Talus caves are formed when masses of piled boulders or other rock debris arrange themselves in such a way as to leave negotiable openings beneath and between the rock. In order to form talus caves, rocks must be large to leave enough space for human movement. Typically found in badland topography with steep slopes and little vegetation to hold the soil in place. Water flows down the surface of the rock, usually sandstone or another conglomerate. The cement that holds the particles together is dissolved, and the individual particles get washed down to the base of the rock.
Some cement is left on the rock, and gets drawn upward by capillary action.
This forms a harder area, which becomes the ceiling. Rock shelters at cliff bases are common examples of erosion caves. They are often associated with ancient human activity, such as the Anasazi in the American southwest. They tend to be small. Chemically formed caves Chemically formed caves must involve a substance either changing state or composition .