Wander around a desert most anywhere in the world, and eventually you’ll notice dark-stained rocks, especially where the sun shines most brightly and water trickles down or dew gathers. In some spots, if you’re lucky, you might stumble upon ancient art—petroglyphs—carved into the stain. For years, however, researchers have understood more about the petroglyphs than the mysterious dark stain, called rock varnish, in which they were drawn.
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Source: Phys.org