Johnstown Flood National Memorial 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2008/049 

National Park Service 

U.S. Department of the Interior 

Natural Resource Program Center 
ON THE COVER: 

Remnants of the South Fork Dam abutments–

Johnstown Flood National Memorial, Pennsylvania 

NPS Photo 
Johnstown Flood National Memorial 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2008/049 

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.
The Natural Resource Publication series addresses natural resource topics that are of interest 

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www.nature.nps.gov/publications/NRPM/index.cfm) or by sending a request to the address on 

the back cover. 

Please cite this publication as: 

Thornberry-Ehrlich, T. 2008. Johnstown Flood National Memorial Geologic Resource 

Evaluation Report. Natural Resource Report NPS/NRPC/GRD/NRR—2008/049. National 

Park Service, Denver, Colorado. 

NPS D-67, September 2008 
 JOFL Geologic Resource Evaluation Report iii

Table of Contents 

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

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

Introduction ................................................................................................................... 3

Purpose of the Geologic Resource Evaluation Program ............................................................................................3

Geologic Setting .........................................................................................................................................................3

Geologic Issues............................................................................................................. 6

Geologic Hazards.......................................................................................................................................................6

Water Issues and Flooding.........................................................................................................................................6

Recreational Demands ...............................................................................................................................................7

Acid Mine Drainage ....................................................................................................................................................7

General Geology, Paleontology, and Miscellaneous Issues.......................................................................................7

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

Geology and history connections..............................................................................................................................10

South Fork Dam and Lake Conemaugh ...................................................................................................................10

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

Johnstown Flood Geologic Map Unit Notes..............................................................................................................15 

Map Unit Properties Table........................................................................................................................................17

Geologic History.......................................................................................................... 19

Glossary....................................................................................................................... 25

References................................................................................................................... 29

Appendix A: Geologic Map Graphic .......................................................................... 32

Appendix B: Scoping Summary................................................................................. 35

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

List of Figures 

Figure 1: Johnstown Flood National Memorial Location Map.........................................................................................2 

Figure 2: Map of Pennsylvania with physiographic provinces ........................................................................................5

Figure 3: Satellite image of Johnstown Flood National Memorial...................................................................................9

Figure 4: Destruction of Johnstown by 1889 flood .......................................................................................................11

Figure 5: Historic photograph of the destruction of the town of South Fork..................................................................12

Figure 6: Historic map of Johnstown showing flood damaged areas ...........................................................................13

Figure 7: Photograph of remnants of South Fork Dam.................................................................................................14

Figure 8: Photograph of the lakebed of former Lake Conemaugh ...............................................................................14

Figure 9: Geologic time scale.......................................................................................................................................22

Figure 10: Evolution of the landscape in the Johnstown Flood National Memorial area ..............................................23

Figure 11: Diagram showing frost wedge weathering ..................................................................................................24
 JOFL Geologic Resource Evaluation Report 1

Executive Summary 

This report accompanies the digital geologic map for Johnstown Flood National Memorial 

in Pennsylvania, which the Geologic Resources Division produced in collaboration with its 

partners. It contains information relevant to resource management and scientific research. 

The story at Johnstown Flood National Memorial is a 

tragic one. On May 31, 1889, 2,209 people lost their lives 

in one of the worst disasters in United States history. The 

memorial is dedicated to this loss. In addition to this 

cultural resource, the memorial preserves a portion of 

the Allegheny Mountains section of the Appalachian 

Plateaus physiographic province. Geologic resources and 

the natural history of the area are integral parts of the 

landscape and their interpretation can enhance the 

visitor’s experience. 

The rocks in the Appalachian Plateaus province are 

relatively flat-lying and undeformed in contrast to the 

rocks of the Valley and Ridge province just to the east of 

Johnstown. These two provinces are separated by the 

steep Allegheny Front. In the Johnstown area, surface 

exposures consist primarily of Devonian through 

Pennsylvanian age sedimentary rocks. These include 

sandstones, limestones, clay stones, conglomerates, 

dolomites, and shales. These rocks contain fossils, coal, 

iron-rich minerals, and other clues to past environments. 

The geologic substrate, as well as surficial processes 

(geologic, hydrologic, and biologic) determine the 

features of every landscape. Understanding the geology 

of central Pennsylvania enhances understanding of the 

unique relationship between geology, the environment, 

and the cultural history of the area. Geologic processes 

give rise to rock formations, mountains, and valleys, 

escarpments and ridges, dissected plateaus, and soils. 

The landscape played a prominent role in the history of 

south-central Pennsylvania, including the development 

of historic recreational facilities. At Johnstown Flood 

National Memorial, the South Fork Dam was repaired in 

1862 to preserve a recreational paradise for the South 

Fork Fishing & Hunting Club. Heavy rains and 

subsequent lake level rise toppled the dam and resulted 

in catastrophic flooding (fig. 1). 

The following issues (described further in the “Geologic 

Issues” section) have geologic importance and a high 

level of management significance within the park: 

• Geologic Hazards 

The steep terrain that characterizes the Allegheny 

Front region surrounding the park is prone to 

landslides, slumps, and rockfalls. In particular, areas 

containing resistant rock units undercut by erosion of 

weaker units, or human land use are susceptible to 

hazardous failure. Friable clays may underlie the 

abutments of the remnants of the South Fork Dam and 

could cause damage to the structure if they become 

water saturated. Though uncommon, seismic activity 

is possible in central Pennsylvania. Even minor seismic 

tremors could trigger massive landslides and debris 

flows on steep or water-saturated slopes. 

