The gradual coating of exposed rock surfaces in arid regions with a thin, dark patina of clay minerals cemented by manganese and iron oxides, built up over long periods as windblown dust settles on the rock and is chemically altered, likely with the aid of surface microorganisms. The varnish's slow, steady accumulation has made it a subject of study for dating rock surfaces and the petroglyphs and geoglyphs desert peoples have carved through it.
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1. Desert Varnish Formation (Wikipedia)
Wikimedia FoundationIntroductionQuote, Introduction
Desert varnish or rock varnish is an orange-yellow to black coating found on exposed rock surfaces in arid environments. Desert varnish is approximately one micrometer thick and exhibits nanometer-scale layering.
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