NH Photography: Desert Varnish Around Moab UT

Travel to most any desert region and you will observe black, brown or orange streaks or crusts on the surface of many rocks.  These stains on the rock have been called desert or rock varnish.  The origins have been scrutinized for many years and still are not fully understood. I will explore some of what we do know about desert varnish and what it can tell us about the age of rocks in a future article, but for now I thought I would share some pictures I took of desert varnish in the southwestern US this past summer.

Desert varnish on rocks around arch in Hunters Canyon near Moab UT.  Image: Joel Duff

Desert varnish on rocks around a small arch in Hunters Canyon near Moab UT. Image: Joel Duff

Rock varnish is nearly ubiquitous on sandstone cliffs in southeastern UT.  Here we see some particularly dark streaks.  The darker the varnish the longer it likely taken to form.  This varnish is an accumulation of iron and/or manganese oxides that accumulates on the surface of the rock.  The iron and manganese atoms do not derive from the rock itself but are obtained from the environment and presumably from rainwater.

Hiking back from Corona Arch near Moab UT.  Notice the desert varnish on the walls but not on the horizontal rocks.  A couple of my kids are in the foreground for scale.  Image: Joel Duff

Hiking back from Corona Arch near Moab UT. Notice the desert varnish on the walls but not on the horizontal rocks. A couple of my kids are in the foreground for scale. Image: Joel Duff

In the picture above we are hiking back from Corona arch near Moab UT and the Colorado river.  Notice that there is a good bit of desert varnish on the walls but the sandstone rock we are walking on has none.  This is because erosion is occurring to quickly on horizontal surfaces for the varnish to build up there.  Only where there is very very low erosive forces but some moisture at least periodically can the desert varnish “grow.”

Sandstone cliffs overlooking the Colorado River just west of Moab UT.  Photo: Joel Duff

Sandstone cliffs overlooking the Colorado River just west of Moab UT. Photo: Joel Duff

The huge sandstone walls overlooking the Colorado River just west of Moab show an impressive display of desert varnish.  There are some surfaces that must be very very old as they have very thick layers of varnish but as pieces of the wall have fallen or experience faster erosion they show up as lighter portions.  The black portions have experienced essentially NO erosion at all potentially for 10s of thousands if not a 100,000 years since the varnish is very resistant to erosion and actually grows in place over time.

Close-up of sandstone wall along the Colorado River showing where a piece of the rock has fallen off revealing sandstone without desert varnish.  The relative ages of the rock surfaces can be gauged by the amount of varnish on the surface.  Image credit: Joel Duff

Close-up of sandstone wall along the Colorado River showing where a piece of the rock has fallen off revealing sandstone without desert varnish. The relative ages of the rock surfaces can be gauged by the amount of varnish on the surface. Image credit: Joel Duff

In this close-up of a section of that same wall you can see where a piece of the rock face has fallen away revealing surface that has not been exposed before.  That surface has no discernible rock varnish.   It may be a thousand years or more before this spot begins to show any signs of varnish on it.

Desert varnish on sandstone walls just to the east of Corona Arch near Moab UT.  I Iiked this image because it looks like a face.  Image credit: Joel Duff

Desert varnish on sandstone walls just to the east of Corona Arch near Moab UT. I liked this image because it looks like a face.  For scale I am just taller than the rock in the foreground at the bottom of the image.  Image credit: Joel Duff

Desert varnish on wall of Gatherers Canyon just west of Moab UT.

Desert varnish (the black streaks) on wall of Gatherers Canyon just west of Moab UT.

A close-up image of sandstone rock that has a dark layer of desert varnish and a portion of that rock that has broken off showing the light colored rock beneath.  Image Credit: Joel Duff

A close-up image of sandstone rock that has a dark layer of desert varnish and a portion of that rock that has broken off showing the light colored rock beneath. Image Credit: Joel Duff

I don’t have a my lens cover here for scale but it would fill almost half this image.  This was right on a hiking trail. The lighter color in the middles is where a piece of sandstone was chipped off but the other lighter patches are from hiking books wearing on the desert varnish overtime. The varnish is really really tough stuff but once damaged it will take many hundreds if not thousands of years to recover this small spots.

Petroglyphs etched into desert varnish along the Colorado river just west of Moab UT.  Image Credit: Joel Duff

Petroglyphs etched into desert varnish along the Colorado river just west of Moab UT. Image Credit: Joel Duff

How do we know that rock varnish doesn’t grow very quickly. One was we know for sure is that we can still make out petroglyphs that were created by scratching the desert varnish down to the rock beneath.  Some of these petroglyphs may only be 500 years old but many are a thousand to possibly 10 thousand years old.   Still, they are easily visualized because new varnish has barely begun to cover this scratches.   In the example above this cliff is along the Colorado River and the bottom 15 feet of light sandstone rock used to be covered in soil before it was eroded.  Since that erosion long ago the varnish has yet to grow to the extent that it is above.  When the soil was much higher the rock was reachable by Native Americans who made the petroglyphs.  It is because these sit so high above the ground today that we can be certain that the petroglyphs are authentic and must be very old.   These petroglyphs are a testimony to the fact that the landscape of this desert region has barely changed over thousands of years.

A large boulder that has fallen into Hunters Canyon near Moab UT.  A couple of my kids are in the picture for scale.  Photo credit: Joel Duff

A large boulder that has fallen into Hunters Canyon near Moab UT.  The source of the boulder is not visible in this picture.  A couple of my kids are in the picture for scale. Photo credit: Joel Duff

Above you can see a very large piece of sandstone that has broken off and fallen to the canyon floor.  Unfortunately I didn’t get a picture of the source of the rock which was visible as a lighter section on the canyon wall.   This rock has obviously been here a long time as streaks of desert varnish have developed on its side. These would not have been present on the rock before it fell.

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