Cyanobacteria and green algae make up a large component of biological crusts in semiarid and arid regions of the U.S. Biological crust characteristics are quite variable. Color, surface topography, and surface coverage varies. Mature crusts of the Great Basin and Colorado Plateau are usually darker than the surrounding soil. The dark color is due in part to the density of the organisms and to the often dark color of the cyanobacteria, lichens, and mosses. The presence or absence of a crust is partly determined by soil texture and conductivity, pH, moisture, and possibly temperature.
Some crusts increase surface topography by forming pinnacles or pedicles.
Other crusts are merely rough or even smooth and flat. The process of creating surface topography, or pinnacling, is due largely to the presence of filamentous cyanobacteria and green algae. They swell when wet and migrate out of their sheaths. After each migration, new sheath material is exuded, thus extending sheath length. Repeated swelling leaves a complex network of empty sheath material that maintains soil structure after the organisms have dehydrated and decreased in size.