Natural History

Desert pavement occurs only in the drier parts of the Sonoran Desert and consists of large, flat areas that are free of vegetation and covered with tightly packed, small stones. Pavement forms when large rocks on the surface weather away to form smaller stones. These smaller stones then tend to accumulate in the lower depressions on the soil surface eventually forming a flat pavement.
 

Desert pavements are usually dark in color due to a thin coating of desert varnish on the surface of the stones. Desert varnish consists of clay minerals and manganese oxides which impart the dark color. How varnish forms is not well understood. It is thought that bacteria which reside on the surface of the rock plays a major role in concentrating the mineral ingredients of varnish, perhaps as a means to protect themselves from the extreme environment of the desert..