desert varnish

A thin, dark, lustrous coating of manganese and iron oxides that forms on exposed rock surfaces in arid environments over thousands of years. Desert varnish gives canyon walls, cliff faces, and boulder fields their characteristic dark sheen — a patina of deep brown, black, or reddish-purple layered onto the underlying stone by bacteria and chemical processes not yet fully understood. Petroglyphs are carved through the varnish to expose the lighter rock beneath; the contrast between dark surface and pale incision is what makes them visible.