RECENT THEORIES OF CAVERN DEVELOPMENT

In recent years there has been an important transition from the physiographic theorizing and qualitative reasoning of the ”classic” theories to a more quantitative process approach. Many recent studies have investigated the geologic setting, the hydrology, and the chemical and mechanical weathering and erosional processes associated with cavern and karst development. The reader is referred to the comprehensive works by Jennings (1985), Sweeting (1973), Ford and Cullingford (1976), White (1988) and Ford and Williams (1989).

 According to Ford (1981), it is now recognized that there is not one general case of limestone cavern development that can be precisely defined as older theories would have it. Rather, there are three common cases, the predominantly vadose cave, the deep phreatic cave and the water table cave (Figure 1).



The type or types of common cave development that occurs is governed by the frequency of fissures significantly penetrated by groundwater, and by the joint to bedding plane ratio. Together these characteristics combine to form the concept of hydraulic conductivity. Hydraulic conductivity is a coefficient of proportionality describing the rate at which water can move through a permeable medium (Fetter, 1980). The higher the hydraulic conductivity, the more likely a water table cave will develop. Water table caves are particularly common in flat-lying rocks, where the perching of groundwater occurs because of the presence of more resistant rock layers. Deep penetration of water is inhibited by the presence of shallow open bedding planes which are continuous to springs. Vadose type caves develop where sufficient streams collect above sink points and transport water to the water table or a spring. Deep phreatic caves attain their optimum development in steeply dipping rocks because continuous bedding planes guide water to greater depths.

Palmer (1984) noted that larger passages of many caves show a succession of levels with the youngest, and still active ones, lying at the lowest elevation. The level in which enlargement occurs is concentrated at or near the contemporary river levels. Studies in Kentucky by Miotke and Palmer (1972), have shown that cave patterns are a reflection of numerous changes in base level and climate since the Late Tertiary Period (Figure 2).



Palmer (1984) also noted that cave development is possible above, at, or below the water table. Where caves form is dependent upon local geology, and hydrology, and it is possible for one cave to have passages formed above, at or below the water table. He further point out that a clear relationship in many areas between cave levels and fluvial history, shows the tendency for much of the solutioning to take place at the water table.

 The evidence cited here from various karst researchers makes it clear than an evolution of ideas has occurred pertaining to cavern speleogenesis. The debate among early investigators as to whether caves originate at, below, or above the water table has essentially been resolved. It is now accepted that all three origins are possible depending on local hydrologic and geologic conditions.