Quaternary deposits and landscape evolution of the central Blue Ridge of Virginia

A catastrophic storm that struck the central Virginia Blue Ridge Mountains in June 1995 delivered over 775 mm (30.5 in) of rain in 16 h. The deluge triggered more than 1000 slope failures; and stream channels and debris fans were deeply incised, exposing the stratigraphy of earlier mass movement and fluvial deposits. The synthesis of data obtained from detailed pollen studies and 39 radiometrically dated surficial deposits in the Rapidan basin gives new insights into Quaternary climatic change and landscape evolution of the central Blue Ridge Mountains. The oldest depositional landforms in the study area are fluvial terraces. Their deposits have weathering characteristics similar to both early Pleistocene and late Tertiary terrace surfaces located near the Fall Zone of Virginia. Terraces of similar ages are also present in nearby basins and suggest regional incision of streams in the area since early Pleistocene–late Tertiary time. The oldest debris-flow deposits in the study area are much older than Wisconsinan glaciation as indicated by 2.5YR colors, thick argillic horizons, and fully disintegrated granitic cobbles. Radiocarbon dating indicates that debris flow activity since 25,000 YBP has recurred, on average, at least every 2500 years. The presence of stratified slope deposits, emplaced from 27,410 through 15,800 YBP, indicates hillslope stripping and reduced vegetation cover on upland slopes during the Wisconsinan glacial maximum. Regolith generated from mechanical weathering during the Pleistocene collected in low-order stream channels and was episodically delivered to the valley floor by debris flows. Debris fans prograded onto flood plains during the late Pleistocene but have been incised by Holocene stream entrenchment. The fan incision allows Holocene debris flows to largely bypass many of the higher elevation debris fan surfaces and deposit onto the topographically lower surfaces. These episodic, high-magnitude storm events are responsible for transporting approximately half of the sediment from high gradient, low-order drainage basins to debris fans and flood plains. D 2003 Elsevier Science B.V. All rights reserved.