Wiktionary
n. (context geology English) A dark coating found on exposed rock surfaces in arid environments.
Wikipedia
Desert varnish or rock varnish is an orange-yellow to black coating found on exposed rock surfaces in arid environments. Desert varnish is usually around one micrometer thick and represents nanometer-scale layering. Rock rust and desert patina are other terms which are also used for the condition, but less often.
Usage examples of "desert varnish".
On the cliffs here he found an array of petroglyphs cut through the black desert varnish into the sandstone.
Like I tell the dudes, pictographs were put on the rock with paint, but petroglyphs were chiseled out of the black desert varnish to expose the tan base rock underneath.
Below the tunnel entrance, and spreading like a fan down the cliff, was an area of lighter-colored rock, as if the sun-glazed desert varnish had been erased in a triangle shape, its apex pointing to the threshold of the tunnel.
Angling her binoculars down, she saw the answer: below the lone notch, a section of the cliff face had recently peeled off: the desert varnishthat layer of oxidation built up on sandstone over centurieswas a lighter, fresher color.
The sandstone was blackened and covered with a patina of what Byrne called desert varnish.
Just three rocks, one atop the other, slick with desert varnish and almost pushed over by the Joshua tree growing up beside it.
I specked through my enhancement pressures building in sizzle baked by ultraviolet across the years, wind removing layers of flopsand and powder, sudden cold night air flowing from nearby scarps, pressure within the desert varnish squeezing tiny crystals of quartz .
In places the desert varnish had slipped away, and what stories might have been written there, lost.
The sandstone was also discolored by the rain-leached desert varnish.
Moreover, the so-called 'desert varnish', which has a brown glint after oxidisation, has formed on them.
They seemed to be loosely cemented in place by a coating of some form of desert varnish.