Evolution of the Blue Ridge Escarpment

East of Boone, NC

Edward McDonald

Introduction

The Blue Ridge Escarpment (BRE) is located in the Appalachian

Mountains of the eastern United States, spanning the states of North

Carolina, South Carolina, and Georgia. It marks a rapid transition from low to

high elevation, separating the coastal plain to the east and the Appalachian

Plateau to the west, an elevation change that varies from 300 to 600 m

(Prince, 2010). The high elevation ridge of the escarpment forms the eastern

continental divide, which drains water west into the Gulf of Mexico, and east

into the Atlantic Ocean. This divide is asymmetric, with a generally linear

shape and localized areas of increased headward erosion, moving west into

the escarpment.

Great escarpments are a common feature, existing along 1/3 of the

world’s passive margins (Prince et. al, 2010). They are formed by rift-flank

uplift during tectonically active periods. When a rift develops within

continental plates, they spread apart, generating a rift zone. The topography

from the original continental plates remains on the periphery of growing rift

zones, and eventually results in areas of high topography adjacent to

oceanic basins.

When tectonic activity ceases, fluvial and colluvial systems take

control of the system, seeking equilibrium with base level. Escarpments at

different phases of erosion vary from .3 to 1 km in elevation (Prince et. al,

2010). In the case of the Blue Ridge Escarpment (BRE), this erosion is driving

the escarpment slowly west.

This study aims to concentrate on the hydrology and morphology of

the escarpment east of Boone, NC. By studying and describing the hydrologic

erosive records of streams, we will understand what evidence is present to

explain how and why the escarpment is migrating. The evidence found in

this small scale study could refute, confirm, or tell us little about the

predominant theories of BRE migration.

Background

When tectonic activity ceases along passive margins such as the BRE,

fluvial and colluvial networks begin processes of erosion and deposition. By

expanding and developing stream networks, these forces transport sediment

loads towards equilibrium (Prince et. al, 2011). The behavior of these

escarpments during this time period is difficult to understand because,

despite their apparent similarity as geomorphic structures, they can behave

quite differently. Variations can be seen both in how long escarpments

actively migrate and how quickly they do so. For example, the southeastern

Australia escarpment (Persano et. al, 2002) and southwestern Africa

escarpment (Gilchrist et. al, 1994) have not experienced significant

migration since rifting. Migration of these escarpments ceased ~70 Ma years

ago lasting ~30 Ma years (Persano et. al, 2002; Gilchrist et. al, 1994).

Meanwhile, the BRE (Spotila et. al, 2004) and southeastern Brazil

escarpment (Gallagher et al., 1994) continued actively migrating further into

their development for as long as 250 Ma years. The rate of exhumation for

the BRE and southeastern Australia escarpment are approximately the same

(Spotila et. al, 2004; Persano et. al, 2002), and yet the BRE has preserved a

much steeper, youthful topography. This is difficult to explain, because the

BRE is 200 M. yr. older and should be more equilibrated than the Australian

escarpment (Prince, 2010).

The concavity of profiles of the BRE drainage networks may represent

an equilibrated system, shifting only with external forcing mechanisms such

as climate, isostatic unloading, or resumed tectonic activity. In this case, the

drainage networks would maintain equilibrium, unless some regional,

systematic perturbations occur. Profiles would achieve a concave up profile,

and any variations would be systematic throughout the escarpment. The

profile’s concavity could also be the result of an internal variable, in which

case the topography will adjust transiently. Transient adjustment will result

in localized variations of concavity. If this is true, catchments are controlled

locally by heterogeneous internal characteristics, such as drainage geometry

or lithology, which will be reflected in various ways throughout the

escarpment.

Determining if the landscape is equilibrated or transient will help

contribute to the understanding of nontectonic denudation and

sedimentation (Prince, 2011). The driving forces behind the migration of

these escarpments remain unknown. In the broader context of

geomorphology, study and interpretation of profile concavity can help us

understand landscape response in a post-orogenic setting, and how erosional

mechanisms are distributed through space and time.

Thermo-chronologic data sets give us approximations for long term

timing of the retreat of escarpments, but are not detailed and precise

enough to develop a complex understanding of the full evolutionary history.

They target larger timescales and magnitudes that do not indicate the

internal or external nature of continued escarpment behavior (Prince, 2011).

