An imaginary line or narrow zone connecting the waterfalls on several adjacent nearparallel rivers, marking the points where these rivers make a sudden descent from an upland to a lowland, as at the edge of a plateau; specif. the fall line marking the 1151 boundary between the ancient, resistant crystalline rocks of the Piedmont Plateau and the younger, softer sediments of the Atlantic Coastal Plain in the Eastern United States. It also marks the limit of navigability of the rivers.