ruck 1 properties and mechanisms of transport of …
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PROPERTIES AND MECHANISMS OF TRANSPORT OF COLLUVIAL SEDIMENT IN
RELICT LOBATE LANDFORMS ON HILLSLOPES SOUTH OF THE LAST GLACIAL
MAXIMUM ICE MARGIN, PENNSYLVANIA, AND POSSIBLE ASSOCIATIONS WITH
LATE PLEISTOCENE PERMAFROST
John Gregory Ruck, ‘20
Advisor: Dr. Dorothy J. Merritts
Committee: Dr. Robert Walter, Dr. Timothy Bechtel, Dr. Zeshan Ismat
ENE 490
May 2020
An honors thesis submitted to the Department of Earth and Environment at Franklin and
Marshall College in conformity with necessary requirements
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Table of Contents
COVID-19 Impact …………………………………………………………….…...……………. 4
Abstract ………………………………………………………………………………………….. 5
Acknowledgements ……………………………………………………………………………… 6
Introduction …………………………………………………………………………………….... 7
Background ………………..………………..…………………………………………………… 9
Study Area ……………………………………………...……………………………………… 21
Methods ………………………………………………………………………………………… 26
Topographic Analysis and Field Area Surveying …………………………………….... 28
Sample Collection …..…………..……………………………………………………… 29
Grain Size and Angularity ……………...……………………………………………… 30
Drone Photogrammetry ………………………………………………………………… 31
Cosmogenic Laboratory Sample Preparation ………………………………………….. 32
Cosmogenic Nuclide Sample Analyses ………………………………………...……… 33
GIS Grain Size Distribution Analysis: Point Counts ……………………..……………. 33
GIS for Grain Size Distribution Analysis: Grain Covers ….....……………..………….. 34
Results ……………………………………………………………………….…………………. 35
Grain Size and Angularity Analysis for Samples from ATT Road: Site 1………..……. 35
Using GIS for Grain Size Distribution Analysis-Point Counts ………………..………. 38
ATT Road: Site 1 ………………………………………………………………. 38
ATT Road: Site 3 ………………………………………………………………. 41
Using GIS for Grain Cover Distributions ……………...……………………….....…… 43
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ATT Road: Site 1 ………………………………………………………….…… 43
ATT Road: Site 3 ………………………………………………………………. 45
Cosmogenic Isotope Analysis ………………………………………………………….. 49
Discussion ………………………………………………………………………………...……. 51
Conclusion …………………………………………………………………………...………… 62
References …………………………………………………………………………………….... 64
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COVID-19 Impact
As described in this thesis, the majority of time for this one-year independent study was
used to acquire data from controlled experimentation and modelling of gelifluction processes
occurred in a laboratory on Franklin and Marshall College’s campus in 2019-2020. As
COVID-19 spread globally in late 2019 to early 2020, especially throughout the United States,
the administration of Franklin and Marshall College and the government of the State of
Pennsylvania issued restrictions to student access of academic buildings and laboratories on
campus. Due to these stringent limitations, the scope of my thesis, originally focused on
modelling gelifluction through a series of freeze-thaw cycles in a freezer, was changed
approximately two months before the end of the Spring semester. I adjusted the project goals to
focus on mapping grain size distributions of outcrops of periglacial sediment at two field sites,
and evaluating the sedimentary fabrics and spatial relationships of clasts in these outcrops in
order to evaluate the processes that formed the deposits.
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Abstract
Relict lobate landforms and benches of poorly sorted colluvium are ubiquitous
throughout unglaciated central and southern Pennsylvanian, yet the timing and processes
associated with their formation are not entirely understood. Similar features known as
gelifluction lobes are common in modern cold regions with permafrost, and form during
permafrost thaw as a result of slow downslope movement of water-saturated soil or colluvium
above a seasonally or perennially frozen substrate. Relict lobes preserved south of the Last
Glacial Maximum (LGM) ice margin in Pennsylvania might be indicators of past permafrost
conditions. This study characterizes colluvial sediment within relict periglacial lobes in
Pennsylvania, using cosmogenic nuclides for age control and both sieving and Geographic
Information Systems (ArcGIS) for grain size analysis. The primary objectives are to identify
sediment transport mechanisms that were active on hillslopes during the LGM and
Pleistocene-Holocene transition (PHT), and to determine if they might have been associated with
permafrost conditions. The sedimentary fabrics of colluvium within relict periglacial lobes at a
study site 16 km south of the LGM ice margin in eastern Pennsylvania change from
clast-supported to matrix-supported in a downslope direction, with increasing distance from the
probable bedrock source area of boulders within the sediment. Maps of grain (i.e., clast) cover
from drone photogrammetry indicate that colluvium becomes finer-grained and more stratified
downslope. In situ cosmogenic 10Be concentration data for multiple samples from depths of ~1
to 5.4 m near the terminus of one relict lobe are consistent with near-surface exposure during the
last glacial cycle. They are also consistent with rapid erosion and deposition, and with minimal
reworking of sediment since it was deposited. It is concluded that the relict, lobate features
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studied here are likely gelifluction lobes that were active during the LGM and possibly the PHT,
and were produced by freezing and thawing associated with regional permafrost.
Acknowledgements
This research was performed as part of a regional, multi-year effort by Dr. Dorothy
Merritts, numerous Franklin and Marshall College students, and other collaborators, to evaluate
the impact of cold-climate conditions, particularly those associated with permafrost, on
landscapes in the mid-Atlantic US. This work is the first of that regional effort to apply
cosmogenic nuclide analysis in evaluating the age of periglacial sediment. Cosmogenic nuclide
analysis constrains the near surface exposure histories of rocks and sediments based on the
accumulation of cosmogenic nuclides produced by cosmic ray bombardment in the uppermost
few meters of Earth’s surface (Lal, 1991). The director of the NSF-funded University of
Vermont (UVM) Community Cosmogenic Facility (CCF), Dr. Paul Bierman, and facility
manager Dr. Lee Corbett, collaborated on this aspect of the work and assisted first in the
sampling protocol, and then by guiding me and another student from Franklin and Marshall
College, Nic Hertzler, to extract silica and cosmogenic nuclide aliquots at the NSF/UVM
Facility. Dr. Merritts’ unparalleled guidance and support throughout the course of this study is
greatly appreciated, and I am truly privileged to have experienced her novel and innovative
perspectives. Dr. Robert Walter’s expertise in radionuclide geochemistry also was helpful for
this part of the research. The guidance and input provided by Dr. Douglas Jerolmack (University
of Pennsylvania), Dr. Frank Pazzaglia (Lehigh University), Dr. Jill Marshall (University of
Arkansas), and Joanmarie Del Vecchio (graduate student, Pennsylvania State University) on
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periglacial landforms, gelifluction mechanics, and mass movement on hillslopes was incredibly
valued and appreciated. Julia Carr’s (Pennsylvania State University) methods of using ArcGIS
for grain size data collection have been integral to the success of this study. The editing
expertise and guidance of Jim Gerhart (USGS, retired) have been influential in writing and
editing this thesis. I am grateful to Mr. Ron Gilbert for his permission to work on land that he
owns along the newly excavated road built for an ATT cell tower on Chestnut Ridge; his
kindness is greatly appreciated. Craig Robertson’s and Jane Woodward’s generosity and
donation to the Moss Ritter fund to support field work and cosmogenic analysis was essential,
and without it this research could not have happened. I would furthermore like to thank all of
those who have donated and supported the Hackman Fund at Franklin and Marshall College, as
well as Dr. Robert Walter, Dr. Timothy Bechtel, and Dr. Zeshan Ismat (all Franklin and Marshall
College) for agreeing to be on my thesis committee. Without their benevolence and passion for
the geosciences, I would have not been provided the opportunity to perform research as a
student-scholar with leading researchers in periglacial processes, for which I am incredibly
grateful.
