This report describes basic data used to develop Geographic Information System data sets of bedrock geology, sinkholes and closed depressions, and spring and well locations attributed with hydrogeologic and water-quality data in the Pequea and Mill Creek watersheds, a 210square-mile area in Lancaster and Chester Counties, Pa. The data sets, which do not contain hydrogeologic interpretations, were developed by the use of ARC/INFO software during 1990-93 by the U.S. Geological Survey, in cooperation with the Pennsylvania Department of Environmental Resources. Associated data sets of groundwater levels, unsaturated-zone thickness, and regolith thickness incorporate hydrogeologic interpretations and are described along with associated basic data in a companion report. The U.S. Environmental Protection Agency proposes to use these noninterpretive and interpretive data sets, and those from other sources, to aid in the assessment of groundwater vulnerability to pesticides in the Pequea and Mill Creek watersheds. which this report is based. The U.S. Environmental Protection Agency provided incentive and overall coordination for the project. The Pennsylvania Department of Environmental Resources actively participated in the design of the project, furnished essential data, and, through the USGS Federal-State Cooperative Water-Resources Program, provided financial support for the project. Many well owners and several well drillers furnished essential information and permitted measurement of water levels and collection of water samples from wells.