Valley Forge National Historical Park 

Geologic Resources Inventory Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2010/236 

National Park Service 

U.S. Department of the Interior 

Natural Resource Program Center 
THIS PAGE: 

Reconstruction of a hut built

by soldiers of Washington’s 

Continental Army to survive 

the winter of 1777-1778. 

Copyrighted image by M.J.

Ticcino for National Park

Service, courtesy

Kristina Heister (Valley Forge

National Historical Park) 

ON THE COVER:

The Grand Parade ground of

Valley Forge National Park, 

view from the Huntington 

Camp. Mount Joy rises in the 

background.

National Park Service photograph by Megan Carfioli, 

courtesy Amy Ruhe and Kristina Heister (Valley Forge 

National Historical Park). 
Valley Forge National Historical Park 

Geologic Resources Inventory Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2010/236 

Geologic Resources Division 

Natural Resource Program Center 

P.O. Box 25287 

Denver, Colorado 80225 

September 2010 

U.S. Department of the Interior 

National Park Service 

Natural Resource Program Center 

Ft. Collins, Colorado
The National Park Service, Natural Resource Program Center publishes a range of reports that 

address natural resource topics of interest and applicability to a broad audience in the National 

Park Service and others in natural resource management, including scientists, conservation and 

environmental constituencies, and the public. 

The Natural Resource Report Series is used to disseminate high-priority, current natural 

resource management information with managerial application. The series targets a general, 

diverse audience, and may contain NPS policy considerations or address sensitive issues of 

management applicability. 

All manuscripts in the series receive the appropriate level of peer review to ensure that the 

information is scientifically credible, technically accurate, appropriately written for the 

intended audience, and designed and published in a professional manner. This report received 

informal peer review by subject-matter experts who were not directly involved in the 

collection, analysis, or reporting of the data. 

Views, statements, findings, conclusions, recommendations, and data in this report do not 

necessarily reflect views and policies of the National Park Service, U.S. Department of the 

Interior. Mention of trade names or commercial products does not constitute endorsement or 

recommendation for use by the U. S. Government. 

Printed copies of this report are produced in a limited quantity and they are only available as 

long as the supply lasts. This report is available from the Geologic Resources Inventory website 

(http://www.nature.nps.gov/geology/inventory/gre_publications.cfm) and the Natural 

Resource Publications Management website (http://www.nature.nps.gov/publications/

NRPM). 

Please cite this publication as: 

Thornberry-Ehrlich, T. 2010. Valley Forge National Historical Park: geologic resources 

inventory report. Natural Resource Report NPS/NRPC/GRD/NRR—2010/236. National Park 

Service, Ft. Collins, Colorado. 

NPS 464/105493, September 2010 

ii NPS Geologic Resources Division 
VAFO Geologic Resources Inventory Report iii

Contents

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

Executive Summary ....................................................................................................... v

Acknowledgements...................................................................................................... vi

Credits........................................................................................................................................................................ vi

Introduction ................................................................................................................... 1

Purpose of the Geologic Resources Inventory ...............................................................................................................1

Geologic Setting ..........................................................................................................................................................1

Atlantic Coastal Plain Province......................................................................................................................................1

Piedmont Province .......................................................................................................................................................2

Blue Ridge Province......................................................................................................................................................2

History of Valley Forge .................................................................................................................................................2

Geologic Issues .............................................................................................................. 5

Introduction.................................................................................................................................................................5

Karst Issues..................................................................................................................................................................5

Water and Sediment Issues ..........................................................................................................................................6

Paleontological Resources ............................................................................................................................................8

Slope Processes............................................................................................................................................................8

Disturbed Lands and Mineral Development ..................................................................................................................9

Geologic Influences on Cultural Resources....................................................................................................................9

Geologic Features and Processes................................................................................ 15

Cambrian-Triassic Unconformity .................................................................................................................................15

Caves and Karst .........................................................................................................................................................15

Historic Connections with Geology.............................................................................................................................16

Map Unit Properties .................................................................................................... 21

Geologic History .......................................................................................................... 27

Precambrian and Early Paleozoic Era: Ancient Mountain Building and Iapetus Ocean Formation ..................................27

