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.




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
Desert Varnish:
Comet spatter can frequently be found on Clovis spear points. Comets:
Comets were formed beyond the heliosphere, so the Niger primitive achondrites have a large cosmic ray exposure, but no solar exposure.
Comets have a rocky crust over an icy core with hard tar on the surface. The rocky crust composed of shattered rock crystals on the order of 1 micrometer has random-sized metallic and ice inclusions in a raisin-bread--like composition. Naturally the ice melts on impact leaving voids. Since voids are never found in asteroid meteorites, having been formed far closer to the sun, millions of tons of comet-crust meteorites have been overlooked as iron-furnace slag. Comets were formed in two stages: first the ice core followed by the rocky crust of shattered mafic crystals and iron chunks. Perhaps a binary companion to our sun was itself a close binary that repeatedly collided over millions of years shedding first its hydrogen, then its ice and finally its rock and liquid-iron outer core before the inner cores finally stuck together with their iridium. (Comets have less than 1 ppb iridium.) Originally, our solar companion was closer to our sun, perhaps at the outer edge of the Kuiper Belt, before migrating outward to conserve angular momentum as it spewed jets of ice then rock as the close binary orbs rubbed shoulders in multiple collisions over millions of years. The companion presently is calculated to orbit between 1000 AU at periapsis and 4000 AU at apoapsis with a period of 25,000 years.[1] In this way, it swept out the space between the Kuiper Belt/scattered disc and the Inner Oort Cloud (Hills Cloud) and bombarded the inner solar system with comets over the last 4.5 Ga, resulting in many of the structural basins on earth including Witwatersrand Basin, Cuddapah Basin, Athabasca Basin (and their mineral ores) as well as The Great Unconformity. Comets are likely responsible for initiating the lava flows of the Deccan and Siberian Traps, for dislodging asteroids (such as the Chicxulub impact) and for bringing on most of the major and minor extinctions on the planet.
Understanding Precession of the Equinox: Evidence our Sun may be part of a long cycle binary system Walter Cruttenden and Vince Dayes (2003)
Dave Carlson Philadelphia, PA See comet-related images at: http://picasaweb.google.com/dave19128
Posted by: Dave Carlson | August 1, 2009 3:11 AM