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them as fractured outcrops, unmantled by
soil and regolith. In this framework, boul-
der field longevity is controlled by the
resistance of boulders to erosion over time.
Although most prior research suggests
that boulder fields result from periglacial
activity (Braun, 1989; Clark and Ciolkosz,
1988), extant cosmogenic data are largely
agnostic as to the timing of boulder gen-
eration. The absence of LGM histories
among the 52 Hickory Run samples we
analyzed could indicate a lack of new boul-
der generation during the most recent cold
period. Conversely, the absence of LGM
histories may reflect pre-exposure of boul-
ders, at depth if they are unroofed, or
upslope if they moved downslope from
source outcrops. Comparison of the cumu-
lative probability distribution of all boulder Observatory. M.W. Caffee was supported by
analyses (Fig. 6) to the marine oxygen
isotope record of climate shows no obvious
correlation of boulder histories with climate
except that the mode of boulder histories at
Hickory Run is generally consistent with
the Illinoian cold period (130–190 ka,
MIS 6). Either the complexity of boulder
histories (flipping, erosion, exhumation)
blur any coherent time signal in the data or
perhaps boulder field generation is not
strictly a periglacial phenomenon.
Hickory Run is mapped within the
Illinoian glacial margin (Sevon and
Braun, 2000) and, if mapping and dating
of the Illinoian are correct, would have
been under glacial ice ca. 150 ka (Fig. 1A).
The absence of erratics within the field
and the presence of boulders with mini-
mum histories far exceeding 150 k.y. sug-
gest that the “Illinoian” in this part of
Pennsylvania is likely older than previ-
ously assumed, a possibility given the
lack of quantitative age constraints on old
glaciations (Sevon and Braun, 2000).
Alternately, if the mapping were correct,
then any overriding Illinoian ice must
have been cold-based and non-erosive, as
the boulder field was preserved rather
than eroded. The preservation of block
streams under cold-based ice is possible
(Kleman and Borgström, 1990), and por-
tions of the southern Laurentide ice sheet
were likely cold-based (Colgan et al.,
2002; Bierman et al., 1999, 2015).
High concentrations of cosmogenic
nuclides in samples collected from Hickory
Run highlight the stability and persistence
of this landform, which has survived at
least one, and likely several, glacial/inter-
glacial cycles. Cosmogenic nuclide
measurements provide limited information
about the timing of boulder field activity
(insufficient to confirm it is a periglacial
feature), but clearly indicate that Hickory
Run and at least some other boulder fields
throughout the world are ancient, dynamic,
multigenerational features, the longevity
of which appears to be controlled by the
resistance of their boulders to erosion.
ACKNOWLEDGMENTS
We thank N. West and M. Bruno for field
assistance. We also thank our anonymous
reviewers, who greatly improved the quality of
this manuscript, as well as Noel Potter for his
input on this mysterious feature! This research
was supported by NSF-EAR1331726 (S. Brantley)
for the Susquehanna Shale Hills Critical Zone
NSF-EAR1560658.
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