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10/1/10 8:07 PMDesert Varnish : Encyclopedia of GeographyPage 1 of 4http://www.sage-ereference.com/geography/Article_n276.htmlDesert VarnishDesert varnish is but one of more than a dozen rock coatings (Table 1) that drastically alter the appearance ofrock surfaces. The better term is rock varnish because this coating occurs in virtually all environments, includingalpine, antarctic, arctic, desert, periglacial, stream, temperate, and tropical settings. This paper-thin accretion ischaracterized by extremely high concentrations, typically more than 10%, of manganese oxides that give it acharacteristic black to dark brown appearance. Clay minerals, however, make up the bulk of rock varnish alongwith iron oxides.There are four general explanations for how rock varnish accretes on top of rock surfaces, but all these modelsreject the old idea that the constituents of varnish derive from the underlying rock. The model that has not yetbeen falsified is the polygenetic model of rock varnish form...