Field work on the Ordovician limestones and shales of the Lexington area, Kentucky, has shown that no appreciable quantity of ground water is transmitted through interstitial openings in these rocks. Ground-water movement is restricted to joint planes and, to a lesser extent, bedding planes that have been enlarged by solution. Studies of these planes indicate that solution has been effective only to a limited depth below the land surface. The bottom of the solutional zone is an exaggerated replica of the land surface, showing a greater depth of the solutional zone below stream valleys. The greater depth is attributed to the greater accumulation of ground water, the dissolving agent, in those areas. Modifications of the ideal condition are effected by the less soluble beds and the configuration of the land surface.The underground drainage systems of the area develop in a manner similar to rectilinear stream patterns, showing a definite alignment along the two joint sets of the area. Directional solution has taken place where one of these joint sets has been developed to the exclusion of the other.