During the large Middle Pleistocene glaciations, surface runoff from glacial meltwaters would have fed directly into the river channel network because much of the upland limestone terrain was covered by ice, and many conduits in the karst would have been choked with sediment or permanently frozen. As a result, the glacier snouts came close to the modern valley bottoms. In contrast, during interglacial and interstadial periods, more effective coupling occurred between the surface drainage network and the internal karst drainage system. Since limestone dissolves as the water percolates through its pores, an extended underground drainage system has developed, with caves and channels that enlarge with time when their roofs collapse, producing rocky exposures and perpendicular slopes, which is also the reason why the water is scarce. Only when an impenetrable stratum is met, does water appear on the surface.