SHALLOW GEOPHYSICAL SURVEYS OF FOUR TERRACES ALONG THE SOUTH FORK OF THE NEW RIVER IN THE BLUE RIDGE PHYSIOGRAPHIC PROVINCE

The alluvial stratigraphy of a set of stream terraces between 2 and 25 m above the South Fork of the New River immediately upstream from the confluence of Roan Creek in Ashe Country, NC was imaged with corresponding transects of direct current electrical resistivity and ground-penetrating radar (GPR). Each terrace consists of a bedrock bench that is broadly horizontal, buried by up to 15 m of alluvium. Far-travelled fluvial cobbles of quartzite and vein quartz indicate alluvium transported by the New River rather than a tributary stream. On the highest terrace (T4), quartz-rich bedrock (high resistivity) stands 8 m above differentially eroded amphibolite (low resistivity). Alluvium filled this irregular surface with gravel to cobble lenses capped by a fining-upward sequence to produce a gently sloping landform. T3 has 7.5 m of continuous, overlapping gravel that appears to lack fine-grained sediment deposited on eroded amphibolite. T2 is composed of 5 m of fine-grained alluvium with isolated medium to coarse-grained channel deposits. T1, 2 m above the modern river level appears to be the coarsest grained with 4 m of gravel above the amphibolite bedrock. Backhoe trenches on the T4 and T2 confirm the alluvial stratigraphy interpreted from the geophysical transects. An archaeological excavation on T2 yielded buried artifacts from the late Pleistocene (ca. 13,000 BP) through the late prehistoric Holocene (ca. 650 BP). The timing suggests that the terrace set formed in response to climate change that modified the sediment supply and stream hydraulics in the South Fork of the New River valley. Glacial periods dominated by permafrost in the Blue Ridge produced a coarse-grained braid plain as recorded on T3 that was then replaced by fine-grained sediment deposited by a meandering stream as the New River down-cut to the level of T2. T1 has been influenced by Holocene climate change as well as land use changes within the tributary drainage basin.