UPSTREAM TERRACES ON THE NEW RIVER IN NORTH CAROLINA: ILLUSIVE LANDFORMS WITH A CLIMATE HISTORY?
Far-travelled cobbles of quartzite and vein quartz were common on the surface of higher terraces. The alluvial stratigraphy of a terrace set from each fork was imaged with direct current electrical resistivity and ground-penetrating radar (GPR). Geophysical interpretations were confirmed by backhoe trenching. Each terrace consists of a bedrock bench that is broadly horizontal buried by up to 15 m of alluvium. On the T4, alluvium consists of gravel to cobble lenses capped by a fining-upward sequence to produce a gently sloping landform.
An OSL date of 24.2 ka + 0.1480 was measured on alluvial sand within the T4. T3 has 7.5 m of continuous, overlapping gravel that lacks fine-grained sediment. T2 is composed of 5 m of fine-grained alluvium with isolated medium to coarse-grained channel deposits. An archaeological excavation on T2 yielded buried artifacts from late Pleistocene (ca. 13,000 BP) through the late prehistoric Holocene (ca. 650 BP). Similar terrace stratigraphy was observed on the North Fork suggesting that these patterns are regional. The timing suggests that terraces formed in response to climate change that modified the sediment supply and stream hydraulics in the drainage basin. Glacial periods dominated by permafrost in the Blue Ridge produced coarse-grained braid plains that filled the valley as recorded on the T3 and dated on the T4. This regime was replaced by fine-grained sediment deposited by meandering streams as the New River downcut to the level of the T2. The lower terraces have been influenced by Holocene climate change as well as land use changes within tributary drainage basins.