Geology of Arches National Park

To form sandstone arches, specific conditions must be present. These conditions include the presence of brittle sandstone that has been jointed through faulting activity, a dry climate, and proximity to salt anticlines undergoing dissolution. Most of the arches in Arches National Park, such as those in Devils Garden and Klondike bluffs, are found within the Entrada Sandstone.

The Entrada Sandstone is characterized by nearly spherical grains, resulting in a highly porous rock. Underneath the Entrada Sandstone lies the Carmel layer, composed of sand and clay. The smaller clay grains in the Carmel layer pack together tightly, creating a less porous rock. This reduced porosity prevents efficient water absorption, leading to pooling at the base of the Entrada Sandstone and consequent erosion.

The unique forms seen in the Entrada Sandstone at Arches National Park were created through a process of uplifts and collapses. Over time, the rock layers above the sandstone faded, leaving behind cracks. Through the action of water and wind erosion, these cracks expanded, forming fin-like structures. Despite only receiving 8-10 inches of precipitation annually, the park experiences enough erosion throughout the year. When rainwater enters the sandstone fins, it collects above the less permeable Carmel layers, dissolving the natural cement that holds the sandstone together. In winter, the water that seeps into the cracks freezes, causing them to widen. This ongoing cycle of erosion leads to the eventual collapse of the rock formations, rendering them impermanent. The 2008 collapse of the Wall Arch serves as an example of the ever-changing nature of these arches.