It’s fascinating to consider the creation of Arches NP. It lies atop an underground salt bed which is responsible for the formation of the arches, spires, balanced rocks, sandstone fins, and eroded monoliths. Over millions of years, the salt bed was covered with debris. During the Early Jurassic Period desert conditions prevailed and the vast Navajo Sandstone was deposited (c200m years ago).
An additional sequence of stream laid and windblown sediments, the Entrada Sandstone, was deposited on top of the Navajo (c140m years ago). Over 5,000 feet (1,500 m) of younger sediments were deposited and have been mostly eroded away.
The arches of the area are developed mostly within the Entrada formation. The weight of this cover caused the salt bed below it to liquefy and thrust up layers of rock into salt domes. As this subsurface movement of salt shaped the landscape, erosion removed the younger rock layers from the surface. Except for isolated remnants, the major formations visible in the park today are the salmon-colored Entrada Sandstone, in which most of the arches form, and the buff-colored Navajo Sandstone. These are visible in layer cake fashion throughout most of the park.
Over time, water seeped into the surface cracks, joints, and folds of these layers. Ice formed in the fissures, expanding and putting pressure on surrounding rock, breaking off bits and pieces. Winds later cleaned out the loose particles. A series of free-standing fins remained. Wind and water attacked these fins until, in some, the cementing material gave way and chunks of rock tumbled out. Many damaged fins collapsed. Others, with the right degree of hardness and balance, survived despite their missing sections. These became the famous arches.
Hiking the Devil’s Garden area to sights such as Double O Arch is breathtaking, walking the back’s of the strange fins that remain after millions of years of erosion, following the small cairns through the landscape and breathing in the desert landscapes is wonderful experience.
As mentioned previously Canyonlands NP covers a huge area and preserves a wilderness of rock at the heart of the Colorado Plateau. Water and gravity, this land’s prime architects, cut flat layers of sedimentary rock into hundreds of canyons, mesas, buttes, fins, arches, and spires. At centre stage are two canyons carved by the Green and Colorado rivers.