Preservation of Valley and Ridge topography via delivery of resistant, ridge-sourced boulders to hillslopes and channels, Southern Appalachian Mountains, U.S.A.

Abstract

Discrete bands of resistant strata found along ridgelines in lithologically controlled, geologically heterogeneous landscapes are generally credited with maintaining high topography in these settings. However, the mechanisms by which these “ridge-formers” prevent erosion and preserve topography are not well described. Here we propose a new way to conceptualize the landscape-scale geomorphic function of ridge-forming lithologies, not only as units whose intermittent resistant outcrops slow erosion of ridgelines themselves, but as producers of large, durable boulders that litter adjacent hillslopes and channels, extending the caprock's resistant qualities to weaker underlying units and inhibiting channel incision over laterally extensive portions of the landscape. This conceptual model is based on observations of boulder distributions in the slowly-eroding Valley and Ridge province of the south-central Appalachian Mountains, and is in agreement with recent numerical models exploring the role of large boulders in channel and hillslope evolution.Though boulders are widely observed in the Valley and Ridge, boulder distributions have not generally been mapped, nor has their presence been linked to topographic persistence. To examine the potential link between resistant, ridge-derived boulders and relief preservation in the Valley and Ridge, we use UAV imagery and field observations to create detailed maps of boulder distributions along representative hillslopes and channels in SW Virginia, and document boulder appearance, size, and strength to make inferences about longevity and geomorphic function. We find that boulders cover significant portions of hillslopes (up to 9%) and are concentrated in channels (up to 90%). Boulders are very large (average size >1 m and up to 6 m diameter), indicating fluvial immobility and channel armoring, and often trap sediment upslope, indicating hillslope shielding. Boulders are entirely sourced from durable ridgeline lithologies, and exhibit features indicating longevity (e.g., fracturing, extensive lichen coverage, rounding). We interpret boulders to be relict from Pleistocene periglacial conditions, when intense freeze-thaw cycles likely enhanced boulder production. Our results are consistent with processes described in numerical models of boulder function and suggest that ridge-sourced boulders in this region play an important role in topographic preservation by stabilizing hillslopes and channels, challenging the idea that ridge-forming units primarily maintain relief by stabilizing the ridgelines themselves. In addition, the linkage of boulder production to a colder Pleistocene climate highlights a mechanism by which past climatic conditions may continue to impact landscape evolution after the transition to a new climatic regime.