Introduction
Periglacial processes and gelifluction, the slow downslope movement of water-saturated
soil or colluvium above a seasonally or perennially frozen substrate, are of critical importance in
understanding the response of landscapes in cold regions to modern global warming. Periglacial
processes occur where the ground is frozen seasonally or year-round, but not covered by glacial
ice. Distinctive lobate and terrace-like landforms produced on hillslopes by gelifluction, called
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gelifluction lobes, are common in both formerly and modern periglacial landscapes (Fig. 1;
Benedict, 1976; Johnsson et al, 2012).
Average modern global temperatures are increasing at an unprecedented rate, with
greatest rates of increase at higher altitudes and latitudes where glacial and periglacial processes
and landscapes are predominant. As frozen ground thaws, saturated soil and boulders on
hillslopes can become unstable, moving downslope via different types of mass movement
processes and subsequently altering the morphology of landscapes (Gooseff et al, 2009). These
areas can pose significant risks to inhabitants, as land sinks, cracks, and drains, becoming a
weak, loosely consolidated mush. Thawing of frozen ground in Arctic coastal villages, for
example, has eroded shorelines and streambanks, undermining schools, homes, and pipelines
necessary for water and waste transport.
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Two primary purposes of this study are to characterize colluvial sediment in slope
stratified deposits within relict periglacial lobes south of the last glacial maximum (LGM)
Laurentide ice sheet margin in Pennsylvania, and to assess the possibility that this colluvium was
transported by mass movement in association with permafrost thaw, possibly during the
Pleistocene Holocene transition (PHT) circa 16,000 to 11,650 yrs BP. By mapping and
characterizing periglacial sediments, and determining the processes associated with their
deposition, this research has implications for modern landscapes that are responding to warming
and thawing of frozen ground.
An hypothesis evaluated here is that LGM conditions were sufficiently cold for intense
frost cracking to produce loose sediment that became bound in continuous permafrost south of
the LGM ice margin, and that this sediment subsequently was transported downslope via mass
movement over a relatively impermeable frost table during permafrost thaw. Prior work by Dr.
Merritts and her students has shown that intense frost cracking produced ubiquitous thermal
contraction polygons in shale bedrock south of the LGM ice margin in Pennsylvania, and the
sandy infill within polygonal cracks has been dated to the beginning of the PHT (Merritts et al,
2015, 2017; Gross et al, 2017). These polygons are diagnostic indicators of continuous
permafrost in modern cold regions. This study seeks to determine if colluvial lobes in
Pennsylvania are also an indicator of continuous permafrost.
Background
Earth’s warming climate is posing notable threats to permafrost landscapes. Permafrost
is ground composed of rock, sediments, and ice that form a cohesive soil aggregate that remains
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frozen for two or more consecutive years. Permafrost is composed of multiple different layers,
including the uppermost active layer, which changes seasonally by freezing during the winter
and thawing during the summer, permafrost, which is frozen for at least two years, and talik,
which is unfrozen ground beneath permafrost. Increased ground temperatures and accelerated
permafrost thaw have been documented at many locations in the Northern Hemisphere, Alaska,
Siberia, Canada, and Greenland (Liu et al. 2010). This phenomenon is an anomaly with respect
to Earth’s climatic history. In the Arctic region, for example, permafrost temperatures on
Alaska’s North Slope permafrost reached 11 degrees Fahrenheit in 30 years (Rozell, 2019).
Permafrost is sensitive to changes in atmospheric temperature, and general circulation models
(GCMs) suggest that warming, and as a consequence, permafrost thaw will be greater in Arctic
regions and more pronounced at higher altitudes (Smith, 2004). As permafrost landscapes
continue to warm and thaw, enhanced levels of near surface permafrost degradation will threaten
the stability of hillslopes, induce rapid soil movement, and change the dynamics of surrounding
landscape processes (Haeberli and Burn, 2002).
The commencement of glacial cycles in the Northern Hemisphere occurred
approximately 2.8 million years ago, marking the beginning of the Pleistocene Epoch (Raymo,
1994). Induced by climate cooling, glacial cycles were characterized by glacial ice sheet
expansion and retreat, and have been linked to variations in the Earth’s orbital parameters (Hays
et al. 1976). In North America, the Laurentide ice sheet reached mid-latitudes multiple times,
extending into what is now northern Pennsylvania (Fig. 2). The Laurentide ice sheet scoured
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the underlying landscape and bedrock terrains, producing massive amounts of glacial till, silt,
and deep glacial lakes. The two most recent cold periods with associated glacial advances in
eastern North America are referred to as the Illinoian, ~191 to 130 ka, and Wisconsinan, ~70 to
11.6 ka (Braun, 2006a). Numerous glacial episodes occurred prior to these, as evidenced by
glacial deposits, however their ages are poorly constrained. Repeated glacial advances produced
features in the landscape that have since been masked or removed by later glacial advances.
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Glacial deposits from these advances indicate that ice sheets advanced farther into the eastern
and western parts of what is now Pennsylvania, perhaps controlled by topographic features such
as the Valley and Ridge (Fig. 2). In fact, part of north-central Pennsylvania was never glaciated.
During the Wisconsinan LGM, roughly 20,000 years ago, much of Earth in the northern
hemisphere was covered in ice (Ullman, 2016). After the LGM, global warming led to shrinkage
of the Laurentide ice sheet during a period known as the Pleistocene and Holocene deglacial
transition (PHT) that began approximately 15 ka (Alley, 2000; Zielinski & Mershon, 1997). The
Holocene epoch, modern warm, interglacial cycle, officially began 11,650 years ago. 10Be
chronology data indicate that complete Laurentide ice sheet deglaciation occurred approximately
6,700 years ago (Ullman et. al, 2015).
Near the northern border of Pennsylvania, Late Wisconsinan ice was present for
approximately 9,000 years, but existed for only 2,000 –3,000 years near the glacial terminus to
the south (Braun, 2006b). South of full glacial ice margins in what is now Pennsylvania,
periglacial landscapes were affected by a variety of cold-climate processes. Paleobotanical
evidence indicates that the LGM periglacial climate was cooler than present contemporary boreal
regions, as alpine tundra occupied the higher Appalachian summits approximately 15,000-18,000
years ago (Jackson et al., 1997). Landscape response to periglacial climatic conditions depended
on hillslope orientation (e.g., south-facing slopes get more solar energy than north-facing), depth
of frost penetration, local lithology, physical properties of bedrock such as joints and bedding,
and many other variables. Permafrost probably existed, but details of spatial distribution,
thickness, and other attributes are poorly known.
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Figure 3 indicates that the extent of possible LGM permafrost, referred to as a
“speculative limit,” was notably wide in the eastern U.S., possibly the result of relatively less
snow and hence deeper frost penetration (French & Millar, 2014). This map of LGM periglacial
features shows different types of patterned ground, including frost-cracked polygons, rock
streams, and rubble sheets within the zone inferred to have had some type of permafrost (i.e.,
continuous, discontinuous, or sporadic).
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The evidence of relict thermal contraction polygons with ice- and sand-wedge ‘casts’ in
the eastern United States remains the most important diagnostic criterion for the former existence
of continuous permafrost. Thermal-contraction of rocks and sediment, and the formation of ice
(in moist conditions) or sand (in dry conditions) wedges in the resultant cracks, occurs today in
both continuous and discontinuous permafrost regions, but is far more common where climates
are coldest and continuous permafrost exists (Mackay 1974; Burn 1990; Mackay & Burn 2002).
Previous studies of modern permafrost and ground-cracking indicate that continuous permafrost
exists at a mean annual temperature of at most approximately -6ºC, and the temperature of the
ground at the time of cracking of between -13 and -20ºC. Relicts of thermal contraction
polygons in Pennsylvania are diagnostic evidence for the existence of permafrost (Gardner et al,
1991; French & Millar, 2014; Merritts et al, 2015, 2017; Gross, 2017; Gross et al, 2017).
