Desert Canvas

by Pat Flanagan

Hiking through Joshua Tree National Park the eye is drawn to what appears to be a uniform layer of dark paint covering many of the different rock formations. In addition, those lucky enough to find Indian rock art sites, often discover that the images or symbols are made by scratching through this covering. This rocky canvas is called rock varnish and besides presenting Indians with a durable surface to display their thoughts, has long intrigued scientists. Recent discoveries have started to shed light on this mysterious stuff.

The coating, in colors ranging from a deep black to a reddish brown, to a light gold are just fractions of a millimeter in thickness and contain about 70% clay with the remainder being manganese and iron oxides. While the manganese and iron were originally thought to come from within the rock they are now known to be air born along with the clay particles.

This is where the story becomes wondrous. If ever there were a place that appeared impossible for life to exist it would be the exposed surfaces of desert rocks. Where could life find a comfort zone on a surface that is dry for months on end, exposed to the searing sun and ultraviolet radiation, whipped by desiccating winds, and enveloped by freezing winter storms and nightly temperatures? Life can and did make its own comfort zone.

Colonies of microscopic bacteria living on the surface of a rock for thousands of years absorb trace amounts of the manganese and iron minerals and deposit them along with cemented clay particles on the rock. The mineral rich clay umbrella helps to shield the bacteria from the extremes of temperature and the harmful solar rays. As important, the bacteria use the manganese and iron oxides in the same way we use glucose, to generate energy. The bacteria eat the oxides. Living on the surface of desert rocks is not a lively process. Research indicates that perhaps 10,000 years are required for a complete coat of varnish to form; the darker the varnish the longer the process has been going on.

It is thought provoking to note that on some rock art panels there are images which appear to be fresh (whatever that means) while others have begun the process of filling in the scraped lines with a new layer of varnish. Scientists are looking for a way to date the varnish, without touching the rock art, in order to date the art itself.

David Wayne from the Los Alamos Research Laboratories, in collaboration with scientists from the University of Nevada and Eastern Washington University has reported that rock varnish might be useful as a passive environmental monitoring tool. Along with manganese and iron oxides, the varnish making process also captures heavy metals like lead, arsenic, zinc, cobalt, uranium, and tungsten. (I wonder if they give the bacteria indigestion.) These heavy metals are distributed into the atmosphere through natural processes but also through industrial activities like mining, smelting, oil refining, chemical processing, and nuclear plant operations.

For instance, desert varnish collected near Fallon NV contained 10 to100 times more tungsten than the varnish from other localities, and the top-facing surface of entirely varnish-coated pebbles from the San Juan River contained 5 to 10 times more lead, arsenic, and cadmium that the bottom-facing surfaces. Wayne believes that if desert varnish is as good at capturing and preserving airborne heavy metals and other elements as it appears, scientists could analyze it to infer what sorts of activities have been going on nearby.

Desert varnish is indeed a canvas. Not only did it provide a surface for the thoughts and stories of earlier peoples on this continent, it appears to hold the evidence for other tales as well.