The 1:100,000-scale 3-D surficial geology map of the Mad River and Little Darby Creek watersheds depicts the thickness and stratigraphic sequence of unconsolidated materials from the land surface down to and including the uppermost bedrock unit. Data for the map came mainly from county soil surveys, Ohio Department of Transportation and Ohio EPA geotechnical logs, water-well records and logs, and theses. Mapping was partially funded by an Ohio EPA Nonpoint-Source Pollution grant. The map area for the Mad River and Little Darby Creek watersheds covers 26 7.5-minute quadrangles and encompasses portions of 10 Ohio counties: Champaign, Clark, Delaware, Franklin, Greene, Logan, Madison, Miami, Montgomery, and Union. The map area is significant for both its surface geomorphic features and its deeply buried preglacial topography. Prominent geomorphic features present in the map area include: the Bellefontaine Upland; the Mad River Interlobate Plain; the London, Bloomingburg, Esboro, Glendon, Reesville, and Cuba Moraines of the Scioto Lobe of the Wisconsinan glacier; and the Farmersville and Springfield Moraines and boulder belt of the Miami Lobe of the Wisconsinan glacier. The Mad River Interlobate Plain formed by the coalescence of the Miami Lobe from the west and the Scioto Lobe from the east. The interlobate area also contains the Cable Moraine Complex, the Kennard Outwash Plain, the Siegenthaler-Kaestner Esker, and numerous fens, of which Cedar Bog is the most prominent. Beneath the mantle of mainly Wisconsinan glacial deposits lies the preglacial valley of the Teays River system. The Teays River Valley follows a southeast-to-northwest course through the map area, as delineated by well and geophysical data. This map is an invaluable tool to help water-well drillers provide better service and for planners, developers, engineers, emergency response agencies, and geologists to better understand and anticipate how human activities occurring on the surface can affect the unconsolidated subsurface environments.