• Water Issues and Flooding 

Associated with the South Fork of the Little 

Conemaugh River and various small streams, springs, 

and seeps at Johnstown Flood National Memorial is 

the risk of damage and loss of resources due to erosion 

and the flood activity that is inherent to this river 

system. Intense seasonal thunderstorms and high 

runoff have historically caused flooding along these 

waterways. This flooding threatens human life as well 

as many of the historical structures and features of the 

landscape at the park. In order to make informed 

management decisions it is crucial for park resource 

managers to understand the park’s hydrogeologic 

system. Increased understanding of surface and 

subsurface water flow will help more accurately 

predict environmental changes that occur from 

hydrologic processes. 

• Recreational Demands 

The memorial preserves natural lands for recreational 

use including the remnants of the South Fork Dam and 

the meadowlands of the former Lake Conemaugh. 

Overuse of these areas may lead to contamination 

from human waste and trash and degradation of the 

ecosystem such as trampling and removal of 

vegetation, soil compaction and erosion. 

• Acid Mine Drainage 

Johnstown Flood National Memorial is located in 

south-central Pennsylvania, an area known for coal 

and iron mining. Abandoned and inactive mines pose 

health, safety, and environmental problems in the 

vicinity of the park. Foremost among these is acid 

mine drainage. The potential for acid mine drainage as 

well as heavy metal contamination that accompanies 

low pH runoff is a serious resource management 

concern. The South Fork of the Little Conemaugh 

River is devoid of fish due to contamination from 

upstream mining activities. Alluvial sediments and 

floodplain deposits are rich in heavy metals 

precipitated from contaminated runoff. 

 JOFL Geologic Resource Evaluation Report 3

Introduction 

The following section briefly describes the National Park Service Geologic Resource 

Evaluation Program and the regional geologic setting of Johnstown Flood National 

Memorial. 

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 

Prior to the failure of the South Fork Dam, Conemaugh 

Lake provided fishing, hunting, boating, and swimming 

opportunities for the South Fork Fishing and Hunting 

Club, an exclusive resort on the lakeshore. On May 31, 

1889, at 3:10 a.m., the South Fork Dam failed and 2,209 

people lost their lives as torrential floodwaters from 

Conemaugh Lake destroyed the working class city of 

Johnstown in south-central Pennsylvania (Bern 1999). 

This event is one of the worst natural disasters in United 

States history and no news was bigger during the last few 

decades of the nineteenth century. Johnstown Flood 

National Memorial (designated August 31 1964 during 

the presidency of John F. Kennedy) commemorates this 

tragedy and preserves portions (165 acres) of the remains 

of the South Fork Dam and former Lake Conemaugh. 

This area is now managed as a wet meadow with 

approximately seven acres of biodiverse wetlands within 

the former lakebed. 

Johnstown Flood National Memorial is located in southcentral Pennsylvania about 16 km (10 miles) northeast of 

Johnstown. The South Fork of the Little Conemaugh 

River flows through the park toward its confluence with 

the Little Conemaugh River and ultimately the 

Conemaugh River that flows through the city of 

Johnstown. 

The park is located in southwestern Cambria County. 

Here the mountains range in elevation from 346–734 m 

(1,135–2,408 ft) above sea level. Historically, the 

mountains proved a significant obstacle to boat trade of 

the Pennsylvania Mainline Canal. The memorial is 

located near the junction of the Appalachian Plateau and 

the Valley and Ridge physiographic provinces. These 

provinces are divided into local sections and thus more 

specifically, the park is within the Allegheny Mountain 

section (fig. 2). Rolling hills and ridges characterize 

topography in the park. The ridgetops are composed of 

resistant Paleozoic sandstones and the valleys are 

underlain by less resistant units such as carbonates and 

shales that have preferentially eroded through time. 

A general east to west description of several of the 

different physiographic provinces of the Appalachian 

Mountains follows. 

Piedmont Province 

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

where the softer, less consolidated sedimentary rocks of 
 NPS Geologic Resources Division 4 

the Atlantic Coastal Plain to the east intersect the harder, 

more resistant metamorphic rock 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 Plateau 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. This province 

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 uplifted during several orogenic events to form the 

steep, rugged terrain. Resistant Cambrian quartzite forms 

most of the high ridges, whereas Precambrian 

metamorphic rocks underlie the valleys (Nickelsen 

1956). 

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 an 

average height 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 

known as the “Ridge and Valley province”). The valleys 

formed in areas of more easily eroded shale and 

carbonate formations and ridges are commonly 

composed of more resistant sandstones. The province 

contains strongly folded and faulted sedimentary rocks 

in central Pennsylvania. The Valley and Ridge province 

averages 80 km (48 miles) in width. The eastern part of 

the Valley and Ridge is part of the Great Valley, a rolling 

lowland formed on folded carbonates and shales. 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 300–900 m (1,000-3,000 ft) above the 

adjacent landscape. Maximum elevations along this front 

are generally greater than those of the ridges in the Valley 

and Ridge province. In Pennsylvania, elevations range 

from 530–900 m (1,750–3,000 ft). Deep ravines carved 

into the nearly horizontal sedimentary rock layers 

characterize the topography of this province. Local 

geologic units are typically repetitious sequences of 

shale, coal, limestone, and sandstone, called cyclothems. 

