Configuration of the base of the upper permeable zone of the Tertiary limestone aquifer system, southeastern United States

The Tertiary limestone aquifer system of the southeastern United States is a thick sequence of carbonate rocks that vary in age and that are hydraulically connected in varying degrees. The aquifer system consists, in large part, of two major permeable zones separated by a less-permeable unit. A map is presented that shows the altitude and configuration of the base of the upper permeable zone of the system; and the age and lithology of the different low permeability materials that mark the base are delineated and briefly described. Several types of geological structures that affect the configuration of the base of the upper permeable zone may be readily recognized. (USGS)