Desert varnish is a dark coating on rocks found in arid regions. The coating is composed dominantly of fine-grained clay minerals. Within the clays are black manganese oxide and red iron oxide. A more general term is rock varnish which applies to dark coatings on rocks in general.
Varnish can be a prominent feature in many landscapes. It often coats canyon walls, particularly in the areas where water flows down the the sides of canyons such as this canyon wall in Canyonlands National Park, Utah, USA, coated with varnish.
Desert varnish plays an important role in archeology. Many petroglyphs are created by chipping through a dark coat of desert varnish to expose a lighter colored underlying rock. This petroglyph is in Indian Petroglyph State Park, New Mexico, USA.
Desert varnish is commonly seen coating rocks in deserts. On the east side of Death Valley, you can observe canyons with rock slides of different ages distinguished by the degree of varnish development. The older slides have a more mature coating of varnish.
Well-developed coatings can form in the splash zone of rivers in arid regions. The example of a manganese oxide coating in the splash zone of the Rio Grande River, New Mexico, USA, contains clays and manganese oxide similar to that found in desert varnish.
Desert varnish is dominantly clay. The clay minerals represent the clays found locally in the region where the varnish develops. In the clay layer, black manganese oxide (the mineral birnesite) and red iron oxide (the mineral hematite) add color.
A cross section through desert varnish shows the contact on the lower right with the colorless quartz and feldspar grains and black iron oxides of the rock with the deep red varnish layer. Layering in the clays can be seen to follow the contour of the underlying rock.
The clays appear to be derived from airborne dust. Varnish has a sharp contact with the underlying rock. In this SEM image layers of clay are seen lying on the underlying rock. The sample was prepared by fracturing a thin slab of a varnish-coated rock. The exposed surface filling most of the frame is a fracture through the varnish layer. A portion of the underlying rock surface can be seen in the lower left portion of the image. Note how the varnish smoothly goes around the rounded mineral grain in the bottom center-right side of the image.