The Sherwood Sandstone aquifer in the Vale of York is generally confined beneath thick clay drift in the west and by thin drift plus an increasing thickness of Mercia Mudstone towards the east. Published geological maps show a gentle easterly dipping succession cut by one major east-west fault passing through Stamford Bridge. The junction between the Sherwood Sandstone and the Mercia Mudstone is shown by a north-south trending line south of this fault. Recent boreholes for water supply have shown that the structure is more complex however, and reliable predictions of depths to the aquifer frequently cannot be made without pre-drilling surveys. Consequently the results of over 50 resistivity sounding have been used in conjunction with all available well logs to redraw the important hydrogeological boundary between the Sherwood Sandstone and the Mercia Mudstone across the central part of the Vale. The results have shown that depths to the aquifer, up to around 200 m, can generally be predicted prior to drilling to within 100y surface resistivity methods, except where the ‘Mudstone’ is very thin in the ‘feather-edge’ zone. The results have also shown the presence of many persistent east-west or northeast trending faults with vertical throws up to 80 m and some strike faults or monoclines in this part of the Vale. The significance of these faults to groundwater flow and water extraction are discussed. Wireline logs at six wells suggest the occurrence of at least two thick units of gypsum and anhydrite within the Mercia Mudstone. These cause high, 100-150 ohm m, depth averaged resistivity values to be computed for the Mercia Mudstone.