Paleozoic Era: Appalachian Mountain Building ...........................................................................................................27

Mesozoic Era: Rifting, Atlantic Ocean Formation, and Mountain Erosion.....................................................................28

Cenozoic Era: Karst Development and Ice Ages ..........................................................................................................28

Glossary........................................................................................................................ 32

Literature Cited............................................................................................................ 37

Additional References ................................................................................................. 41

Appendix A: Overview of Digital Geologic Data....................................................... 43

Appendix B: Scoping Meeting Participants................................................................ 45

Attachment 1: Geologic Resources Inventory Products CD
iv NPS Geologic Resources Division 

List of Figures

Figure 1. Map of Valley Forge National Historical Park......................................................................................................3

Figure 2. Physiographic provinces of Pennsylvania ............................................................................................................4

Figure 3. Wetlands are present near the contact between the Triassic sandstones and Cambrian dolomite......................11

Figure 4. Cross-sections of Skolithos (worm burrow) trace fossils....................................................................................12

Figure 5. Exposure of Ledger Formation in abandoned dolostone quarry near the visitor center......................................13

Figure 6. Fossil jaw of gracile sabretooth (Smilodon gracilis) recovered from Bone Cave..................................................17

Figure 7. Fragmentary jaw of a large tapir (Tapirus copei) discovered at Bone Cave.........................................................17

Figure 8. Generalized cross-sectional view of the development of a karstic landscape through dissolution and collapse...18

Figure 9. View looking west from the Statue of General Anthony Wayne at Valley Forge National Historical Park ...........19

Figure 10. Rolling karst landscape as seen from Huntington Camp (looking west)...........................................................19

Figure 11. Recreation of cabins at Muhlenberg Camp ....................................................................................................20

Figure 12. Geologic timescale ........................................................................................................................................30

Figure 13. Evolution of the landscape in the area of the Valley Forge National Historical Park .........................................31
VAFO Geologic Resources Inventory v

Executive Summary

This report accompanies the digital geologic map data for Valley Forge National Historical 

Park in Pennsylvania, produced by the Geologic Resources Division in collaboration with its 

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

This document incorporates preexisting geologic information and does not include new data 

or additional fieldwork.

In December 1777, after a series of battles, British forces

occupied Philadelphia and George Washington sought a 

suitable winter encampment site for his tired troops. 

Washington chose Valley Forge, an easily defended 

location on the southern banks of the Schuylkill River. 

This position buffered the colonial army against British 

surprise attacks, while providing them enough proximity 

to apply constant pressure to the British forces. Harsh 

conditions and lack of adequate provisions tested the

resolve and endurance of the army, eventually hardening 

them and inspiring the French to join the revolution 

toward American independence from Britain. Valley 

Forge National Historical Park commemorates the spirit 

of independence and educates the public about this 

pivotal moment in American history.

Valley Forge is located in the eastern Piedmont 

physiographic province, adjacent to the Atlantic Coastal 

Plain and near the junction of the Gettysburg-Newark 

Lowland, Piedmont Upland, and Piedmont Lowland 

sections. The rocks in this area range from 

metamorphosed and deformed Precambrian gneisses 

and Paleozoic metamorphosed sedimentary rocks to the 

gently tilted, much younger sedimentary strata of the 

Mesozoic rift basins. Within the park, Triassic-age red 

shales lay juxtaposed against Cambrian carbonatebearing units in an approximately 300-million-year

unconformity. These rocks contain fossils and hold clues 

to the geologic history of the area. Like its historical 

resources, the park’s geologic resources deserve 

emphasis and interpretation. 

Geology provides the foundation for the entire 

ecosystem. Geologic processes initiate complex 

responses that produce rock formations, topographic 

expression, surface and subsurface fluid movement, and 

soils. At Valley Forge, human land-use disturbances are

also apparent. Prospectors dug pits and quarries in 

search of limestone, altering the topographic expression 

of the landscape. Slopes and meadows have been logged 

and cleared for agricultural use, and dredged riverbottom sediments now line riparian zones. The following 

issues have a high level of resource management 

importance at Valley Forge National Historical Park: 

• Karst Issues. Karst processes and features are 

important evolutionary factors for the landscape of 

this historical park. Karst landscapes are produced by 

the dominant geomorphic processes of chemical 

erosion and weathering of limestone or dolomite. 

