Alison R. Denn¹*, Paul R. Bierman¹, Susan R.H. Zimmerman², Marc W. Caffee³, Lee B. Corbett⁴, Eric Kirby⁵

1 Department of Geology, University of Vermont, Burlington, Vermont 05405, USA
2 Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California 94550, USA
3 Department of Physics and Astronomy, Purdue University, West Lafayette, Indiana 47907, USA, and Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, Indiana 47907, USA
4 Department of Geology, University of Vermont, Burlington, Vermont 05405, USA
5 College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA

Abstract

Boulder fields are found throughout the world; yet, the history of these features, as well as the processes that form them, remain poorly understood. In high and mid-latitudes, boulder fields are thought to form and be active during glacial periods; however, few quantitative data support this assertion. Here, we use in situ cosmogenic ¹⁰Be and ²⁶Al to quantify the near-surface history of 52 samples in and around the largest boulder field in North America, Hickory Run, in central Pennsylvania, USA.

Boulder surface ¹⁰Be concentrations (n = 43) increase downslope, indicate minimum near-surface histories of 70–600 k.y., and are not correlated with lithology or boulder size. Measurements of samples from the top and bottom of one boulder and three underlying clasts as well as ²⁶Al/¹⁰Be ratios (n = 25) suggest that at least some boulders have complex exposure histories caused by flipping and/or cover by other rocks, soil, or ice. Cosmogenic nuclide data demonstrate that Hickory Run, and likely other boulder fields, are dynamic features that persist through multiple glacial-interglacial cycles because of boulder resistance to weathering and erosion. Long and complex boulder histories suggest that climatic interpretations based on the presence of these rocky landforms are likely oversimplifications.

* Now at Pinnacle Potash International, Ltd., 111 Congress Ave, Suite 2020, Austin, Texas 78701, USA

Manuscript received 31 Mar. 2017. Revised manuscript received 21 Aug. 2017. Manuscript accepted 4 Oct. 2017. Posted online 20 Dec. 2017

© The Geological Society of America, 2017. CC-BY-NC.

10.1130/GSATG340A.1