It is not necessary for a cave to have an opening to the surface. Defining caves in terms of human accessibility is called explorational bias , and this bias must be recognized when we think about the geology of caves. Mechanically formed caves are produced from the physical action of one substance against another. Sandstone caves are often found in desert regions, including the American southwest. Eolian caves are also formed in loess , a fine soil composed primarily of silt.
Eolian caves tend to be small.
Sea caves are formed from the constant washing of tidal waters over coastal shore. Also called littoral caves. 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.