When this dissolution occurs underground, voids and 

conduits can develop into a vast karst network. 

Cambrian limestones were quarried extensively in the 

area, and dissolution of these limestones formed caves 

and sinkholes, such as Bone Cave (Port Kennedy 

Cave). Karst processes are still active at Valley Forge 

National Historical Park and may threaten park 

infrastructure and visitor safety.

• Water and Sediment Issues. The Schuylkill River and 

several tributaries flow through the historical park. 

Surrounding urbanization and industrial activities

threaten the surface and groundwater, as well as the 

sediments within the area’s streambeds. It is crucial 

that park management be knowledgeable about the 

area’s hydrogeologic system. Recent studies have 

focused on understanding the multilayered aquifer 

system underlying Valley Forge National Historical 

Park. A nearby Superfund site is a point source of 

groundwater contaminants. Other research within the 

park has examined contaminant-laden sediment loads 

in local streams and potential sources of contaminants. 

• Paleontological Resources. Fossils can record 

information that allows researchers to reconstruct past 

depositional environments and paleoclimates. They 

may also provide information on post-burial 

taphonomic conditions, including geochemical 

changes, deformational regimes, and changes in 

bedding orientation. Several geologic units in the park 

are fossiliferous, containing stromatolites, Skolithos 

burrows, trilobites, and conodonts. One of the most 

important Pleistocene-age fossil deposits in North 

America is located in a karst feature within park 

boundaries. This feature contains unique species and a 

rich record of numerous plants, insects, reptiles, and 

mammals. Currently buried beneath industrial fill, the 

site’s historic significance is related to paleontology 

and limestone quarrying. 

Other issues of resource management concern at Valley 

Forge National Historical Park include slope processes, 

disturbed lands and mineral development, and geologic 

influences on the park’s history. Since American Indians 

first inhabited the region, humans have come to Valley 

Forge to take advantage of the natural resources, 

including geological resources. George Washington 

chose Valley Forge as the encampment site of the 

colonial army because of landscape features shaped by 

geologic processes.
vi NPS Geologic Resources Division 

Acknowledgements

The Geologic Resources Inventory (GRI) is one of 12 inventories funded by the National 

Park Service Inventory and Monitoring Program. The GRI is administered by the 

Geologic Resources Division of the Natural Resource Program Center.

The Geologic Resources Division relies heavily on 

partnerships with institutions such as the U.S. Geological 

Survey, Colorado State University, state geologic surveys, 

local museums, and universities in developing GRI 

products.

Special thanks to: Vincent Santucci (George Washington 

Memorial Parkway) who provided a number of images 

used in the report. Amy Ruhe and Kristina Heister 

(Valley Forge National Historical Park) also provided 

images and information. 

Credits

Author

Trista Thornberry-Ehrlich (Colorado State University)

Review

William Kochanov (Pennsylvania Bureau of Topographic 

and Geologic Survey)

Jason Kenworthy (NPS Geologic Resources Division)

Editing

Jennifer Piehl (Write Science Right)

Digital Geologic Data Production

Georgia Hybels (NPS Geologic Resources Division)

Andrea Croskrey (NPS Geologic Resources Division)

Stephanie O’Meara (Colorado State University)

Digital Geologic Data Overview Layout Design

Phil Reiker (NPS Geologic Resources Division)

John Gilbert (Colorado State University)
VAFO Geologic Resources Inventory Report 1

Introduction

The following section briefly describes the National Park Service Geologic Resources 

Inventory and the regional geologic setting of Valley Forge National Historical Park.

Purpose of the Geologic Resources Inventory 

The Geologic Resources Inventory (GRI) is one of 12 

inventories funded by the National Park Service (NPS) 

Inventory and Monitoring Program. The GRI, 

administered by the Geologic Resources Division of the 

Natural Resource Program Center, is designed to 

provide and enhance baseline information available to

park managers. The GRI team relies heavily on 

partnerships with institutions such as the U.S. Geological 

Survey, Colorado State University, state geologic surveys, 

local museums, and universities in developing GRI 

products.

