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SCV Outdoor Report: Desert Varnish: Microbial Waste Or Silica Paste?

By: Wendy Langhans


A closer look at Desert Varnish.

Desert Varnish is a thin black coating that is found on rock formations all over the world, from western Chile to Morocco to our own local Vasquez Rocks at the eastern rim the Santa Clarita Valley.  While it’s easy for a casual observer to dismiss it as dirty smudges on a rock, there’s more to this story than meets the eye.  The origins of Desert Varnish are a scientific mystery, one that intrigued Charles Darwin in the 19th century and is still today suitable for an episode of CSI.  Right now there are two competing theories, which I am calling (with tongue firmly planted in cheek) the Coppertone Theory and the Elmer’s Glue Theory.

Theory #1:  The Coppertone Theory.  

This first theory proposes a biogeochemical process:  Desert Varnish consists of a mixture of metallic oxides (from microbial waste) and windborne clay particles.  This coating of varnish can be either black or red, depending on whether the metallic oxides are manganese (black) or iron (red).  

The clay particles are originally deposited on the surface of exposed rocks by the wind.  
But the metallic oxides come from a living source – the colonies of bacteria that live on the rocks.  Certain types of bacteria absorb trace amounts of manganese and/or iron from the atmosphere.  After being oxidized by the bacteria as part of their biochemical energy cycle, these metallic oxides are excreted onto the surface of the rocks, where they combine with the clay particles to form a cement-like coating.  According to this theory, the coating of Desert Varnish serves a useful purpose, protecting the bacteria from drying out and from damaging solar radiation.  The varnish works like layer of Coppertone suntan lotion.


Theory #2:  The Elmer’s Glue Theory.

This second theory proposes a strictly geochemical process:  Desert Varnish a result of dissolved silica that forms an adhesive glaze.  According to this theory, Desert Varnish is made up mostly of silica, with some aluminum and other trace metals such as manganese thrown in for good measure.  Particles of dust, windborne organic material and manganese along with other dissolved constituents from the atmosphere are deposited on the silica-coated rocks by the wind.  At a conceptual level, Desert Varnish resembles an elementary school art project, made with layers of Elmer’s Glue overlaid with tidbits of construction paper.


Common to Both Theories.


To get a better sense of scale, the arrow points to hikers on the rock.

Both theories agree that it’s a slow process; it can take 10,000 years to produce a thin layer of Desert Varnish.  So the next time you want to go clambering over a rock, please treat it with respect.  That dirty smudge is really quite mysterious, ancient and venerable.


Upcoming Outdoor Events:  (Remember, heavy rain cancels MRCA-sponsored events)

Late winter is a good time to visit Vasquez Rocks, which is owned and operated by the County of Los Angeles.  
Directions:  heading east on Hwy 14, take the exit at Agua Dulce Canyon Road and turn left at the off ramp. The road makes an abrupt right turn and later an abrupt left turn, but the park entrance is straight ahead at 10700 W. Escondido Canyon Road.  For more information call (661) 268-0840.

Saturdays, February 23, and every Wednesday, 8:00 am.  Trail Maintenance Volunteers at Towsley Canyon.
Come join our trail maintenance volunteers for camaraderie and a heart-thumping workout.  For more information call Steve Ioerger at 661-291-1565 or
Sponsored by the Mountains Recreation and Conservation Authority.

Saturday March 8, 4:00-6:00 am. Twilight Hike.  Join us on an easy hike to see how the canyon makes the transition from day to night.  Sponsored by the Mountains Recreation and Conservation Authority.

You can listen to stories like this every Friday morning at 7:10 a.m. on "The Hike Report", brought to you by your hometown radio station KHTS (AM1220) and by the Mountains Recreation and Conservation Authority.

For the complete MRCA hike and activity schedule and for trail maps, go to

SCV Outdoor Report: Desert Varnish: Microbial Waste Or Silica Paste?

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