Cold-climate processes cause extensive mechanical weathering of rock, particularly by
frost-cracking, and the movement of this loose sediment on hillslopes results in a variety of
periglacial landforms (Hales et al, 2007; French and Millar, 2014; Marshall et al, 2017). The
most ubiquitous feature of periglacial landforms south of the LGM ice margin in Pennsylvania is
colluvium, unconsolidated sediment on hillslopes commonly attributed to frost-shattering and
other cold-climate processes (Denny, 1951; Hack, 1965; Ciolkosz et al, 1986; 1990; Braun,
1989; Gardner et al, 1991; Pazzaglia & Cleaves, 1998; Eaton et al, 2003; Newell and DeJong,
2011). The most common and widespread periglacial landforms on hillslopes in Pennsylvania
are lobate and bench-like features comprised of slope-stratified colluvium (shown as
“solifluction deposits and other colluvium,” in Fig 3). Given that other geomorphic features,
particularly thermal contraction polygons, indicate that permafrost existed from the LGM to as
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far south as at least the Maryland border, it is possible that some of these lobes were produced by
gelifluction. Whereas solifluction is the process of slow downslope movement of
water-saturated mass on a hillslope, gelifluction is a type of solifluction that is controlled by
alternate freezing and thawing (Washburn, 1980; Ballantyne & Harris, 1994; French, 1996;
Matsuoka, 2001). Frozen ground limits drainage of water during thaw and ice melt, leading to
higher pore pressures and hence lower soil strength in the ground that has thawed, typically the
active layer atop the permafrost table. As a consequence, gelifluction is common in areas of
modern permafrost thaw (Gooseff et al, 2009; Johnsson et al, 2012). Both solifluction and
gelifluction, however, can produce step-like topography with lobes. Lobes and steps on colluvial
hillslopes are thought to result from variable rates of downslope motion of colluvium, sometimes
resulting in the overriding of near-surface sediments above older deposits (Benedict, 1976).
Active and relict periglacial lobes south of glacial ice margins in North America and
elsewhere (e.g. northern Europe) have been studied by other researchers, but outcrops of the
internal stratigraphy of colluvium on hillslopes are rare and most studies do not describe the
composition and internal sedimentary fabric of periglacial lobes (see for example, Johnsson et al,
2012; Del Vecchio et al, 2018). One reason is that outcrops of the internal stratigraphy of
colluvium on hillslopes are relatively rare. In fact, the few studies that describe subsurface
stratigraphy relied upon digging into hillslope colluvium (Benedict, 1976; Pazzaglia & Cleaves,
1998), or on erosion of relict Pleistocene toe of slope colluvium along the edges of modern
streams (Smoot, 2004; Eaton et al, 2003). Furthermore, the few studies of slope stratified
colluvium that describe internal sedimentary structure typically do not link the sediments to the
lobate landforms in which they possibly occur. The reason for this disconnect might be that it
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generally is difficult to identify relict lobate features in tree-covered terrain without LiDAR data,
which can be used to filter vegetation in order to generate bare earth topographic data and reveal
lobes beneath tree covers (Merritts et al, 2014, 2015). Bare-earth digital elevation models from
LiDAR data, particularly when used to generate slopeshades that emphasize changes in hillslope
gradient, reveal that relict lobes are ubiquitous on Pennsylvania hillslopes, as discussed below.
An example of a detailed analysis of the stratigraphy of three slope stratified deposits is
the work of J. Smoot, who described poorly sorted, matrix-supported fabrics within offlapping
wedges of Pleistocene colluvium in the Blue Ridge Mountains, northern Virginia (Smoot, 2004).
Smoot interpreted each wedge as representing a distinct deposit, with higher (younger) wedges
prograding out over those that are lower (older) in the stratigraphic section. At one of these sites,
named Hoover Camp, erosion into the toe of slope by the Rapidan River revealed 5 m of slope
stratified colluvium with sediment ranging from clay to boulder size (Fig. 4). Smoot (2004)
identified several sedimentary features, such as boulder-cobble clusters, matrix-rich pebble and
cobble layers, and variable packaging of sediments that are consistent with mass movement by
solifluction processes.
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Smoot (2004) observed that elongate clasts of pebble- to larger-sized particles are
oriented parallel to wedge boundaries except at the toes of the wedges, where they are nearly
vertical. He suggested that isolated, vertical boulders might represent former ploughing blocks
that moved slowly compared to finer-grained materials during thawing of ground ice (Smoot,
2004). Although he interpreted these sediments as the result of transport and deposition during
alternate freezing and thawing of ground ice, he also noted that some fine layers within outcrop
were the result of intermittent sheetwash between thaw events (Smoot, 2004).
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At other nearby sites in the Blue Ridge Mountains of Virginia, Eaton et al (2003) mapped
and described similar exposures of stratified slope deposits that are also up to many meters thick.
Both Smoot (2004) and Eaton et al (2003) note that the deposits they described might have
formed in association with permafrost and be part of solifluction deposits, but neither clearly
linked the sediments at their sites to specific lobate landforms.
Gelifluction is a type of solifluction process that describes generally slow mass
movement controlled by alternative freezing and thawing (Washburn, 1979; Matsuoka, 2001).
Frozen ground limits drainage of water during thaw and ice melt, leading to higher pore
pressures and hence lower soil strength. Both solifluction and gelifluction can produce step-like
topography with lobes (see Fig. 1). The lobes are thought to result from variable rates of
downslope motion, and even the overriding of near-surface sediments above older deposits (e.g.,
Benedict, 1976).
Landforms and sedimentary deposits resulting from gelifluction are strongly dependent
on climatic conditions and can be used as indicators of paleoclimatic conditions (Matsuoka,
2001). Gelifluction lobes associated with frozen ground are found where there are certain types
of moisture conditions and temperatures, influenced heavily by climate. (Matsuoka, 2001; Rapp,
1960; Smith, 1992). During seasonal freezing, high moisture availability from rain or meltwater
enhances seasonal frost heaving, raising the moisture content of the thawed layer and promoting
gelifluction during thawing periods (Matsuoka, 2001). Alternatively, low moisture availability
minimizes the likelihood of mass movement due to the intrinsic lower viscosity in the active
layer (Matsuoka, 2001). The presence of permafrost may encourage gelifluction by improving
moisture availability during seasonal thawing, contributing moisture content with the potential to
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increase thaw action (Matsuoka, 2001). Soils favorable for these conditions and processes are
sandy to silty soils having low liquid limits, where raised moisture content may induce soil
deformation at a pre-failure stress level resulting in slow downslope displacement of the soil
mass (Harris, 1989).
Other studies have found that gelifluction is induced when seasonal thawing causes a
plastic soil layer to deform downslope resulting primarily from frictional flow or creep (Harris et
al., 1997). A decrease in the effective strength of a particular soil unit due to ice segregation
creates a physical separation of soil particles during freezing and high thaw-consolidation ratios
that cause excess porewater pressures, allowing elasto-plastic deformation (Smith, 2004). Rates
of downslope movement are contingent on environmental and physical controls on the hillslope,
such as soil moisture, vegetation cover, slope gradient, grain size, etc.
Gelifluction lobes typically transition into benches further downslope as they coalesce.
This merging seems to be a function of decreasing slope gradient (Matsuoka, 2001). Both
landforms can originate from the overturning of superficial soil due to reduced velocity, and thus
develop most extensively where gradient decreases downslope (Matsuoka, 2001). The presence
of frost-shattered boulders and ploughing blocks that might be frozen at greater depths, below
the active layer for example, can also inhibit the overall velocity of the mass movement, and
create characteristic convex topographic profiles of the surfaces of lobes and benches (Smith,
2004).
