Allegheny Portage Railroad National Historic Site 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2008/045 

National Park Service 

U.S. Department of the Interior 

Natural Resource Program Center 
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Park trail, Allegheny Portage Railroad NHS 

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Skew arch bridge, Allegheny Portage Railroad NHS 

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Allegheny Portage Railroad National Historic Site 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2008/045 

Geologic Resources Division 

Natural Resource Program Center 

P.O. Box 25287 

Denver, Colorado 80225 

September 2008 

U.S. Department of the Interior 

Washington, D.C.
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Printed copies of reports in these series may be produced in a limited quantity and they are only 

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index.cfm) or by sending a request to the address on the back cover. 

Please cite this publication as: 

Thornberry-Ehrlich, T. 2008. Allegheny Portage Railroad National Historic Site Geologic Resource 

Evaluation Report. Natural Resource Report NPS/NRPC/GRD/NRR—2008/045. National Park Service, 

Denver, Colorado. 

NPS D-112, September 2008 
 ALPO Geologic Resource Evaluation Report iii

Table of Contents 

List of Figures .............................................................................................................. iv

Executive Summary ...................................................................................................... 1

Introduction ................................................................................................................... 2

Purpose of the Geologic Resource Evaluation Program ............................................................................................2

Geologic Setting .........................................................................................................................................................2

History of the Allegheny Portage Railroad..................................................................................................................3

Geologic Issues............................................................................................................. 7

Mining and Energy......................................................................................................................................................7

Water..........................................................................................................................................................................7

Geologic Hazards.......................................................................................................................................................8

Recreational Demands ...............................................................................................................................................8

General Geology, Paleontology, and Miscellaneous Issues.......................................................................................9

Geologic Features and Processes............................................................................. 10

Connections between Geology and History..............................................................................................................10

The Allegheny Front .................................................................................................................................................10

Map Unit Properties .................................................................................................... 13

Map Unit Properties Table........................................................................................................................................15

Geologic History.......................................................................................................... 17

Glossary....................................................................................................................... 24

References................................................................................................................... 28

Appendix A: Geologic Map Graphic .......................................................................... 30

Appendix B: Scoping Summary................................................................................. 33

Appendix A: Geologic Map Graphic .......................................................................... 30

Attachment 1: Geologic Resource Evaluation Products CD
 NPS Geologic Resources Division iv 

List of Figures 

Figure 1. Map of Pennsylvania showing physiographic setting. .....................................................................................4

Figure 2. Map showing the 10 inclined planes of the Allegheny Portage Railroad.........................................................5

Figure 3. Historic Staple Bend Tunnel............................................................................................................................6

Figure 4. Inclined plane number 8................................................................................................................................11

Figure 5. Historic Lemon House inn and tavern ...........................................................................................................12

Figure 6. Geologic time scale.......................................................................................................................................20

Figure 7. Evolution of the landscape in the area of the Allegheny Portage Railroad National Historic Site..................21

Figure 8. View of railroad grade ...................................................................................................................................22

Figure 9. Diagram showing frost-wedge weathering ....................................................................................................23
 ALPO Geologic Resource Evaluation Report 1

Executive Summary 

This report accompanies the digital geologic map for Allegheny Portage Railroad National 

Historic Site in Pennsylvania, which the Geologic Resources Division produced in 

collaboration with its partners. It contains information relevant to resource management 

and scientific research. 

Allegheny Portage Railroad National Historic Site 

commemorates the pioneering lifestyle and highlights 

the 19th-century engineering effort that resulted in the 

construction of a railroad through the Allegheny 

Mountains. This railroad included 10 inclined planes and 

the nation’s first railroad tunnel. The Allegheny Portage 

Railroad allowed the transport of boats from the 

Pennsylvania canal over the Allegheny Mountains. 

The park protects the landscape in part of the Allegheny 

Front in central Pennsylvania. The park is located along 

the boundary between the Valley and Ridge and the 

Appalachian Plateaus physiographic province. This area 

displays a sharp contrast between the folded and faulted 

rocks on the eastern edge of the mountains versus the 

eroded, horizontal beds (plateaus) of the western side. 

The geologic substrate, as well as surficial processes 

(geologic, hydrologic, and biologic), determine the 

features of every landscape. Geologic processes gave rise 

to rock formations, mountains and valleys, escarpments 

and ridges, dissected plateaus, and coal deposits at 

Allegheny Portage Railroad National Historic Site. The 

landscape played a prominent role in the history of 

central Pennsylvania with respect to early settlement, 

mining activity, and the development of trade and 

transportation routes. Knowledge of the park’s natural 

and cultural resources, including the geology, directly 

influences resource management decisions in the park. 

Geologic processes and the features these processes 

produce can be interpreted to enhance the visitor 

experience. The park’s richness in geological, biological, 

historical, and cultural resources are key elements in 

land-use planning and in planning visitor uses in the 

park. A detailed geologic map, interpretive exhibits, and 

a road or trail log would enhance the visitor’s 

appreciation of the geologic history and dynamic 

processes that created the park’s natural landscape. 

In addition to natural processes, human activities have 

played a significant role in determining the surficial 

features at the park, as well as affecting the ecological 

response to natural changes. Mining, railroad 

construction, tunnel blasting, and human settlement 

have changed the landscape and its response to erosion 

and other surficial processes and can create noticeable 

changes to the landscape within a human’s life span. 

The following features, issues, and processes (discussed 

in the “Geologic Issues” section of this report) have 

geological importance and a high level of management 

significance within the park: 

• Mining and Energy 

Allegheny Portage Railroad is located in areas of 

central Pennsylvania known for coal mining. 

Abandoned and inactive mines pose health, safety, and 

environmental problems in the vicinity of the park. 

Geologic Resources Division (GRD) records indicate 

that the park has at least four abandoned mine land 

(AML) sites with 17 openings. All known shafts and 

adits in the park are sealed and do not present an 

immediate hazard (John Burghardt GRD, personal 

communication 2008). However, there is a history of 

and the potential for subsidence due to the collapse of 

underground mine workings in the park. Future 

construction activities need to consider the location of 

underground mines to minimize this hazard. 

Additionally, the acid mine drainage and heavy metal 

contamination that accompanies low pH runoff from 

sources outside the park is a serious resource 

management concern. 

• Water 

The historic site protects large parts of several 

watersheds, including important tributaries to the 

Ohio and Susquehanna Rivers. Intense seasonal 

thunderstorms and high runoff cause flooding along 

these waterways. This flooding threatens many of the 

historical structures and natural features at the park. It 

is crucial for resource management staff to understand 

the hydrogeologic system at the park, including 

ground-water movement. 

• Geologic hazards 

The steep terrain that characterizes the Allegheny 

Front region is prone to landslides, slumps, and 

rockfalls. In particular, areas containing resistant rock 

units undercut by erosion of weaker underlying units 

or areas altered by human use are susceptible to 

geologic hazards. Though not common, seismic 

activity is possible in central Pennsylvania. Even minor 

seismic tremors can trigger massive landslides and 

debris flows on steep and/or water-saturated slopes. 

• Recreational demands 

The site preserves natural lands for recreational use. 

Without proper management, overuse of certain areas 

can lead to contamination from human waste and 

trash, and degradation of the ecosystem, such as 

trampling and removal of vegetation, soil compaction, 

and increased erosion. 
 NPS Geologic Resources Division 2 

Introduction 

The following section briefly describes the National Park Service Geologic Resource 

Evaluation Program and the regional geologic setting of Allegheny Portage Railroad 

National Historic Site. 

Purpose of the Geologic Resource Evaluation Program 

The Geologic Resource Evaluation (GRE) Program is 

one of 12 inventories funded under the NPS Natural 

Resource Challenge designed to enhance baseline 

information available to park managers. The program 

carries out the geologic component of the inventory 

effort from the development of digital geologic maps to 

providing park staff with a geologic report tailored to a 

park’s specific geologic resource issues. The Geologic 

Resources Division of the Natural Resource Program 

Center administers this program. The GRE team relies 

heavily on partnerships with the U.S. Geological Survey, 

Colorado State University, state surveys, and others in 

developing GRE products. 

The goal of the GRE Program is to increase 

understanding of the geologic processes at work in parks 

and provide sound geologic information for use in park 

decision making. Sound park stewardship relies on 

understanding natural resources and their role in the 

ecosystem. Geology is the foundation of park 

ecosystems. The compilation and use of natural resource 

information by park managers is called for in section 204 

of the National Parks Omnibus Management Act of 1998 

and in NPS-75, Natural Resources Inventory and 

Monitoring Guideline. 

To realize this goal, the GRE team is systematically 

working towards providing each of the identified 270 

natural area parks with a geologic scoping meeting, a 

digital geologic map, and a geologic report. These 

products support the stewardship of park resources and 

are designed for non-geoscientists. During scoping 

meetings the GRE team brings together park staff and 

geologic experts to review available geologic maps and 

discuss specific geologic issues, features, and processes. 

The GRE mapping team converts the geologic maps 

identified for park use at the scoping meeting into digital 

geologic data in accordance with their innovative 

Geographic Information Systems (GIS) Data Model. 

These digital data sets bring an exciting interactive 

dimension to traditional paper maps by providing 

geologic data for use in park GIS and facilitating the 

incorporation of geologic considerations into a wide 

range of resource management applications. The newest 

maps come complete with interactive help files. As a 

companion to the digital geologic maps, the GRE team 

prepares a park-specific geologic report that aids in use 

of the maps and provides park managers with an 

overview of park geology and geologic resource 

management issues. 

For additional information regarding the content of this 

report and up to date GRE contact information please 

refer to the Geologic Resource Evaluation Web site 

(http://www.nature.nps.gov/geology/inventory/). 

Geologic Setting 

Allegheny Portage Railroad National Historic Site is 

located at the junction of the Appalachian Plateaus and 

the Valley and Ridge physiographic provinces. These 

provinces are divided into sections. The park is located 

within the transition zone between the Appalachian 

Mountain and Allegheny Front sections (fig. 1), 

extending from the eastern edge of the Appalachian 

Plateaus province (the Allegheny Front) to the summit of 

Cresson Ridge. 

Stream valleys dividing high, rounded ridges characterize 

the topography in the park. The ridgetops are composed 

of resistant Paleozoic sandstone. Valleys separate the 

ridgetops and are underlain by less-resistant units, such 

as carbonate rocks and shale that have been 

preferentially eroded. 

A general east-to-west description of several of the 

different physiographic provinces of the Appalachian 

Mountains follows. This information is relevant to 

understanding the geologic history of Allegheny Portage 

Railroad National Historic Site. 

Piedmont Province 

The “Fall Line,” or “Fall Zone,” marks a transitional zone 

where the softer, less- consolidated sedimentary rocks of 

the Atlantic Coastal Plain to the east intersect the harder, 

more resistant metamorphic rocks to the west forming 

an area of ridges, waterfalls, and rapids. Examples of the 

transition are present in the Potomac Gorge of the 

Chesapeake and Ohio Canal National Historic Park. The 

Piedmont physiographic province encompasses the Fall 

Line westward to the Blue Ridge Mountains. The 

eastward-sloping Piedmont formed through a 

combination of folds, faults, uplifts, and erosion. The 

resulting eastern landscape of gently rolling hills starting 

at 60 m (197 ft) in elevation becomes gradually steeper 

westward toward the western edge of the province and 

reaches 300 m (984 ft) above sea level. The Piedmont 

Plateau is composed of hard, crystalline igneous and 

metamorphic rocks, such as schist, phyllite, slate, gneiss, 

and gabbro. 

