiked a total of 7 miles today, enjoying Double Arch, the Windows Arches and Turret Arch, Balanced Rock, Sand Dune Arch, Broken Arch, Skyline Arch, Tunnel Arch, Pine Tree Arch, and lastly, Landscape Arch. There were great overlooks as well including the Firey Furnance, Delicate Arch, Courthouse Towers, and Park Avenue. We stood on the hill overlooking the Moab Fault valley and saw a 40 million year difference between the Entrada Sandstone we were standing on and the Wingate Sandtone exposed across the valley. All in all, a fantastic day--no higher than 70 degrees and sunny with a slight breeze.

Landscape Arch