Surface exposure dating using cosmogenic nuclides and isotopic analysis is a powerful
tool for constraining the near surface histories of periglacial colluvium and landforms.
Cosmogenic nuclides accumulate with time in minerals exposed to cosmic rays as the earth is
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bombarded with protons and alpha particles (Ivy-Ochs, Kober 2008). Measuring the
concentrations of certain cosmogenic nuclides that accumulate in minerals allows determination
of how long rocks or sediment have been exposed at or near the surface of the Earth (Lal 1991;
Gosse & Phillips 2001). In order to accurately constrain both burial and exposure ages for clast
and matrix samples, 10Be and 26Al in quartz are useful isotopes. This is due to quartz’s ubiquity
and mineral resistance, its ability to be consistently cleaned of meteoric 10Be produced in the
atmosphere, and 26Al having a relatively high production rate (Ivy-Ochs, Kober 2008). A recent
cosmogenic study of these isotopes in quartz samples from a 9-m core of colluvium at the base
of a sandstone hillslope in unglaciated central Pennsylvania, for example, revealed that sediment
was deposited in possibly two pulses since 340 ± 80 ka, a time period that spans multiple glacial
episodes (Del Vecchio et al, 2018). One of the possible models of colluvial deposition based on
this cosmogenic data is consistent with the younger pulse occurring since ~80,000 yrs BP, and
hence being Wisconsinan in age. The authors interpreted these results as indicative of complex
histories of surficial sediments and landforms, with likely remobilization during successive cold
periods. We currently know of no cosmogenic analysis that has determined LGM ages for
production and deposition of boulder colluvium within individual solifluction lobes in
Pennsylvania.
New tools with Geographic Information Systems (GIS) can be used to analyze both
landforms and the colluvium that comprises them. Drone photogrammetry can provide
orthoimages for grain size analysis with GIS tools. Ph.D. student Julia Carr at Pennsylvania
State University, for example, uses ArcGIS for grain size data collection with drone orthoimages
of river bed gravels in Taiwan, and her procedure is adapted for use here. Carr implements
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traditional point counts in GIS by measuring the intermediate axis of each intersecting grain. By
characterizing how the coarse fraction of sediment changes after different flood events, her
measurements effectively show how the different size fractions in gravel patches change over
time. Carr also shows that constructing grain cover maps for each grain size is a promising
approach for assessing variations in particle size within a deposit.
Study Area
The field area investigated here is located on Chestnut Ridge, just north of the Blue Ridge
Mountain Ski Resort, near the towns of Palmerton and Danielsville, Pennsylvania. This study
focuses on outcrops of colluvium within lobate landforms along a new road (designated here as
ATT Road) excavated for the development of an AT&T cellular signal tower (Fig. 5). The field
area is approximately 16 km south of the southernmost extent of the LGM ice sheet margin (see
Fig. 2). The approximately 2-km road excavated from valley bottom to ridge crest created
exposures of colluvium up to 10 m high that reveal the transformation and down-slope transport
of sandstone, siltstone, and shale colluvium from bedrock source to valley bottom over an
elevation of 180 m. This roadcut provides unprecedented opportunities to study the internal
stratigraphy and sedimentology of periglacial deposits.
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Similarly to other ridges within the Appalachian Ridge and Valley physiographic
province, Chestnut Ridge is formed by tightly folded and deformed Middle Devonian
conglomerate, sandstone, and siltstone within an anticlinal nose that transitions to a syncline to
the north (Fig. 6). Intensely fractured, cleaved, and vertically bedded sandstone of the Palmerton
Formation is juxtaposed with conglomeratic, ridge-forming Oriskany sandstones and
well-cross-bedded, coarse, fossiliferous siltstones of the Schoharie and Esopus Formations.
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Poorly sorted, weakly stratified colluvium comprised of mixtures of silt- to boulder-sized
sediment was exposed at many outcrops along the new roadcut, and even was observed to a
depth of several meters at the ridge crest where a large hole was excavated for the foundation of
the cell tower. Some patches of highly fractured bedrock also were exposed along the road, as
discussed below. Siliceous and calcareous sandstones of the Palmerton and Oriskany
Formations have been excessively leached in places, forming semi-consolidated friable sand that
was mined at a quarry on the western end of Chestnut Ridge (Geological Survey Research,
1969). It is thought that these deposits were formed during pre-Wisconsinan weathering and
stripped away from their bedrock source areas as a result of periglacial processes during the
Wisconsinan glacial advance and retreat (Geological Survey Research, 1969).
The sites chosen for this study within this field area are of interest due to the presence of
many relict lobate features and exposures of colluvium and bedrock. The recently excavated
ATT Road has 4 switchbacks that reveal three-dimensional views of colluvium within multiple
lobes. The sites, described below, each provide different exposures, and five outcrops of interest
identified along the roadcut were given numbers 1-5 (Fig. 7). Sites 1 and 3 are the primary focus
of this study. Sites 1 is the field sampling site and is referenced extensively in this study. Site 1
reveals the stratigraphy within a relict lobe (Fig. 8) and clearly shows at least two strata. The
lower stratum contains multiple boulders of Palmerton Formation conglomeratic sandstone with
>1 m intermediate axes that might have been ploughing blocks at the time of downslope
sediment transport. Site 2 is a large exposure of bedrock of the Schoharie and Esopus formations
at the first switchback in the new road. Site 3 sits at the second switchback along the road, and
was chosen for its abundance of clearly exposed, clast-supported boulders. A short distance to
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the east of the third switchback sits Site 4, a large unvegetated boulder field. Site 5 lies on the top
of Chestnut Ridge, where excavation occurred for the cell tower. Of note is that pebbles,
cobbles, and boulders from the Palmerton Formation exist in lobes and benches along the entire
length of the newly excavated road, even though the outcrop of this formation is limited to the
uppermost part of the slope above Site 1.
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Methods
The methods used in this investigation included drone photogrammetry, field mapping,
sampling, cosmogenic isotope measurements, and geographic information systems analysis of
topographic and clast size data. Characterizing the sedimentology and sedimentary fabric within
lobes of slope stratified deposits, in combination with cosmogenic nuclide analysis, is used to
investigate the sediment transport mechanisms active on Pennsylvania hillslopes during the
LGM and PHT. The following sections outline the tasks and procedures used to investigate the
topography and geomorphology of the Chestnut Ridge field area, and to evaluate differences in
sedimentary fabrics at ATT Rd: Site 1 and ATT Rd: Site 3. Site 1 can be seen in Fig. 8,
discussed in Section I of the Methods, and Site 3 can be seen in Fig. 9. In addition, this section
describes the sampling and analysis of quartz clasts for cosmogenic isotopes from Site 1.
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I. Topographic Analysis and Field Area Surveying
In order to identify periglacial landforms such as gelifluction lobes and thermal
contraction polygons throughout the study area, we examined recent orthoimagery and used a
3.2-foot resolution Digital Elevation Model from the 2014 PAMAP lidar survey of Pennsylvania
(DCNR PAMAP Program, 2014) to generate slopeshades and hillshades (see Fig. 5). Using a
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Geo7x RTK GPS unit, we acquired x, y, and z coordinate data along the newly excavated road
developed for an AT&T cellular tower.
II. Sample Collection
Samples from the face of the outcrop at Site 1, as well as the top of the lobe (i.e., the
tread) at the same site, were collected for grain size and cosmogenic isotope analysis. Matrix,
boulder chips, and clast samples were collected from base to top of the exposure of colluvium, a
slope distance of ~7 m, on 06/03/19 and 12/21/19 (Fig. 10). The face of each sample location
was scraped with a masonry spade to remove any cover sediment that might have originated
from upslope. A total of 38 sediment samples was collected in Ziploc bags for grain size and
cosmogenic analysis from the lobe at Site 1, with 8 matrix and 24 clast samples collected from
slope depths of 0.1-6.61 m below the top of the lobe along the 45º sloping face of the roadcut,
and 6 samples of boulders from the tread of the lobe above the roadcut. For the boulders, chips
were sampled from 6 different boulders mantling the vegetated tread of the lobe using a sledge
hammer and chisel. Each sample location was surveyed with a Trimble Geo7x RTK GPS unit.
