Sunday, January 22, 2012

Grooving In The Yellow Knolls

It can’t be helped. During the winter I find myself drawn to all sorts of hidden corners of the Red Cliffs Desert Reserve. If you have ever been to southern Utah you will understand why. 

IMG_0667Yellow Knolls
Yellow Knolls In the Red Cliffs Desert Reserve

You can swing a cat from atop the cliffs of nearby Zion National Park and it will most likely land somewhere in the Desert Reserve. That relatively short distance translates into the same rock formations outcropping in both Zion and the Desert Reserve. I’m talking here about the Navajo Sandstone. 

Even if you don’t know its name or what it actually represents, you probably recognize the Navajo Sandstone when you see it. Its massive sheer walls are distinctive and in Zion Canyon tower several thousand feet above the canyon floor. 

IMG_1084HangingValleyInNavajo SandstoneZion NationalPark
Hanging valley in cliffs of Navajo Sandstone, Zion National Park

Hanging valleys, blind arches, desert varnish, cross–bedding, horizontal and vertical fractures and grooves, and fresh–water seeps all characterize the fossilized sand dune desert that is the soul of the Navajo Sandstone in Zion. I have heard it referred to as a vast Early Jurassic erg or sea of sand, and it existed for nearly 10 million years, from around 190–180 million years ago. 

IMG_0680Navajo Sandstone_RedCliffs DesertReserve
Navajo Sandstone in Yellow Knolls – JO for scale

In many areas of the Desert Reserve, however, these features tend to be more subdued. The scale is smaller and more compact, the cliffs nowhere near as massive. Nonetheless, the dramatic geometry of grooves incised into the sandstone along the Yellow Knolls trail can still take your breath away and leave you standing there staring, utterly speechless. It certainly does for me, and I see this stuff a lot because I live here. 

Chopped Off And Shifted
Picture in the back of your mind the vastness of constantly shifting dunes of White Sands National Monument in New Mexico, or the Sahara Desert in Africa. In Utah’s Red Cliffs Desert Reserve, shallow horizontal grooves in the fossil dunes of the Navajo Sandstone are formed by differential weathering along layers of coarser sand, where previously deposited dunes had been truncated or chopped off by the shifting of new dunes as they swept across the ancient desert. 

IMG_0729Elephant SkinTextureNavajo Sandstone
Cross bedding and grooves in Navajo Sandstone

Perpendicular to these horizontal grooves are vertical sets of grooves formed in more recent shallow fractures that were themselves formed by expansion and contraction of the surface sandstone. It is reminiscent of the wrinkled skin of an elephant. 

Elephant skin texture in cross–bedded Navajo Sandstone

This expansion and contraction is due to any number of natural processes including freezing & thawing, heating & cooling, and wetting & drying. A jaw–dropping display of polygonal fractures occurs where bedding or layering is parallel to the exposed ground surface. 

Walking Along And Wondering Why
Basalt boulders the size of washing machines tumble off steep hillsides of Navajo Sandstone in the Yellow Knolls area. The trail is wide open and easy to follow at first but narrows quickly as it traverses up and across a small saddle in between the knolls. Cross–bedding in the hillsides displays the direction that ancient winds once blew across this vast sea of sand. Blue–black desert varnish disguises the inherent grayness of the crystal—rich basalt. 

IMG_0686Basalt FlowObscures NavajoSandstone
Desert varnished basalt tumbles down Navajo Sandstone

On the other side of the saddle the view opens up once again. Filling the landscape are rusty white sandstone surfaces of steep inclines and gentle slopes composed of some of nature’s finest geometric artistry. Stress due to thermal expansion and contraction is the explanation of how these fractures and grooves formed. As I walked around I could not help but wonder in amazement at why they formed. What is it about nature’s physical laws that make five sides the best configuration for this particular sandstone to take? With four or six equal sides there are no wasted spaces, but when you think about it even this design in sandstone is as extraordinary as thunder in a snowstorm. 

IMG_0694Shallow Grooves_Navajo Sandstone
Yellow Knolls trail

Polygons in Navajo Sandstone - JO for scale

IMG_0705Polygongrooves_Navajo Sandstone
JO completing her assault on the polygons
IMG_0711Bedding LayersInNavajo Sandstone
Polygons form when bedding or layering is parallel to exposed ground surface

IMG_0700Sandstone Polygons
Width of polygon from lens cap to upper right is about 2.5 feet

We see irregular five–sided shapes a lot in nature. Cooled columns of basalt such as that seen at Devil’s Tower National Monument often display it. It is a common feature of mud cracks, formed as the mud of receding floods dries out and curls up. Starfish and sea urchins are symmetrically five–sided.

I suppose there is a scientific explanation for this design but it would be far beyond where this blog wants to go. It is human nature to search for the why. For now I am content to gaze in admiration at things I may never fully understand, realizing that I know just enough to be dangerous. 

IMG_0719Lichen AndMossIn Grooves_Navajo Sandstone
Lichen and moss in sandstone groove - everybody has a job to do.


Checking it out:
Biek, R.F.,, Willis, G.C., Hylland, M.D., and Doeling, H.H., 2000, Geology of Zion National Park, Utah in Geology of Utah’s National Parks and Monuments, Utah Geological Assoc. Publication 28, pg. 106–138.
Hamilton, W.L., 1984, The Sculpturing of Zion with Road Guide to the Geology of Zion National Park, Zion Natural History Assoc. (Mine is the older edition)
Kocurek, G., and Hunter, R.E., 1986, Origin of polygonal fractures in sand, uppermost Navajo and Page sandstones, Page, Arizona, Journal of Sedimentary Research, v. 56 no. 6 p. 895-904 (abstract).


  1. Navajo sandstone is special. I don't think I've seen the polygonal structure of it. Hmmmm, something to explore next season. I miss the red rock country and look forward to next weekend's fix at the South Rim.

  2. Gaelyn - I don't remember ever seeing polygon structures like this anywhere else, either. The Desert Reserve is full of surprises! I hope you get to spend some time there in the future.

  3. I love rocks and caves so much, and I'm so curious about the reasons why the land looks as it does, that I have thought about taking a few geology courses just for fun. Looking at your pictures is adding to my desire to know more! Thanks for sharing. :D

  4. Hi TourAbsurd -

    The reason I went back to school at age 51 to study geology is that I needed to find out "How do they KNOW all this stuff?" I love sharing what I learn about the geology of anywhere I go.

    So glad you enjoyed the post. Let me know if you ever do take any geology courses!

  5. That one rock really does look like elephant skin!!

    1. Hi dtravelsround - Yes it does! It's really remarkable. We have Elephant Arch so this should be called "Elephant Skin Knoll!"