Erosion of these units created a rugged, jumbled 

topographic surface. The northern parts of the province 

in Pennsylvania and New York display more rounded 

hills with gentler slopes. 
 JOFL Geologic Resource Evaluation Report 5

Figure 2. Map of Pennsylvania showing the physiographic setting of Johnstown Flood National Memorial. Red lines indicate 

boundaries between major physiographic provinces. Northern terminus of Blue Ridge province is located by the black arrow. The 

location of park is denoted by a green circle. Map information modified from Pennsylvania Geological Survey map 13, 2000). Graphic 

by Trista L. Thornberry-Ehrlich (Colorado State University). 
 NPS Geologic Resources Division 6 

Geologic Issues 

A Geologic Resource Evaluation scoping session was held for Johnstown Flood National 

Memorial 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. 

Geologic Hazards 

The steep terrain that created a seemingly ideal location 

for construction of the South Fork Dam is prone to 

several geologic hazards including landslides, slumps, 

and rockfalls. Steep slopes along valleys and ravines 

between ridges and rolling hills are characteristic of the 

Allegheny Mountains region. Historic features such as 

the remains of the dam abutments 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 weaken and deteriorate when 

saturated with water and are prone to fail when exposed 

on steep slopes. These types of units may underlie the 

abutments of the remnants of the South Fork Dam at 

Johnstown Flood National Memorial. When more 

resistant rock units such as sandstone and limestone 

overlie less resistant rock units, preferential erosion may 

undercut the more resistant units and result in rockfalls. 

A site investigation could determine if any friable clays 

are locally present in conjunction with the Ames 

Limestone (a unit present at the boundary between the 

clay rich Pennsylvanian age Glenshaw and Casselman 

Formations). If these units are present together, potential 

slide hazards exist. 

There is limited knowledge regarding moderate or low 

levels of seismicity and movement along specific faults in 

the eastern United States (Jacob 1989). Though not 

common, seismic activity is possible in central 

Pennsylvania. Some historic earthquakes in the area were 

caused by blasting at local mines (in 1893 and 1939), 

additionally a natural ~5 magnitude quake occured 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. Assessing vulnerability to seismicly 

induced geologic hazards may be beneficial for 

Johnstown Flood National Memorial. 

Inventory, Monitoring, and Research Suggestions for 

Geologic Hazards 

• Conduct site investigations of clay rich layers and 

friable limestones exposed on slopes to determine a 

vulnerability index to slope failure. 

• Inventory the areas along steep slopes of the South 

Fork of the Little Conemaugh River valley to locate 

historic landslides. 

Water Issues and Flooding 

Johnstown Flood National Memorial was designated to 

memorialize a devastating flood, thus water is an obvious 

resource management issue at the park. Although Lake 

Conemaugh and the dam that once held it back are now 

gone, there is continued risk of damage and loss of 

resources due to future flooding associated with the 

South Fork of the Little Conemaugh River and other 

small streams, springs, and seeps in the vicinity of the 

park. Floods are most common during spring runoff, and 

seasonal storm events. Flood events when they occur 

threaten historic foundations in the river and flood 

plane. 

In addition to the devastating flood of 1889 (fig. 3), the 

Johnstown area has experienced two other major flood 

events in 1936 and 1977. The 1977 flood occurred when 

the hydraulic fill (fill in which the materials are deposited 

in situ by flowing water) Laurel Run Dam, located on a 

tributary of the Little Conemaugh River, failed. The dam 

was constructed in 1915–1918 to supply water to a 

system of navigational canals for local steel producing 

operations. In 1960, the dam was purchased by the City 

of Johnstown to act as a component of the municipal 

water system. Its malleable clay core failed after 15–30 

cm (6–12 inches) of rain fell in 6–8 hours on the night of 

July 19–20, 1977 (Knight 1989). This event further 

demonstrates the vulnerability of the system to floods. 

An understanding of the hydrogeologic system at the 

park is crucial for resource managers. Hydrogeologic 

systems determine groundwater flow, and ultimately the 

pathways of contaminants and other wastes. The 

movement of nutrients and contaminants through the 

hydrogeologic system can be modeled by monitoring the 

composition of system inputs, such as rainfall, and 

outputs, such as streamflow. Other contaminant sources 

include wind, surface runoff, groundwater transport, 

sewage outfalls, landfills, and fill dirt. Streams integrate 

the surface runoff and groundwater flow of their 

watersheds and provide a cumulative measure of the 

health of the watershed’s hydrologic system. Consistent 

measurement of these parameters is crucial to 

establishing baselines for comparison. 

Inventory, Monitoring, and Research Suggestions for Water 

Issues and Flooding 

• Develop emergency response procedures to 

implement in case of extreme flood events along park 

rivers and streams. 
 JOFL Geologic Resource Evaluation Report 7

• Consult Pennsylvania Geological Survey water reports 

to obtain further data on the surface and groundwater 

hydrology in the park. 

• Map and quantify groundwater recharge zones. 

• Work with the Water Resources Division of the NPS 

to develop an appropriate water quality monitoring 

plan. 

Recreational Demands 

Recreational use at Johnstown Flood National Memorial 

includes hiking, picnicking, cross-country skiing, bird 

watching, and photography. 

The park receives many visitors, especially during the 

summer months. In 2007, 126,066 visitors made 

recreational visits to Johnstown Flood National 

Memorial. These visitors place increasing demands on 

the resources of the park. Resource management 

concerns vary from trail erosion (especially near the 

remnants of the South Fork Dam), protection of 

wetlands and water quality, to slope failures. 

