HICKORY RUN BOULDER FIELD - AN ANCIENT, ROCKY CRITICAL ZONE

Boulder fields are found throughout the world; yet, the history of these features, which make up part of the critical zone in cold and formerly cold regions, as well as the processes that form them, remain poorly understood. Boulder fields are thought to form and be active during glaciations but outside the glacial limit; however, few fields have been dated and little data support this assertion of activity during cold periods. Here, we report on measurements of in-situ cosmogenic ¹⁰Be and ²⁶Al to quantify the near-surface history of 56 samples near and in the largest boulder field in North America, Hickory Run, in central Pennsylvania.

Boulder surface ¹⁰Be concentrations (n = 43) increase downfield and indicate minimum near surface histories of 70 to 600 ka; they are not correlated with boulder lithology or size. Measurements of samples from the top and bottom of one boulder and underlying clasts as well as ²⁶Al/¹⁰Be ratios suggest that some boulders have complex exposure histories the result of flipping and/or cover by other rocks, soil, or ice. Our new data demonstrate that Hickory Run, and other boulder fields, are dynamic features that resist erosion and persist on the landscape through multiple glacial-interglacial cycles. These enigmatic and scenic features of the critical zone likely reflect boulder resistance to weathering and erosion. Long and complex boulder histories suggest that climatic interpretations based on the presence of these rocky landforms are at best, oversimplifications.