Monday, April 07, 2014

Into the Heart of Monument Valley

I recently led a group of amateur archaeologists into Monument Valley. This landscape icon never fails to impress. It truly is an awesome place.

Taken from John Ford Point looking north. Let's meet the rock layers of Monument Valley.

There are four rock layers that make up a typical red rock butte in Monument Valley and from oldest to youngest (bottom to top) they are labeled above. The Organ Rock Shale is often overlooked since it only forms the "apron" upon which the "monument" stands. It is however, one of my  favorite rock layers as it represents the outwash deposits from the Ancestral Rocky Mountains in Colorado (follow it northeast and the clasts get larger). It is time-equivalent to the Hermit Shale in Grand Canyon. Next, forming the massive walls of the buttes in Monument Valley is the De Chelly Sandstone, an eolian deposit left in a giant sand sea or erg. It was, of course, first described in Canyon De Chelly to the southeast of here. Both of these deposits are Permian in age. Cryptically exposed on the tops of the buttes are two thin remnants of Triassic age deposits, the Moenkopi Formation and the Shinarump Conglomerate Member of the Chinle Formation. They are both fluvial (river) deposits, with the Shinarump being coarser-grained. Some uranium exploration was completed within the Shinarump during the Cold War on top of some buttes in Monument Valley (not shown). Not exposed but making up the floor of the valley is the Cedar Mesa Sandstone. Once these monuments have eroded away, incision into the Cedar Mesa Sandstone/Halgaito Shale couplet below may renew the process again and perhaps forming a Monument Valley II. (Or perhaps Monument Valley III if the process also happened previous to this one, in layers now completely gone today). Wow!

We were a group of 18 and had a wonderful day of exploration.

Cisco from Kayenta served as our local Navajo guide.

There are a few Navajo families that call the valley home and their hogans (earthen homes) can be seen occasionally.

Although the floors are dirt, the walls and roof are made from beautifully fashioned juniper logs. Here is a loom for weaving rugs.

A scene reminiscent of another era.

The massive walls of De Chelly Sandstone are composed of cemented grains of quartz sand. These massive quartz bodies will often yield conchoidal fractures that are scalloped-shaped upon the walls of the cliff.

Boulders of De Chelly Sandstone lie on the valley floor where they are ultimately shaped by wind and sand. This area, known as Pancake Alley,  gets its name from the way the boulders tend to fracture along the bedding planes in the sandstone, yielding rounded planes like a stack of pancakes.

Where the Organ Rock Shale trends beneath the valley floor, the De Chelly Sandstone walls rise straight up, providing a natural palette for the prehistoric artist.

And what rock art there is! This is the "famous" Big Horn Panel near the Eye in the Sky. Members in our group noticed how the large animal appears to be in motion

The Eye in the Sky

In a remote corner of Monument Valley, an Anasazi ruin appears in an alcove.

More rock art is seen nearby. These are recumbent kokopelli's or flute players.

Cross0bedding in the De Chelly is well developed. However, I know of a place where these angled beds shows signs of disruption, perhaps from a gravity slide that may have been initiated during an earthquake. Look at how these beds seem to have slumped downhill along one of the planes and then ruptured as they slid. The horizontal line at the level of Ronnie's head is a small scale fault plane where the beds above were thrust to the right over the lower beds.

Close-up of the same feature showing how small blocks of sand remained coherent as they slid downhill. This may suggest that the sand was partially lithified when it happened. Note that the lower bed is not disrupted and still retains the original angle of the cross-bedding.

Ear of the Wind arch. It seems like many body parts are covered in the naming of the arches here.

A final view of the valley from Artist Point. Monument Valley is an awesome place and I never get tired of seeing it.

4 comments:

Dr. Jack Share said...

"WOW" is right! Fabulous!

Karron Joy said...

Wonderful photos. Makes me miss living in Arizona and seeing beautiful geological formations everywhere.

Karron Joy said...

Wonderful photos. Makes me miss living in Arizona and seeing beautiful geological formations everywhere.

Al Tomas said...

Is the "observation deck" at John Ford Point DeChelly over Organ rock Shale?