Rock Varnish

When you look at rock cliffs and large boulders, they sometimes look like they've been painted dark red, brown, or black. These rocks have been exposed to harsh conditions in desert and arid landscapes. The coatings are also found in mountains and tropical environments. This reddish-brown coating forms on rock over thousands of years. It's called 'desert varnish' or 'rock varnish.'

The pictures below show the appearance of rock varnish.

What Is It, Really?

Rock varnish is made up of clay particles combined with minerals. Wind blows these materials onto the rock. All the ingredients are cemented to the rock surface by living bacteria. The bacteria lives in the microscopic layers of varnish. Some people think the varnish is used by the microbes to protect themselves. The minerals may act like a sunscreen for the rock-dwelling bacteria.

Rock varnish forms very slowly. The varnish becomes thicker and darker as it ages. Some varnished rock surfaces are tens of thousands of years old.

Geologists measure and study the varnish. They use it to find out how long the rock surface has been undisturbed.

Ancient peoples chipped designs through the rock varnish using rock tools (Left photo: 'Moab Man' petroglyph, Utah). Many of these petroglyphs can still be found.

Over time, rock varnish can slowly cover the petroglyphs. This ancient art can become difficult to see.

More Information


Note: All photographs taken with a digital camera in Utah.
Developed by Annette Lamb and Larry Johnson, 04/02.