Blue Ridge Province 

The Blue Ridge province is located along the eastern 

edge of the Appalachian Mountains and includes the 

highest elevations in the Appalachian Mountain system, 

in Great Smoky Mountains National Park in North 

Carolina and Tennessee. Precambrian and Paleozoic 

igneous, sedimentary, and metamorphic rocks were 
 ALPO Geologic Resource Evaluation Report 3

uplifted during several orogenic events forming the 

steep, rugged terrain. Resistant Cambrian quartzite forms 

most of the high ridges, whereas Precambrian 

metamorphic rocks underlie the valleys. 

The elongate belt of the Blue Ridge stretches from 

Georgia to Pennsylvania. Eroding streams have 

narrowed the northern section of the Blue Ridge 

Mountains into a thin band of steep ridges that rise to 

heights of approximately 1,200 m (3,937 ft). 

Valley and Ridge Province 

Long, parallel ridges separated by valleys 100−200 m 

(330−660 ft) below the ridges characterize the landscape 

of the Valley and Ridge physiographic province (also 

referred to as the “Ridge and Valley province”). The 

valleys formed in areas of more easily eroded shale and 

carbonate formations, and the ridges are commonly 

composed of more resistant sandstone units. The 

province contains strongly folded and faulted 

sedimentary rocks in central Pennsylvania. The Valley 

and Ridge province averages approximately 80 km (50 

mi) in width. The eastern part of the Valley and Ridge 

province is the Great Valley section, a rolling lowland 

formed on folded carbonate rocks and shale. It is 

connected to the Piedmont province by streams that cut 

through the Blue Ridge Mountains. 

Appalachian Plateaus Province 

Compared to the eastern Appalachian physiographic 

provinces, the Appalachian Plateaus province is relatively 

undeformed. Instead of the highly folded and inclined 

strata of the Valley and Ridge province, the rock layers 

are nearly flat. A steep scarp known as the Allegheny 

Front bounds the plateau on the east. This escarpment 

rises abruptly from 300 to 900 m (1,000 to 3,000 ft). 

Maximum elevations at this front are generally greater 

than those of the ridges in the Valley and Ridge province. 

In Pennsylvania, elevations range from 530 to 900 m 

(1,750 to 3,000 ft). Deep ravines carved into the 

horizontal sedimentary rock layers characterize the 

topography of this province. Geologic units are typically 

repetitious sequences of shale, coal, limestone, and 

sandstone. Erosion of these units has created a rugged, 

jumbled topographic surface. The northern parts of the 

province in Pennsylvania and New York are typically 

more rounded hills with gentle slopes. 

History of the Allegheny Portage Railroad 

Allegheny Portage Railroad National Historic Site 

preserves parts (1,296 acres) of the 1834−54 inclinedplane railroad system. This type of railroad was the first 

constructed over the Allegheny Mountains of 

Pennsylvania. The inclined-plane railroad (with 10 

planes) helped establish the interior of the United States 

as a trade and settlement center (fig. 2). 

The main unit, designated as a national historic site on 

August 31, 1964, is located approximately 19 km (12 mi) 

west of Altoona, Pennsylvania, and the Staple Bend 

Tunnel unit is located approximately 6 km (4 mi) east of 

Johnstown, Pennsylvania. The historic Staple Bend 

Tunnel was the first railroad tunnel in the country (fig. 

3). The site’s boundaries changed November 10, 1978, 

and December 19, 2002. 

The park stretches 59 km (37 mi) through the Allegheny 

Mountains in Blair and Cambria Counties. The 

mountains there range in elevation from 346 m (1,135 ft) 

to 734 m (2,408 ft) above sea level. Prior to construction 

of the railroad, the mountains were a significant obstacle 

to boat trade between the eastern and western divisions 

of the Pennsylvania Mainline Canal. After the railroad 

was built, trains portaged the boats across mountain 

slopes of more than 5 degrees. 

The main unit of the park lies on the divide between the 

Chesapeake Bay watershed to the east and the Ohio 

River basin on the west. Blair Gap Run flows from 

Cresson Summit towards the Juniata River, and Bradley 

Run flows towards the West Branch of the Susquehanna 

River on the Chesapeake Bay side and on the Ohio River 

side water flows parallel to the Little Conemaugh River. 
 NPS Geologic Resources Division 4 

Figure 1. Map of Pennsylvania showing physiographic setting of Allegheny Portage Railroad National Historic Site (National Historic 

Site). Red lines indicate boundaries between major physiographic provinces. Northern terminus of Blue Ridge province is located by 

the black arrow. Location of park is indicated by a green circle. Map information modified from Pennsylvania Geological Survey (map 

13, 2000). Graphic by Trista L. Thornberry-Ehrlich (Colorado State University). 
 ALPO Geologic Resource Evaluation Report 5

Figure 2. Map showing the 10 inclined planes (numbered) of the Allegheny Portage Railroad (red line) over the Cresson Summit of the 

Allegheny Front between the Valley and Ridge and Appalachian Plateaus physiographic provinces. Note locations of Staple Bend 

Tunnel, Conemaugh Viaduct, Skew Arch Bridge, and Lemon House. Map is not to scale. Graphic revised from 

http://www.cr.nps.gov/nr/twhp/wwwlps/lessons/23allegheny/23visual1.htm by Trista L. Thornberry-Ehrlich (Colorado State University). 
 NPS Geologic Resources Division 6 

Figure 3. Historic Staple Bend Tunnel at Allegheny Portage Railroad National Historic Site before and after its restoration in 2000−2001. 

The tunnel linked inclined planes numbers 1 and 2. Top photograph is courtesy of the Railroad Museum of Pennsylvania. Bottom 

photograph is from http://www.todayinsci.com/cgi-bin/indexpage.pl?http://www.todayinsci.com/3/3_18.htm (accessed August 24, 2006) 
 ALPO Geologic Resource Evaluation Report 7

Geologic Issues 

A Geologic Resource Evaluation scoping session was held for Allegheny Portage 

Railroad National Historic Site on June 22, 2004, to discuss geologic resources, address 

the status of geologic mapping, and assess resource management issues and needs. This 

section synthesizes the scoping results, in particular those issues that may require 

attention from resource managers. 

Mining and Energy 

Although there are no current mining operations in 

Allegheny Portage Railroad National Historic Site, coal 

mining has long been associated with central 

Pennsylvania. The Pennsylvanian Allegheny Group 

includes the commercially viable Freeport, Kittanning, 

and Brookville-Clarion coal seams within the park. 

Extensive underground coal mines in the park, including 

4 sites with 17 openings, produce acid mine drainage and 

have the potential for subsidence and collapse of mine 

workings. The Pennsylvania Department of 

Environmental Protection maps show mines and 

document subsidence associated with the historic 

mining. In 1992 it was reported that subsidence related to 

historic mining occurred between Lemon House, a 

historic structure, and Highway22. Future construction 

activities need to consider the location of underground 

mines to minimize this hazard. It would also be prudent 

for park staff to develop contingency plans for 

temporary fencing, warning signs, and backfilling to 

reduce hazards following subsidence. 

GRD records indicate that all known shafts and adits in 

the park are sealed and do not present an immediate 

hazard (John Burghardt GRD, personal communication 

2008). However, a formal AML inventory is needed to 

identify potential threats. 

Abandoned and inactive mines pose environmental and 

health problems for the Allegheny Portage Railroad area. 

Foremost among these is acid mine drainage and residual 

heavy metal contamination of ground water, surface 

water, and soil. Mine areas within and upstream from the 

Allegheny Portage Railroad units produce acid mine 

drainage. Fluvial sediments (sediments carried by 

streams and rivers) can contain metals and other waste 

materials. Springs and seeps in the Staple Bend Tunnel 

unit are being monitored for acid mine drainage due to 

their proximity to local coal mines. 

The acidity of coal-mine drainage is caused primarily by 

the oxidation of the mineral pyrite, which is found in 

coal, coal overburden, and mine waste piles. The rate of 

pyrite oxidation depends on the following: reactive 

surface area of the pyrite, the oxygen concentration and 

pH of the water, the forms of pyrite, and the presence of 

iron-oxidizing bacteria (Thiobacillus ferroxidans) 

(http://energy.er.usgs.gov/health_environment/acid_min

e_drainage/index.htm Accessed September 2008). 

Acid mine drainage can leach major elements such as 

aluminum, calcium, and magnesium and trace elements 

such as copper and zinc from surrounding rocks. Newlyformed acidic, metal-rich waters can then flow into 

receiving streams. These metals are transported from the 

vicinity of a mine by ground and surface water as 

dissolved ions, suspended sediment, or as part of the 

bedload in a stream (Madison et al. 1998). 

Iron ore was locally mined from the Silurian Clinton 

Group rock units, where it was present in limestone as 

siderite (iron carbonate). Limestone necessary for 

smelting was also available and locally abundant. Effects 

of this mining are unknown at the park. The area also has 

local sandstone quarries for building materials. Stones 

from these quarries were used in the construction of 

Staple Bend Tunnel. 

In addition, research that GRD conducted in 1996 

indicates that two oil and gas pipelines cross Allegheny 

Portage National Historic Site. Pipeline locations should 

be considered when planning park construction or 

resource management activities (Kerry Moss, GRD 

personal communication 2008) 

Inventory, Monitoring, and Research Suggestions for Mining 

Issues 

• Determine pathways for the contaminants and work 

with mine owners, regulatory agencies and others to 

remediate the acid mine drainage and other problems 

associated with the upstream mines. 

• Work closely with state officials and owners of 

adjacent mining operations to ensure that blasting 

activities and off-site acid mine drainage do not 

increase the likelihood of subsidence or resource 

damage in the park. 

• Continue mine remediation in the park. 

• Monitor biota (e.g., aquatic insects) for heavy-metal 

contamination. 

• Incorporate Pennsylvania Department of 

Environmental Protection maps of mine features into a 

GIS database. 

• Have a trained inspector investigate mine features 

(shafts and drainage) at all park units, including 

collapsed features at Staple Bend Tunnel unit. 

Water 

It is crucial for resource management to understand the 

hydrogeologic system at the park. Understanding 

ground-water flow is necessary to predict hydrologic 

response to inputs such as contaminants and other 
 NPS Geologic Resources Division 8 

wastes. The movement of nutrients and contaminants in 

aquifers can be modeled by monitoring the composition 

of hydrologic inputs, such as rainfall, and outputs, such 

as streamflow. Other sources of inputs include 

windblown materials, surface runoff, sewage outfalls, 

landfills, and fill dirt. Surface and ground-water flow are 

interconnected, and contamination in one part of the 

system may be transferred to, and detected in, other 

parts of the hydrogeologic system. Thus, streams provide 

a measure of the chemical status of the hydrologic system 

in a watershed. Consistent measurement of these 

parameters is crucial to establishing baseline conditions 

that can be used during monitoring. 

Blair Gap Run is a mountain stream that is regularly 

monitored by the park. The headwaters of this 

Susquehanna River tributary are within the main unit of 

the park. The trace of the railroad followed Blair Gap 

across the mountains. Water quality of the Little 

Conemaugh River at the Staple Bend Tunnel unit has 

been degraded by acid mine drainage from sources 

within and adjacent to parklands. Park staff are currently 

taking steps to remediate the contamination of water 

flowing through the park. 

