Figure 1. A (above), Index map of the Eastern United States showing the location of the field trip (star) near Great Falls of the Potomac River. Regional features include the Fall Zone, separating the Piedmont and Coastal Plain provinces, and the Pleistocene glacial borders north of the Potomac River drainage basin. B (right), Location map of the Potomac River in the vicinity of Great Falls, showing the areas to be visited on Days 1 and 2 of this field trip.