Studies have found escarpments such as the southeastern Australia

escarpment (Persano et al., 2002) had phases of initial rapid erosion that

slowed, while others, including the BRE (Spotila, 2004) and southeastern

Brazil escarpment (Gallagher et al., 2004), have had continued late-stage

retreat. Further thermo-chronologic data indicates that typical topography of

escarpments ceases to exhume sediment within 30-40 Ma years, while the

BRE is ~200 Ma years old and shows exhumation as recently as 20 Ma years

ago (Gunnell et. al, 2010). The youthful topography of the BRE indicates

some post-orogenic erosional energy must have been introduced, to drive

the continued exhumation of sediment. The question remains whether it is

from an internal or external source. Because there are multiple escarpments

that have undergone this late-stage retreat (i.e. BRE and Brazil escarpment),

we have reason to believe it could be the result of an internal forcing

mechanism. These thermo-chronologic experiments provide a long term

understanding of escarpments and a need for targeted field study of

erosional trends to determine the mechanisms of rejuvenation on these

shorter time scales (Prince, 2011).

Analysis of sediment provenance and location in first order streams of

the BRE indicates that erosional mechanisms are not controlled by changes

in lithology and strength (Prince et. al, 2011). Multiple studies have agreed

that the evolution of the BRE landscape depends little on the underlying

lithological units. This is mostly evident because the eroding streams seem

to not be controlled by these variable subsurface conditions (Gallen, 2011;

Prince, 2010). The gradient and location of knickpoints also fails to follow

lithological transitions (Gallen, 2011). Since erosion is controlled by a

balance of resistive and erosive forces, the unexplained variable seems to

manifest as a change in erosional potential.

One study points towards external sources, specifically tectonic

rejuvenation, as a source of this new potential (Gallen et. al, 2013). By

comparing changes in drainage steepness both east and west of the

escarpment, the study found comparable forces maintaining the landscape

on either side of the escarpment (Gallen et. al, 2013). This could imply some

post-orogenic force, released by isostatic unloading, which would periodically

raise the erosive potential of the entire escarpment, and create knickpoints

that are observed (Gallen, 2013). An older study attempted to correlate

tectonic reactivation of the local Brevard fault zone in Virginia and North

Carolina (White, 1950), but it was contradicted by others, which claim

erosional evidence is not large enough to indicate this reactivation (Spotila

et. al, 2004).

There is a strong body of research that supports an internal transient

variable maintaining topography. Localized behavior of the westward

evolution of gorges into relict plateaus indicates some kind of internal

variable driving evolution of a landscape, because it displays great spatial

variability (Prince et. al, 2011.) Gorges gradually erode headwardly, carving

into the escarpment, leaving behind narrow, high elevation plateaus of the

relict Appalachian Plateau. Some gorges, immediately adjacent and similar

to others, are able to erode much further west, past the escarpment. The

local differences in behaviors amongst gorges of the BRE (Springer et al.,

1997; Granger et al., 2001; Hancock and Kirwan, 2007) indicate an internal

variable because it does not affect the adjacent profiles equally. For

instance, if the variable were external, such as rejuvenating tectonic activity

or climate change, it would affect immediately local erosion similarly.

Streams in the east draining Roanoke Basin and west flowing New River

Basin have displayed a particularly extreme spatial variation in erosion rates

(Prince et. al, 2011), allowing some gorges to progress westward much more

rapidly than others, pointing towards some internal variable still playing out

its role in the landscape.

The BRE exists in a climate where erosional forces are driven

principally by fluvial and colluvial mechanisms. Variation in drainage network

shape, size, and orientation with regard to the continental divide also

controls how these systems migrate over time. Drainage rearrangement is a

predominant theory to explain the internal variability controlling this

migration (Prince, 2010). Drainage rearrangement is the transfer of a

drainage area from one watershed to another. Headwardly eroding streams

intersect, cut off, and capture the flow of another drainage system. This new

potential flow path diverts the flow, greatly increasing the upstream drainage

area and erosive potential of the initially headwardly eroding stream. The

diversion can happen very quickly or gradually, depending on the geometry

of the captured watershed, and how quickly it is reincorporated in a new

direction (Bishop, 1995).