There is a risk of damage and loss of resources due to 

flooding associated with the rivers and streams at 

Allegheny Portage Railroad National Historic Site. 

Floods are most common during spring runoff and 

seasonal storm events. 

There are several springs and seeps within Allegheny 

Portage Railroad National Historic Site. Seeps within the 

Staple Bend Tunnel unit are fed from collapsed mines 

and have high concentrations of heavy metals. These 

seeps are associated with the formation of iron mounds 

on park property. 

Inventory, Monitoring, and Research Suggestions for Water 

Issues 

• Research the influence of bedrock and topography on 

local watersheds at Allegheny Portage Railroad 

National Historic Site. 

• Develop a park-wide flood-warning system and 

evacuation plan. 

• Consult Pennsylvania Geological Survey water reports 

to obtain further data on the hydrologic system in the 

park. 

Geologic Hazards 

The steep terrain in the Allegheny Front that prompted 

construction of the Allegheny Portage Railroad is prone 

to geologic hazards, including landslides, slumps, and 

rockfalls. Historic features in the park, including the 

railroad and Staple Bend Tunnel, are at risk of damage 

from slope failures, frost wedging, and seismic activity. 

The geologic units in the park are a heterogeneous mix of 

shale, sandstone, siltstone, limestone, dolomite, 

conglomerate, and mudstone. Clay-rich units such as 

shale and mudstone may deteriorate when saturated with 

water and are prone to fail when exposed on steep 

slopes. When more resistant rock units such as 

sandstone and limestone overlie weaker units 

preferential erosion may undercut the resistant rocks 

and result in rockfalls. 

The relationship between, active faulting, moderate or 

low levels of seismicity, and the risks to structures and 

human interests in the eastern United States is poorly 

established (Jacob 1989). Though not common, there is 

seismic activity in central Pennsylvania. Some historic 

earthquakes in the area have been attributed to blasting 

during mining (in 1893 and 1939). However, a natural ~5 

magnitude quake did occur in northwest Pennsylvania in 

1986 (Scharnberger, 1987). Even minor seismic tremors 

can trigger massive landslides and debris flows on steep 

and/or water-saturated slopes. Risks associated with 

seismic activity should be assessed at Allegheny Portage 

Railroad. 

Inventory, Monitoring, and Research Suggestions for 

Geologic Hazards 

• Investigate clay-rich layers exposed on slopes for 

vulnerability to slope failure. 

• Monitor steep slopes along the Allegheny Front for 

signs of landslides (seeps, springs, cracks, or scarps). 

• Monitor potential for roof fall in Staple Bend Tunnel; 

remediate potential problem areas. 

• Work with geologic societies and others to encourage 

seismic monitoring in the park and throughout central 

Pennsylvania. 

Recreational Demands 

Allegheny Portage Railroad National Historic Site 

provides numerous recreational opportunities: hiking, 

cross-country skiing, bird watching, picnicking, 

photography, and other activities. The park receives 

many visitors, especially during the summer months. As 

many as 123,215 people visited the park in 2007. These 

visitors place increasing demands on the resources of the 

park, and management concerns include trail erosion, 

waste treatment, water quality, and potential danger 

from slope hazards (landslides, rockfalls). 

Many trails wind through preserved biological, 

historical, and geological environments at the park. 

Many of these are especially fragile, and the wandering 

of visitors off trails may degrade park resources, placing 

delicate ecosystems at risk for contamination from 

human waste and garbage. To minimize the impact, the 

park directs visitors to designated trails and established 

picnic areas. 

Several streams, including Blair Gap Run and the Little 

Conemaugh River, enhance the natural beauty of the 

park. As with hiking, overuse of certain areas can lead to 

degradation of the ecosystem and increased erosion of 

riparian areas. 
 ALPO Geologic Resource Evaluation Report 9

Inventory, Monitoring, and Research Suggestions for 

Recreational Demands 

• Inventory and monitor geologic and other natural 

resources to further identify human impacts to 

resources, including springs, wetlands, and marsh flora 

within the park. 

• Design wayside exhibits to encourage responsible use 

of park resources. 

General Geology, Paleontology, and Miscellaneous 

Issues 

The unique geology of the Allegheny Front region and 

the Valley and Ridge physiographic province has sparked 

research interest among geologists for many years. 

Railroad cuts and other developments have exposed 

excellent outcrops for geologic research (Inners 1989). 

Additional research would help resource managers at the 

park better understand the geology and the relationships 

among physical processes in the ecosystem. 

Rocks in the park area contain fossils that record the 

beginnings of ancient life (fossil algae called 

stromatolites) through the evolution of land plants 

(Stoher 1992), and the mass extinction following the 

Permian Period. These paleontological resources could 

be displayed at the park for the education of visitors. 

In addition to its interest for general research, the park 

serves as an outdoor classroom and provides an 

opportunity to interpret the engineering efforts and 

reasons behind the construction of Allegheny Portage 

Railroad. This area has been a trade corridor since 

prehistoric times and continues today as part of the 

larger Susquehanna Valley corridor (Suciu et al. 1992). 

Inventory, Monitoring, and Research Suggestions for 

General Geology and Miscellaneous Issues 

• Map the geomorphology of the park. 

• Study cultural resources to better understand 

landscape evolution, focusing on Native American 

sites, early settlement activities, and trade 

(transportation). 

• Study paleontological resources within the park. 

(Interested parties can refer to Vince Santucci’s report 

on Eastern Rivers and Mountains Network 

paleontology.) 
 NPS Geologic Resources Division 10 

Geologic Features and Processes 

This section describes the most prominent and distinctive geologic features and processes 

in Allegheny Portage Railroad National Historic Site. 

Connections between Geology and History 

One of the major goals of the park is to preserve the 

historical context of the area; this includes preserving 

and restoring the inclined railroad planes (fig. 4), historic 

buildings, and the landscape around them. Maintaining 

the cultural landscape often means controlling natural 

processes, which presents several management 

challenges. 

Geologic processes that affect slopes, such as landsliding, 

slumping, chemical weathering, block sliding, and slope 

creep, constantly change the landscape at the park. 

Runoff erodes sediments from open areas and carries 

them down streams and gullies. Erosion naturally lowers 

higher areas, and the lower areas are filled in with 

sediments, modifying the historic context of the 

landscape. 

The history of the park is heavily influenced by its 

geology. The valleys and ravines carved into the 

Allegheny Front region made travel and farming difficult. 

This natural barrier led to many transportation 

innovations that allowed safe and relatively easy passage 

through the mountains. Transportation growth 

continues in the present day with development of the 

Pennsylvania interstate corridors. 

Allegheny Portage Railroad National Historic Site 

highlights the engineering marvel of the construction of 

10 inclined railroad planes that allowed the portage of 

boats from the Pennsylvania canal over the Allegheny 

Mountains (fig. 4). This was the first rail crossing of the 

Allegheny Mountains, and Staple Bend Tunnel, at 275 m 

(900 ft), was the first railroad tunnel in the country. 

The portage railroad and tunnel were part of a larger 

system of canals and railroads that connected the trade 

route between Philadelphia and Pittsburgh and operated 

until 1900. Coal and iron mines dotted the landscape, as 

well as regional furnaces to smelt the iron ore. These 

features document the early industrial development of 

the area. However, they pose environmental and safety 

hazards in the park and region (see “Geologic Issues” 

above). 

The history of the area begins long before the 

development of the railroad in the mid-1800s. Native 

American settlements and tool quarries predate local 

European settlement. Earliest known evidence of Native 

American activity nearby dates from 10,000 years ago at 

Sheep rock, near Huntingdon, Pennsylvania. Even the 

name “Allegheny” stems from the mound-building 

Alligewi, or Allegheny Indians, from western 

Pennsylvania (Wells 1973). 

The Native Americans left artifacts and morphological 

changes in the landscape, such as terraced campsites, 

tool quarries, and other archaeological sites in the 

Susquehanna River Valley and the Allegheny Mountains 

(Suciu et al. 1992). 

It is a challenge to balance the management of cultural 

and natural resources. For example, the restoration of a 

historic building (such as the Lemon House or the 

Engine House) may require altering or removing 

surrounding terrain, or planting non-native plant species. 

In order to preserve cultural sites in the park, such as the 

Lemon House (fig 5), Engine House, Skew Arch Bridge, 

and Staple Bend Tunnel, geologic and other natural 

physical processes may be affected. 

The Allegheny Front 

The Allegheny Front is a sharp escarpment that divides 

the Appalachian Plateaus and the Valley and Ridge 

physiographic provinces. This topographic feature 

extends from northern Pennsylvania into southeastern 

West Virginia. Overall, the front has an elevation change 

of roughly 610 m (2,000 ft) and is approximately 280 km 

(174 mi) long. Historically, the Allegheny Front was an 

imposing barrier to east-west trade and transportation 

routes. Thus, this topographic feature was the reason for 

the construction of the Allegheny Portage Railroad. 

The eastern side of the Allegheny Front consists of 

rounded and linear hills dissected by narrow valleys. 

These hills step up to the top of the escarpment and are 

composed of rocks that are highly folded, faulted, and 

deformed as a result of the Alleghenian orogenic event. 

In stark contrast, the western side of the front has 

undulating hills sloping away from the escarpment and is 

composed of relatively undeformed, horizontally layered 

rocks that resemble the sediments as they were 

deposited. 

Predominant rock types of the Allegheny Front include 

sandstone (with some local conglomerate), siltstone, and 

shale (Whitfield et al. 2001). Geologic features associated 

with the front in Pennsylvania include Blue Knob, Horse 

Shoe Curve, Hunter Rocks, Turtle Rocks, Wolf Rocks, 

and Wopsononock Lookout. 
 ALPO Geologic Resource Evaluation Report 11

Figure 4. Inclined plane number 8 of the Allegheny Portage Railroad., Painting by George Storm, date unknown; photograph is 

courtesy of the National Park Service. 
 NPS Geologic Resources Division 12 

Figure 5. Historic Lemon House inn and tavern near the top of inclined plane number 6 at Allegheny Portage Railroad National Historic 

Site. Photograph is courtesy of the Railroad Museum of Pennsylvania. 
 ALPO Geologic Resource Evaluation Report 13

Map Unit Properties 

This section identifies characteristics of map units that appear on the Geologic Resource 

Evaluation digital geologic map of Allegheny Portage Railroad National Historic Site. 

The accompanying table is highly generalized and is provided for background purposes 

only. Ground-disturbing activities should not be permitted or denied on the basis of 

information in this table. 

Geologic maps facilitate an understanding of Earth, its 

processes, and the geologic history responsible for its 

formation. Hence, the geologic map for Allegheny 

Portage Railroad National Historic Site informed the 

“Geologic History,” “Geologic Features and Processes,” 

and “Geologic Issues” sections of this report. Geologic 

maps are essentially two-dimensional representations of 

complex three-dimensional relationships. The various 

colors on geologic maps represent rocks and 

unconsolidated deposits. Bold lines that cross and 

separate the color patterns mark structural features such 

as faults and folds. Point symbols indicate features such 

as dipping strata, sample localities, mine features, wells, 

and cave openings. 

Incorporation of geologic data into a geographic 

information system (GIS) increases the utility of geologic 

maps and clarifies spatial relationships to other natural 

resources and anthropogenic features. Geologic maps 

are indicators of water resources because they show 

which rock units are potential aquifers and are useful for 

finding seeps and springs. Geologic maps do not show 

soil types and are not soil maps, but they do show parent 

material, a key factor in soil formation. Furthermore, 

resource managers have used geologic maps to make 

correlations between geology and biology; for instance, 

geologic maps have served as tools for locating 

threatened and endangered plant species. 

