This data set is a polygon coverage created in ARC/INFO that represents the saturated thickness of the Minnelusa aquifer, Black Hills area, South Dakota. The unconfined zone of the Minnelusa 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 Minnelusa aquifer was subtracted from the top of the Minnelusa Formation, and the boundary of the unconfined zone was created where the calculation results equalled zero. The saturated thickness of the aquifer in the zone was determined by subtracting the elevation of the top of the Madison Limestone from the potentiometric surface elevation of the Minnelusa aquifer. The potentiometric surface and the tops of the formation were 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.