Hiking west on the National Trail you encounter areas of rock carved by erosion into fantastical shapes. Colossal boulders stack one on another and it’s easy to imagine human profiles, animal shapes and space ships. At Fat Man’s Pass squeeze through a narrow gap between mansion-size boulders and slide over massive rock faces shiny with wear.
Telegraph Pass is roughly midpoint in the mountain range. To the west rugged cliffs and chiseled outcrops of schist, granite and gneiss resist the pull of time. Although the shapes differ dramatically, much of the South Mountains’ rock surface appears the same, mostly black and polished to a sheen.
This is because of a process that’s been called Nature’s smallest sedimentary formation. Thin as a piece of paper, a layer accumulates on exposed rock in arid locations creating a coating called desert varnish.
Materials that make up this varnish are composed of fine grain clay from surrounding rock and earth material carried in airborne dust. The colors themselves come from black manganese oxide and red iron oxide. The oxidation is accomplished by bacteria that thrive on rock surfaces in areas with low levels of organic material and high evaporation rates. These microorganisms facilitate the chemical reaction between the clay elements and heat. Wind abrasion hones the varnish to a patina. This thin layer can take thousands of years to form.
In ancient times Native Americans recorded events and messages with rock art images chipped through the desert varnish to the lighter rock below. Today thousands of these intriguing petroglyphs in the South Mountains depict people, animals, gods, celestial events and ways of life. Current day Pimas, descendents of the Akimel O’odham peoples of long ago, call the range Muhadag Du’ag or Greasy Mountain, an apt description for the effects of desert varnish.
Landscape of the Spirits by Todd Bostwick