Stromboli's Rolling Log: Day 89, Thurs. Sep. 1, 2011 At Arches National Park near Moab UT

Day 89, Thursday September 1, 2011 Arches National Park

Geologic lesson on the formation of the arches in Arches N.P.

We visited Arches National Park north of Moab, UT today and found it to be most pictorial, as well as being great look at the landscape of this portion of he SW USA. The red sandstone (Entrada and Navajo) is the outstanding feature in almost all rocks and monoliths. The following brief scientific explanation can be a help to understand the pictures that accompany the article. I hope you will find time to read to and understand the formation of these rocks. Tomorrow we will do another National Park called Canyon- lands. Enjoy the article and a few of the pictures that we took today..-Jim

Arches 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 (about 210 Ma.,The unit of time, geologically) 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 (140 Ma), was deposited on top of the Navajo. Over 5000 feet (1500 m) 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 (760 m) displacement, the Moab Fault, is seen from the visitor center. (See pIcture)

(This is the Moab fault that dropped sme 2400 feet . Look across the valley on the left center of the picture. The visitor center of the Park is at bottom center. The Colorado River is the cut.) (See picture at right)

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. (See pictures)

An example of the fins, now worn and well runded in th Park.

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.

I hope that wasn’t too scientific for you , and helps with understanding the process over many millennia. Here are a few more ictures from the beautiful Park.