This illustration shows how headwardly eroding streams capture and divert watersheds East off of the escarpment. The process occurs repeatedly over time, and gorges indicating the behavior will come in and out of existence, at various points in the cycle.(Prince et. al, 2010)

In the case of the BRE, drainage capture would indicate east flowing streams headwardly

eroding gorges into the BRE, which sporadically capture west flowing streams, as shown in the

illustration above. Eastern flowing streams eroding into the steep escarpment have been capable

of reaching and capturing watersheds which previously drained to the west and the Gulf of

Mexico (Prince, 2010). This increased energy potential, caused by additional drainage area, is

redirected to the east. This could be a major factor causing the streams to incise more rapidly and

increase the rate of erosion atop the escarpment. Because of the variation in stream geometry and

locations available for capture, capture events will be sporadic and transient, determined by the

pre-existing geometry of streams. These punctuated events, and subsequent erasure of

topographic evidence, could explain a transient behavior of the escarpment.

Networks which are incorporating captured energy from relief create reflecting patterns

of knickpoints, which have been recently studied (Crosby and Whipple, 2005; Harkins et al.,

2007.) A reflecting pattern of knickpoints is when one tributary downstream in the network has a

knickpoint which gradually moves up and becomes present in all tributaries upstream of that

point. The knickpoints in these situations migrate horizontally through connected tributary

networks, seeking equilibrium wherever they can. They tend to maintain their vertical elevation.

Because of this, knickpoints created by stream capture can be identified by their common-

elevation reflection through tributary networks, which indicates the elevation of original capture.

Methods

Three primary methods were applied to test the equilibrium and erosional mechanisms of

the BRE in this area. First, digital elevation models (DEM) of the area were examined for

topographic features, including streams, waterfalls, buttes and relict sections of plateau east of

the escarpment. The drainage networks identified flowed east down the escarpment, where active

erosion would be occurring. Second, longitudinal profiles of five stream networks flowing East

off of the escarpment were generated. Multiple tributary profiles were generated for each

network. These profiles were studied in the context of their watersheds and junctions to study

concavity and its implications of equilibrium throughout the streams. Third, maps of both

bedrock lithology and faults were examined. Changes in lithology and faults were compared to

the location of observed knickpoints, since either could potentially cause the disturbance in the

profile.

Topographic Analysis

We began the study with USGS quads of the study area, from Boone, Deep Gap,

Glendale Springs, Maple Springs, and Horse Gap. We identified five drainage networks on the

escarpment, all of which had tributaries reaching or passing the crest of the escarpment, and

identified local geographic features to reference. Once the study area of east flowing streams was

established, Light Detection and Ranging (LIDAR) Elevation Grids were downloaded for GIS

analysis from the NC Department of Transportation for Watauga, Ashe, and Wilkes County.

These LIDAR files were converted to feature classes using the 3D-Analyst toolbox in

ArcCatalog. The 3D-Analyst toolbox was then used to create new terrain files and digital

elevation models (DEM) from the feature classes. The three counties were then mosaicked

together using ArcMap 10.1. Using this elevation model, we observed the incision of the

drainage networks for unique buttes and gorges.

Longitudinal Profiles

Our method for generating profiles was based on methods developed by Whipple

(Whipple et al., 2007) and Richardson (and Richardson, 2013.) The DEM data was first filled, to

get rid of any “holes” which would distort the approximations of accumulation. ArcMap found

and eliminated any pixels with negative or 0 elevation. Then, based off of the slope, flow

direction and flow accumulation rasters were developed using the spatial analyst toolbar. Their

accumulation rasters was adjusted until it illustrated the streams on our observable maps

(Whipple et al., 2007.) This meant approximating the level of precipitation in our area, until it

matched with the stream networks we can see in the field.

At this point, we began to use new methods developed by Richardson, utilizing the new

ArcHydro Tools 2.0 provided by ESRI. We used the stream definition tool on our accumulation

raster. The stream to feature tool in ArcHydro Tools was then used to create shapefiles out of

these defined streams. The 3D line tool allowed us to assign z-axis coordinates to our shapefiles,

which were exported to Excel using the Arc3D Profiler Toolbar.

These profiles were examined for changes in concavity, and evidence of knickpoints. We

noted whenever knickpoints of similar elevation were found reflected through a connected

stream network.

Lithological and Fault Analysis

Characteristic shape files of NC bedrock and faulting were downloaded from USGS

statewide geologic maps database. These were then clipped to our study area and compared to

our map of knickpoints, to determine whether lithological changes and fault variations play some

role in creating our observed knickpoints. We noted each knickpoint near a lithological

transition, as well as the type of transition. We also noted if any knickpoints were located near

faults.

