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.
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.
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.”
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.
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.
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.
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.
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.
Naturalis Historia 