the general outline of the formation of these caves is invariably the same. the entrance is a vertical shaft some 3 feet (1 m) in diameter falling, on an average, to a depth of 60 feet (20 m). the depth is regulated by the depth of the chalk layer from the surface, although chalk can be found within a few feet, or even inches, from the surface. a depth of from 45 to 80 feet or more, is a characteristic feature.