Maria Brumm has a Master's degree... in Science! She wrote her thesis on 
If you kick a dark pebble in the middle of the desert, you will sometimes find that it is light underneath. What this means is that you have disturbed a pebble that has been sitting there untouched for thousands of years. During that time, it accumulated a thin coating of windblown gunk - mainly clay dust, and manganese and iron oxides - known as desert varnish.
Desert varnish is not difficult to scratch through, and petroglyphic sgraffito is a popular artistic medium for native desert-dwellers and idiot tourists alike.
Desert varnish has a complex internal structure; there are thin sections below the fold.
These entirely gratuitous micrographs are from a paper in the latest issue of Geology. Three scientists at Arizona State took a careful look at some sections of desert varnish picked up in the Sonoran desert, and find that most of the things that happen to large-scale sediments also happen to desert varnish: chemicals are dissolved at one place and precipitated at another, cracks heal, microbes might eat the delicious manganese.
This all makes it difficult to give a precise age for a varnished surface.
This is a false color image of the distribution of iron and manganese in a desert varnish sample. Red is iron, green is manganese, and darker areas are mostly silicates (clay). The segregation of iron and manganese indicates that something wacky is going on.
Environment
Physical Science
Comments
Why did you have to mention manganese!?! Now I'm starving!
Posted by: Andrew Bleiman | February 29, 2008 3:21 PM
The Geo Dept is often brought false meteorites (or "meteor-wrongs") by members of the public, and we nicely examine and/or test their rock or other object, show them why it is not a meteorite, and usually try to figure out what it really is. Something that is commonly mistaken for a meteorite with a fusion crust is a rock with desert varnish. Then one has to explain that sandstones don't come from space.
Posted by: Ellery | February 29, 2008 10:16 PM
Desert varnish is one of those things that drives home the concept of deep time to me. "Nanometer-scale complexity" makes me think of that Dr. Suess book where there was a whole world on a pollen speck.
Posted by: Andrew | March 1, 2008 12:58 AM
Great post! I grew up in canyonlands, where cliffs coated in desert varnish abound. And yes, lots of petroglyphs, too! Thanks for the brief glimpse of home.
Posted by: Kiri | March 1, 2008 1:30 AM