I also visited the web and found, and borrowed material from, a new USDI Technical Reference. Biological soil crusts are commonly found in semiarid and arid environments throughout the world. In the U.S., they have been identified as a prominent feature in the Great Basin, Colorado Plateau, Sonoran Desert, and the lower Columbia Basin. They are also found in agricultural areas and native prairies. Biological soil crusts are known by several names, including microbiotic, cryptogamic, cryptobiotic, and microphytic crusts.
Biological crusts are different than chemical or physical crusts.
Biological crusts are formed by living organisms and their by-products, creating a crust of soil particles bound together by organic materials. Liverworts, fungi, and bacteria can also be important components. 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.