Chemical and Hydrologic Factors in the Excavation of Limestone Caves
Various hydrologic and chemical processes have been investigated in an attempt to explain the development of caves in the region just beneath the water table (shallow-phreatic zone). Special attention was paid to the differences between the Darcy flow of granular aquifers and the laminar or turbulent flow of limestone aquifers, and to the ways in which water in the limestone aquifer may become undersaturated with respect to calcite. The pattern of flow in a limestone aquifer, although dependent on the boundary conditions, is similar under Darcy, laminar, or turbulent flow, provided the aquifer is wide relative to its depth and the permeability distribution is homogeneous. Although the vadose water which supplies an aquifer is often saturated or supersaturated with respect to calcite, such water may become undersaturated by being cooled or by being mixed with water which is in equilibrium with a lower P co 2 . The water of surface streams may be undersaturated with respect to calcite and may enter the aquifer by capture or by backflooding from effluent streams. The supply of vadose water to a limestone aquifer is commonly irregular. In an area where no water crosses the water table, flow in the aquifer will be nearly horizontal and water from sources adjacent to the area will follow shallow flow-paths. If such water is undersaturated with respect to calcite, caves will be excavated. Mammoth Cave, Kentucky, was probably formed by undersaturated water spilling from an impermeable bed. After reaching the water table, this water followed shallow-phreatic paths in the area beneath the impermeable bed. Backflooding from the Green River may have aided in the excavation of the cave.