The park concentrates visitor use along designated trails 

and at picnic areas. However, visitor use in other areas of 

the park increases the area of impact and could place 

delicate ecosystems at risk of contamination from human 

waste and overuse. 

The South Fork of the Little Conemaugh River enhances 

the natural beauty of the park. Overuse of certain 

streambanks can lead to contamination from human 

waste, trash, and increased stream edge erosion. 

Inventory, Monitoring, and Research Suggestions for 

Recreational Demands 

• Inventory and monitor human impacts to the wetlands 

and marsh flora within the former Lake Conemaugh. 

• Design and construct interpretive wayside exhibits to 

encourage responsible use of park resources. 

Acid Mine Drainage 

Coal and iron mining (siderite in limestones) have long 

been associated with central Pennsylvania. The 

Pennsylvanian-age Freeport, Kittanning, and BrookvilleClarion commercially viable coal seams all exist in the 

vicinity of the park. Extensive underground, mining 

around the park has resulted in lasting environmental 

effects ranging from acid mine drainage to potential 

subsidence and collapse of underground mine workings. 

No abandoned mine features have been identified within 

the boundaries of Johnstown Flood National Memorial. 

However, nearby mine features in St. Michael and 

Beaverdale are causing major pollution problems 

upstream of the park in the South Fork of the Little 

Conemaugh River. The Pennsylvania Department of 

Environmental Protection has maps showing mined 

areas and has documented subsidence associated with 

mines. 

Abandoned and inactive mines pose environmental and 

health problems for the Johnstown Flood National 

Memorial area. Foremost among these is acid mine 

drainage and residual heavy metal contamination of 

ground water, surface water, and soil. Mine areas 

upstream from the park produce acid mine drainage. 

Fluvial sediments (sediments carried by water) can 

contain metals and other waste materials. 

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). 

Drainage from mines near the upstream communities of 

Saint Michael and Beaverdale flow through the memorial 

creating a significant resource management problem. 

This flow is concentrated in the Little Conemaugh River 

where high levels of aluminum, sulfates, and iron have 

contributed to white, orange, and yellow coloring of the 

riverbed and surrounding floodplains. The water is toxic 

to fish and other aquatic species as well as riparian 

vegetation along its banks. Some bacteria, algae, and 

insects inhabit the polluted waterway. Remediation 

efforts are underway at the park, but coordination with 

upstream communities is critical to prevent further 

contamination and resource damage. 

Inventory, Monitoring, and Research Suggestions for 

Abandoned Mine Drainage 

• 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. 

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

contamination. 

• Incorporate Pennsylvania Department of 

Environmental Protection maps of mine features into a 

GIS database. 

General Geology, Paleontology, and Miscellaneous 

Issues 

The contrast between the nearly horizontal, undeformed 

layers of the Appalachian Plateaus province and the 

folded and faulted Valley and Ridge physiographic 

provinces has sparked research interest among geologists 

for many years. 
 NPS Geologic Resources Division 8 

Railroad cuts and other development activities have 

exposed rock layers for geologic research (Inners 1989). 

More local research would help resource managers at the 

park improve understanding of the geology and the 

relationships among physical factors in the ecosystem. 

The rock units in the park area contain vast 

paleontological resources that record life present during 

the Devonian through Pennsylvanian periods. These 

resources present an opportunity for display and 

interpretation at the park. The park also serves as an 

excellent outdoor classroom for the history of 

engineering and natural disasters. 

Inventory, Monitoring, and Research Suggestions for 

General Geology and Miscellaneous Issues 

• Promote geomorphologic mapping at the park to 

better understand landslide hazards. 

• Promote land use evolution studies, especially 

concerning any possible Native American sites, early 

settlement activities, and trade (transportation) to 

enhance the park’s interpretive materials. 

• Promote paleontological research in the park. Refer to 

the Eastern Rivers and Mountains Network 

paleontology report by Vince Santucci. 

• Promote historical engineering studies related to 

construction and maintenance of the South Fork Dam. 

This information can be used to create interpretive 

materials. 
 JOFL Geologic Resource Evaluation Report 9

Figure 3. Satellite image of Johnstown Flood National Memorial area courtesy of The Pennsylvania State University. Highstand lake 

level of 1889 is indicated as well as a regular stand level from 1853. Floodwaters flowed out of the natural topographic basin of Lake 

Conemaugh, in to the narrow gorge of the South Fork of Little Conemaugh River towards the confluence with the North Fork. The 

floodwaters surged towards Johnstown after temporarily being redammed behind a viaduct near the oxbow (not on figure). Where the 

river straightens just before Johnstown, the flow increased in velocity before reaching and devastating the city. Graphic by Trista L. 

Thornberry-Ehrlich (Colorado State University). 
 NPS Geologic Resources Division 10 

Geologic Features and Processes 

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

in Johnstown Flood National Memorial. 

Geology and history connections 

On May 31, 1889, following 8 inches of rainfall, the South 

Fork Dam that contained Lake Conemaugh collapsed, 

sending a 23 m (76 ft) high wall of 20 million gallons of 

water rushing (60 km/hr, 40 mph) down a narrow gorge 

towards a town of 30,000 unsuspecting people and killing 

2,209 (fig’s 4–6). The dam was constructed in the 1850’s, 

and its lake was the mountain retreat and summer 

playground for some of the wealthiest men of the 

Industrial Age including Andrew Carnegie, Robert 

Pitcairn, Howard Hartley, H.C. Yeager, and Andrew 

Mellon (Earnest 2002; Issenberg 2004). Two more major 

floods in 1936 and 1977 inundated Johnstown killing 264 

and 85 people, respectively. 

