HIGH TERRACE GRAVELS IN THE APPALACHIAN REGION AND IMPLICATIONS FOR THE STUDY OF CENOZOIC DRAINAGE EVOLUTION

High-level terraces provide the best evidence for the study of drainage evolution. Observation suggests, however, that for terraces in the Appalachians much higher than 50 m above modern river level (AMRL), little or no original surface remains; terraces have been transformed into rolling, irregular topography. The destruction of terrace topography means that confident recognition of former high-level terraces must depend upon the presence of alluvial gravels. The maximum height of terrace that can be recognized thus depends on how long these gravels can survive. Quartz clasts last the longest, but eventually even these are weathered away or eroded and removed from the hilltops. The thinness of original flood-plain deposits in the Appalachians contributes to the limited duration of high-level alluvial gravels. Where the long-term stream incision rate is known, gravel deposits can be approximately dated by dividing their heights by the incision rate. The maximum elevations AMRL at which such gravels occur thus set a limit on the antiquity of abandoned stream courses that can be reconstructed. A compilation of maximum gravel heights in the Appalachians shows that, except for the New River, where deposits reach elevations AMRL of 300 m, stream deposits in the Appalachians rarely exceed 150 m AMRL. Dividing the latter height by typical Appalachian stream incision rates of 20-60 m/m.y. indicates ages of 2.5 - 7.5 Ma. The 300-m-high deposits of the New River, which has an incision rate of 27 m/m.y., date to 11.1 Ma. It thus appears that alluvial deposits older than Late Miocene are unlikely in this region. Claims of gravel deposits of Mesozoic or early Cenozoic therefore lack credibility. Accounts of ancient gravels having been let down by erosion of the underlying bedrock cannot be disproved, but the best topographic evidence of antiquity, a high elevation AMRL, has been lost.