Southern Utah Rocks

Moab, Utah

Moab is a city in the eastern part of Southern Utah. It is the county seat of Grand County. Moab hosts a large number of tourists every year, mostly visitors of the Arches and Canyonlands National Parks, which are located close by. The town is also a popular base for mountain bikers who come to ride the extensive network of trails including the famed Slickrock Trail, and off-roaders who come for the annual Moab Jeep Safari.


In recent years Moab has also seen a surge of second-home owners. The relatively mild winters and enjoyable summers have attracted many people to build such homes throughout the area.

Sunset Magazine's March 2009 issue listed Moab as one of the "20 best small towns in the West".

 

Arches National Park


Arches National Park is a U.S. National Park in eastern part of Southern Utah. It is known for preserving over 2,000 natural sandstone arches, including the world-famous Delicate Arch, in addition to a variety of unique geological resources and formations.

 
The park is located just outside of Moab, Utah, and is 119 square miles in size. Its highest elevation is 5,653 feet at Elephant Butte, and its lowest elevation is 4,085 feet at the visitor center. Since 1970, forty-three arches have toppled because of erosion. The park receives 10 inches of rain a year on average.


The national park lies atop an underground evaporite layer or salt bed, which is the main cause of the formation of the arches, spires, balanced rocks, sandstone fins, and eroded monoliths in the area. This salt bed is thousands of feet thick in places, and was deposited in the Paradox Basin of the Colorado Plateau some 300 million years ago when a sea flowed into the region and eventually evaporated. Over millions of years, the salt bed was covered with debris eroded from the Uncompahgre Uplift to the northeast. During the Early Jurassic, desert conditions prevailed in the region and the vast Navajo Sandstone was deposited. An additional sequence of stream laid and windblown sediments, the Entrada Sandstone, was deposited on top of the Navajo. Over 5,000 feet of younger sediments were deposited and have been mostly eroded away. Remnants of the cover exist in the area including exposures of the Cretaceous Mancos Shale. 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. The evaporites of the area formed more unusual salt anticlines or linear regions of uplift. Faulting occurred and whole sections of rock subsided into the areas between the domes. In some places, they turned almost on edge. The result of one such 2,500-foot displacement, the Moab Fault, is seen from the visitor center.

 
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.