Central and Southern Appalachian water and wind gap origins: Review and new data

Abstract

The origin and evolution of transverse drainage were stumbling blocks to classical, historical geomorphologists, and many problems still remain. Décollements that upramp laterally-transported, at-depth, tectonic sheets in the central and southern Appalachians have produced structural geomorphic fronts that are prominent features, often of regional or subregional extent (cf. Blue Ridge, North Mountain, Allegheny). As these structural-geomorphic fronts likely were features that developed significant cover relief during the Alleghanian orogeny, they may hold clues to incipient landscape development during and soon after orogenesis, unless buried by thick sediment cover. Where these fronts are crossed by transverse drainage lines, spectacular water gaps that frequently expose complex structural features are produced. There are many other transverse water gaps not located across décollement structures in the region. These gaps display complex features that shatter bedrock, and must also be accounted for in any comprehensive hypothesis of transverse water gap origin. Lineaments, including both cross-strike structural discontinuities and disturbed zones are present in the Appalachians, and coincide with a number of transverse water gaps. The positioning of weakened bedrock zones high on structures where downcutting streams had access to them permits a localizing mechanism such as structural ensnarement to operate. Local sedimentary patterns may also have played roles in drainage localization. Major unsolved problems include: post-Alleghanian tectonic effects, rates and timings of uplift, erosion and deposition, whether lithotectonic weaknesses are most pronounced in gap areas, and establishing a numerical chronology for drainage development. Whereas lithotectonic influence upon drainage development is stressed in this chapter, the importance of other factors (e.g., climate, catastrophic events), as documented by others in this volume is fully recognized.