Although geologic maps do not show where future 

earthquakes will occur, the presence of a fault indicates 

past movement and possible future seismic activity. 

Geologic maps will not show where the next landslide, 

rockfall, or volcanic eruption will occur, but mapped 

deposits show areas that have been susceptible to such 

geologic hazards. Geologic maps do not show 

archaeological or cultural resources, but past peoples 

may have inhabited or been influenced by various 

geomorphic features that are shown on geologic maps: 

alluvial terraces may preserve artifacts, and inhabited 

alcoves may occur at the contact between two rock units. 

The features and properties of the geologic units in the 

following table correspond to the accompanying digital 

geologic data. Map units are listed from youngest to 

oldest. Please refer to the geologic time scale (fig. 6) for 

the age associated with each time period. This table 

highlights characteristics of map units such as 

susceptibility to hazards; the occurrence of fossils, 

cultural resources, mineral resources, and caves; and the 

suitability as habitat or for recreational use. The 

following are source data for the GRE digital geologic 

map: 

Whitfield, T.G. and others. 2001. Digital Bedrock Geology 

of Pennsylvania, Johnstown and Tyrone 30' x 60' 

quadrangles, Pennsylvania. Scale 1:250,000. PAGS 

30’x60’ Digital Quadrangle maps. Pennsylvania 

Geological Survey. 

McElroy, T.A. 1998. Groundwater resources of Cambria 

County, Pennsylvania. Scale 1:50,000. Water Resource 

Report W 67. Pennsylvania Geological Survey. 

Using ESRI ArcGIS software, the Geologic Resource 

Evaluation team created a digital geologic map from this 

source. GRE digital geologic-GIS map products include 

data in ESRI personal geodatabase, shapefile, and 

coverage GIS formats, layer files with feature symbology, 

FGDC metadata, a Windows HelpFile that contains all of 

the ancillary map information and graphics, and an ESRI 

ArcMap map document file that easily displays the map. 

GRE digital geologic data are included on the attached 

CD and are available through the NPS Data Store 

(http://science.nature.nps.gov/nrdata/). 

Allegheny Portage Railroad National Historic Site lies 

near the boundary between the Valley and Ridge and 

Appalachian Plateaus physiographic provinces. The 

oldest rocks in the park area are the Cambrian 

Waynesboro, Pleasant Hill, Warrior, and Gatesburg 

Formations. These units are deformed sedimentary 

rocks, including dolomite, limestone, sandstone, and 

shale. The limestone and dolomite units locally contain 

fossil algae (stromatolites) from early Cambrian seas 

(Berg et al. 1980; Whitfield et al. 2001). 

Ordovician units such as the Nittany, Stonehenge/Larke, 

Bellefonte, Axemann, Coburn through Loysburg, 

Reedsville, Bald Eagle, and Juniata Formations were 

deposited atop the Cambrian units. Cherty dolomite, 

shale, and fossiliferous limestone dominate the lower 

units. Siltstone, sandstone, and conglomerate (in the Bald 

Eagle Formation) become more abundant upwards in the 

Ordovician stratigraphic sequence. 

Following the Taconic orogeny, Silurian units such as the 

Tuscarora Formation, Clinton Group, Bloomsburg, 

Mifflintown, and Wills Creek Formations recorded the 

gradual return to tectonic quiescence and intense 

erosion of the newly formed Taconic highlands. 

Sediments changed from largely terrigenous sandstone 

to mud, and eventually to marine carbonate units with 
 NPS Geologic Resources Division 14 

occasional storm-deposited layers as sea level rose and 

the mountains were beveled (Cotter 1983). 

The Keyser and Tonoloway Formations contain 

fossiliferous limestone and mud-cracked shale that 

record the transition between the Silurian and Devonian 

Periods. Marine deposition continued throughout the 

Devonian as seen in the calcareous shale, siltstone, chert, 

and fossiliferous limestone of the Onondaga and Old 

Port Formations, the Hamilton Group, and the Braillier 

and Harrell Formations. 

Sandstone and mudstone become more prevalent in the 

Scherr, Foreknobs, and Catskill Formations, whereas the 

argillaceous sandstone and shale with local conglomerate 

of the Rockwell and the Shenango through Oswayo 

Formations recorded the transition from the Devonian 

to the Mississippian Periods (Berg et al. 1980; Whitfield 

et al. 2001). 

Map units from the Mississippian and Pennsylvanian 

Periods include the Pocono Formation, Burgoon 

Sandstone, Loyalhanna, and Mauch Chunk Formations, 

the Pottsville and Allegheny Groups, and the Glenshaw 

and Casselman Formations. These units contain 

abundant sandstone and siltstone, some conglomerate 

layers, shale, and limestone; they are also fossiliferous, 

containing remains of vast marshy peat swamps and 

wetlands. They contain significant amounts of coal, 

including the commercially valuable Freeport, 

Kittanning, and Brookeville-Clarion coals (Whitfield et 

al. 2001). 

Mesozoic and all but the most recent Cenozoic rock 

units are missing from the landscape surrounding 

Allegheny Portage Railroad. Recent, Quaternary 

alluvium deposits were deposited in the river valleys. 

These consist of unconsolidated clay, silt, sand, gravel, 

and sparse boulder deposits (McElroy 1998). 

The following pages present a tabular view of the 

stratigraphic column and an itemized list of features for 

each map unit. Map units are listed from youngest to 

oldest; please refer to the geologic time scale (fig. 6) for 

the age associated with each time period. This table 

includes properties of each map unit such as map 

symbol, name, description, resistance to erosion, 

suitability for development, hazards, potential 

paleontologic resources, cultural and mineral resources, 

potential karst issues, potential for recreational use, and 

global significance. 
 ALPO Geologic Resource Evaluation Report 17

Geologic History 

This section describes the rocks and unconsolidated deposits that appear on the digital 

geologic map of Allegheny Portage Railroad National Historic Site, the environment in 

which those units were deposited, and the timing of geologic events that created the 

present landscape. 

Allegheny Portage Railroad National Historic Site is on 

the ridge that forms the drainage divide between the 

Chesapeake Bay and Ohio River watersheds. This area 

corresponds with the boundary between the Valley and 

Ridge and Appalachian Plateaus physiographic provinces 

in central Pennsylvania. Physiographic features 

associated with the long geologic history of the 

Appalachian Mountains and the evolution of the eastern 

coast of the North American continent can be seen in the 

park. A regional perspective is presented here to connect 

the landscape and geology of the park to its 

surroundings. 

The recorded history of the Appalachian Mountains 

begins in the Proterozoic (fig. 6). During the Grenville 

orogeny (mid-Proterozoic), a supercontinent formed 

that consisted of most of the continental crust in 

existence at that time. This included the crust of North 

America and Africa. Sedimentation, deformation, 

plutonism, and volcanism are all apparent in the 

metamorphic granite and gneiss in the core of the 

modern Blue Ridge Mountains to the south and east of 

Allegheny Portage Railroad (Harris et al. 1997). These 

rocks formed over a period of approximately 100 million 

years and are more than a billion years old, making them 

among the oldest rocks in this region. They were later 

uplifted and thus exposed to erosion for hundreds of 

millions of years. Their leveled surface forms a basement 

upon which all other rocks of the Appalachians were 

deposited (Southworth et al. 2001). 

The late Proterozoic, roughly 800−600 million years ago 

(Ma), brought extensional rifting to the area. The crustal 

extension created fissures through which massive 

volumes of basaltic magma were extruded (fig. 7A). The 

volcanism lasted tens of millions of years and alternated 

between flood basalt flows and ash falls. The volcanic 

rocks covered the granitic/gneissic basement in central 

Pennsylvania. 

Continued crustal extension caused the supercontinent 

to break up, and a basin formed that eventually became 

the Iapetus Ocean. This basin subsided, and sediments 

that would eventually form the rock units of the 

Appalachian Mountains were deposited (fig. 7B). Some 

of the sediments were deposited as alluvial fans, large 

submarine landslides, and turbidity flows, which today 

preserve their depositional features. 

These late Proterozoic sediments are exposed 

throughout eastern Pennsylvania. There is a good 

exposure of these rocks on South Mountain some 100 

km (60 mi) southeast of the site (Chilhowee Group: 

Loudoun Formation, Weverton Formation, Harpers 

Formation, and Antietam Formation) (Southworth et al. 

2001). 

Extensive sand, silt, and mud deposited in near-shore, 

deltaic, barrier island, and tidal-flat areas were associated 

with the shallow marine setting along the eastern 

continental margin of the Iapetus Ocean (Schwab 1970; 

Kauffman and Frey 1979; Simpson 1991). In addition, 

widespread carbonate sediments, sand, and mud, such as 

the Cambrian Waynesboro, Pleasant Hill, and Gatesburg 

Formations in the Allegheny Portage Railroad area, were 

deposited on top of the Chilhowee Group. They 

represent part of a grand platform that thickened to the 

east and that persisted during the Cambrian and 

Ordovician Periods (545−480 Ma) (Means 1995). 

Ordovician units such as the Nittany, Bellefonte, 

Reedsville, and Juniata Formations recorded the 

transition from a carbonate platform to a more nearshore terrestrial deposition associated with the beginning 

of the Taconic orogeny. 

Igneous plutons intruded the sedimentary rocks along 

the eastern edge of the continent intermittently (540, 

470, and 360 Ma). During several episodes of mountain 

building and continental collision (described below), the 

entire sedimentary section, intrusive rocks, and basalt 

were deformed and metamorphosed into schist, gneiss, 

marble, slate, and migmatite (Southworth et al. 2000). 

Taconic Orogeny 

From Early Cambrian through Early Ordovician time, 

orogenic activity along the eastern margin of the 

continent began again. The Taconic orogeny (≈440−420 

Ma in the central Appalachians) was associated with a 

volcanic arc−continent convergence. Oceanic crust and 

the volcanic arc from the Iapetus basin were thrust onto 

the eastern edge of the North American continent. The 

Taconic orogeny resulted in the closing of the ocean, 

subduction of oceanic crust, creation of volcanic arcs, 

and uplift of continental crust (Means 1995). Initial 

metamorphism of the basalt of the Catoctin Formation 

into metabasalt and metarhyolite, as well as the 

Chilhowee Group rocks into quartzite and phyllite, 

occurred during this orogenic event. 

The crust bowed downwards in response to the 

overriding plate that was thrust westward onto the 

continental margin of North America, creating a deep 

basin that filled with mud and sand eroded from the 

highlands to the east (fig. 7C) (Harris et al. 1997). This 

so-called Appalachian basin was centered on what is now 

West Virginia. These in-filling sediments covered the 
 NPS Geologic Resources Division 18 

Cambrian carbonate platform and are today represented 

by the Ordovician (505−440 Ma) units (Southworth et al. 

2001). 

The oceanic sediments of the shrinking Iapetus Ocean 

were thrust westward along the Pleasant Grove fault 

during the Late Ordovician onto other deep-water 

sediments of the western Piedmont. Sand, mud, silt, and 

carbonate sediment of the Tuscarora Formation, Clinton 

Group, Bloomsburg Formation, and other Silurian rocks 

were then deposited in a shallow marine to deltaic 

environment of the Appalachian basin. These rocks, now 

metamorphosed, underlie the Valley and Ridge 

physiographic province (Fisher 1976). 

