Newly recognized en echelon fall lines in the Piedmont and Blue Ridge provinces of North Carolina and Virginia; with a discussion of their possible ages and origins

Abstract

zones along streams within Blue Ridge and Piedmont provinces of North Carolina and Virginia are distributed systematically. Nearly all occur along seven curvilinear trends (fall lines) that parallel regional tectonic fabric and gravitational gradient of Appalachian orogen. These seven fall lines are defined and named, and processes that could have formed them are discussed. The limited available evidence favors a neotectonic origin. The fall lines described here probably formed within last 2 million years, because late Pliocene fluvial terraces that accumulated along major Piedmont riverways appear to be warped in tandem with adjacent fall zones across at least two of these lines. This indicates that modern fall lines developed in latest Pliocene or Pleistocene time, after terrace deposits had accumulated. Introduction Although fall zones are an important source of water power in Piedmont of North Carolina and Virginia, they have been a serious impediment to navigation since earliest European exploration and settlement. Exploring upstream, colonial era ocean-going ships quickly found their way blocked by major rapids or falls along nearly every major coastal river. These rapids and falls define a prominent linear trace that has been called Fall Line or Fall Zone for over a hundred years (for example, Darton, 1894; Clark and others, 1912; Renner, 1927; Dietrich, 1970; Reed, 1981; Hack, 1982; Johnson and others, 1987). This fall line consistently has been located at or near western edge of Atlantic Coastal Plain physiographic province (Figure 1, labeled TFL). The origin of the Line has been ascribed to tectonic warping (for example, McGee and others, 1891; Hack, 1957; Mixon and Newell, 1977; Reed, 1981), though variation in world eustatic sea level in response to Quaternary glaciation also has played a significant role in shaping its present prominence as a geomorphic feature (Hack, 1957).