Published by the American Geophysical Union as part of the Field Trip Guidebooks Series, Volume 232. This field trip is an introduction to the hydrogeology of the Washington, D.C., area. The area contains two distinctly different physiographic provinces: the Piedmont and the Coastal Plain. The Fall Line (fig.1) separates the Piedmont on the west from the Coastal Plain on the east. The Piedmont rocks are composed of schists, gneiss, phyllite, quartzite, greenstone, and granite of late Proterozoic and early Paleozoic age. The Coastal Plain rocks are composed of unconsolidated sand, gravel and clay of Cretaceous, Tertiary and Quaternary age. Part of the Piedmont west of Washington, D.C., is underlain by downfaulted rocks of early Mesozoic age that contain sandstone, siltstone, shale, and limestone conglomerate that have been locally thermally metamorphosed by diabase intrusives in the Culpeper Mesozoic basin. The information presented in this guide is based on published information (see references).