The park memorializes the devastating flood of 1889. 

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

historical context of the area; this includes preserving the 

remains of South Fork Dam and former Lake 

Conemaugh, historic buildings and the landscape 

surrounding them (fig.s 7–8). The visible impacts of the 

flood on the landscape are now softened by erosion and 

weathering. Maintaining this landscape often means 

resisting natural geologic changes, which presents several 

resource management challenges. 

Geologic slope processes such as landsliding, slumping, 

chemical weathering, rockfalls and slope creep are 

constantly changing the landscape at the park. Runoff 

erodes sediments and carries them downslope. Erosion 

over time lowers higher areas and fills in the lower areas 

changing the historical context of the landscape. 

Issues arise between the sometimes-differing objectives 

of cultural and natural resource management. For 

example, a proposal to restore an historic building may 

consist of removing surrounding natural resources or 

planting exotics plant species. Several areas in the park 

such as Carriage Road, the South Fork Dam remnants, 

the caretaker’s house, the newly acquired South Fork 

Fishing & Hunting Club’s clubhouse and cottages 

(Victorian mansions along the former lakeshore) 

including the Moorehead Cottage are popular historic 

features for visitors that may require management 

actions that affect adjacent natural resources (NPS News 

2004). 

History at Johnstown Flood National Memorial also 

contains ties to the region’s industries. Coal and iron ore 

mines dotted the landscape as well as regional furnaces 

to smelt the iron ore. These features stand as testament 

to the early industrial development of the area. Today 

they pose environmental and safety hazards within and 

near the park (see discussion in Geologic Issues section). 

Early fields, settlements, and tool quarries of Native 

Americans predate European settlements. Earliest 

known evidence of Native American activity dates back 

to 10,000 years ago from Sheep rock, near Huntingdon, 

Pennsylvania. The name “Allegheny” stems from the 

mound building Alligewi or Allegheny Indians from 

western Pennsylvania (Wells 1973). 

Native Americans left artifacts and geomorphological 

changes to the landscape such as terraced campsites, tool 

quarries, and other archaeological sites in the 

Susquehanna River Valley and the Allegheny Mountains 

(Suciu and Bachman 1992). 

South Fork Dam and Lake Conemaugh 

“A dam is the most hazardous of civil engineering 

structures. Its failure can cause loss of life and 

property, the value of which may be many times that of 

the cost of the dam. Moreover, the impact of the 

failure is felt a great distance beyond the site limits.” 

– George F. Sowers, Professor, Georgia Institute of 

Technology (1977) 

When completed in 1853, the 14-year construction of the 

South Fork Dam was the largest earthfill dam in the 

world. It measured more than 110 m (70 ft) tall and 275 

m (900 ft) across (Katkins 1989; Issenberg 2004). It 

included a 21 m (70 ft) wide spillway that was cut 

through sandstone bedrock with cast-iron pipes at the 

base of the dam to control the water level in the lake 

(Katkins 1989). It was originally intended to serve as a 

reservoir for the Pennsylvania Mainline Canal basin in 

Johnstown, but the lengthy construction rendered the 

canal obsolete when Pennsylvania Railroad was 

completed. The railroad company owned the canal and 

operated it for a number of years. During this time, the 

original South Fork Dam failed in 1862 but resulted in 

little flooding (Earnest 2002). 

The dam and reservoir changed hands a number of times 

before they were acquired for $2,000 in a state of 

disrepair by the South Fork Fishing & Hunting Club. The 

club began repairs to the dam in 1879 and completed 

work in 1881. They did not hire qualified engineers to 

oversee the work and the repairs were substandard. 

The culverts, originally installed to control lake level 

were blocked and water could not flow through them. 

The eroded embankment from the 1862 flood was 

repaired by placing an earth and rock berm on the 

upstream face to raise the dam to a height of 22 m (72 ft). 

A 3 m (10 ft) wide spillway channel was cut through one 

abutment 2.5 m (8.2 ft) below the maximum height of the 

dam. A road was installed across the top of the dam, 

causing it to sag in the middle. The subsequent lake, Lake 

Conemaugh, was stocked with fish and served as a resort 
 JOFL Geologic Resource Evaluation Report 11

area for the club between 1881 and 1889 (Katkins 1989; 

Earnest 2002). 

The probability of a major flood in the Johnstown area 

was high before the construction of the South Fork Dam 

due to its the unique topographic setting. The valley 

above the city, where the former Lake Conemaugh was 

located, is a 4 km (2.5 mi) long bowl-shaped basin that 

would easily collect runoff (Issenberg 2004). This valley 

sits atop the narrow gorge of the South Creek of the 

Little Conemaugh River. 

The stream falls steeply below the dam for about 5 km 

(3 mi). Johnstown sits in a narrow floodplain formed at 

the junction of Little Conemaugh River and Stoney 

Creek to form the Conemaugh River (Sowers 1977). 

The heavy rains of May 30–31, 1889 carried a 

considerable amount of debris that jammed at a fish 

screen installed across the spillway. The dam was 

toppled by water in its center and failed (Katkins 1989). 

