Striated Canyon and Desert Varnish

Photographer: Bruce Horrocks
Summary Author and Editor: Bruce Horrocks, Stu Witmer

The above photo showing an intensely striated canyon wall and a surface feature known as desert varnish was taken in Colorado National Monument on October 7, 2009. This canyon, featuring sheer cliffs over 300 ft (91 m) high, was formed over many thousands of years primarily by stream erosion of Wingate Sandstone. The bare surfaces of rock on the lower half of the photo are the result of much more recent water freeze/thaw activity.
Desert varnish is a veneer commonly found on resistant rock surfaces in arid regions. It primarily consists of minerals, oxides and hydroxides of manganese (Mn) and or iron (Fe) which give it the distinctive coloring. If the coating is manganese-rich and smooth, the coloring is typically dark to black and the surface appears shiny; whereas iron-rich varnishes are reddish orange. Tan to light brown surfaces, as shown above, likely contain near equal mixtures. The minerals are wind-blown and, although it hasn't been proven, most research indicates that they are fixed by bacteria living on the rock surface, which are able to concentrate manganese and iron at far higher levels than are found naturally, either in the rock or the surrounding environment. There is also some evidence that some rocks on Mars may contain varnishes. Colorado National Monument GPS: 39.0602°N 108.6992°W.