Shallow marine to fluvial sedimentation continued 

intermittently for a period of about 200 million years 

during the Ordovician, Silurian, Devonian, 

Mississippian, Pennsylvanian, and Permian Periods. This 

resulted in a thick sedimentary section. The source of 

these sediments was the highlands to the east that were 

rising during the Taconian orogeny (Ordovician) and the 

Acadian orogeny (Devonian). The Keyser and 

Tonoloway Formations contain widespread limestone 

and shale that reflect the transition from the Silurian to 

the Devonian periods. 

Acadian Orogeny 

The Acadian orogeny (≈360 Ma) continued the mountain 

building of the Taconic orogeny as the African continent 

drifted towards North America (Harris et al. 1997). The 

Acadian event involved collision of land masses, 

mountain building, and regional metamorphism similar 

to the preceding Taconic orogeny (Means 1995). This 

event was focused north of present-day central 

Pennsylvania. 

The Acadian event caused further uplift of Taconic 

highlands in central Pennsylvania. Erosion of these 

highlands provided more sediments, leading to the basinwide deposition of the Onondaga, Catskill, Foreknobs, 

and Old Port Formations, the Hamilton Group, and 

other Devonian units. 

The tectonic quiescence between the Acadian and 

Alleghenian orogenic events (see below) led to the 

deposition of the vast marsh and wetland deposits of the 

Mississippian and Pennsylvanian periods. Burial and 

compression of these deposits created the vast coalbearing units of the Mississippian Burgoon Sandstone, 

the Pennsylvanian Casselman and Glenshaw Formations, 

and the Allegheny and Pottsville Groups (Berg et al. 

1980; Whitfield et al. 2001). 

Alleghenian Orogeny 

During the Late Paleozoic and following the Acadian 

orogeny, the proto-Atlantic Iapetus Ocean was closed as 

the North American continent collided with the African 

continent. This formed a supercontinent named Pangaea 

and the Appalachian mountain belt we see today. This 

mountain-building episode is called the Alleghenian 

orogeny (≈325−265 Ma), the last major orogeny that 

affected the Appalachians (fig. 7D) (Means 1995). The 

rocks deformed during as many as seven phases of 

folding and faulting, producing the Sugarloaf Mountain 

anticlinorium and the Frederick Valley synclinorium in 

the western Piedmont, the Blue Ridge−South Mountain 

anticlinorium, and the numerous folds of the Valley and 

Ridge province in the Allegheny Portage Railroad area 

(Nickelsen 1983; Southworth et al. 2001). Many of the 

faults and folds associated with the Alleghenian orogeny 

are exposed today around the Allegheny Portage 

Railroad National Historic Site. 

During this orogeny, rocks of the Great Valley, Blue 

Ridge, and Piedmont provinces were transported along 

the North Mountain fault as a massive block (Blue 

Ridge−Piedmont thrust sheet) westward onto younger 

rocks of the Valley and Ridge. The amount of crustal 

shortening was very large: estimates of 20−50 percent 

shortening would amount to 125–350 km (80−220 mi) 

(Harris et al. 1997). 

Deformed rocks in the eastern Piedmont were also 

folded and faulted, and existing thrust faults were 

reactivated as both strike-slip and thrust faults during the 

Alleghenian orogeny (Southworth et al. 2001). 

Paleoelevations of the Alleghenian Mountains are 

estimated at approximately 6,000 m (20,000 ft), 

analogous to the modern-day Himalaya Range in Asia. 

These mountains have been beveled by erosion to 

elevations less than 734 m (2,408 ft) above sea level in the 

Allegheny Portage Railroad area (Means 1995). 

Triassic Extension to the Present 

Following the Alleghenian orogeny, during the late 

Triassic (230−200 Ma), a period of rifting began as the 

deformed rocks of the joined continents started to break 

apart. The supercontinent Pangaea was segmented into 

roughly the same continents that persist today. This 

episode of rifting initiated the formation of the current 

Atlantic Ocean and caused many block-fault basins to 

develop with accompanying volcanism (fig. 7E) (Harris 

et al. 1997; Southworth et al. 2001). 

The Newark Basin system, to the east of Allegheny 

Portage Railroad, is a large part of this tectonic setting. 

Large streams carried debris shed from the uplifted Blue 

Ridge and Piedmont provinces depositing them in 

alluvial fans. These were deposited as non-marine mud 

and sand in fault-created troughs such as the Frederick 

Valley in central Maryland and the Culpeper basin in the 

western Piedmont of central Virginia. Many of these 

rifted areas became lacustrine basins and were filled with 

thick silt and sand deposits. 

Thick deposits of unconsolidated gravel, sand, and silt 

were shed from the eroded Alleghenian mountains. 

These were deposited eastward at the base of the 

mountains as alluvial fans and became part of the 

Atlantic Coastal Plain (Duffy and Whittecar 1991; 

Whittecar and Duffy 2000; Southworth et al. 2001). The 

amount of material that was deposited has been inferred 

from the now-exposed metamorphic rocks in the Blue 

Ridge province to have been immense. Many of the rocks 

exposed at the surface must have been at least 20 km (≈10 
 ALPO Geologic Resource Evaluation Report 19

mi) below the surface prior to regional uplift and 

erosion. Erosion continues to create the present 

landscape, the Susquehanna, Juniata, and Delaware 

rivers and their tributaries are eroding sediments and 

depositing alluvial terraces along the rivers (fig. 7F). 

The North American plate has continued to drift toward 

the west since the breakup of Pangaea and the uplift of 

the Appalachian Mountains. The isostatic adjustments 

that uplifted the continent after the Alleghenian orogeny 

continued at a lesser rate throughout the Cenozoic 

Period (Harris et al. 1997). 

The landscape and geomorphology of the greater 

Allegheny Front area are the result of erosion and 

deposition along the rivers from about the mid-Cenozoic 

Period to the present, or at least the last 5 million years. 

The distribution of floodplain alluvium and ancient river 

terraces reflects the historical development of the local 

drainage systems. It seems the rivers have cut downward 

through very old, resistant rocks, overprinting their early 

drainageways (Southworth et al. 2001). The steep ridges 

and ravines present at Allegheny Portage Railroad attest 

to this downward cutting and overprinting. 

Glaciers from the Pleistocene Ice Ages never reached 

central Pennsylvania. The southern terminus of 

Pleistocene glaciers was at 365−610 m (1,200−2,000 ft) in 

elevation in northwestern and northeastern 

Pennsylvania. In the northern part of Pennsylvania, 

upland surfaces have been glaciated to rounded ridges 

and sand- and gravel-filled valleys (Davies 2005). 

However, the colder climates of the Ice Ages played a 

role in the formation of the landscape at Allegheny 

Portage Railroad. 

The Allegheny Mountains developed periglacial 

conditions that included discontinuous permafrost, 

tundra-like vegetation, and many freeze-thaw cycles due 

to its proximity to the glacial environment as well as its 

high elevation. These freeze-and-thaw cycles led to the 

ice wedging of thousands of boulders and small rocks 

from the mountains. Water would melt during the day 

and seep into cracks, freeze at night, expand, and force 

the rocks apart. Downslope movement of these rocks 

created talus piles, and larger, water-saturated rock 

masses slid over the partially frozen layer below in a 

process known as solifluction (Means 1995). Frost 

wedging continues in the area today (fig. 8 and 9). 

The Ice Age climate may have affected the morphology 

of the river valleys as well. The periglacial conditions at 

high altitudes intensified weathering and other erosional 

processes (Harris et al. 1997). The landforms and 

deposits are probably late Tertiary to Quaternary, when a 

wetter climate, sparse vegetation, and frozen ground 

caused increased precipitation to drain into the ancestral 

river channels, enhancing downcutting and erosion by 

waterways such as Blair Gap Run (Means 1995). The 

several water gaps through the resistant ridges 

throughout the Valley and Ridge province are examples 

of these incised waterways. 
 NPS Geologic Resources Division 20 

Figure 6. Geologic time scale; adapted from the U.S. Geological Survey (http://pubs.usgs.gov/fs/2007/3015/). Red lines indicate major 

unconformities between eras. Included are major events in life history and tectonic events occurring on the North American continent. 

Absolute ages shown are in millions of years. 
 ALPO Geologic Resource Evaluation Report 21

Figure 7. Evolution of the landscape in the area of the Allegheny Portage Railroad National Historic Site from the Precambrian through the 

present. Graphic adapted from Means (1995). Ma, millions of years (mega-annum). Drawings not to scale. 
 NPS Geologic Resources Division 22 

Figure 8. View of railroad grade at Allegheny Portage Railroad National Historic Site. Note talus boulders visible along left edge of 

photograph, which can be the result of frost-wedge weathering, described in “Geologic History” section below. Photograph is courtesy of 

the National Park Service. 
 ALPO Geologic Resource Evaluation Report 23

Figure 9. Diagram showing frost-wedge weathering and subsequent formation of talus piles in a typical outcrop in the Allegheny Mountains. 

Graphic is by Trista L. Thornberry-Ehrlich (Colorado State University). 
 NPS Geologic Resources Division 24 

Glossary 

This glossary contains brief definitions of technical geologic terms used in this report. Not all 

geologic terms used are referenced. For more detailed definitions or to find terms not listed 

here please visit: http://wrgis.wr.usgs.gov/docs/parks/misc/glossarya.html. 

active margin. A continental margin where significant 

volcanic and earthquake activity occurs; commonly a 

convergent plate margin. 

alluvial fan. A fan-shaped deposit of sediment that 

accumulates where a high-gradient stream flows out of 

a mountain front into an area of lesser gradient, such 

as a valley. 

alluvium. Stream-deposited sediment that is generally 

rounded, sorted, and stratified. 

angular unconformity. An unconformity where the strata 

above and below are oriented differently; generally 

caused by structural deformation and erosion prior to 

deposition of the upper bed. 

anticlinorium. A composite anticlinal structure of 

regional extent composed of lesser folds. 

aquifer. Rock or sediment that are sufficiently porous, 

permeable, and saturated to be useful as a source of 

water. 

ash (volcanic). Fine pyroclastic material ejected from a 

volcano (also see “tuff”). 

asthenosphere. Weak layer in the upper mantle below 

the lithosphere where seismic waves are attenuated. 

basement. The undifferentiated rocks, commonly 

igneous and metamorphic, that underlie the rocks of 

interest. 

basin (structural). A doubly-plunging syncline in which 

rocks dip inward from all sides (also see “dome”). 

basin (sedimentary). Any depression, from continental 

to local scales, into which sediments are deposited. 

bed. The smallest sedimentary strata unit, commonly 

ranging in thickness from one centimeter to a meter or 

two and distinguishable from beds above. 

bedding. Depositional layering or stratification of 

sediments. 

bedrock geology. The geology of underlying solid rock 

as it would appear with the sediment, soil, and 

vegetative cover stripped away. 

block (fault). A crustal unit bounded by faults, either 

completely or in part. 

braided stream. A stream, clogged with sediment that 

forms multiple channels that divide and rejoin. 

calcareous. A rock or sediment containing calcium 

carbonate. 

carbonaceous. A rock or sediment with considerable 

carbon, especially organics, hydrocarbons, or coal. 

cementation. Chemical precipitation of material into 

pores between grains that bind the grains into rock. 

chemical sediment. A sediment precipitated directly 

from solution (also called “nonclastic”). 

chemical weathering. The dissolution or chemical 

breakdown of minerals at Earth’s surface via reaction 

with water, air, or dissolved substances. 