There is still debate as to whether the 1889 dam failure 

and resulting flood is solely attributable to the heavy 

rains or from the lack of basic maintenance of the dam by 

the South Fork Fishing & Hunting Club. There is some 

consensus that had the dam been maintained to the 

original specifications, the disaster would never have 

occurred (Issenberg 2004). 

Figure 4. Historic photograph of the destruction of Johnstown by 1889 flood. The people standing on rooftops were likely sightseers. 

The event prompted the first major relief effort by the American Red Cross led by Clara Barton. Photograph is courtesy of the 

Johnstown Flood Museum. 
 NPS Geologic Resources Division 12 

Figure 5. Historic photograph of the destruction of the town of South Fork, Pennsylvania near the confluence of South and North Forks 

of the Little Conemaugh River. Photograph is courtesy of the Johnstown Flood Museum. 
 JOFL Geologic Resource Evaluation Report 13

Figure 6. Historic map of Johnstown showing flood damaged areas (shaded). Photograph is courtesy of the Johnstown Flood Museum.
 NPS Geologic Resources Division 14 

Figure 7. Photograph of remnants of South Fork Dam in the right hand portion of the image. Photograph by Jeffrey Kitsko, 2005. 

Figure 8. Photograph of the lakebed of former Lake Conemaugh, now managed as a wetland – meadow system. Photograph by Jeffrey 

Kitsko, 2005. 
 JOFL Geologic Resource Evaluation Report 15

Map Unit Properties 

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

Evaluation digital geologic map of Johnstown Flood National Memorial. 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 Johnstown 

Flood National Memorial 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. 9) 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. Middletown 

Pennsylvania: Pennsylvania Geological Survey. 

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

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

Report W 67. Middletown Pennsylvania: 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 shapefile and coverage GIS formats, 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 

with appropriate symbology. 

GRE digital geologic data are included on the attached 

CD and are available through the NPS Data Store 

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

Johnstown Flood Geologic Map Unit Notes 

Johnstown Flood National Memorial sits near the 

boundary between the Valley and Ridge and 

Appalachian Plateau 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 consisting of 

dolomite, limestone, sandstone, and shale. The limestone 

and dolomite units locally contain fossil algae called 

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 dolomites, 

shales, and fossiliferous limestones dominate the lower 

units. Siltstones, sandstones and conglomerates (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 record the 

gradual return to tectonic quiescence and intense 
 NPS Geologic Resources Division 16 

erosion of the newly formed Taconic highlands. 

Sediments changed from largely terrigenous sandstones 

to muds, and eventually to marine carbonate units with 

occasional storm layers as sea level rose and the 

mountains were deeply eroded (Cotter 1983). 

The Keyser and Tonoloway Formations contain 

fossiliferous limestones and mudcracked shales 

recording the transition between the Silurian and 

Devonian Periods. Marine deposition continued 

throughout the Devonian comprising the calcareous 

shales, siltstones, chert, and fossiliferous limestones of 

the Onondaga and Old Port Formations, the Hamilton 

Group, 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 

conglomerates of the Rockwell, and Shenango through 

Oswayo Formations record the transition from the 

Devonian to Mississippian Periods (Berg et al. 1980; 

Whitfield et al. 2001). 

Within the park and immediate surrounding area, map 

units from the Mississippian and Pennsylvanian Periods 

are present and 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. These units 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 

Johnstown Flood National Memorial due to regional 

erosion or non-deposition. Recent, Quaternary age 

alluvium deposits line local river and stream valleys. 

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

and occasional 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 (see fig. 9) 

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. 
 JOFL Geologic Resource Evaluation Report 19

Geologic History 

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

geologic map of Johnstown Flood National Memorial, the environment in which those 

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

landscape. 

Johnstown Flood National Memorial sits along the zone 

between the Valley and Ridge and Appalachian Plateau 

physiographic provinces in central Pennsylvania known 

as the Allegheny Front section. The park contains 

features intimately tied with the long geologic history of 

the Appalachian Mountains and the evolution of the 

eastern coast of the North American continent. 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. 9). 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. The sedimentation, deformation, 

plutonism (the intrusion of igneous rocks), and 

volcanism are apparent in the metamorphic granite and 

gneiss in the core of the modern Blue Ridge Mountains 

to the south and east of Johnstown Flood National 

Memorial (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, 

brought extensional rifting to the area. The crustal 

extension created fissures through which massive 

volumes of basaltic magma were extruded (fig. 10A). 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. 10B). 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 sea (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 Johnstown 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 million years ago) (Means 1995). Ordovician 

units such as the Nittany, Bellefonte, Reedsville, and 

Juniata Formations recorded the transition from a 

carbonate platform to a more near-shore 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 million years ago). 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 

million years ago 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. 10C) (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 20 

Cambrian carbonate platform and are today represented 

by the Ordovician (505−440 million years ago) units 

(Southworth et al. 2001). 

The oceanic sediments of the shrinking Iapetus sea 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 million years ago) 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 

landmasses, mountain building, and regional 

metamorphism similar to the preceding Taconic orogeny 

(Means 1995). This event was focused north of presentday 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 

Alleghanian 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). 

Alleghanian 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 Alleghanian 

orogeny (≈325−265 million years ago), the last major 

orogeny that affected the Appalachians (fig. 10D) (Means 

1995). The rocks were 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 Johnstown 

Flood National Memorial area (Nickelsen 1983; 

Southworth et al. 2001). Many of the faults and folds 

associated with the Alleghanian orogeny are exposed 

today around the Johnstown Flood National Memorial. 