Results

Topographic Analysis

Our observation of topographic maps allowed us to target five primary drainage networks

on the BRE east of Boone, NC. These were labelled and studied based on the name of their

headwater tributary, going NE to SW: Wildcat Creek, Osborne Creek, Stanley Branch, North

Prong Lewis Fork, and Stone Creek (figure 1). Five primary tributaries were defined for each

drainage network, chosen based on how far they extended into the escarpment. They are

numbered 1-5 in each catchment. Wildcat, Osborne, and Stanley connect off of the escarpment,

eventually reaching the Kerr Scott Reservoir in the NE. North Prong Lewis Fork and Stone

Creek connect and drain to Rhodhis Lake in the SW. There are evident abandoned buttes and

gorges, which can be seen in the light brown branching arms of high elevation, and the low

elevation dark brown gorges of the digital elevation model (figure 1).

The three northern catchments (Wildcat Creek, Osborne Creek, and Stanley Branch) have

gorges at ~280 m. The two southern catchments (North Prong Lewis Fork and Stone Creek) have

gorges at ~320 m. The two southern catchments have unique buttes at ~600 m.

fig. #1

This map shows the study area, identified stream networks, and their headwaters.

Longitudinal Profiles

The longitudinal profiles contained a number of knickpoints. Though high in number,

these knickpoints demonstrate few trends (figures 2-6). They are often very subtle, and occur at

intermediate elevations, not atop the escarpment. The only significant trend amongst knickpoints

occurs in the North Prong Lewis Fork profiles. All five tributaries have separate knickpoints

occurring at ~600 m.

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600 Wildcat Branch Profiles

Wildcat Branch Profile 1Wildcat Branch Profile 2Wildcat Branch Profile 3Wildcat Branch Profile 4Wildcat Branch Profile 5

Profiles to Common Point (m)

Elevation (

m)

fig #2

Wildcat Branch contained four knickpoints. Tributaries 1 and 3 have a shared point at 376 m. Tributary 2 has a point at 382 m. Tributary 4 has two points at 410 and 490 m. Only the 410 m point was distinct.

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Osborne Creek Profile 1Osborne Creek Profile 2Osborne Creek Profile 3Osborne Creek Profile 4Osborne Creek Profile 5

Profiles to Common Point (m)

Elevation (

m)

fig #3

Osborne Creek contained two knickpoints. All of the profiles shared a point at 297 m. Tributary 5 has a point at 545 m.

0.00 5000.00 10000.00 15000.00 20000.00 25000.00200

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Stanley Branch Profile 1Stanley Branch Profile 2Stanley Branch Profile 3Stanley Branch Profile 4Stanley Branch Profile 5

Profiles to Common Point (m)

Elevation (

m)

fig. #4

Stanley Branch contained four knickpoints. Tributary 1 had points at 523 m and 792 m. Tributary 4 had a point at 559 m. Tributary 5 had the only distinct point at 364 m.

0 2000 4000 6000 8000 10000 12000 14000 16000 18000200

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North Prong Profile 3

North Prong Profile 2

North Prong Profile 5

North Prong Profile 1

North Prong Profile 4

Profiles to Common Point (m)

Elevation (

m)

fig. #5

The North Prong contained 7 knickpoints. All of the tributaries shared a point at 371 m. Tributaries 1 and 3 share a point at 518 m. Tributary 2 has a point at 578 m. Tributary 3 has a point at 628 m. Tributary 4 has a point at 603 m. Tributary 5 has two points at 595m and 509 m.

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1400 Stoney Creek Profiles

Soney Creek Profile 1

Stoney Creek Profile 2

Stoney Creek Profile 3

Stoney Creek Profile 4

Stoney Creek Profile 5

Profiles to Common Point (m)

Elevation (

m)

fig. #6

Stone Creek contained four knickpoints. Tributaries 1, 2, 3, 4, and 5 shared a point at 390 m. Tributaries 1, 2, 3, and 4 shared two points at 526 m and 600 m. Tributary 5 has a point at 582 m.

