Limestones and other carbonate rocks which underlie approximately 50 per cent of the Tennessee Valley are extensively dissolved. Solution has been in progress a long, but undeterminable, time geologically. Caves, sinks, and other manifestations of bedrock solution abound throughout the region. Deep core borings made in the subsurface exploration of dam sites and large open cuts made at dam construction projects reveal that channels and other solutional openings, some of which are of cavernous dimensions, are present to depths of hundreds of feet below the present water table. In the development of such cavities, geologic structure is of greater importance than the chemical purity of the limestone.