clastic. Rock or sediment made of fragments or preexisting rocks. 

clay. Clay minerals or sedimentary fragments the size of 

clay minerals (>1/256 mm). 

cleavage (rock). The tendency of rock to break along 

parallel planes that correspond to the alignment of 

platy minerals. 

concordant. Strata with contacts parallel to the attitude 

of adjacent strata. 

conglomerate. A coarse-grained sedimentary rock with 

clasts larger than 2 mm in a fine-grained matrix. 

continental crust. The type of crustal rocks underlying 

the continents and continental shelves; having a 

thickness of 25−60 km (16−37 mi) and a density of 

approximately 2.7 grams per cubic centimeter. 

continental drift. The concept that continents have 

shifted in position over Earth (see and use “plate 

tectonics”). 

continental rise. Gently sloping region from the foot of 

the continental slope to the abyssal plain. 

continental shelf. The shallowly submerged part of a 

continental margin extending from the shoreline to the 

continental slope with water depths of less than 200 m 

(656 ft). 

continental shield. A continental block of Earth’s crust 

that has remained relatively stable over a long period 

of time and has undergone only gentle warping 

compared to the intense deformation of bordering 

crust 

continental slope. The relatively steep slope from the 

outer edge of the continental shelf down to the more 

gently sloping ocean depths of the continental rise or 

abyssal plain. 

convergent boundary. A plate boundary where two 

tectonic plates are moving together (i.e., a zone of 

subduction or obduction). 

craton. The relatively old and geologically stable interior 

of a continent (also see “continental shield”). 

cross-bedding. Uniform to highly varied sets of inclined 

sedimentary beds deposited by wind or water that 

indicate distinctive flow conditions. 

cross section. A graphical interpretation of geology, 

structure, and/or stratigraphy in the third (vertical) 

dimension based on mapped and measured geological 

extents and attitudes depicted in an oriented vertical 

plane. 

crust. The outermost compositional shell of Earth, 10−40 

km (6−25 mi) thick, consisting predominantly of 

relatively low-density silicate minerals (also see 

“oceanic crust” and “continental crust”). 

crystalline. Describes the structure of a regular, orderly, 

repeating geometric arrangement of atoms 

debris flow. A rapid and often sudden flow or slide of 

rock and soil material involving a wide range of types 

and sizes. 
 ALPO Geologic Resource Evaluation Report 25

deformation. A general term for the process of faulting, 

folding, shearing, extension, or compression of rocks 

as a result of various Earth forces. 

delta. A sediment wedge deposited at a stream’s mouth 

where it flows into a lake or sea. 

dike. A tabular, discordant igneous intrusion. 

dip. The angle between a structural surface and a 

horizontal reference plane measured normal to their 

line of intersection. 

disconformity. An unconformity at which the bedding of 

the strata above and below are parallel. 

discordant. Having contacts that cut across or are set an 

angle to the orientation of adjacent rocks. 

divergent boundary. A tectonic plate boundary where 

the plates are moving apart (e.g., a spreading ridge or 

continental rift zone). 

drainage basin. The total area from which a stream 

system receives or drains precipitation runoff. 

eustatic. Relates to simultaneous worldwide rise or fall 

of sea level in Earth’s oceans. 

evaporite. Chemically precipitated mineral(s) formed by 

the evaporation of solute-rich water under restricted 

conditions. 

exfoliation. The breakup, spalling, peeling, flaking, etc., 

of layers or concentric sheets from an exposed rock 

mass due to differential stresses resulting from thermal 

changes or pressure unloading. 

extrusion. The emission of relatively viscous lava onto 

the Earth’s surface; also, the rock so formed. 

extrusive. Of or pertaining to the eruption of igneous 

material onto the surface of Earth. 

facies (metamorphic). The pressure-temperature regime 

that results in a particular, distinctive metamorphic 

mineralogy (i.e., a suite of index minerals). 

facies (sedimentary). The depositional or environmental 

conditions reflected in the sedimentary structures, 

textures, mineralogy, fossils, etc. of a sedimentary 

rock. 

fault. A subplanar break in rock along which relative 

movement occurs between the two sides. 

formation. Fundamental rock-stratigraphic unit that is 

mappable and lithologically distinct from adjoining 

strata and has definable upper and lower contacts. 

fracture. Irregular breakage of a mineral; also any break 

in a rock (e.g., crack, joint, fault) 

frost wedging. The breakup of rock due to the expansion 

of water freezing in fractures. 

geology. The study of Earth including its origin, history, 

physical processes, components, and morphology. 

igneous. Refers to a rock or mineral that originated from 

molten material; one of the three main classes or 

rocks—igneous, metamorphic, and sedimentary. 

intrusion. A body of igneous rock that invades older 

rock. The invading rock may be a plastic solid or 

magma that pushes its way into the older rock. 

island arc. A line or arc of volcanic islands formed over 

and parallel to a subduction zone. 

isostasy. The process by which the crust “floats” at an 

elevation compatible with the density and thickness of 

the crustal rocks relative to underlying mantle. 

isostatic adjustment. The shift of the lithosphere of the 

Earth to maintain equilibrium among units of varying 

mass and density; excess mass above is balanced by a 

deficit of density below, and vice versa. 

joint. A semi-planar break in rock without relative 

movement of rocks on either side of the fracture 

surface. 

karst topography. Topography characterized by 

abundant sinkholes and caverns formed by the 

dissolution of calcareous rocks. 

lacustrine. Pertaining to, produced by, or inhabiting a 

lake or lakes. 

lamination. The finest stratification or bedding as seen in 

shale and siltstone (syn: lamina or laminae) or the 

formation of laminae. 

landslide. Any process or landform resulting from rapid 

mass movement under relatively dry conditions. 

lava. Magma that has been extruded out onto Earth’s 

surface, both molten and solidified. 

levees. Raised ridges lining the banks of a stream; may be 

natural or artificial. 

limbs. The two sides of a structural fold on either side of 

its hingeline. 

lineament. Any relatively straight surface feature that can 

be identified via observation, mapping, or remote 

sensing, commonly representing tectonic features. 

lithification. The conversion of sediment into solid rock. 

lithology. The description of a rock or rock unit, 

especially the texture, composition, and structure of 

sedimentary rocks. 

lithosphere. The relatively rigid outmost shell of Earth’s 

structure, 50−100 km (31−62 mi) thick, that 

encompasses the crust and uppermost mantle. 

loess. Silt-sized sediment deposited by wind, generally of 

glacial origin. 

mafic. A rock, magma, or mineral rich in magnesium and 

iron. 

magma. Molten rock generated within the Earth that is 

the parent of igneous rocks. 

mantle. The zone of Earth’s interior between crust and 

core. 

matrix. The fine-grained interstitial material between 

coarse grains in porphyritic igneous rocks and poorly 

sorted clastic sediments or rocks. 

meanders. Sinuous lateral curves or bends in a stream 

channel. 

mechanical weathering. The physical breakup of rocks 

without change in composition (syn: physical 

weathering). 

member. A lithostratigraphic unit with definable contacts 

that subdivides a formation. 

metamorphic. Pertaining to the process of 

metamorphism or to its results. 

metamorphism. Literally, “change in form.” 

Metamorphism occurs in rocks through mineral 

alteration, genesis, and/or recrystallization from 

increased heat and pressure. 

mid-ocean ridge. The continuous, generally submarine, 

seismic, median mountain range that marks the 

divergent tectonic margin(s) in the world’s oceans. 

mineral. A naturally occurring, inorganic crystalline solid 

with a definite chemical composition or compositional 

range. 
 NPS Geologic Resources Division 26 

mud cracks. Cracks formed in clay, silt, or mud by 

shrinkage during subaerial dehydration. 

nonconformity. An erosional surface preserved in strata 

in which crystalline igneous or metamorphic rocks 

underlie sedimentary rocks. 

normal fault. A dip-slip fault in which the hanging wall 

moves down relative to the footwall. 

obduction. The process by which the crust is thickened 

by thrust faulting at a convergent margin. 

oceanic crust. Earth’s crust formed at spreading ridges 

that underlies the ocean basins. Oceanic crust is 6−7 

km (3−mi) thick and generally of basaltic composition. 

orogeny. A mountain-building event, particularly a wellrecognized event in the geological past (e.g., the 

Laramide orogeny). 

outcrop. Any part of a rock mass or formation that is 

exposed or “crops out” at Earth’s surface. 

overbank deposits. Alluvium deposited outside a stream 

channel during flooding. 

paleogeography. The study, description, and 

reconstruction of the physical geography from past 

geologic periods. 

paleontology. The study of the life and chronology of 

Earth’s geologic past based on the phylogeny of fossil 

organisms. 

Pangaea. A theoretical, single supercontinent that 

existed during the Permian and Triassic Periods (also 

see “Laurasia” and “Gondwana”). 

parent (rock). The original rock from which a 

metamorphic rock or soil was formed. 

passive margin. A tectonically quiet continental margin 

indicated by little volcanic or seismic activity. 

pebble. Generally, small, rounded rock particles from 4 

to 64 mm in diameter. 

permeability. A measure of the ease or rate that fluids 

move through rocks or sediments. 

plateau. A broad, flat-topped topographic high of great 

extent and elevation above the surrounding plains, 

canyons, or valleys (both land and marine landforms). 

plate tectonics. The theory that the lithosphere is broken 

up into a series of rigid plates that move over Earth’s 

surface above a more fluid aesthenosphere. 

pluton. A body of intrusive igneous rock. 

plutonic. Describes igneous rock intruded and 

crystallized at some depth in the Earth. 

porosity. The proportion of void space (cracks, 

interstices) in a volume of a rock or sediment. 

Principal of Original Horizontality. The concept that 

sediments are originally deposited in horizontal layers 

and that deviations from the horizontal indicate postdepositional deformation. 