The goals of the GRI are to increase understanding of the 

geologic processes at work in parks and to provide sound 

geologic information for use in park decision making. 

Sound park stewardship requires an understanding of 

the natural resources and their role in the ecosystem. 

Park ecosystems are fundamentally shaped by geology.

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 these goals, the GRI team is systematically 

conducting a scoping meeting for each of the 270 

identified natural area parks and providing a parkspecific digital geologic map and geologic report. These 

products support the stewardship of park resources and 

are designed for nongeoscientists. Scoping meetings 

bring together park staff and geologic experts to review 

available geologic maps and discuss specific geologic 

issues, features, and processes. 

The GRI mapping team converts the geologic maps 

identified for park use at the scoping meeting into digital 

geologic data in accordance with their Geographic 

Information Systems (GIS) Data Model. These digital 

data sets bring an interactive dimension to traditional 

paper maps. The digital data sets provide geologic data 

for use in park GIS and facilitate the incorporation of 

geologic considerations into a wide range of resource 

management applications. The newest maps contain 

interactive help files. This geologic report assists park 

managers in the use of the map and provides an overview 

of park geology and geologic resource management 

issues.

For additional information regarding the content of this 

report and current GRI contact information please refer 

to the Geologic Resources Inventory web site 

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

Geologic Setting

Precambrian crystalline rocks, Lower Paleozoic 

metamorphosed sedimentary rocks (metasediments) and 

sedimentary rocks, and Triassic sedimentary rocks 

underlie the landscape at Valley Forge National 

Historical Park. The Triassic-age sediments overlap the 

Precambrian rocks from the north in the northwestern 

corner of the park. To the south, Lower Paleozoic rocks 

cover the Precambrian complex. The older, Paleozoic

geologic units in the Valley Forge area are faulted and 

deformed, with varying degrees of metamorphism 

preserved in outcrop exposures. The younger Triassicage rocks are generally flat-lying and undeformed, with 

surface variations due to erosion and downcutting of 

rivers through the horizontal layers.

The Schuylkill River flows eastward through the 

northern reaches of the park, and rolling hills dominate 

the southern park landscape (fig. 1). The eastern 

termination of the North Valley Hills is at Mount Misery 

and a small portion of Chester Valley to the south. 

Mount Joy is located east of the confluence of Valley 

Creek and the Schuylkill River. Other tributaries to the 

Schuylkill flow through the park, including Trout Creek, 

Myer’s Run, and Lamb Run. Within park boundaries, 

elevations range from less than 25 m (80 ft) above sea 

level along the banks of the Schuylkill River to just over 

120 m (400 ft) above sea level atop Mount Joy. 

Valley Forge National Historical Park is located near the 

junction of the Atlantic Coastal Plain and the Piedmont 

province, described below (fig. 2). Three Piedmont 

subprovinces meet in this area; from west to east, these 

are the Gettysburg-Newark (Triassic) Lowland Section,

the Piedmont Lowland Section, and the Piedmont 

Upland Section. A general east-to-west description of 

several physiographic provinces of the Appalachian 

Mountains follows. 

Atlantic Coastal Plain Province 

The Atlantic Coastal Plain province is predominantly flat 

terrain, with elevations ranging from sea level to less than 

100 m (300 ft) above sea level in the southeastern corner 

of Pennsylvania. This province extends from New York 

to Mexico, and from the Fall Line east to Chesapeake 

Bay and the Atlantic Ocean. It was formed by the 

intermittent deposition of a wedge-shaped sequence of 

sediments eroding from the Appalachian Highlands to 

the west, during periods of elevated sea level in the past 

100 million years. The sediments are more than 2,400 m 

(8,000 ft) thick at the Atlantic coast. These deposits were 

reworked by fluctuating sea levels and the continual 

erosive action of waves along the coastline. The province 

continues 120 km (75 miles) to the east as the submerged 

Continental Shelf. Coastal Plain surface soils are 

commonly well-drained sands or sandy loams. Large 

streams and rivers, including the Delaware and Schuylkill
2 NPS Geologic Resources Division 

rivers, transport sediment that continues the eastward 

expansion of the Coastal Plain.

