Tag Archives: Arches National Park

I have a couple of degrees in geology, and although I am not a geologist by profession (I graduated with my MS degree at the wrong time), I am still totally enthralled by geology and geologic processes.

Utah is an earth sciences treasure trove. The few photos here that I captured along Hwy 191, at Anticline Overlook, and in Arches National Park are just the tip of the geologic iceberg.

The factoids in this post were taken from the internet as well as three different publications:

Roadside Geology of Utah, by Halka Chronic

Canyonlands Country, by Donald L. Bars

Geology Unfolded, by Thomas H. Morris et al

Travel with me as I depart Monticello, UT and head toward Arches National Park, along Hwy 191.

Sitting by itself, all rounded and monumental, Church Rock, along Hwy 191 heading north from Monticello to Moab, is an erosional remnant (a bedrock formation that remains after extensive erosion).

Anticline Overlook, some 32 miles west-northwest of Hwy 191 along a scenic byway (15 miles of which are well-tended gravel), is so named for the curved, uplifted shape of the Cane Creek Anticline visible across the Colorado river (the left portion of this photo).

Anticline Overlook sits upon a promontory with views of the Colorado River, Dead Horse Point State Park, and Kane Creek Canyon, pictured here (yeah, I don’t get the difference in spellings either, but that’s how they appear on the internet).

One of the first arches one sees along Hwy 191 toward Moab is Wilson Arch, which  formed from massive sandstone eroded on both sides by water and wind into a “fin”.  Further erosion on both sides of the fin along joints in the rock formed  an alcove, then a cave, then ultimately the arch seen here.

Subsurface magma intrusions squeezed in between rock layers to form dome-shaped igneous  “laccoliths”. The overlying sediments were eroded away, exposing these laccoliths to become what we call the La Sal Mountains.

I couldn’t quite get the big picture and it took me a bit of puzzling to figure out exactly where the Moab Fault is located (I mean, relative to me.  I know the Moab fault is located near Moab, UT).  After re-reading the sign at the Moab Fault overlook, right inside the park, I finally got it.  Looking at the photo of the area across the highway from where I stood (I was at the “you are here” part of the sign) is the upthrown fault block, while the area on my side of the highway is the downthrown fault block.  The fracture line is basically parallel to the highway. The fault displacement (how much it’s gone down/up) is about 2500 feet!

Arches in the making.  With continued erosion via water and wind, those holes you see now will eventually become arches….but not in my lifetime….or your lifetime…..or your kids’ lifetimes…..or….well, you get it.

Those squiggly rock layers along the bottom of this big sandstone structure are collectively called the Dewey Bridge Member.  A “member” is a distinctive rock within a formation (a formation is a distinctive, mappable rock unit).

The Dewey Bridge Member erodes far more easily than the sandstones sitting atop it.  This is called differential erosion.  The pinnacle known as Balanced Rock was formed because of differential erosion.

Another example of a pinnacle, the Dewey Bridge Member, and differential erosion.

Salt Valley does indeed consist of salt deposits. Hundreds of millions of years ago, this entire area was a sea. Layers of salt thousands of feet thick were deposited right here. Salt domes were formed, creating uplift in the land. Huge cracks occurred in the uplifted layers, water poured in, salt leached out leaving empty spaces, and collapse ensued, creating this valley. OK, it’s a simplistic explanation, but I’m writing this for mostly non-geologists and this is indeed what happened.

The above photos are looking over Salt Valley toward Devils Garden (consisting of a bunch of those “fins” I described earlier).

These two photos are looking the other way, across Salt Valley toward the La Sal Mountains and the Windows Section of the park.

Here’s a nice example of weathering by water (frost and rain) and erosion.

All arches struggle with the pull of gravity, and Landscape Arch is no exception. This was proved back in 1991 when a 60-foot slab of this arch fell to the ground (that’s 180 tons of rock debris, according to the sign near this arch). There is no longer a path leading to a view beneath the arch.  It’s all fenced off now, although I’m pretty sure some photographers still risk it to get that perfect image.  The thing is, nobody can predict there won’t be more slabs of rock sloughing off from this arch unexpectedly.  Who knows? Maybe in my lifetime, that arch will indeed totally collapse.

Heading out of Moab, UT on toward Grand Junction, CO, I stopped to photograph this view. I later discovered this very same scene had been published in one of the books referenced at the beginning of this post: a real-life stratigraphic column of the Jurassic-age (140 – 200 million years ago) rock found within Arches National Park.  I just originally photographed it because I thought it was really cool, with all those differing layers of sandstone….and I figured it would make a great addition to a geology blog post I was thinking of writing

These few photos show just a little bit of the wonderful geology found in Utah.  You don’t have to be a geologist or a geology student to totally understand the processes that created all of these wonders.  All you really need is an observant eye and an appreciation of the geologic results.