Some of these samples collected from Site 1 on 06/03/19 were processed at the
University of Vermont for cosmogenic nuclide sample preparation, as discussed below. A subset
of those samples with sufficient weight for final analysis was sent to the PRIME lab at Purdue
University for nuclide concentration measurements.
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III. Grain Size and Angularity
The 8 matrix samples were placed in aluminum foil tins and dried using a heat lamp
source in a lab at Franklin and Marshall College. Once dried, the matrix samples were weighed
and lightly crushed to separate cohesive clusters using a mortar and pestle. The <2 mm portion
of eight matrix samples collected from Site 1 was sieved with a Ro-Tap in the Department of
Earth and Environment at Franklin and Marshall College in order to evaluate weight percentages
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finer than the following sieve sizes: 2000 μm, 833 μm, 600 μm, 500 μm, and 250 μm. These
grain sizes were then plotted for sample weight percent finer than each sieve size with
cumulative particle size distribution curves.
Of the clasts >2 mm in particle size diameter of these eight matrix samples, ten were
randomly sampled from each, a total of 80 clasts, and assessed for angularity. Angularity is a
measure of smoothness and rounding of particles, and is related to both clast size and transport
distance. Each clast was compared with a standard chart of clast angularity and given an index
number that best represented its angularity. This metric assigned a numeric value to respective
angularity. Clasts that were very angular were assigned 1, angular: 2, subangular: 3, subrounded:
4, rounded: 5, and well-rounded: 6. Intermediate values (1.5, 2.5, 3.5, 4.5, 5.5) indicate an
unclear angularity, and are, for example, very angular-angular or subangular-subrounded.
IV. Drone Photogrammetry
High resolution, three dimensional, point clouds and digital elevation models of the
outcrops along the ATT Road at Site 1 and 3 were developed using a DJI Mavic 2 Pro model
drone and Agisoft Metashape software. Approximately 150 photos were acquired for each of the
two sites with the drone, in a sweeping fashion that took pictures of the hillslopes from a
multitude of angles, altitudes, and proximities. These photos were then subsequently imported
into Agisoft where a tie point cloud was generated from identified key points. A high quality
dense point cloud was derived from classified ground points. A digital elevation model and
mesh of the hillslope were then developed from the dense point cloud. These models were then
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supplemented by importing sample GPS location points acquired in the field during the drone
flights.
V. Cosmogenic Laboratory Sample Preparation
The 38 samples collected from ATT Road Site 1 were assigned FM numbers and
processed for cosmogenic analysis in the sedimentology lab at Franklin and Marshall College.
All sample information is provided in an FM database for samples from this project. CCF
procedures are described in the References section at the end of this thesis (see NSF/UVM
Community Cosmogenic Facility Methods.) In accordance with the CCF procedures, all samples
for cosmogenic analysis must be finer than 833 μm and coarser than 250 μm in grain size in
order to be processed and analyzed. As a result, matrix samples must be sieved )as described
above), and clasts and boulder chips must be crushed and sieved. As noted above, 24 samples
were whole clasts, 8 were matrix samples, and an additional 6 were chips from surface boulders
on the tread of the lobe. Each was processed for cosmogenic isotope analysis in slightly
different ways.
Clast and boulder chip samples were prepared in the rock crushing laboratory at Franklin
and Marshall College using a rock hammer and pulverizer. These rock fragments were likewise
sieved into 2mm, 833 μm, 600 μm, 500 μm, and 250 μm fractions. As with the matrix samples,
crushed grains between 833 μm and 250 μm were separated and amalgamated for shipment to
the CCF.
The boulder chip samples are being held in reserve for cosmogenic isotope analysis,
pending future funding for this project, but all other samples were shipped to the CCF.
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VI. Cosmogenic Nuclide Sample Analyses
At the University of Vermont, samples underwent two 6N HCl etches for 24 hours each
in order to remove Al, Fe, and carbonate coatings from grains as well as to dissolve iron filings
from the grinding process and remove adhered meteoric 10Be. Samples then were leached by
three HF/HNO3 etches for 24 hours each in order to keep fluoride in solution and remove almost
all other minerals but quartz. Following these etching procedures, the samples were leached in a
72 hour etch and week-long etch in weak HF/HNO3. Following three etches, each sample was
dried in an oven at ~60° C and stored in 50 mL vials for Al and Be extraction.
VII. GIS Grain Size Distribution Analysis: Point Counts
Methods for developing spatial data related to grain size distributions at Site 1 and Site 3
were adapted from procedures developed by Julia Carr, a Ph.D. student at Pennsylvania State
University. First, an orthomosaic developed from drone photogrammetry was created in Agisoft
Metashape and then imported into ArcGIS. A polygon was created around the hillslope to define
the areas of analysis, and a new feature class was created for the “patch” that would be
processed. Using Create Fishnet, an ArcGIS tool, a grid was drawn at a specified scale, which in
this instance was 1m x 1m, about the size of the coarsest clasts in the outcrops. A new polyline
feature class was then developed, serving as the point count feature class. Polylines were then
drawn over the intermediate axis (B-axis) of any grain that was found at the intersection of the
grid and orthomosaic image. Here, it is important to note that the point count is limited by the
resolution of the imagery, and the resolution threshold of the smallest resolvable grain size (2x)
should be taken note of in accounting for the sizes less than this limit. Once the entire area of
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interest was mapped, these SHAPE_Length (B-axis) measurements were exported into Microsoft
Excel where cumulative distribution functions (CDF) were developed, producing a
representative grain size distribution for the outcrop at each site. These CDFs plotted B-axis
lengths with respect to their counts and cumulative percentages.
VIII. GIS for Grain Size Distribution Analysis: Grain Covers
Methods for developing spatial data relating to grain cover and size distributions, similar
to a facies analysis, were also adapted from a procedure that Julia Carr developed with ArcMap
tools. Similar to methods performed in constructing point counts, the AgiSoft metashape for
both Site 1 and 3 was imported to ArcMap. A new polygon feature class was created and a field
called “Cover” was added, in which specific types of cover were mapped and stored. These
features were outlined using the polygon editing tool and polygonal shape files. Types of cover
discerned for each field site and stored as classes for distribution data include large boulders,
boulders, small boulders, cobbles, pebbles, granules, organic material, debris apron, and road.
Large boulders are ≥ 0.25 m, boulders are < 0.25 m and ≥ 0.15 m, small boulders are < 0.15 m
and ≥ 0.08 m, cobbles are < 0.08 m and ≥ 0.064 m, pebbles are < 0.064 m and ≥ 0.004 m, and
granules are < 0.004 m and ≥ 0.001 m. Fine-grained material, that is material finer than the
0.001 m limit, was grouped in one, all-encompassing polygon for the entire exposure.
As each of the observable cover types was mapped, attributes of that shape were recorded
in the shape file. Specific attention was given to SHAPE_Area, which provided the area of each
mapped polygon in square meters. In order of decreasing size, each cover type was assigned a
cover type name dependent on the size of the SHAPE_Area. The threshold resolution of x2 here
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exhibits an approximate 0.001 m margin of error. To extract this data in order to construct
graincover distribution diagrams, the Summary Statistics tool was used in ArcGIS. In the
Statistics field, the Statistic type for SHAPE_Area was changed to SUM, and the selected case
field was “cover.” This tool effectively sums the area of each cover type and saves it to a new
table. This data was plotted in a distribution-style graph, displaying cover types and their
respective frequencies and composition percentage with regard to the entire field site.