Piedmont Province

The Piedmont physiographic province extends from the 

Fall Line to the Blue Ridge Mountains. The “Fall Line,” 

or “Fall Zone,” marks a transitional zone of ridges, 

waterfalls, and rapids where the softer, less-consolidated 

sedimentary rocks of the Atlantic Coastal Plain intersect 

the harder, more-resistant metamorphic rocks of the 

Piedmont province . Examples of the rapids formed by 

Piedmont rocks are present in the Potomac Gorge of 

Maryland’s Chesapeake and Ohio Canal National 

Historical Park, and at Great Falls Park in Virginia. 

The eastward-sloping Piedmont formed through a 

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

resulting eastern landscape of gently rolling hills 

becomes gradually steeper to the west; elevations range 

from 60 m (200 ft) to 300 m (1,000 ft) above sea level. The 

Piedmont Plateau is composed of hard, crystalline 

igneous and metamorphic rocks, such as schist, phyllite, 

slate, gneiss, and gabbro. Soils in the Piedmont Plateau 

are highly weathered and generally well-drained.

A series of Triassic-age extensional basins are 

superposed on the Piedmont. These basins were formed 

by normal faults during crustal extension (pulling apart). 

The faults opened basins (grabens), which were rapidly 

filled with roughly horizontal layers of sediment. 

Examples include Frederick Valley in Maryland, the 

Newark-Gettysburg Basin in Pennsylvania, and Culpeper 

Valley in northern Virginia. In general, depositional 

contacts define the eastern boundary of the basins with 

the Piedmont Plateau. The western boundaries of the 

basins, which border the Blue Ridge province, are 

defined sharply by normal faults.

Blue Ridge Province

The Blue Ridge province extends from Georgia to 

Pennsylvania along the eastern edge of the Appalachian 

Mountains. It contains the highest elevations in the

Appalachian Mountain system, at Mt. Mitchell near 

North Carolina’s Great Smoky Mountains National 

Park. Precambrian and Paleozoic igneous and 

metamorphic rocks were uplifted during several 

mountain-building events to form the steep, rugged 

terrain. Resistant Cambrian quartzite forms Blue Ridge, 

Bull Run Mountain, South Mountain, and Hogback 

Ridge, whereas Precambrian metamorphic rocks 

underlie the valleys. South and Catoctin mountains, both 

anticlines, are two examples of the pervasive folding in 

the Blue Ridge province.

Eroding streams have caused the narrowing of the 

northern Blue Ridge Mountains into a thin band of steep 

ridges, reaching elevations of more than 1,200 m 

(3,900 ft) above sea level. The Blue Ridge province is 

typified by steep terrain covered by thin, shallow soils, 

resulting in rapid runoff and low groundwater recharge 

rates.

History of Valley Forge

In the winter of 1777-1778, General George Washington 

and the Continental Army, some 11,000 men, wintered 

for six months at Valley Forge. The site was about 32 km 

(20 mi) northwest of then British-occupied Philadelphia,

close enough to the capital to maintain pressure on the 

British forces, yet far enough away to avoid a surprise 

attack. Conditions were harsh; the army faced a severe 

winter, rampant disease, and an overall lack of 

provisions. Preserved earthworks, archaeological sites, 

and historic structures document the army’s struggle to 

overcome this adversity. Resourceful soldiers from all 13 

original states foraged supplies, built some 2,000 log 

cabins in planned military avenues, dug miles of 

trenches, constructed a bridge over the Schuylkill River, 

built five earthen redoubts, and defended the camp. The 

army matured into a more professional military force 

during their tenure at Valley Forge, which ultimately 

impressed the French into recognizing the United States 

as a sovereign power and joining the fight against the 

British. As Washington noted in a February 16, 1778 

letter to Governor George Clinton, 

“Naked and starving as they are, we cannot enough 

admire the incomparable patience and fidelity of the 

soldiery.”

