Desert Varnish EarthCache
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There is a self guided walking tour at Chasm view on the North Rim of the Black Canyon. A brochure is available at the trail head. Please take time to enjoy the entire tour.
Desert varnish is a smooth black coating that accumulates slowly on rock formations ranging from small boulders to cliffs hundreds of feet high.
In general, about 70% of a varnish coat consists of clay minerals. The remainder is composed of oxides of iron (Fe) and manganese (Mn). Variations in the ratios of these two oxides, probably due to variations in climate during their accumulation, cause variations in varnish color, ranging from dark bluish-black (Mn rich) to dark brownish-black (Fe rich).
Most of the coating collects from sources outside the rock rather than from material leached out of it, as many geologists had believed. One reason for this conclusion is that varnish is found covering non-manganese or iron bearing quartz crystals. Although some rocks may contribute oxides through weathering, the primary source seems to be wind deposited particles.
Fine, windblown clay particles are a critical ingredient in forming the varnish which first forms on rough, porous surfaces. These surfaces allow dew and other moisture to collect, depositing a thin film of clay when the water evaporates. This film of sediment on the rock's surface encourages water to migrate through tiny pores inside the film, depositing traces of manganese and iron as the water evaporates.
The formation of desert varnish is interdependent upon the clay and oxides. The dry, fluffy clay particles depend on the oxides to form a resistant cementing agent. The oxides, in turn, require clay particles for transportation and deposition. This is the underlying reason why all desert varnish that was examined contained both clay and manganese and iron oxides -- never one without the other.
These ingredients are cemented to the rock surface by living bacteria. The bacteria reside within and beneath the microscopic layers of varnish, and are usually absent from the exposed surfaces.
The development rate is, however, relatively slow, i.e., measured in years, decades, and millennia. The varnish becomes thicker and darker as it ages. There are some varnished rock surfaces that have been untouched for tens of thousands of years. Many older deposits become almost black. By closely examining and measuring the varnish coat, geologists are able to measure how long the rock's surface has been undisturbed
To claim this cache answer this question: Based on your observations of the opposing canyon wall at the coordinates and your understanding of the narrative of this listing, do you think the desert varnish is Fe rich or Mn rich?
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