UPSTREAM TERRACES ON THE NEW RIVER IN NORTH CAROLINA: ILLUSIVE LANDFORMS WITH A CLIMATE HISTORY?

Hugh Mills conducted pioneering work on unpaired fluvial terraces along the New River in the Valley and Ridge province of SW Virginia. Terrace elevations above the modern river level (AMRL) were correlated with relative age and a hypothesis of stream incision driven by climate change was advanced. Recently, we have investigated terraces along the North and South Fork of the New River near the headwaters in the Blue Ridge province of North Carolina. Although low elevation terraces (T1,T2) are common, only 4 sets >T3 were identified. Three of these were identified on the South Fork at 87, 96, and 120 km and a fourth on the North Fork, 33 km downstream of the headwaters. Terrace elevations range from 41 m (T6) to 4 m AMRL (T1).

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.