DESERT VARNISH & THE COLORADO PLATEAU

On my first trip to western Colorado and eastern Utah I was somewhat amazed at all of the red rocks exposed virtually “everywhere”.   Coming from Kansas with its gray and cream limestones I had trouble understanding how “those rocks got so red”.  Equally fascinating was the fact that many exposures were covered with some sort of a dark to black coloring.  Upon stopping near Moab, Utah, I made the discovery (personal) that early Native Americans had chipped pictures on the rocks and from that point forward I was hooked on “red rocks”.  That curiosity has existed to this day and I hope it never disappears.

I later found out that red rocks with black surficial coloring confuse many people as they travel through “Red Rock Country”.  What I observed, the black coloring, is termed Desert Varnish, the subject of this small piece.  Back in the “olden days”, at least back in the days when I was a ranger at Dinosaur National Monument, “we” thought Desert Varnish came from dark-colored manganese that appeared on the surface of the rock via water “seeping” out.  Since that time, numerous articles and research have given me a different story about Desert Varnish.

Desert Varnish is just one type of coating that geologists generally refer to as Rock Coatings, and all are actually types of geochemical sediments deposited on the surfaces of various rocks.  Naturalists also, at one time, believed that rock coatings were restricted to desert environments; however, Dorn (1981, 2004, 2014) has shown the feature may be present in a variety of environments and may form on a variety of rocks.  It is just that Desert Varnish is so observable in arid environments.

 

The Navajo Formation, an old dune field.

Great scenery in the Colorado Plateau.

The Colorado Plateau (CP) is one of the most fascinating and beautiful areas in the U.S. and Colorado is fortunate to have the region essentially in our backyard.  The world’s foremost geological authority (and my good friend) on the CP, Ron Blakey of Northern Arizona University, has best described the region: The brilliantly colored rocks of the Colorado Plateau comprise a truly spellbinding landscape.  Within this 130,000 square miles of rocky bliss there are magnificent escarpments, graceful flat-topped mesas, surreal towers and monuments, and multitudes of deep, sinuous canyons, all framed by an azure sky.  Unlike some other notable landscapes on our planet, the Colorado Plateau is dominated by rocks.  Not just any rocks, mind you, but colorful, stratified rocks containing subtle textures and minute details that bear witness to the environments in which they originated.  These rocks, rich in various shades of red, yellow, orange, brown, tan, purple, and green, are composed of many familiar rock types—limestone, shale, sandstone, conglomerate, mudstone—which manifest as long lines of colorful cliffs or stupendous stair-stepped canyons.  When geologists think of paradise on earth, the Colorado Plateau is what comes to mind (2008). 

 

Map showing location of the Colorado Plateau.  Public Domain photo

The CP is interesting, geologically speaking, from several points of view.  I will mention but two: 1) the extreme stability of the region since the latest part of the Precambrian (last ~600 million years).  The CP is surrounded by tectonically active areas, that is, mountain ranges such as the Uinta and Wasatch Mountains, and to the west the Basin and Range.  However, the CP has not experienced major building events such as the compressional Laramide Orogeny (the tectonic event uplifting the Rocky Mountains with ocean-continent plate conversion) or the tensional (stretching) orogeny of the Basin and Range.  The CP seems to have existed as a somewhat rigid block undergoing faulting, volcanism, rotation, tilting, intrusions, and vertical movement (both up and down) but not severe mountain building periods.  During much of the Paleozoic and Mesozoic Eras (~546-66 Ma) the CP was near, at, or below sea level and so marine sedimentary rocks were commonly deposited (although this is a vast over simplification and numerous terrestrial [land] formations such as the great Navajo Formation dune field) exist (see above photo; and 2) the large exposures of red (but also yellow and orange) rocks. 

The first question then becomes, what causes the dominantly red to orange colors in rocks of the CP?  The simple answer is iron oxide, Fe₂O₃, a combination of iron and oxygen in a mineral called hematite.  In some instances the iron oxide imparts a brownish or yellow color, due to the “mineral” limonite (iron oxide with water in the chemical formula).  Geologists also know that varying amounts of the iron minerals can cause significant color changes in the rocks, and that it takes a minor amount of iron oxide to turn a sandstone bright red or orange. 

 

Sketch of late Paleozoic Ancestral Rockies.  The two highest ranges were Frontrangia (~location of modern Front Range) and Uncompahgria (~location of the modern Uncompahgre Uplift).  The Ouachita Orogenic Belt is thought to play a critical role in the uplift of these ancient mountains. Courtesy of Burchfield and others, 2002.

