Sipapu Bridge, Natural Bridges National Monument, near Blanding, Utah, USA.
Desert varnish streaks Sipapu Bridge, in Natural Bridges National Monument, near Blanding, San Juan County, Utah, USA. White Canyon Creek has cut Sipapu Natural Bridge with a span of 225 feet (with a height of 144 feet, width of 41 feet, and thickness of 53 feet, says www.naturalarches.org) through a meander of white Permian sandstone of the Cedar Mesa Formation. Manganese-rich desert varnish requires thousands of years to coat a rock face 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.Add to Cart Add to Lightbox Download