Originally established as a public park in 1893, Valley 

Forge was administered by the State of Pennsylvania 

until 1976. Valley Forge National Historical Park was 

authorized on July 4, 1976, during the administration of 

Gerald R. Ford, and subsequent boundary changes were 

made on June 28, 1980. This park preserves the historic 

landscape at the site of the Continental Army’s winter 

encampment. It covers approximately 14 km2 (3,465

acres), about 12.4 km2 (3,067 acres) of which are Federal 

lands, in Lower Providence and West Norriton 

townships in Montgomery County. For further 

information on the history and setting of the park, 

consult the park’s web site: http://www.nps.gov/vafo.
VAFO Geologic Resources Inventory Report 3 Figure 1. Map of Valley Forge National Historical Park. National Park Service map.
4 NPS Geologic Resources Division 

Figure 2. Physiographic provinces of Pennsylvania. The map shows the physiographic setting of Valley Forge National Historical Park. The green circle marks the location of the park and 

historic site. Red lines indicate boundaries between major physiographic provinces. The black arrow locates the northern terminus of the Blue Ridge province. Graphic by Trista L. Thornberry-Ehrlich (Colorado State University) after Sevon (2000).
VAFO Geologic Resources Inventory Report 5

Geologic Issues

The Geologic Resources Division held a Geologic Resources Inventory scoping session for

Valley Forge National Historical Park on June 22–24, 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. Contact the Geologic Resources 

Division for technical assistance.

Introduction

Issues in this section are presented in relative order of 

resource management significance, as identified by the 

scoping participants and literature-based research. The 

most critical issues for Valley Forge National Historical 

Park are listed first. Potential research projects and 

topics of scientific interest are presented at the end of 

each section.

Karst Issues

The term “karst” refers to a characteristic terrain 

produced by the chemical erosional processes of 

limestone or dolostone (carbonate rocks) over a large 

area (Palmer 1984). These carbonate rocks are common 

constituents of the rock units within and surrounding 

Valley Forge National Historical Park. They dissolve in 

the process of karst formation, when acidic water reacts 

with rock surfaces along cracks and fractures. Over 

hundreds of thousands of years, dissolution occurs 

between intergranular pores and along fractures and 

joints to create progressively larger voids. Many 

Cambrian- and Ordovician-age units (e.g., Ledger and 

Elbrook formations) in the park contain enough 

limestone to have been significantly affected by karstic 

dissolution (Rader 1951; Berg et al. 1980). 

Because of the increased permeability and porosity 

associated with karst processes, these features have a 

significant impact on the hydrogeologic system of an 

area. This is described in greater detail in Kochanov 

(1999). In extreme cases, streams and rivers flow 

underground into subterranean passages. Springs appear 

where the percolating water intersects a less permeable 

unit and flows along the boundary to the Earth’s surface. 

Karst features include caves and caverns, sinkholes, 

solution holes, pinnacles, and cave formations 

(speleothems). Karst topography underlies two-thirds 

(about 850 ha or 2,100 acres) of Valley Forge National 

Historical Park. The park contains at least five small 

caves and at least 100 sinkholes, although an 

comprehensive inventory of these features has not yet 

been undertaken (Kristina Heister, Valley Forge 

National Historical Park, Natural Resource Manager, 

personal communication, September 12, 2010). Karst 

processes such as subsidence, sinkhole formation, 

intermittent stream disappearance, and dissolution also 

affect the park and region. For example, a hole in the 

wall of a nearby quarry is emitting a stream of air due to 

pressure differences between the quarry and another 

opening. The location of this other opening is unknown 

(GRI scoping meeting notes 2004). 

Sinkholes are subsidence features that are most 

commonly circular, but may also be elliptical, linear, or 

irregular in shape (Kochanov 1999). In general, the 

surface expression of a sinkhole or depression is caused 

by the water transport of residual material and soil 

through subsurface pathways established by chemical 

dissolution (Kochanov and Reese 2003). These features

may form quickly when overlying material collapses into

subsurface cavities. Sinkholes in Pennsylvania range 

from 1 to 6 m (4–20 ft) in both diameter and depth. 

Neighboring sinkholes may coalesce to form a single 

large sinkhole (Kochanov 1999). The Grand Parade 

grounds are underlain by carbonate rocks and contain 

many sinkholes (Kristina Heister, Valley Forge National 

Historical Park, Natural Resource Manager, personal 

communication, September 12, 2010).

Sinkholes such as Bone Cave—a paleo-sinkhole, 

probably formed during the Pleistocene approximately 

670,000 to 600,000 years ago (Bell et al. 2004)—continue 

to form in the park today (National Park Service 2010). 

