What is Desert Varnish?
The desert varnish coating is the result of airborne particles of certain minerals, such as iron oxide and manganese oxide, that settle on the surface of the rock. In a complex natural process involving microbial and bacterial action, the dust particles are turned into a concrete-like substance that adheres to the surface. This takes centuries. Once the varnish is removed, new varnish immediately begins to form in the scar. The amount of varnish that has built up over a petroglyph is one of the means archeologists use to date them. The petroglyphs in the Agua Fria National Monument are typically seven to eight hundred years old.