Principle of Superposition. The concept that sediments 

are deposited in layers, one atop another, i.e., the rocks 

on the bottom are oldest with the overlying rocks 

progressively younger toward the top. 

progradation. The seaward building of land area due to 

sedimentary deposition. 

provenance. A place of origin. The area from which the 

constituent materials of a sedimentary rock were 

derived. 

radioactivity. The spontaneous decay or breakdown of 

unstable atomic nuclei. 

radiometric age. An age in years determined from 

radioisotopes and their decay products. 

recharge. Infiltration processes that replenish ground 

water. 

red beds. Sedimentary strata composed largely of 

sandstone, siltstone, and shale that are predominantly 

red due to the presence of ferric oxide (hematite) 

coating individual grains. 

regression. A long-term seaward retreat of the shoreline 

or relative fall of sea level. 

relative dating. Determining the chronological 

placement of rocks, events, fossils, etc. from geological 

evidence. 

reverse fault. A contractional, high-angle (>45o), dip-slip 

fault in which the hanging wall moves up relative to the 

footwall (also see “thrust fault”). 

rift valley. A depression formed by grabens along the 

crest of an oceanic spreading ridge or in a continental 

rift zone. 

ripple marks. The undulating, subparallel, usually smallscale ridge pattern formed on sediment by the flow of 

wind or water. 

rock. A solid, cohesive aggregate of one or more minerals 

or mineraloids. 

roundness. The relative amount of curvature of the 

“corners” of a sediment grain, especially with respect 

to the maximum radius of curvature of the particle. 

sandstone. Clastic sedimentary rock of predominantly 

sand-sized grains. 

scarp. A steep cliff or topographic step resulting from 

vertical displacement on a fault or by mass movement. 

seafloor spreading. The process by which tectonic plates 

diverge and new lithosphere is created at oceanic 

ridges. 

sediment. An eroded and deposited, unconsolidated 

accumulation of lithic and mineral fragments. 

sedimentary rock. A consolidated and lithified rock 

consisting of detrital and/or chemical sediment(s). 

sequence. A major informal rock-stratigraphic unit that 

is traceable over large areas and defined by a major sea 

level transgression-regression sediment package. 

shale. A clastic sedimentary rock made of clay-sized 

particles that exhibit parallel splitting properties. 

silt. Clastic sedimentary material intermediate in size 

between fine-grained sand and coarse clay (1/256−1/16 

mm). 

siltstone. A variably lithified sedimentary rock composed 

of silt-sized grains. 

slickenside. A smoothly polished and commonly striated 

surface representing deformation of a fault plane. 

slope. The inclined surface of any geomorphic feature or 

rational measurement thereof (syn: gradient). 

slump. A generally large, coherent mass movement with 

a concave-up failure surface and subsequent backward 

rotation relative to the slope. 

soil. Surface accumulation of weathered rock and 

organic matter capable of supporting plant growth and 

commonly overlying the parent rock from which it 

formed. 

spring. A site where water flows out at the surface due to 

the water table intersecting the ground surface. 

strata. Tabular or sheetlike masses or distinct layers (e.g., 

of rock). 
 ALPO Geologic Resource Evaluation Report 27

stratigraphy. The geologic study of the origin, 

occurrence, distribution, classification, correlation, 

age, etc. of rock layers, especially sedimentary rocks. 

stream. Any body of water moving under gravity flow 

and confined within a channel. 

strike. The compass direction of the line of intersection 

that an inclined surface makes with a horizontal plane. 

strike-slip fault. A fault with measurable offset where the 

relative movement is parallel to the strike of the fault. 

subduction zone. A convergent plate boundary where 

oceanic lithosphere descends beneath a continental or 

oceanic plate and is carried down into the mantle. 

subsidence. The gradual sinking or depression of part of 

Earth’s surface. 

suture. The linear zone where two continental 

landmasses become joined due to obduction. 

syncline. A fold of which the core contains the 

stratigraphically younger rocks; it is generally concave 

upward. 

synclinorium. A composite synclinal structure of regional 

extent composed of lesser folds. 

tectonic. Relating to large-scale movement and 

deformation of Earth’s crust. 

tectonics. The geological study of the broad structural 

architecture and deformational processes of the 

lithosphere and aesthenosphere (also see “structural 

geology”). 

terraces (stream). Step-like benches surrounding the 

present floodplain of a stream due to dissection of 

previous flood plain(s), stream bed(s), and/or valley 

floor(s). 

terrane. A region or group of rocks with similar geology, 

age, or structural style. 

terrestrial. Relating to Earth or Earth’s dry land. 

theory. A hypothesis that has been rigorously tested 

against further observations or experiments to become 

a generally accepted tenet of science. 

thrust fault. A contractional, dip-slip fault with a 

shallowly dipping fault surface (<45o) where the 

hanging wall moves up and over relative to the 

footwall. 

topography. The general morphology of Earth’s surface 

including relief and location of natural and 

anthropogenic features. 

trace (fault). The exposed intersection of a fault with 

Earth’s surface. 

trace fossils. Sedimentary structures, such as tracks, 

trails, burrows, etc., that preserve evidence of 

organisms’ life activities, rather than the organisms 

themselves. 

transgression. Landward migration of the sea due to a 

relative rise in sea level. 

trend. The direction or azimuth of elongation of a linear 

geological feature. 

type locality. The geographic location where a 

stratigraphic unit is well displayed, is formally defined 

as a typical section, and derives its name. 

unconformity. A surface within sedimentary strata that 

marks a prolonged period of nondeposition or 

erosion. 

uplift. A structurally high area in the crust, produced by 

movement that raises the rocks. 

volcanic. Related to volcanoes; describes igneous rock 

crystallized at or near Earth’s surface (e.g., lava). 

water table. The upper surface of the saturated (phreatic) 

zone. 

weathering. The set of physical, chemical, and biological 

processes by which rock is broken down in place. 
 NPS Geologic Resources Division 28 

References 

This section lists references cited in this report as well as a general bibliography that may 

be of use to resource managers. A more complete geologic bibliography is available from 

the National Park Service Geologic Resources Division. 

Alterman, I. B. 1972. Structure and tectonic history of the 

Taconic allochthon and surrounding autochthon, eastcentral Pennsylvania. PhD diss., Columbia University. 

Becker, T., W. A. Thomas, S. D. Samson, and G. E. 

Gehrels. 2003. Characterization of the late Paleozoic 

Alleghenian Orogen by detrital zircons from the 

synorogenic clastic wedge. Geological Society of 

America Abstracts with Programs 35 (6): 389. 

Berg, T. M., W. E. Edmunds, A. R. Geyer, and others, 

compilers. 1980. Geologic map of Pennsylvania. Scale 

1:250,000, from source maps (PAGS Map 61) mapped 

at 1:62,500. 4th ser., Map 1.Middletown, PA: 

Pennsylvania Geological Survey. 

Cotter, E. 1983. Silurian depositional history. In Silurian 

depositional history and Alleghenian deformation in the 

Pennsylvania Valley and Ridge, ed. R. P. Nickelson and 

E. Cotter, 48:3−28. Guidebook for the Annual Field 

Conference of Pennsylvania Geologists. 

Cotter, E., and S. G. Driese. 1998. Incised-valley fills and 

other evidence of sea-level fluctuations affecting 

deposition of the Catskill Formation (Upper 

Devonian), Appalachian foreland basin, Pennsylvania. 

Journal of Sedimentary Research 68 (2): 347−361. 

Coyle, P. R., and T. H. Anderson. 2002. Structural and 

stratigraphic controls on the distribution of siliceous 

iron ore, Allegheny Plateau, west central Pennsylvania. 

Geological Society of America Abstracts with Programs 

34(6): 133. 

Cressler, W. L., S. E. Scheckler, J. F. Basinger, and D. 

Lindsay-Postnikoff. 2002. Wetlands paleoecology of 

the Middle and Late Devonian. Geological Society of 

America Abstracts with Programs 34 (6): 210. 

Davies, W. E. 2005. Physiography. 

http://www.cagenweb.com/quarries/articles_and_boo

ks/min_res_appalachian_region/physiography.html 

(accessed March 3, 2006). 

Duffy, D. F., and G. R. Whittecar. 1991. Geomorphic 

development of segmented alluvial fans in the 

Shenandoah Valley, Stuarts Draft, Virginia. Geological 

Society of America Abstracts with Programs 23 (1): 24. 

Erney, B. M., J. Schneider, A. VanDyne, and T. Schiappa. 

2003. Use of stratigraphic analysis to reconstruct the 

Pennsylvanian depositional history of western 

Pennsylvania. Geological Society of America Abstracts 

with Programs 35 (3): 92. 

Evans, M. A. 2000. Characterizing the deformation 

associated with cross-strike tear faults in south central 

Pennsylvania. Geological Society of America Abstracts 

with Programs 32 (1): 16. 

Fisher, G. W. 1976. The geologic evolution of the 

northeastern Piedmont of the Appalachians. Geological 

Society of America Abstracts with Programs 8 (2): 

172−173. 

Harris, A. G., E. Tuttle, and S. D. Tuttle. 1997. Geology of 

National Parks. Dubuque, IA: Kendall/Hunt 

Publishing Company. 

Hower, J. C., J. R. Levine, J. W. Skehan, E. J. Daniels, A. 

Davis, R. J. Gray, and S. P. Altaner. 1992. Appalachian 

anthracites. Annual Meeting of the Society for Organic 

Petrology Abstracts and Programs 9: 118−120. 

Inners, J. D. 1989. Historical geography, stratigraphy, and 

economic geology of the Horseshoe Curve area, westcentral Pennsylvania. Geological Society of America

Abstracts with Programs 21 (2): 25. 

Jacob, K. H. 1989. Earthquake hazards in the eastern 

U.S.; Research topics and hazards mitigation strategies. 

Geological Society of America Abstracts with Programs 

21 (2): 25. 

Kauffman, M. E., and E. P. Frey. 1979. Antietam 

sandstone ridges; exhumed barrier islands or faultbounded blocks? Geological Society of America 

Abstracts with Programs 11 (1): 18. 

Kochel, R. C., L. S. Eaton, and A. D. Howard. 1996. 

Catastrophic debris flows in the northern and central 

Appalachians; considerations of their role in the 

evolution of Piedmont geomorphology. Geological 

Society of America Abstracts with Programs 28 (7): 299. 

Laughrey, C. D., D. A. Billman, and M. R. Canich. 1999. 

Geology of the Council Run gas field, north central 

Pennsylvania; implications for energy resource 

development in the Eastern Plateaus Province of the 

central Appalachian Basin. AAPG Bulletin 83 (8): 1369. 

Madison, J. P., J. D. Lonn, R. K. Marvin, J. J. Metesh, and 

R. Wintergerst. 1998. Abandoned-inactive mines 

program, Deer Lodge National Forest; Volume IV, 

Upper Clark Fork River drainage. Open-File Report. 

Montana Bureau of Mines and Geology. 

McElroy, T. A. 1998. Groundwater resources of Cambria 

County, Pennsylvania. Scale 1:50,000. Water Resource 

Report W 67. Pennsylvania Geological Survey. 
 ALPO Geologic Resource Evaluation Report 29

Means, J. 1995. Maryland’s Catoctin Mountain parks; an 

interpretive guide to Catoctin Mountain Park and 

Cunningham Falls State Park. Blacksburg, VA: 

McDonald & Woodward Publishing Company. 

Miller, J. D., and D. V. Kent. 1986. Synfolding and 

prefolding magnetizations in the Upper Devonian 

Catskill Formation of eastern Pennsylvania; 

implications for the tectonic history of Acadia. Journal 

of Geophysical Research 91 (B12): 12791−12803. 

Moore, J. N., and W. W. Woessner. 2000. Geologic, soil 

water and groundwater report—2000, Grant-Kohrs 

Ranch National Historic Site. National Park Service. 

Nickelsen, R. P. 1983. Aspects of Alleghenian 

deformation. In Silurian depositional history and 

Alleghenian deformation in the Pennsylvania Valley and 

Ridge. Guidebook for the Annual Field Conference of 

Pennsylvania Geologists, ed. R. P. Nickelson and E. 

Cotter, 48: 29−39. Guidebook for the Annual Field 

Conference of Pennsylvania Geologists. 

Potter, N., Jr. 1985. Colluvial and alluvial gravels, 

carbonate weathering, and the preservation of a 

Cenozoic erosional history in the Great Valley, 

southcentral Pennsylvania. Geological Society of 

America Abstracts with Programs 17 (1): 59. 

Prave, A. R., and W. L. Duke. 1991. Sedimentology and 

sequence stratigraphic framework of the Middle 

Devonian Mahantango Formation in central 

Pennsylvania. In Geologic evolution of the eastern 

United States, ed. A. Schultz and E. Compton-Gooding, 

2:207−228. Geological Society of America Field Trip 

Guidebook NE-SE, 1991. Virginia Museum of Natural 

History. 

Sasowsky, I. D., R. A. Kempthorne, B. Crowell, S. M. 