The second question is: what is the origin of the iron oxide in the rocks?  It appears that most of the iron can be traced to weathering of minerals eroded from the intracratonic mountains of the Ancestral Rockies.  I have mentioned these mountains before as a group of uplifts with accompanying basins centered near the Four Corners Region.  Uplift commenced perhaps 320 Ma in the early part of the Pennsylvanian Period and the ranges did not completely disappear (from erosion) until sometime in the Mesozoic.  The two major ranges are referred to as the Ancestral Front Range Uplift and the Ancestral Uncompahgre Uplift and seem related to compressional events along the southeastern margin of North America (the Ouachita and Marathon Orogenies with continent-continent plate convergence).  At any rate, after the early and middle Paleozoic, rocks (mostly marine) were eroded off the ranges as sediments into the accompanying and adjacent basins, while the basement rock, granites and other igneous rocks, were exposed to erosion.  A “typical” granite is composed of light-colored quartz and feldspar, and black biotite or hornblende; the latter are often called ferromagnesian minerals due to their composition (high in iron).  As these ferromagnesian minerals chemically weather, they turn into hematite ---the red color is produced!  The quartz, almost immune to chemical weathering, produces “sand-sized” grains, perfect for the rock sandstone.  The feldspar either chemically weathers to clay minerals, or physically breaks down into small fragments incorporated into the sandstone.  These sediments (unconsolidated) are then cemented together into sedimentary rocks, are often eroded themselves, and the cycle starts over.  So, a “little bit of hematite” can cause “an awfully lot of red rocks”.

 

Iron oxide coloring red sandstones, shales and siltstones in the San Rafael Swell

That answer leads us back to Desert Varnish, a rock coating that is common in the red rocks of arid areas where scarce vegetative cover, rock surface stability, mild winds, and a source of clay “dust” exist---the CP.  Extensive research by Dorn (1981, 2004, 2014) has shown that the formation of Desert Varnish is an extremely complex process--- like so many of the features that seem rather simple but upon detailed research turns out to be quite complex!   Clay minerals are the dominant ingredient in Desert Varnish and arrive by wind.  The dark color, and the “cementing agent, is manganese and iron supplied by chemical breakdown of both bacteria and some of the clay minerals.  The end result is that large parts of the red rocks, especially the sandstone cliffs, are covered by the dark-colored Desert Varnish.

The CP has been home to several different groups of Native Americans over the millennia but among the more interesting (in terms of the rock drawings) were members of the Fremont Culture.  The Fremont were a pre-Columbian (ca. 700-1300 A.D.), Ancestral Puebloan people, and a group with a strong relationship to the better known Ancient Ones (Anasazi).  They subsisted by hunting (deer, sheep, small mammals), gathering edible wild plants (such as Pinon Nuts and berries), and growing corn, squash and beans along river/creek bottoms.  The Fremont also were fine artists and left behind an amazing record of rock art, predominantly petroglyphs (picked or chipped into the rock surface), but also some pictographs (painted on the rock surface).  And, their favorite place to chip a petroglyph was on an orange/red sandstone wall covered by black Desert Varnish.  The chipping went through the thin layer of varnish and exposed the red sandstone underneath to provide a dramatic contrast in color.

Pictographs, rock painting, attributed to members of the Barrier Canyon Culture, perhaps 2000 years old.  Buckhorn Wash Pictograph Panel, San Rafael, Utah.

Some of the more interesting petroglyphs include the “human-like” figures, almost always male, trapezoidal in shape with arms, legs, often fingers, and adorned with ornaments like necklaces and headdresses.  The lizards are spectacular, and commonly the art includes sheep, deer, handprints, concentric circles, and fish.  My favorite is the flute player, “Kokopelli”.

The Fremont rock art is fascinating and I have spent hundreds of hours scouring the back country of the CP trying to locate sites.  At one time, I hiked around and attempted to photograph every known petroglyph and pictograph in Dinosaur National Monument.  If readers are interested in Fremont rock art they may drive to many sites in the San Rafael Swell (central Utah), Nine Mile Canyon (central Utah), and Dinosaur National Park (northeastern Utah).  Perhaps closer to home, but more difficult to access, are sites in northwestern Colorado.

 

Fremont petroglyphs at Dinosaur National Monument.

Lizard petroglyph at Dinosaur National Monument.

Flute player petroglyph at Dinosaur National Park.  Note how some of the Desert varnish has spalled off since construction of the petroglyph.

So, it all seems to come together:  deposition, mountain building, erosion, sun, microbes, wind, clay, climate, culture, petroglyphs, plate tectonic activities in the Ouachita Orogen 320 Ma ago leading to an Ancestral Puebloan chipping away, to a kid from Kansas amazed at finding a projectile point. 

It’s like déjà vu all over again (Yogi Berra).

REFERENCES CITED

Blakey, R. and W. Ranney, 2008, Ancient Landscapes of  the Colorado Plateau:  Grand Canyon Association.

Dorn, R.I. 2014,  Rock Varnish (desert varnish): An Internet Primer for Rock Art Research: http://alliance.la.asu.edu/dorn/VarnishPages/VarnishPrimerIntro.html  

Dorn, R.I. 2004. Desert Varnish, in Encyclopedia of

Geomorphology , ed. A.S. Goudie, Routledge: London

 Dorn, R.I. and Oberlander, T.M. 1981. Microbial origin of desert varnish. Science 213.