Joints are more numerous and determine the orientation of most caves. For the most part, cave fill is either old, pre-entrance red clay and kaolinite, or modern black soils eroded from the surface probably because of overgrazing. The BFZ is closely related to but separated from the Edwards Plateau by a strip where the Edwards Group limestones have been erosionally removed. It is one of the largest continuous karst areas in the United States. Many of the caves, some quite extensive, are confined to small vertical intervals, either by lithology of the confining beds or by still-stands of nearby rivers.
Vertical caves are less common.
The Edwards Plateau can be divided into subregions principally on the basis of stratigraphy and hydrology. Fractures may be related to underlying fault zones. Passage orientations are determined by the local hydraulic gradients and are weakly guided by fractures along those flowpaths. Central Edwards Plateau North of the Devils River Trend, cavernous rocks become progressively thinner to the north. Stream incision into the soluble carbonates is much younger in most of this area, and fewer well-integrated caves have formed.