Walko, E. J. Larock, and W. Harbert. 2000. Clastic 

sediments in a long-term karst trap, Valley and Ridge, 

central Pennsylvania. Geological Society of America 

Abstracts with Programs 32 (7): 27. 

Scharnberger, C. K. 1987. Some questionable events in 

the earthquake history of south-central Pennsylvania. 

Seismological Research Letters 58 (4): 104. 

Schwab, F. L. 1970. Origin of the Antietam Formation 

(late Precambrian?, lower Cambrian), central Virginia. 

Journal of Sedimentary Petrology 40 (1): 354−366. 

Simpson, E. L. 1991. An exhumed Lower Cambrian tidalflat; the Antietam Formation, central Virginia, U.S.A. 

In Clastic tidal sedimentology. eds. D. G. Smith, B. A. 

Zaitlin, G. E. Reinson, and R. A. Rahmani. Canadian 

Society of Petroleum Geologists Memoir 16:123−133. 

Southworth, S.; D. K. Brezinski; R. C. Orndorff; K. M. 

Lagueux; and P. G. Chirico. 2000. Digital geologic map 

of the Harpers Ferry National Historical Park. U. S. 

Geological Survey Open-File Report OF 00-0297, 1 

disc. 

Southworth, S., D. K. Brezinski, R. C. Orndorff, P. G. 

Chirico, and K. M Lagueux. 2001. Geology of the 

Chesapeake and Ohio Canal National Historical Park 

and Potomac River Corridor, District of Columbia, 

Maryland, West Virginia, and Virginia. CD-ROM (Disc 

1: A, geologic map and GIS files; Disc 2: B, geologic 

report and figures). Open-File Report OF 01-0188. 

Reston, VA: U. S. Geological Survey. 

Stoher, P. 1992. A review of the fossil evidence for the 

existence of land plants from Silurian rocks of 

Pennsylvania. Geological Society of America Abstracts 

with Programs 24 (3): 79. 

Suciu, D. L., D. C. Bachman, and R. W. Owens. 1992. 

Pathway to history; the cultural geology of the U.S. 

11/15 corridor in central Pennsylvania from 

prehistoric occupation to the 21st century. Geological 

Society of America Abstracts with Programs 24 (3): 79. 

Trexler, B. D., Jr., D. A. Ralston, D. A. Reece, and R. E. 

Williams. 1975. Sources and causes of acid mine 

drainage. Pamphlet 165. Moscow, ID: Idaho Bureau of 

Mines and Geology. 

Wells, R. B. 1973. Historical sketch; Juniata and 

Susquehanna rivers: Structure and Silurian and 

Devonian stratigraphy of the Valley and Ridge 

Province in central Pennsylvania. In 38th Annual Field 

Conference of Pennsylvania Geologists, ed. R. T. Faill, 

51−52. [place of publication: publisher]. 

Whitfield, T. G., and others. 2001. Digital Bedrock 

Geology of Pennsylvania, Johnstown and Tyrone 30' × 

60' quadrangles, Pennsylvania. Scale 1:250,000. 

Pennsylvania Geological Survey 30' × 60' Digital 

Quadrangle Maps.. 

Whittecar, G. R., and D. F. Duffy. 2000. Geomorphology 

and stratigraphy of late Cenozoic alluvial fans, Augusta 

County, Virginia, U.S.A. In Regolith in the Central and 

Southern Appalachians, eds. G. M Clark, H. H. Mills, 

and J. S. Kite. Southeastern Geology 39 (3−4): 259−279.
 NPS Geologic Resources Division 30 

Appendix A: Geologic Map Graphic 

The following page is a snapshot of the geologic map for Allegheny Portage Railroad 

National Historic Site. For a poster-size PDF of this map or for digital geologic map 

data, please see the included CD or visit the Geologic Resource Evaluation publications 

Web page (http://www.nature.nps.gov/geology/inventory/gre_publications.cfm). 
 ALPO Geologic Resource Evaluation Report 33

Appendix B: Scoping Summary 

The following excerpts are from the GRE scoping summary for Allegheny Portage 

Railroad National Historic Site. The scoping meeting was on June 22, 2004; therefore, 

the contact information and Web addresses referred to in this appendix may be 

outdated. Please contact the Geologic Resources Division for current information. 

Executive Summary 

A geologic resources evaluation workshop was held for 

Allegheny Portage Railroad National Historic Site 

(ALPO) and Johnstown Flood National Memorial 

(JOFL) on June 22, 2004, to view and discuss the parks’ 

geologic resources, to address the status of geologic 

mapping for compiling both paper and digital maps, and 

to assess resource management issues and needs. 

Cooperators from the NPS Geologic Resources Division 

(GRD), NPS ALPO and JOFL units, as well as local 

geologic experts, were present for the workshop. 

Geologic Mapping 

Existing Geologic Maps and Publications 

After the bibliographies were assembled, a separate 

search was made for any existing surficial and bedrock 

geologic maps for ALPO and JOFL. 

The bounding coordinates for each map were noted and 

entered into a GIS to assemble an index geologic map. 

Separate coverages were developed based on scales 

(1:24,000, 1:100,000, etc.) available for the specific parks. 

Numerous geologic maps at varying scales and vintages 

cover the area. Index maps were distributed to each 

workshop participant during the scoping session. 

It was decided to use Pennsylvania Map 61 for Blair 

County with focus on the Cresson, Hollidaysburg, and 

Frankstown quadrangles. Map 61 is presented at 

1:62,500 scale but was originally mapped at 1:24,000. This 

covers the main unit of ALPO. 

No water report has ever been done for Blair County 

since a 1945 folio on Holidaysburg and Huntington. 

Use the Pennsylvania map W-67 for Cambria County; 

this covers Staple Bend Unit and JOFL; some of ALPO 

main will be included as part of this map at 1:50,000 

scale. 

Additionally, for coal information discussion focused on 

map m-96 by the Pennsylvania Geological Survey (PA 

GS) for only areas containing coal. The GRE would need 

interpretation from PA GS before digitizing. 

A small-scale digital map of Pennsylvania exists and is 

available from PA GS website. 

Other Topics of Discussion 

A discussion of geologic resource management issues 

followed the mapping discussion touching upon the 

following features and/or processes: 

• Aeolian: NA 

Fluvial (surface water) Processes: ALPO (Staple Bend 

Unit) runs down Little Conemaugh River; ALPO main 

unit is eastern slope of Blair Gap Run; JOFL is South 

fork of Little Conemaugh; obviously, theme is in park 

name; foundations in river may be threatened 

• Groundwater: NA; but can reference the PA GS water 

reports as well 

• Hazards: Landslides, slumps, rockfalls prevalent in 

western PA 

• Clays that disintegrate when they become water 

saturated may be underlying JOFL dam abutments; 

should conduct a site investigation if Ames Limestone 

is present 

• Occasional earthquakes but not of significant 

magnitude; largest in PA was ≈5M in northwest PA 

• Roof fall in Staple Bend tunnel 

• Floods 

• No CO3 sinkholes 

• Paleontology: See Vince Santucci’s report on ERMN 

paleontology. 

• Mineral Extraction: See disturbed lands, but coal and 

Fe known to have been mined 

• Caves / karst: Carbonate beds often too thin to form 

karst, thus a non-issue 

• Glacial: NA 

• Coastal/Marine: NA 

• Geologic interpretation: Allegheny Front was reason 

for AL Portage and Staple Bend tunnel 

• Coal mine remediation 

• Mahoning sandstone quarry is big part of park’s story 

• JOFL topography is essential to park’s story 

• Unique geologic features: Allegheny Front 

• Staple Bend tunnel was first blasted through solid rock 

in U.S.; took 3 years 

• Oxbows at Little Conemaugh viaduct 

• Geothermal: NA 

• Disturbed Lands−mining subsidence: Coal Mining 

subsidence at ALPO and JOFL; ALPO Staple Bend 

Unit and Main unit have acid mine drainage issues; 

JOFL has acid mine drainage initiating external to the 

park; PA Department of Environmental Protection has 

maps showing mined areas and they document mine 

subsidence events; should be GIS layer. 

• The maps are quad-based. They are a regulatory 

agency; PA GS is a research agency. 
 NPS Geologic Resources Division 34 

• Mine shafts are an issue at ALPO main unit; at Staple 

Bend unit they appear to be collapsed but haven’t been 

investigated to date; should submit TA request to GRD 

for Burghardt or Cloues assistance 

• Only mined lands need reclaimed in both parks 

• Fe-mining of siderite in limestone supplanted local 

steel industry; effects unknown 

Monitoring Issues 

Monitor for landslides along steep hillsides of Allegheny 

Front. 

Meeting attendees and contact information 

Connors, Tim, NPS, Geologic Resources Division (303) 

969-2093 e-mail: tim_connors@nps.gov 

Harper, John, Pennsylvania Geological Survey (412) 442-

4230 e-mail: jharper@state.pa.us 

Heise, Bruce, NPS, Geologic Resources Division (303) 

969-2017 e-mail: Bruce_Heise@nps.gov 

Inners, John, Pennsylvania Geological Survey (717) 702-

2034 e-mail: jinners@state.pa.us 

Marshall, Matt, ERMN (814) 863-0134 e-mail: 

matt_marshall@nps.gov 

Penrod, Kathy, NPS ALPO & JOFL (814) 886-6128 email: kathy_penrod@nps.gov 

Piekielek, Nate ,ERMN (814) 863-2320 e-mail: 

nathan_piekielek@nps.gov 

Santucci, Vince, NPS, Geologic Resources Division (307) 

877-4455 e-mail: Vincent_Santucci@nps.gov 
The Department of the Interior protects and manages the nation’s natural resources and cultural heritage; provides scientific and other 

information about those resources; and honors its special responsibilities to American Indians, Alaska Natives, and affiliated Island 

Communities. 

Allegheny Portage Railroad National Historic Site 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2008/045 

NPS D-112, September 2008 

National Park Service 

Director • Mary A. Bomar 

Natural Resource Stewardship and Science 

Associate Director • Bert Frost 

Natural Resource Program Center 

The Natural Resource Program Center (NRPC) is the core of the NPS Natural Resource Stewardship and Science 

Directorate. The Center Director is located in Fort Collins, with staff located principally in Lakewood and Fort 

Collins, Colorado and in Washington, D.C. The NRPC has five divisions: Air Resources Division, Biological Resource 

Management Division, Environmental Quality Division, Geologic Resources Division, and Water Resources Division. 

NRPC also includes three offices: The Office of Education and Outreach, the Office of Inventory, Monitoring and 

Evaluation, and the Office of Natural Resource Information Systems. In addition, Natural Resource Web 

Management and Partnership Coordination are cross-cutting disciplines under the Center Director. The 

multidisciplinary staff of NRPC is dedicated to resolving park resource management challenges originating in and 

outside units of the national park system. 

Geologic Resources Division 

Chief • Dave Steensen 

Planning Evaluation and Permits Branch Chief • Carol McCoy 

Credits 

Author • Trista Thornberry-Ehrlich

Review • Lisa Norby 

Editing • Diane Lane, Carol McCoy, and Melanie Ransmeier 

Digital Map Production • Stephanie O'Meara and Trista Thornberry-Ehrlich 

Map Layout Design • Melanie Ransmeier
Geologic Resources Division 

Natural Resource Program Center 

P.O. Box 25287 

Denver, CO 80225 

http://www.nature.nps.gov/geology/inventory/ 

(303) 969-2090 

EXPERIENCE YOUR AMERICATM 

National Park Service 

U.S. Department of the Interior