Lithological and Fault Analysis

While there were no knickpoints located close to faults, some did lie in close proximity to

lithological changes. These are marked by white spots on figure 7. The 297 m knick in Osborne

Creek lies on a transition from gneiss to metasedimentary rock. The 382 m knick in Wildcat

Creek lies on a transition from gneiss to quartz diorite. The 490 m knick in Wildcat Creek lies on

a transition from granitic gneiss to gneiss. The 410 m knick in Wildcat Creek lies on a transition

from gneiss to quartz diorite.

fig. #7

This map illustrates the bedrock geology of the study area. Catchment areas are illustrated, and points indicate knickpoints along the profiles.

EPCrestQ=2.47 h2.50

Equation 1. Discharge rate

N

Discussion

Our data suggests erosional forces actively working throughout the BRE, but the trends in

profiles are not strong enough to conclusively determine the root cause as internal or external

force. Numerous knickpoints with no relationship to lithology do indicate recent landscape

rejuvenation seeking equilibrium. We only see one instance of similar elevation knickpoints

reflected through connected tributaries. This occurs at ~600 m in all five North Prong Lewis

Fork profiles (figure 5). The clarity of knickpoint migration at similar elevations indicates some

form of transient erosion, at an intermediate elevation. It is transient because we see no similar

behavior in adjacent catchment areas.

Unlike other studies observing knickpoint migration on the BRE (Richardson, 2013,

Prince, 2010), our knickpoints were at more intermediate elevations (578-628 m), instead of atop

the escarpment. This would suggest that, while the BRE is maintaining its prominent elevation

elsewhere, some locations (including the North Prong of Lewis Fork) adjust transiently to

significantly lower elevations. The most significant instances of intermediate elevation

knickpoints exists in the North Prong Lewis Fork Profiles. Profiles 1-5 have a reflected

knickpoint at approximately 600m (figure 5). This may suggest that a capture event occurred in

this network at 600m, upstream of where it joins Stone Creek, reflecting rejuvenation solely in

the lower elevation reaches of the North Prong of Lewis Fork.

Our results found that, while lithological controls did not account for all of our knicks,

some points did lie over lithological transitions. This is clearly apparent in Wildcat and Osborne

Creek. Most of the knickpoints in both of these catchment areas lie directly on lithological

transitions. This indicates that these catchment areas may not be experiencing rejuvenation, or

Q=2.47 h2.50

Equation 1. Discharge rate

have already transitioned to an equilibrated state. This is contrary to most research, which

contradicts lithological dependence of faults along the BRE.

The topographic analysis of the BRE reveals stream profiles with no significant or

systematic high elevation knickzones throughout the study area. This result may indicate that the

remnant high elevation escarpment has transitioned from transience to equilibrium. Meanwhile,

the North Prong of Lewis Fork demonstrates a transient catchment, reflecting erosive

rejuvenation at ~600 m through an intermediate elevation knickzone network in all five

tributaries. Seemingly lithologically controlled and simple concave up profiles of Stone Creek

and Wildcat Creek indicate catchments which have achieved continuous equilibrium within the

study area.

Conclusions

Our study within the BRE details how transience is currently reflected in stream profiles.

This transience, preserved in the concavity of stream networks, indicates new and significant

behavior in the long term pattern of escarpment retreat. Profiles in the north (Stone Creek and

Wildcat Creek) demonstrate catchments of equilibrated erosion, and concavity influenced by

lithologic erodability. Because this is so heavily contested in literature, it warrants further field

analysis to provide verification of digital analysis. These catchments are a good example of why

it is difficult to understand the driving mechanisms of escarpment evolution. While transient

signals could have influenced these catchments historically, the evidence has simply been

removed. We observe the abandoned buttes and gorges indicating headward erosion, but are

unable to observe relevant patterns differentiating them in different networks at high elevations.

Locations of more recent or active stream capture need to be observed to determine its

Q=2.47 h2.50

Equation 1. Discharge rate

importance in the migration of the escarpment. The North Prong of Lewis Fork provides our best

indication a reflected knickzone in a transient tributary network. This knickzone, however, only

accounts for intermediate rejuvenation of the escarpment. This is a new process in our

understanding of how the escarpment migrates, because transient erosive signals now show the

potential to adjust to lower elevations, instead of maintaining a constant elevation atop the

escarpment.

In the future, drainage networks to the north and south of this study area should be

studied. Indications of transient erosion could be anywhere along the escarpment. The Linville

Gorge are, in particular, to the south, represents a significant amount of erosive action. It would

be interesting to study the extent of this erosion, and where it is demonstrates equilibrated or

transient profiles.

Q=2.47 h2.50

Equation 1. Discharge rate

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