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 

Alleghanian 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 

Johnstown Flood National Memorial area (Means 1995). 

Triassic Extension to the Present 

Following the Alleghanian orogeny, during the late 

Triassic (230−200 million years ago), a period of rifting 

began as the deformed rocks of the joined continents 

began 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. 

10E) (Harris et al. 1997; Southworth et al. 2001). 

The Newark Basin system, to the east of Johnstown 

Flood National Memorial, 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 nonmarine 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 
 JOFL Geologic Resource Evaluation Report 21

exposed at the surface must have been at least 20 km (≈10 

mi) below the surface prior to regional uplift and 

erosion. The erosion continues to create the present 

landscape, the Susquehanna, Juniata, and Delaware 

rivers and their tributaries eroding sediments and 

depositing alluvial terraces along the rivers (fig. 10F). 

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 Alleghanian 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 reflect 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 Johnstown Flood National 

Memorial attest to this downward cutting and 

overprinting. 

Glaciers from the Pleistocene Ice Ages never reached 

central Pennsylvania. The southern terminus 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 

Johnstown Flood National Memorial. 

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. 11). 

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 22 

Figure 9. 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. 
 JOFL Geologic Resource Evaluation Report 23

Figure 10. Evolution of the landscape in the area of Johnstown Flood National Memorial area 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 24 

Figure 11. Diagram showing frost wedge weathering and subsequent formation of talus piles in a typical outcrop in the Allegheny

Mountains. Graphic by Trista L. Thornberry-Ehrlich (Colorado State University). 
 JOFL Geologic Resource Evaluation Report 25

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. 
 NPS Geologic Resources Division 26 

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. 
 JOFL Geologic Resource Evaluation Report 27

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). 
 NPS Geologic Resources Division 28 

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. 
 JOFL Geologic Resource Evaluation Report 29

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. 34 (6), Columbia University, 

Palisades, NY: Doctoral Dissertation. 

Becker, T.; Thomas, W.A.; Samson, S.D.; and Gehrels, 

G.E. 2003. Characterization of the late Paleozoic 

Alleghanian Orogen by detrital zircons from the 

synorogenic clastic wedge. Abstracts with Programs–

Geological Society of America, 35 (6): 389. 

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

compilers. 1980. Geologic map of Pennsylvania. 

Pennsylvania Geological Survey, 4th ser., Map 1, 

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

mapped at 1:62,500 scale. 

Bern, Louise. 1999. Flooding; the Great Johnstown flood. 

In The geology of Pennsylvania, ed. Shultz, Charles H., 

758-761. Harrisburg – Pittsburgh, PA: Geological 

Survey of Pennsylvania and Pittsburgh Geological 

Society. 

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

depositional history and Alleghanian deformation in 

the Pennsylvania Valley and Ridge. Guidebook for the 

Annual Field Conference of Pennsylvania Geologists, 

ld. Nickelsen, R.P.; and Cotter, E. 48: 3-28 

Cotter, E.; and Driese, S.G. 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 Anderson, T.H. 2002. Structural and 

stratigraphic controls on the distribution of siliceous 

iron ore, Allegheny Plateau, west central Pennsylvania. 

Abstracts with Programs–Geological Society of America

34(6): 133. 

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

Lindsay-Postnikoff, D. 2002. Wetlands paleoecology 

of the Middle and Late Devonian. Abstracts with 

Programs–Geological Society of America 34 (6): 210. 

Davies, W.E. 2005. Physiography. 

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

ks/min_res_appalachian_region/physiography.html 

(accessed November 4, 2005). 

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

development of segmented alluvial fans in the 

Shenandoah Valley, Stuarts Draft, Virginia. Abstracts 

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

Earnest, J. 2002. The Dam and Lake. 

http://www.nps.gov/jofl/thedam.htm (accessed 

November 29, 2005). 

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

2003 Use of stratigraphic analysis to reconstruct the 

Pennsylvanian depositional history of western 

Pennsylvania. Abstracts with Programs–Geological 

Society of America 35 (3): 92. 

Evans, M.A. 2000. Characterizing the deformation 

associated with cross-strike tear faults in south central 

Pennsylvania. Abstracts with Programs–Geological 

Society of America 32 (1): 16. 

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

northeastern Piedmont of the Appalachians. Abstracts 

with Programs–Geological Society of America 8 (2): 172-

173. 

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

National Parks. Kendall/Hunt Publishing Company. 

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

Davis, A.; Gray, R.J.; and Altaner, S.P. 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. Abstracts with Programs–

Geological Society of America 21 (2): 25. 

Issenberg, A. 2004. After the Flood. Special to the 

Washington Post 11-17-2004: C02, 

http://www.washingtonpost.com/wpdyn/articles/A55686-2004Nov16.html (accessed 

December 1, 2005). 

Jacob, K.H. 1989. Earthquake Hazards in the Eastern 

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

Abstracts with Programs–Geological Society of America

21 (2): 25. 

Kaktins, U. 1989. Stop #10; Johnstown Flood National 

Memorial. In Geology in the Laural Highlands of 

southwestern Pennsylvania, ed. Harper, John A., 224-

227. Harrisburg, PA: Field Conference of Pennsylvania 

Geologists. 

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

sandstone ridges; exhumed barrier islands or faultbounded blocks? Abstracts with Programs–Geological 

Society of America 11 (1): 18. 
 NPS Geologic Resources Division 30 

Knight, F. James. 1989. Stop #5; Laurel Run Dam and the 

Johnstown flood of 1977. In Geology in the Laural 

Highlands of southwestern Pennsylvania, ed. Harper, 

John A., 211-213. Harrisburg, PA: Field Conference of 

Pennsylvania Geologists. 

