You see Desert Varnish almost anywhere you go in the deserts of the Southwest. But its development is not limited to the our region. It is found in El Azizia, the hottest desert on earth, where temperatures reach as high as 136 degrees and in the dry valleys of Antarctica, the coldest desert, where temperatures hover as low as -128. In the American Southwest it is a major component of paleolithic rock-art.
Desert varnish forms in arid and semi-arid regions as a film coating made of alternating layers of materials, each with different compositions. Some of these layers are rich in windblown clays and dust-sized particles, while others are rich in iron and manganese oxides and hydroxides. Usually we find smaller amounts of various metals, organic molecules, and bacteria.
The “varnish” grows, or is deposited, as a sort of patina on the surface of the rock. It’s growth is unimaginably slow – from 1 to 40 millionths of a millimeter for every thousand years. It is the slowest known accumulating, sedimentary deposit on earth.
Despite its widespread presence, and over a century of scientific investigation, we know little more than what’s in it, but we don’t really know how it happens. Mineralogists and other rock-types tend to favor the idea that desert varnish is an accumulation of dust and other materials held together by compounds that readily form on rocks as a result of weathering and other forces. But the fact that desert varnish always contains microbes, or their remains, encourages biologists to believe that the microbes create the coating from dust and minerals they capture from the atmosphere.
All of the single-agent theories have significant problems. The abiotic theories cannot account for the consistent element content of desert varnishes regardless of location. The biotic theorists, on the other hand, cannot prove a causal connection between the bacteria and the coating.
Lately we are considering the possibility that both biological and chemical processes are at work. In this hypothesis bacteria sequesters the manganese, iron, phosphate, carbonates and other materials that seem always to be present. The dust and clay form an adhesive layer which sticks everything to the underlying rock. And the layers themselves may be held together with silica and byproducts of dead bacteria. It’s an attractive idea, but we’re still not sure.
Regardless of how it is formed, desert varnish is extremely durable and fade resistant which makes it an ideal material on which to create rock-art. And our ancient ancestors did just that. Their work can be found throughout the Trans-Pecos. Many painted on rocks using natural pigments. Others scraped or chipped the desert varnish off, leaving the lighter underlying color to form the drawing.
Here in the Trans-Pecos we have a rock-art tradition that represents the work of a single cultural group of Indians over an extended period of time. The paintings show the beginning, a slow refining of style, and the dying of a tradition that was completely isolated from outside artistic influences.
Though it is durable, Desert Varnish is not indestructible. Vandalism and natural weathering of the rock are the most common causes of rock-art deterioration. Weathering can loosen the underlying rock and dislodge the varnish. Many sites have been destroyed in natural landslides. Graffiti obscures the underlying figures and unfortunately, the materials used by vandals often damage the underlying art beyond repair.