Desert varnish in Grand Wash, Capitol Reef National Park, Utah, USA.
Desert varnish in Grand Wash, Capitol Reef National Park, Utah, USA. Capitol Reef National Park is centered upon the 100-mile-long Waterpocket Fold, the steep eastern limb of the Circle Cliffs Uplift, formed in Late Cretaceous time, during the Laramide Orogeny. Manganese-rich desert varnish requires thousands of years to coat a rock face that is protected from precipitation and wind erosion. The varnish likely originates from airborne dust and external surface runoff, including: clay minerals, oxides and hydroxides of manganese (Mn) and/or iron (Fe), sand grains, trace elements, and usually organic matter. Streaks of black varnish often occur where water cascades over cliffs protected from wind. Varnish color varies from shades of brown to black. Manganese-poor, iron-rich varnishes are red to orange, and intermediate concentrations are shaded brown. Manganese-oxidizing microbes may explain the unusually high concentration of manganese in black desert varnish, which can be smooth and shiny where densest. Steeply tilted Triassic and Jurassic rocks form the hogbacks of the Waterpocket Fold and Capitol Reef, which is built of dark-red dune-formed Wingate Sandstone, thinly bedded river deposits of the Kayenta Formation, crested by the massive, white, dune-formed Navajo Sandstone.Add to Cart Add to Lightbox Download