Results
I. Grain Size and Angularity Analysis for Samples from ATT Road: Site 1
The following pertains to the eight matrix samples collected from ATT Road: Site 1 on
06/03/19. It addresses grain size distributions for the <2 mm fraction, and angularity for 80
randomly sampled clasts from the >2 mm fraction. Not that many more samples were collected
than analyzed for grain size and cosmogenic analysis. The D50 (median) grain size for the
samples shown in Fig. 11 ranges from approximately 0.8 mm to 1.5 mm, indicative of a coarse to
very coarse sand. Figure 12 is a distribution of the angularity of 80 pebble to cobble-sized clasts
collected from Site 1.
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Of the 80 samples evaluated for angularity (10 clasts from each of the 8 matrix samples),
none were well-rounded (an angularity of 6). Fifteen percent of the clasts were
subrounded-rounded (angularity of 3 to 4). The majority of these clasts have angularity indices
of 2 to 3.5, indicative of being angular to subangular-subrounded. An angularity index of 3
(subangular) is the frequent value.
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II. Using GIS for Grain Size Distribution Analysis-Point Counts
2.1 ATT Road: Site 1
Point counts and cumulative distribution functions were created for ATT Road: Site 1.
The previous section described grain size analysis of 8 samples of matrix, whereas this section
evaluates particles of all sizes within the outcrop, using a point count approach from an
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orthoimage and ArcMap software as discussed in the Methods section. The blue point count
lines shown in the orthoimage in Fig. 13 represent the length of the B-axis of each clast that was
counted. The lengths of these lines were used to construct a cumulative distribution function
(CDF) for Site 1 that is shown in Fig. 14.
In Fig. 14, it is evident that 78% of the B-axis lengths are between 0 and 10 cm, with a
respective count of 137 of the total 175 axes that were measured. Approximately 15% of the
B-axis lengths are between 10 and 20 cm, with a respective count of 26. Continuing this
decreasing trend in B-axis length, ~4.5% of the B-axis lengths are between 20 and 30 cm.
Approximately 98% of the B-axis lengths are from 0 to 30 cm, representing 171 of the 175 axes
that were measured. One B-axis measurement was between 110 and 120 cm, however this was
treated as an outlier because of its much larger size than all other particles.
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2.2 ATT Road: Site 3
Point counts and cumulative distribution functions were created below for ATT Road:
Site 3.
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The point count lines shown in Fig. 15 represent the intermediate length of the B-axis of
each clast that was counted in a grid overlay for Site 3.
In Fig. 16, a CDF is displayed representing a count of 366 B-axis measurements. About
56% of these measurements have B-axis lengths of 0 to 10 cm, and ~26% have measurements
between 10 and 20 cm. B-axis lengths between 20 and 30 cm account for approximately 10% of
the count total. The majority of the 366 counts can be attributed to axes lengths of 0-20 cm,
comprising ~82% of all measured clasts (301 of 366). Only 7 clasts have B-axes with lengths
between 40-50 and make up approximately 2% of the total measurements.
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III. Using GIS for Grain Cover Distributions
3.1 ATT Road: Site 1
Graincover distribution maps were developed in order to characterize spatial variations in
grain size (Fig. 17) between different cover types. All grains less than a certain size can be
mapped as one cover type, and those of other size ranges can be mapped as other cover types.
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Ten cover types were mapped for the exposure at Site 1. The organic-rich soil at the top
of the outcrop, debris apron along the base of the outcrop, and road were each treated as single
map units but were not included in analyses of grain sizes. They are mapped so as to remove
them from subsequent analysis. The boundaries of particles finer than 1 cm cannot be resolved
due to their small particle sizes relative to the resolution of the point cloud, and they are mapped
as one unit called “fines”. Different shades of blue in Fig. 17 represent different sizes of mapped
cover types that range from pebbles to large boulders. A total of 351 features of interest were
mapped using these procedures, and 346 were measured.
Each cover type was plotted with respect to its frequency of observation and total
percentage composition of the outcrop. In Fig. 18, it is evident that 5 patches of fine-grained
sediment account for approximately 86.09% of the entire outcrop. Other than pebbles and fines,
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all other cover types have a count frequency of occurrence less than 15. Clasts mapped as
pebbles represent 260 of the 346 measured features, but only ~6% in terms of total outcrop area.
Seven large boulders account for ~4% of the total outcrop area. Fifty-two granules with
boundaries distinct enough to be traceable were observed in the outcrop, however they only
account for 0.2% of the total area.
3.2 ATT Road: Site 3
Fig. 19 is the grain cover distribution map for ATT Road: Site 3. The cover types
represented in this distribution correspond to the same colors as at Site 1. There was no
discernable debris apron in Site 3. For Site 3, a total of 555 observable features were assigned to
9 cover types.
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Fig. 20 displays the grain cover distribution for Site 3. Here, there are 7 main patches of
fine-grained sediment, which account for ~77% of the total mapped area. Cover types with a
count frequency greater than 15 include granules, pebbles, and small boulders, with counts of
103, 379, and 43, respectively. The 103 granules account for only 0.4% of the total area. The
379 pebbles account for ~11% of the total area. The 43 small boulders account for ~7% of the
total area.
Fig. 21 displays the graincover distribution for both Site 1 and 3, directly comparing the
data described above. Combined with cumulative data on grain size from Figures 14 and 16, the
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similarities and differences between the outcrops exposed at these two sites include the
following:
● Despite the apparent prominence of boulders, most of the area of both outcrops at
Sites 1 and 3 is covered with fines and pebbles (~92% of the total area of Site 1,
and ~88% of the area of Site 3).
● The prominence of fines and pebbles is in accordance with the cumulative
frequency data for particle sizes, which indicates that the B-axes are <20 cm for
93% of the sediment at Site 1, and <20 cm for 82% of the sediment at Site 3 (see
Figures 14 and 16).
● Sieving of 8 matrix samples from Site 1 indicates that the matrix is a coarse to
very coarse sand (see Figure 11), so the sediment in this outcrop, overall is a
sandy pebbly colluvium.
● A greater percentage of the area of Site 1 is covered with fines compared to Site 3
(~86% versus ~77%).
● Compared to Site 1, a greater percentage of the area of Site 3 is covered with
pebbles (~11% versus ~6%), small boulders (~7% versus ~2%), and boulders
(~3% versus ~1%).
● A slightly greater percentage of the area of Site 1 is covered with large boulders
than Site 3 (~4% versus ~1%).
● Granules and cobbles are rare at both sites (~1% or less).
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● Sedimentary fabrics can be discerned at both sites, but that at Site 1 is more
distinct and clearly slope stratified, and sediments at Site 1 are all
matrix-supported, wheres most are clast-supported at Site 3.
● At least two strata can be clearly identified at Site 1, with an upper finer-grained
stratum and a lower stratum containing for more small to large boulders.
IV. Cosmogenic Isotope Analysis
In order to constrain the timing of formation of relict lobate landforms in the mid-Atlantic
region, surficial exposure and burial histories can be determined through cosmogenic isotope
analysis. In the fall of 2019, Lee Corbett of the University of Vermont sent 11 cathodes from
Site 1 samples to the Purdue Rare Isotope Measurement Lab (PRIME) for 10Be and 26Al analysis,
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however final accelerator mass spectrometry results showed that this particular batch of samples
had a high blank for aluminum. As a result, we are only able to present the 10Be data at this time.
In situ cosmogenic 10Be concentrations for clasts and matrix samples (sand-sized) from this
analysis constrained the near surface residence time of the material collected from depths of ~1
to 7 m on the face of the outcrop (Fig. 22, Ruck et al, 2020). Below ~5 m, 10Be concentrations
for clasts and matrix are similar (35,000 to 50,000 atoms/g), but are 3 to 9x lower than samples
collected above.
