Biological soil crust is as the name describes – a living soil that creates a crust
over the landscape. Biological soil crust is found throughout the world, from the
Colorado Plateau’s high desert to the arctic, In many places, soil crust comprises
over 70 percent of all living ground cover. The knobby, black crust here includes
lichen, mosses, green algae, microfungi, and bacteria,
but is dominated by cyanobacteria.
https://www.nps.gov/articles/seug-soil-crust.htm
Don't Bust the Crust