Desert Varnish: Why Some Rock Faces Turn Dark
Have you seen a pale cliff covered with dark streaks? The rock may not be dark all the way through. Its surface may carry a thin coating known as desert varnish.
A coating on the rock
Desert varnish is a broad name for coatings seen on many exposed rocks in dry regions. What looks like one simple layer may actually contain several kinds of surface deposits.
Water moving over rock, windblown dust, and organisms living on the surface can all contribute material. The coating has a different composition from the rock beneath it.
Why the colours vary
Iron-rich components can produce red, orange, or brown colours. Manganese-rich rock varnish is often darker. Different mixtures help explain why one cliff shows several shades.
Streaks may follow routes where water runs down a wall. This does not mean the whole cliff is made of the minerals found in its coating.
Look at the surface and the setting
When studying a photograph, ask whether the colour follows a rock layer or runs across several layers. That simple question can help separate the rock's structure from later changes to its surface.
Take photographs without scraping or cleaning the coating. Where rock art is present, leave the entire surface untouched. The coating and the marks on it are part of the site's history.
Source: National Park Service: Desert Varnish.
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