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At depths above 5 m, in two colluvial beds, nuclide concentrations are similar for clasts and
matrix (130,000 to 300,000 atoms/g) (Ruck et al., 2020).
In February of 2020, Lee Corbett sent another 19 cathodes for 10Be and 26Al analysis of
Site 1 to the PRIME lab, including 16 unknowns, 2 blanks, and 1 quality control for each
isotope. Depth locations of the 16 samples are shown as black circles (clasts) and triangles
(matrix) in Fig. 22 (Ruck et al., 2020). Of these 16 samples, 11 are replacements for the first 11
samples from Site 1 in order to get both 10Be and 26Al data for the same samples. Six of the
eleven samples came from the original June 3, 2019 sampling, and it was necessary to return to
the field to get enough samples for the other five analyses. As of this writing, because the
PRIME lab had to shut down during the COVID-19 pandemic, we do not yet have results of this
second batch of analyses.
Discussion
The primary purposes of this study were 1) to characterize colluvial sediment in slope
stratified deposits within relict periglacial lobes south of the LGM ice sheet margin in
Pennsylvania, and 2) to assess the possibility that this colluvium was transported by mass
movement in association with permafrost thaw, possibly during the Pleistocene-Holocene
transition (PHT) circa 16,000 to 11,650 yrs BP. Previous sections characterized colluvium in
slope stratified deposits exposed within relict lobes at two sites on the south-facing slope of
Chestnut Ridge, 16 km south of the LGM ice margin in east-central Pennsylvania. Site 1 is at
the outer (downslope) end of the love, and Site 3 is at the upper end of a lobe, close to the
bedrock source area of fractured sediment. Data from these sites, as described above, is used
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here to evaluate the mechanism and timing of sediment transport and deposition. Given that
gelifluction is slow downslope movement of water-saturated soil or colluvium above a
seasonally or perennially frozen substrate during times of thaw, we evaluate evidence that the
colluvium studied here is consistent with a mass movement origin above permafrost.
Regarding the timing of exposure and deposition of sediment at Site 1 on Chestnut Ridge,
both matrix and clasts from all but the lowest samples (below ~5.5 m) yielded 10Be
concentrations consistent with LGM exposure. The concentrations of 10Be for samples between
depths of 1 and ~5.5 m are similar to those for other cosmogenic studies that determine that
samples are from the LGM (e.g., Corbett et al, 2017). Cosmogenic nuclide concentrations
increase with duration of surface exposure, but also can decrease if sediment is buried and cut off
from cosmogenic ray bombardment.
We conclude that gelifluction is likely the mass movement mechanism that transported
these sediments down slope, for the following reasons. Close examination of a LiDAR-derived
slopeshade reveals contrasts between steeper and gentler slopes, and indicates that dozens of
lobate structures oriented oblique to the valley axis cover the slopes of Chestnut Ridge, from
near the ridgecrest to valley bottom (Fig. 23).
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These lobes are characteristic of gelifluction because of their large size (up to many
meters in height) and the presence of boulders (from small to large) embedded in a sandy pebble
matrix. As noted above, however, gelifluction is a type of solifluction, the latter of which
encompasses a broad range of mass movement types that can form lobes and do not all require
permafrost to occur. However, other sources of evidence indicate that permafrost probably
existed at the time of downslope mass movement at Site 1 on Chestnut Ridge. The cosmogenic
data indicate, for example, that sediment had relatively short residence times, consistent with
exposure during the LGM , but there is no evidence of this sediment having moved since
deposition. This suggests that deposition was somewhat short-lived and the result of an event
such as permafrost thaw.
The processes associated with freezing and thawing in areas with permafrost are used
here to evaluate the possibility that permafrost existed at Chestnut Ridge at the time of lobe
formation at Sites 1 and 3. As material freezes and thaws, sediment sorting by frost heave
segregates larger grain sizes from finer grain sizes because finer sediment is easily entrained in
expanding ice. When this entrainment is combined with gravitational segregation of larger grain
sizes, patterns of boulder, cobble, and pebble distributions become components of slope stratified
colluvial deposits (Smoot, 2004). In particular, Smoot (2004) noted the presence of multiple
poorly sorted, matrix-supported strata at the sites he studied in Virginia, and these are similar to
what is observed at Site 1 in this study. In addition, the relative angularity of the sampled clasts
at Site 1 indicates mass movement such as slow gelifluction under saturated conditions was
likely, with little abrasion over the course of sediment transport downslope. At Site 1, the
majority of the clasts have angularity indices from 2 to 3.5, indicative of angular to subangular
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angularity. At Site 3, samples were not collected to evaluate angularity, but field work and
photos of the outcrop show that all clasts other than pebbles weathering from conglomeratic parts
of the Palmerton are angular.
Intense frost-cracking during the LGM, indicated by the presence of thermal-contraction
polygons with sand and ice-wedge casts in nearby areas in Pennsylvania and New Jersey, is a
likely mechanism for producing the angular, shattered boulders mapped at Sites 1 and 3, and in
fact can be observed along the entire ATT Road on Chestnut Ridge. Evidence of permafrost
and/or its thaw includes extensive networks of these polygons on crests and side slopes of shale
hills. Analysis of LiDAR imagery throughout the same areas with evidence of thermal
contraction polygons also reveals multiple gelifluction sheets and lobes on quartzite and
sandstone ridges throughout unglaciated Pennsylvania, and these are similar to the Chestnut
Ridge sheets and lobes (Merritts et al., 2015).
Point counts and their respective cumulative distribution functions for both Sites 1 and
Site 3 are consistent with Site 3 being a primary source of sediment from shattered bedrock that
is carried downslope as mass movement during times of permafrost thaw. From Site 3 to Site 1,
there is a decrease in the median boulder size and count of observable boulders with increased
interstitial matrix content and distance from the ridgeline. Sediment at Site 3 is generally coarser
than at Site 1, and many of the boulder-sized clasts at Site 1 appear to be in place (i.e., bedrock),
but are so shattered and slightly rotated that they also appear loosely jumbled. Whereas fines at
Site 1 are disseminated throughout the matrix of all parts of the outcrop, fines at Site 3 are often
within boulder fractures that appear to have formed in place. In other words, they are not
disseminated throughout a matrix of fines. The greater amount of fines at Site 1 might be the
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result of winnowing fines from sediment transport from Site 3 to Site 1. Whereas Site 1 has 40
measurable B-axes that intersect the fishnet grid, Site 3 has 179 measurable B-axes and much
better clasts because the fines are much more disseminated at Site 1. Fig. 13 and Fig. 15 show
B-axes measurements that are significantly shorter for Site 1 than Site 3.
The overall morphology of the landscape at Sites 1 and 3 is also consistent with slow
mass movement downslope from a bedrock source area (at or close to Site 3) where frost
shattering occurred to the terminus of a lobe marked by ploughing blocks and braking blocks (at
Site 3). LiDAR data shows that Site 1 is at the terminus of a small lobe, near an intersection of
two small lobate features. A sub-horizontal band of boulders exposed in the roadcut at Site 1,
likely ploughing blocks, is interpreted to be the distal end of a gelification lobe. As noted by
Smoot, 2004, both frost heave and gelifluction are conducive to producing the formation of
stone-banked lobes.