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

Catastrophic debris flows in the Northern and Central 

Appalachians; considerations of their role in the 

evolution of Piedmont geomorphology. Abstracts with 

Programs–Geological Society of America 28 (7): 299. 

Laughrey, C.D.; Billman, D.A.; and Canich, M.R. 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.; Lonn, J.D.; Marvin, R.K.; Metesh, J.J.; and 

Wintergerst, R. 1998. Abandoned-inactive mines 

program, Deer Lodge National Forest; Volume IV, 

Upper Clark Fork River drainage. MBMG Open-File 

Report. 

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

County, Pennsylvania, Pennsylvania Geological Survey, 

Water Resource Report W 67, 1:50,000 scale 

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 Kent, D.V. 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): 12,791-12,803. 

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

Water and Groundwater Report – 2000, Grant-Kohrs 

Ranch National Historic Site. U.S. National Park 

Service, Department of the Interior. 

Nickelsen, R.P. 1983. Aspects of Alleghanian 

deformation. In Silurian depositional history and 

Alleghanian deformation in the Pennsylvania Valley 

and Ridge. Guidebook for the Annual Field 

Conference of Pennsylvania Geologists, ld. Nickelsen, 

R.P.; and Cotter, E. 48: 29-39. 

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

carbonate weathering, and the preservation of a 

Cenozoic erosional history in the Great Valley, 

Southcentral Pennsylvania. Abstracts with Programs–

Geological Society of America 17 (1): 59. 

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

sequence stratigraphic framework of the Middle 

Devonian Mahantango Formation in central 

Pennsylvania. In Geologic evolution of the Eastern 

United States; field trip guidebook NE-SE GSA 1991. 

Guidebook Virginia Museum of Natural History, ed. 

Schultz, A.; and Compton-Gooding, E. 2: 207-228. 

Sasowsky, I.D.; Kempthorne, R.A.; Crowell, B.; Walko, 

S.M.; Larock, E.J.; and Harbert, W. 2000. Clastic 

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

central Pennsylvania. Abstracts with Programs–

Geological Society of America 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. 

Silurian depositional history and Alleghanian 

deformation in the Pennsylvania Valley and Ridge. 

1983. In Guidebook for the Annual Field Conference of 

Pennsylvania Geologists, ld. Nickelsen, R.P.; Cotter, E. 

48. 

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

In Clastic tidal sedimentology. Memoir–Canadian 

Society of Petroleum Geologists, eds. Smith, D G.;, 

Zaitlin, B.A.; Reinson, G.E.; and Rahmani, R.A. 16: 

123-133. 

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

P.G.; and Lagueux, K.M. 2001. Geology of the 

Chesapeake and Ohio Canal National Historical Park 

and Potomac River Corridor, District of Columbia, 

Maryland, West Virginia, and Virginia. A, Geologic map 

and GIS files (disc 1); B, Geologic report and figures (disc 

2). U. S. Geological Survey, Open-File Report: OF 01-

0188. 

Sowers, G. F. 1977. Earth dam failures. In Lectures of the 

Seminar; Failures of large dams, reasons and remedial 

measures, ed. Rouve, G.; Strack, B.; Idel, K. H.; 

Beckmann, J.; Hager, M.; Boucek, B.; Sowers, G. F.; 

Gilg, B., 178-226. Aachen, Federal Republic of 

Germany: Inst. Found. Eng., Soil Mech., Rock Mech., 

Water Ways Constr. 

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

existence of land plants from Silurian rocks of 

Pennsylvania. Abstracts with Programs–Geological 

Society of America 24 (3): 79. 

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

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

11/15 corridor in central Pennsylvania from 

prehistoric occupation to the 21st century. Abstracts 

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

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

Williams, R.E. 1975. Sources and causes of acid mine 

drainage. Idaho Bureau of Mines and Geology, 

Pamphlet 165 
 JOFL Geologic Resource Evaluation Report 31

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. Faill, R.T.: 

51-52. 

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

of Pennsylvania, Johnstown and Tyrone 30' x 60' 

quadrangles, Pennsylvania. Pennsylvania Geological 

Survey, PAGS 30’x60’ Digital Quadrangle maps, 

1:250,000 scale. 

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

and stratigraphy of late Cenozoic alluvial fans, Augusta 

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

Southern Appalachians. Southeastern Geology, eds. 

Clark, G.M.; Mills, H.H.; and Kite, J.S. 39 (3-4): 259-

279. 
 NPS Geologic Resources Division 32 

Appendix A: Geologic Map Graphic 

The following page is a preview or snapshot of the geologic map for Johnstown Flood 

National Memorial. 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). 
 JOFL Geologic Resource Evaluation Report 35

Appendix B: Scoping Summary 

The following excerpts are from the GRE scoping summary for Johnstown Flood 

National Memorial. 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 coverage’s 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 main unit of ALPO 

No water report has ever been done for Blair County 

since 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. 

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 
 NPS Geologic Resources Division 36 

• 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. 

• Mine shafts are an issue at ALPO main unit; 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 limestones 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 e-mail: 

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. 

Johnstown Flood National Memorial 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2008/049 

NPS D-67, 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 and Carol McCoy 

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 

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National Park Service 

U.S. Department of the Interior