Bone Cave, also known as Port Kennedy Cave, is an

historic feature at Valley Forge National Historical Park 

that once contained important Pleistocene fossils 

(Santucci et al. 2001). Discovered in 1871 and lost by 

flooding in 1896, Bone Cave is located in a dolomite 

quarry near Port Kennedy. It is one of a series of caves 

found in the Valley Forge region during the mining of the 

area’s abundant carbonate rocks (McCarthy 1994). The 

cave is approximately 12 m (40 ft) deep, 3 to

6 m (10–20 ft) wide (Santucci et al. 2001), and 9.2 to

12.2 m (30–40 ft) below ground surface (Hojdila et al. 

2005). 

The quarry containing Bone Cave was filled in the early 

1900s with hazardous asbestos-containing slurry waste 

material from a neighboring boiler and insulating 

company (Ehret Magnesia Manufacturing Company) 

(Daeschler et al. 2005). Lack of adequate mapping and 

revegetation obscured its location until recent literature 

searches and noninvasive geophysical survey techniques 

such as microgravity, electrical resistivity imaging, and 

magnetometry identified the quarry walls, quarry fill, and 

the filled cavity of Bone Cave (Hojdila et al. 2005; 

Baughman et al. 2006). These techniques operated on the 

hypothesis that whatever filled the fissure would be less 

dense than the surrounding dolomite of the quarry walls 
6 NPS Geologic Resources Division 

(Bechtel et al. 2005), and would thus appear as an 

anomaly. 

Of particular concern for resource management, karst 

features and processes may affect park infrastructure, 

health, and human safety. Common issues with karst 

features and processes include flooding, subsidence, and 

collapse (Drumheller 1985). For example, the location of 

a recently installed water line behind the park office is in 

an area prone to sinkholes. A site-specific approach to 

each sinkhole within the park is needed, with action 

items customized to meet the site conditions (William 

Kochanov, Pennsylvania Bureau of Topographic and 

Geologic Survey, Senior Geologic Scientist, written 

communication, February16, 2010). Approaches to the

identification and investigation of sinkholes vary from 

visual inspection and excavation of subtle ground 

depressions, to test borings with continuous standard 

penetration testing, shallow subsurface radar technology, 

and electromagnetic profiling (Drumheller 1985; 

Ozdemir et al. 1992; Qubain et al. 1995). Detailed site 

characterization using test boring, consolidation tests, 

and other geophysical techniques can provide invaluable 

information for future planning (Qubain and Sekinsky 

1998). If identified, problematic sinkhole areas can be 

stabilized with an installation of rip rap, various grout 

compositions, geotextiles, concrete plugs, and other fill 

materials (Drumheller 1985; Reifsnyder et al. 1988;

Qubain et al. 1995). 

In addition to many useful geologic publications relevant 

to Valley Forge, the Pennsylvania Bureau of Topographic 

and Geologic Survey produced maps of sinkholes and 

karst-related features of Montgomery County 

(Kochanov 1993). Karst density maps indicate that 

portions of Montgomery County along the Schuylkill 

River contain more than 150 karst features per km2 (400 

karst features per mi2

) (Kochanov and Reese 2003). 

To meet resource management objectives, the park is 

interested in undertaking a comprehensive inventory 

and assessment of cave and karst resources within the 

park. An NPS Project Management Information System

(PMIS) proposal has been submitted to fund such an 

inventory, but has not yet been approved (Kristina 

Heister, Valley Forge National Historical Park, Natural 

Resource Manager, personal communication, September 

12, 2010). Ongoing research within the park, including 

gravity and conductivity surveys, continues to yield vital 

information for resource management. The park is also 

promoting a regional study of any unique cave flora and 

fauna, including plants and bats (GRI scoping meeting 

notes 2004). Toomey (2009) has suggested techniques for 

the monitoring of caves and karst landscapes. As 

discussed below in the “Paleontological Resources” 

section, the karst features at Valley Forge National 

Historical Park also contain fossil remains. 

