This data set is a polygon coverage created in ARC/INFO that represents the saturated thickness of the Madison aquifer, which includes the entire thickness of the Madison Limestone and Englewood Formation in the Black Hills area, South Dakota. The unconfined zone of the Madison aquifer is the area where the potentiometric surface of the aquifer is below the top of the formation. The elevation of the top of the potentiometric surface of the Madison aquifer was subtracted from the top of the Madison Limestone, and the boundary of the unconfined zone was created where the calculation results equalled zero. The saturated thickness of the aquifer in the unconfined zone was determined by subtracting the elevation of the top of the Deadwood Formation from the potentiometric surface elevation of the Madison aquifer. The potentiometric surface and the tops of the formations where determined from water levels and geologic logs from selected wells drilled as of 1998. The thickness coverage contains polygons that have single integer values representing a range of thickness.