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A 78˚W 76˚W B C
Hickory Run C
43˚N

42˚N D
F
Last Glacial Maximum (26 ka) 100 km E 250 m
Illinoian (130 ka?) E
Pre-Illinoian (>770 ka)

D F

Figure 1. Study site. (A) Hickory Run location in relation to the extent of the Last Glacial Maximum (LGM) (26 ka, Corbett et al., 2017b), Illinoian (130 ka?),
and pre-Illinoian glaciations, after Sevon and Braun (2000). Hickory Run is 2 km south of the LGM boundary and is mapped within the Illinoian and pre-
Illinoian glaciations. (B) Locations of photographs; (C) tors on a ridgeline 700 m NE of the field; (D) elongate, angular, large boulders upslope; (E) small,
rounded boulders downslope; and (F) massive, angular conglomeritic boulders in the SE sub-field.

mostly subrounded and underlain by small, Peters, 1967). Nuclides build up over time production rates before they were
polished clasts with a red weathering rind and can be used to provide age control for exhumed, when they were covered by
(Fig. 1E). There is a distinct subsection of surficial deposits; however, such dating other boulders, and/or when they flipped
the field to the southeast with boulders requires that at the time of initial surface during transport.
mostly >5 m long; these appear to be bed- exposure, rock contained few if any
rock shattered along bedding planes (Fig. nuclides (Lal, 1991). This is not the case for If rock surfaces experience burial before,
1F). The field is surrounded by coniferous boulder fields because both models of devel- during, or after exposure, by flipping or
forest with stony loam soils (NRCS, opment (see Introduction) include initial cover with soil, snow, ice, or other boulders,
2014). cosmic-ray exposure before incorporation of such complex histories can be detected by
blocks into the field (on cliffs or below a measuring two isotopes with different
Glacial erratics are found south of weathered regolith mantle). half-lives in the same sample (Bierman et
Hickory Run (Pazzaglia et al., 2006; al., 1999; Nishiizumi et al., 1991). Such
Sevon and Braun, 2000), indicating that it The pertinent question becomes, analyses most commonly employ 26Al and
was covered by ice at least once, although “Where were the sampled boulders when 10Be, which are produced in quartz at a
the timing of ice advances is not well they received the cosmic ray dosing that ratio of ~7:1 (Argento et al., 2013; Corbett
known (Braun, 2004), and we found no accounts for the 10Be and 26Al concentra- et al., 2017a). Because the 26Al half-life,
obvious erratics in the field. The last tions they contain today?” This question 0.71 m.y. (Nishiizumi et al., 1991), is about
glaciation to override Hickory Run is arises because there is no unique and half that of 10Be, 1.38 m.y. (Chmeleff et al.,
mapped as Illinoian (ca. 150 ka; Fig. 1A), agreed upon process model for boulder 2009; Korschinek et al., 2010), if an
though it is possible that it was 400 ka field development. If boulders were exposed sample is buried, the 26Al/10Be
(Braun, 2004). South of the boulder field, sourced from outcrops upslope of the field ratio will decrease; if that sample is re-
reversed magnetic polarity deposits indi- and moved downfield, they inherited exposed, production of nuclides begins
cate that the oldest, most extensive glacia- nuclides from exposure on the outcrops. If again and the ratio increases. Because of
tion was in the early Pleistocene (likely boulders originated in place, they inherited the relatively long half-lives of 26Al and
>900 ka); there is another event mapped nuclides from subsurface exposure. In 10Be, the 26Al/10Be ratio is only sensitive to
between the Illinoian event and the >900 either case, measured nuclide concentra- burial by meters of material for >100 k.y.
ka event, distinguished by proglacial lake tions do not allow direct dating of the time (Lal, 1991).
sediments of normal polarity, likely <740 any boulder became exposed as part of the
ka (Braun, 2004). boulder field; rather, they allow for the Published measurements of cosmogenic
calculation of minimum total near-surface nuclides, made on samples collected from
APPLICATION OF COSMOGENIC histories for each sampled boulder. Such rock surfaces in high-latitude boulder
NUCLIDES TO BOULDER FIELDS histories integrate cosmic-ray exposure fields, suggest that some blocks were
and express it as the equivalent of uninter- exposed to cosmic rays relatively recently,
Cosmogenic nuclides are produced pre- rupted surface exposure. These times are while others have concentrations consis-
dominately in the uppermost meters of minima because we know boulders eroded tent with near-surface histories extending
Earth’s surface by cosmic ray bombard- and also experienced less than surface over hundreds of thousands of years.
ment (Gosse and Phillips, 2001; Lal and For example, 36Cl concentrations in 18

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