SPATIAL IMPACTS OF GLACIOFLUVIAL AGGRADATION AND PERIGLACIAL COLLUVIATION ON LATE WISCONSIN TERRACE DEVELOPMENT ON UNGLACIATED RIVERS IN THE UPPER OHIO RIVER BASIN

Late Wisconsin outwash aggradation has been recognized for over a century as a dominant terrace forming agent along the Upper Ohio River and its glaciated tributaries. More recently, geologists have recognized that accelerated colluvial activity under periglacial conditions was a major contemporaneous sediment supply in unglaciated Appalachian portions of the Ohio River fluvial system. The late Quaternary regional chronology of terrace deposits warrants much more study; however, when augmented by terrace profile analysis, soil series correlation, sedimentology, and age-limiting archeological site dates, the record is adequate to assess the variation and relative spatial importance of the two sediment sources throughout the fluvial system.

Periglacial colluvial activity was very significant in high-elevation (>800 m) uplands, where poorly sorted alluvium and diamicton deposits in hybrid terrace-apron landforms tend to slope toward the valley center, rather than downstream. The areal extent of blocky colluvial aprons, slides, and fans far exceeds that of fluvial terraces in canyon reaches along streams such as the New, Gauley, and Cheat rivers. Although most appear to have been somewhat modified and incised during the Late Glacial and Holocene, these colluvial landforms commonly determine channel position within the canyon and the locations of major whitewater rapids.

However, the landscape signature of periglacial deposition diminishes downslope and downstream. Terraces along lower reaches of unglaciated rivers that flow into the Ohio River Valley are quite extensive and have exceptionally low gradients for the region. Alluvial soils and sediments in these extensive terraces are clay and silt rich, indicative of backwater ponding and slackwater conditions caused by outwash aggradation on the Ohio River. The surficial geology at these tributary river mouths indicates that the supply of sediment from periglacial colluvial sources was decidedly less than the supply of sediment from the Ohio River. Terrace profiles and widespread extents suggest that Late Wisconsin Ohio River aggradation governed terrace development for 100 or more km upstream from the mouth of large tributaries, such as the Kanawha and Monongahela rivers.