Old courses of the New River; its late Cenozoic migration and bedrock control inferred from high-level stream gravels, southwestern Virginia

Reconstruction of late Cenozoic courses of the New River are made from the mapped distribution of high-level gravels, within an area in which the course of the New River changes from northeast to northwest across the Valley and Ridge Province. The distribution of these gravels suggests that the major cross-strike course change of the New River was established prior to the late Tertiary when the highest known gravels were deposited. Since then the river cut down more than 275 m. During the latest 125 m of downcutting, the New River has had only relatively minor changes in its course. Higher deposits, however, suggest at least two displacements of as much as 15 km in either its course or that of its major tributary, the Little River. The courses of the New River and its tributaries appear less susceptible to change when wholly floored by carbonate rocks between major water gaps. Greater susceptibility to change occurs when a variable-bedrock floor enhances unroofing of carbonate rocks whose solubility and erodibility promote both cross-fabric and strike-fabric course changes.Previously, the much greater maximum elevation of the New River deposits near Pearisburg, Virginia, relative to those near Radford, Virginia, was attributed to differential uplift of the western Valley and Ridge Province. Present evidence suggests that this difference in elevation results simply from the greater antiquity of the river's course in the Pearisburg area, across a wide expanse of carbonate rocks that were unroofed throughout much of the late Cenozoic, relative to its course near Radford where carbonate rocks were more recently unroofed and where older courses had existed farther east near Blacksburg. Application of a regional erosion rate of 40 mm/1,000 yr and incision rates of 55-100 mm/1,000 yr calculated for two other rivers in the eastern United States indicates an age of 3-7 Ma (late Pliocene-late Miocene) for the highest preserved New River gravels.