The lifecycle of a relict periglacial boulder landscape, southern Appalachians, USA

The deposition of large, resistant boulders on hillslopes and in channels can have an armouring effect on the landscape leading to a decrease in erosion rates, a decrease in the efficiency of downslope sediment transport, and a coeval mismatched increase in slope angle. Such boulder accumulations are a significant component of hillslopes and channels in the southern Appalachian Mountains and influence the landscape's morphology. It has long been speculated that these boulder deposits originated during Quaternary glacial advances under the influence of periglacial processes operating in cold regions south of the maximum extent of the Laurentide Ice Sheet. However, no prior work has tied these features to a specific time or climatically modulated mechanism. By testing and refining the hypothesis of the periglacial origin of these relict boulders and the mechanisms driving their initial deposition and subsequent reworking we hope to contribute to our understanding of the climatically correlated timescales over which contemporary warming can be expected to be a dominating influence on modern boulder armoured periglacial alpine and arctic landscapes.

In this study, we investigated the lifecycle of such boulder deposits by determining cosmogenic ¹⁰Be exposure ages from large boulders on hillslopes and in channels in the Virginia Appalachians, United States. The correlation between the resulting boulder exposure ages (101.7 ± 6.9 ka to 10.8 ± 0.8 ka; n = 23) and the most recent Wisconsin Glacial Stage and subsequent deglaciation (~115 – 11.7 ka) supports their periglacial origin. The lack of exposure ages corresponding to the Last Interglacial Stage or following Wisconsin ice retreat suggests interglacial non-deposition and stability. The absence of exposure ages from the penultimate Illinoian or older Quaternary Glacial Stages suggests that periglacial hillslope processes allow the landscape to be resurfaced with large boulders during each return to cold climate conditions. This cyclic resurfacing of hillslopes and channels is an example of how climatic oscillations insert disequilibrium into the landscape cycle and contributes to our appreciation of the timescales over which climate change may impact boulder landscapes in modern periglacial environments.