Cold-climate conditions associated with permafrost during the LGM would produce these
lobes as the active layer would freeze and thaw annually, inducing displacement and movement
downslope. Approximately 75% of the B-axes measured in Site 1 are between 0 and 20 cm,
while 77.7% of the B-axes measured in Site 3 are between 0 and 20 cm. Count totals for these
sites are 40 and 179 respectively, which is a product of the degree of exposure. At Site 3,
clusters of cobble- to boulder-sized material occur that are dominantly clast-supported. At the
top of the hillslope, elongate clasts with B-axes that are relatively similar in length are in grain
contact (i.e., clast supported) with long axes oriented near vertical and shingled relative to one
another (Fig. 15). These aforementioned clusters juxtaposed with matrix-rich pebble and cobble
layers, are characteristic of periglacial colluvium, similar to sediment described by Smoot (2004)
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and Eaton et al (2003) in the Blue Ridge Mountains of northern Virginia. Underneath these
elongate, vertically oriented clasts in Site 3, the pebbles and cobbles are oriented differently,
with long axes oriented more or less parallel to bedding (seen in Fig. 15).
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These layers of poorly sorted medium- to coarse-grained sand have relatively sharp contacts with
pebbles and cobbles, but appear to be more scattered as transport continues downslope (Fig. 25).
A gradual change in orientation and slow movement suggest colluvial deposits that did not have
velocities fast enough to be characterized as a sheetflow where excess water can be drained from
the pore spaces in the system.
Grain cover distribution maps for Site 1 and Site 3 further reflect this characteristic
sediment fabric. Site 3 is dominantly clast-supported, and Site 1 is matrix-supported. Changing
sedimentary fabrics downslope indicate some form of mass movement capable of changing a
dominant clast-supported sediment as observed in Site 3, to a matrix-supported sediment as
observed in Site 1 (Fig. 24). These colluvial surficial deposits obscure topography and likewise
change with movement downslope (Newell et al., 2011). At Site 1, the graincover types that
compose the majority of the sedimentary features are the fines, namely the medium-coarse sands
and pebbles (Fig. 17). The fines account for roughly 86% of the total composition, and 260
pebbles represent 6.1% of the total composition (Fig. 18). It is clear that the larger boulders at
the terminus of the lobe at Site 1 are not moving today. Boulders, including the three size classes
referenced in Fig. 18, comprise 24 of the 260 total features. Of the 555 traced features in Site 3,
granules, pebbles, and small boulders are the most prominent. A potential type of transport
mechanism associated with a matrix-supported deposit at Site 1 could be an earth flow.
Previous investigations in Maryland, Washington D.C., and northern Virginia (French et
al. 2007) indicate that Late Pleistocene permafrost was likely to have extended south and west of
the LGM ice margin into southern Delaware and central Maryland, and south of latitude 38°N,
where conditions of either discontinuous permafrost or deep seasonal frost prevailed (French &
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Millar, 2014). There is evidence that the sediment in the lobate landforms studied here is
associated with periglacial processes, such as the presence of frost-shattered bedrock, which can
be observed at Site 3, and ploughing blocks, seen in Site 1. Barren boulder fields located just to
the east of Sites 1 and 3 are interpreted as products of a lag of coarse material left behind after
winnowing of finer-grained matrix from the boulders, as has been observed in modern periglacial
landscapes (Matsuoka, 2001).
Field observations, other studies assessing ages of sandstone and quartz landscapes
during the LGM, and preliminary analysis of nuclide concentrations are consistent with
near-surface exposure ages of colluvium that moved downslope via slow, short-lived mass
movement, similar to an earthflow, under water-saturated conditions during the last glacial cycle.
Aside from soil creep induced by gravitational forces and rock fall, little reworking of this
sediment has occurred since mass movement and deposition.
With respect to future studies, it would be beneficial to investigate other outcrops along
similar excavated roadcuts, particularly those upslope, closer to the bedrock source, and those
downslope that have traveled farther. It would be ideal to have additional evidence to support
our conclusions regarding the changing sedimentary fabrics, grain sizes, and grain orientations
observed in surveying hillslopes during this study. Developing point clouds and orthoimagery of
the barren boulder field adjacent to the newly excavated road on Chestnut Ridge would provide
additional, useful evidence. Cosmogenic isotope sampling of these other outcrops would
likewise be beneficial in discerning ages of grains spread across the landscape in order to
understand a more complete picture of temporal variability. Lastly, conducting similar studies in
other periglacial landscapes, particularly in other parts of Pennsylvania, could allow one to
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compare observed sediment fabrics and their spatial variability in great detail. Generating
graincover maps for rare exposures such as these would serve to further constrain the effects of
the LGM in the eastern United States.
Conclusion
At the end of the LGM ~20,000 years ago, the unglaciated part of Pennsylvania south of
the Laurentide ice margin was characterized as a periglacial environment (Braun, 1989, 2006b;
Jackson et al, 1997; Merritts et al, 2014, 2015; Eaton et al, 2003). Previous studies that
identified thermal-contraction polygons and relict gelifluction lobes in this region concluded it
was once cold enough for intense frost cracking to produce loose sediment that became bound in
continuous permafrost (Merritts et al, 2015, 2017; Gardner et al, 1991; French & Millar, 2014,
Gross et al, 2017). In this study, I characterized colluvial slope stratified deposits within relict
periglacial lobes south of the LGM ice sheet margin in Pennsylvania, observed in roadcuts at
Sites 1 and 3 on Chestnut Ridge, to assess the possibility that this colluvium was transported by
mass movement due to permafrost thaw. The evidence presented here supports the hypothesis
that these colluvial hillslope lobes formed due to periglacial processes. These processes most
likely occurred during the late Pleistocene LGM and subsequent Pleistocene-Holocene transition
(PHT) circa 16,000 to 11,650 yrs BP. It is likely that permafrost existed at the southernmost
extent of the LGM in Pennsylvania, and the thawing of this permafrost, accelerated during the
PHT, induced mass movement during summer warm seasons.
In summary, by virtue of a new roadcut with extensive outcrops, this is the first study of
an interior view of relict lobate landforms. The roadcuts expose a nearly complete visual record
Ruck 62
from the zone of boulder production near the ridge crest to a terminal slope-stratified lobe front,
enabling direct observation of the processes that led from boulder production to downslope
movement. These lobate landforms are ubiquitous in the Appalachian Mountains of
Pennsylvania, so that our observations at this outcrop may provide explanations for flow
mechanisms throughout the region.
Given the observations provided here, I conclude that the lobate landforms observed at
the AT&T roadcut are the result of periglacial processes that were known to exist south of the
glacial ice margin in Pennsylvania at the LGM. This evidence includes:
1. Small clast-supported boulders observed in the top portion of Site 3 are likely
products of frost-shattered bedrock and perhaps frost heaving. Elongate clasts
and longer, more consistent B-axes than those observed in Site 1 are in a
clast-supported fabric, with long axes oriented near vertical and organized in a
shingled pattern. This fabric transitions to clusters of pebbles and cobbles that are
oriented more or less parallel to bedding and a matrix-supported fabric
downslope.
2. Ploughing blocks observed in slope stratified colluvium at Site 1 suggests
accumulation downslope through gelifluction and frost heave.
3. In situ cosmogenic 10Be concentrations for clasts and sand (matrix) from isotopic
analysis yield similar concentrations for clasts and matrix below ~5 m (35,000 to
50,000 atoms/g). Shallower than 5 m, in two colluvial beds, nuclide
concentrations are similar for clasts and matrix (130,000 to 300,000 atoms/g).
This is consistent with near-surface exposure during the last glacial cycle and with
Ruck 63
relatively rapid erosion and deposition of colluvium during cold-climate
conditions.
4. Little reworking of sediment exposed at Sites 1 and 3 is demonstrated by the
relative angularity of pebbles and cobbles sampled at Site 1, and field
observations of larger clasts at both Sites 1 and 3. These observations indicate
abrasion with downslope sediment transport.
5. These relict lobes are apparently stable landforms under current climate
conditions, and this work provides supporting evidence that mass movement that
resulted in lobe formation and downslope sediment transport ceased after the
LGM, perhaps as a result of complete permafrost thaw during the PHT.
Ruck 64
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