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Cosmogenic nuclides indicate that boulder fields
are dynamic, ancient, multigenerational features

Alison R. Denn*, Paul R. Bierman, Department of Geology, University of Vermont, Burlington, Vermont 05405, USA; Susan R.H.
Zimmerman, Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California 94550,
USA; Marc W. Caffee, 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; Lee B. Corbett,
Department of Geology, University of Vermont, Burlington, Vermont 05405, USA; and Eric Kirby, College of Earth, Ocean and
Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA

ABSTRACT cold climate periods (Clark and Ciolkosz, erosion, accumulation of unconsolidated
1988) by frost action and mass wasting soil/regolith, and perhaps by periglacial
Boulder fields are found throughout the (periglaciation). These features, particu- action or glaciation during cold periods
world; yet, the history of these features, larly unvegetated boulder fields, boulder (André et al., 2008).
as well as the processes that form them, streams, and talus slopes (areas of broken
remain poorly understood. In high and rock distinguished by differences in mor- Here, we report 52 measurements of
mid-latitudes, boulder fields are thought phology and gradient [Wilson et al., 10Be and 25 measurements of 26Al in boul-
to form and be active during glacial peri- 2016]), are believed to be largely ders and outcrops in and near the Hickory
ods; however, few quantitative data sup- inactive today (Braun, 1989; Clark and Run boulder field. Data show that boulders
port this assertion. Here, we use in situ Ciolkosz, 1988). in the field have moved over time and can
cosmogenic 10Be and 26Al to quantify the have cosmogenic nuclide concentrations
near-surface history of 52 samples in and Boulder fields have been documented equivalent to at least 600 k.y. of near-sur-
around the largest boulder field in North throughout the world, including Australia face history. We conclude that boulder
America, Hickory Run, in central (Barrows et al., 2004), Norway (Wilson et fields survive multiple glacial-interglacial
Pennsylvania, USA. al., 2016), South Africa (Boelhouwers et cycles, calling into question their utility as
al., 2002), the Falkland Islands (Wilson et climatic indicators.
Boulder surface 10Be concentrations al., 2008), Italy (Firpo et al., 2006), Sweden
(n = 43) increase downslope, indicate (Goodfellow et al., 2014), and South GEOLOGIC AND PHYSIOGRAPHIC
minimum near-surface histories of Korea (Seong and Kim, 2003). Hundreds SETTING
70–600 k.y., and are not correlated with of such fields exist in eastern North
lithology or boulder size. Measurements America (Nelson et al., 2007; Potter and Hickory Run boulder field is ~2 km south
of samples from the top and bottom of Moss, 1968; Psilovikos and Van Houten, of the Last Glacial Maximum (LGM)
one boulder and three underlying clasts as 1982; Smith, 1953); however, both the Laurentide Ice Sheet boundary (Pazzaglia
well as 26Al/10Be ratios (n = 25) suggest time scale and mechanism of boulder et al., 2006; Sevon and Braun, 2000) in
that at least some boulders have complex field formation remain poorly understood east-central Pennsylvania, USA (Fig. 1A),
exposure histories caused by flipping because few quantitative data constrain a temperate, forested, inland region of the
and/or cover by other rocks, soil, or ice. the age of boulder field formation or Atlantic passive margin. The field sits on a
Cosmogenic nuclide data demonstrate evolution. low-relief upland surface underlain by
that Hickory Run, and likely other boul- gently folded, resistant Paleozoic sandstones
der fields, are dynamic features that per- Boulder field formation is usually and conglomerates.
sist through multiple glacial-interglacial explained by one of two process models,
cycles because of boulder resistance to both of which invoke periglaciation as a The field is an elongate, 550- by
weathering and erosion. Long and com- catalyst for boulder generation and trans- 150-m-wide, nearly flat (1°) expanse of
plex boulder histories suggest that cli- port (Rea, 2013; Wilson, 2013): (1) boulders boulders in the axis of a small valley
matic interpretations based on the pres- fall from a bedrock outcrop upslope of the (Fig. 1) with ~30 m of relief (Smith, 1953).
ence of these rocky landforms are likely field and are transported downslope by Boulders in the field range from <1 to >10
oversimplifications. ice-catalyzed heaving and sliding (Smith, m long and are hard, gray-red, medium-
1953); or (2) boulders form as corestones grained sandstone and conglomeratic
INTRODUCTION underground, are unearthed by the pro- sandstone from the Catskill formation
gressive removal of surrounding saprolite, (Sevon, 1975), as are the adjacent ridgelines.
Areas outside the maximum extent of and are later reworked (André et al., 2008). Upslope boulders at the northeast end of
Pleistocene glaciation contain landforms However they form, boulder fields are likely the field (Fig. 1D) are generally more
thought to have been produced during altered over time by in situ rock weathering, angular than those downslope to the south-
west (Fig. 1E) (Wedo, 2013), which are

GSA Today, v. 28, doi: 10.1130/GSATG340A.1. Copyright 2017, The Geological Society of America. CC-BY-NC.
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4 GSA Today | March-April 2018