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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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