Some unpaved roads are closed
Recent rains have caused extensive damage to some roads in the Needles District and some of the roads into the Maze District. More »
Safety in Bear Country
Black bears have been seen in the Needles, Maze, and along the Colorado River. Be alert and store food and garbage properly: in hard-sided, latched containers (or your vehicle) when not being prepared or consumed. More »
New backcountry requirements in effect
Hard-sided bear containers are required for backpackers in parts of the Needles District. More »
Desert varnish is the thin red to black coating found on exposed rock surfaces in arid regions. Varnish is composed of clay minerals, oxides and hydroxides of manganese and/or iron, as well as other particles such as sand grains and trace elements. The distinctive elements are Manganese (Mn) and Iron (Fe).
The color of rock varnish depends on the relative amounts of manganese and iron in it: manganese-rich varnishes are black; manganese-poor, iron-rich varnishes are red to orange; those intermediate in composition are usually a shade of brown. Varnish surfaces tend to be shiny when the varnish is smooth and rich in manganese.
Desert varnish consists of clays and other particles cemented to rock surfaces by manganese emplaced and oxidized by bacteria living there. It is produced by the physiological activities of microorganisms which are able to take manganese out of the environment, then oxidize and emplace it onto rock surfaces. These microorganisms live on most rock surfaces and may be able to use both organic and inorganic nutrition sources. These manganese-oxidizing microorganisms thrive in deserts and appear to fill an environmental niche unfit for faster growing organisms which feed only on organic materials.
The sources for desert varnish components come from outside the rock, most likely from atmospheric dust and surface runoff. Streaks of black varnish often occur where water cascades over cliffs. No major varnish characteristics are caused by wind.
Thousands of years are required to form a complete coat of manganese-rich desert varnish so it is rarely found on easily eroded surfaces. A change to more acidic conditions (such as acid rain) can erode rock varnish. In addition, lichens are involved in the chemical erosion of rock varnish.
Did You Know?
Naturally occurring sandstone basins called “potholes” collect rain water and wind-blown sediment, forming tiny ecosystems where a fascinating collection of plants and animals live. Tadpole shrimp, fairy shrimp and many insects can be found in potholes. More...