Water and Sediment Issues

The Schuylkill River and its tributaries are important 

features in the landscape of Valley Forge National 

Historical Park. These waterways and their adjoining 

riparian habitats contribute to Pennsylvania’s scenic 

character and provide a vital sanctuary for many bird 

species. Rivers and streams cut through the rock layers to 

create valleys, ravines, and gullies.

Streams are sites of active geomorphological processes 

such as streambank erosion, mass wasting, and 

downcutting. Downcutting streams often undercut their 

banks, removing material (commonly alluvium) at the 

base of steep terrace slopes. Streambank stabilization is a 

resource management issue in the gap between Mt. Joy 

and Mt. Misery (GRI scoping meeting notes 2004). Some 

stream erosion and flooding within the park threaten 

facilities and resources. Erosion occurs along every 

streambank within the park, most obviously on Valley 

Creek at Washington’s Headquarters (National Park 

Service 2010). Widespread erosion may affect sewer and

water lines, historic structures, and archaeological 

resources along Valley Creek (GRI scoping meeting 

notes 2004).

The effects of stream flow and erosion were also 

evaluated in several runs on the north side of, and 

tributary to, the Schuylkill River, including Meyer’s Run, 

Lamb Run, and Fawn Run (Ellsworth and Noon 2009). 

At the confluence of the tributaries, the floodplain is 

deeply incised. It was concluded that erosion will 

continue on the side slopes of the runs, as a result of 

intense herbivore grazing. Recommendations to reduce 

erosion include reducing the grazing impacts and to 

plant the side slopes of the runs with shrubs and 

herbaceous cover (Ellsworth and Noon 2009).

The Schuylkill River has a long flooding history. While 

little park infrastructure is located within the active 

floodplain, a large-scale flood could still threaten visitor 

safety and the park’s natural and cultural resources. For 

example, a picnic area and restroom on the north side of 

the river are located in an area prone to flooding damage 

(GRI scoping meeting notes 2004). Dredged materials in 

many floodplain areas along the river could contain 

contaminants from distant and local sources (Reif and 

Sloto 1997).

Valley Creek was the subject of a spatial distribution 

study (Fraley et al. 2005) that aimed to understand the 

effects of surrounding urbanization and subsequent 

increases in peak flows (due to the proliferation of 

impervious surfaces in the watershed) on sediment 

storage and transport patterns. The study involved 

extensive channel characterization, surveying and 

resurveying of eroding stream banks and channel crosssections, and field monitoring. Data collection included 

particle-sized spatial distribution patterns, the depth of 

the mobile layer, and the sediment particle sizes 

mobilized as a function of discharge (Fraley et al. 2005). 

Similar studies and subsequent monitoring efforts at 

other sites within Valley Forge National Historical Park 

would help to determine the source(s) of sediment input. 

This input could derive from local erosion, or from 

offsite upstream sources that may be contaminated with 

urban waste that is carried into the Schuylkill River 

corridor. 
VAFO Geologic Resources Inventory Report 7

Fluvial processes include sedimentation and erosion. 

Due to pervasive industrial development and natural 

resource extraction throughout the Schuylkill River 

watershed, the quality of water resources and sediments 

has suffered. The river flows through industrial, 

residential, and agricultural areas that may be sources of 

contaminants (Reif and Sloto 1997). Cooperative efforts 

to remediate the watershed of point and non-point 

sources of pollution are underway. Standard engineering 

practices for contaminant containment include passive 

cut-off walls, chemical immobilization, and top sealing,

as well as active pumping and treatment. Other 

techniques include regrading, surface sealing, 

revegetation, and electrochemical remediation 

technologies. The latter technique passes electrical 

current between electrodes driven into the ground to 

force reduction/oxidation reactions that remediate at the 

pore level (Emrich and Beck1981; Doering et al. 2001). 

The Schuylkill River is a target for remediation of 

contaminated sediments. A 1997 study (Reif and Sloto 

1997) detected metals, pesticides, and semivolatile 

organic compounds adsorbed to particles in Schuylkill 

River sediments within Valley Forge National Historical 

Park. Arsenic, chromium, zinc, iron, manganese, copper, 

and lead were detected. The pesticides DDE, dieldrin, 

chlordane, DDD, DDT, and heptachlor epoxide were 

also present. Samples contained 17 semivolatile organic 

compounds, including fluoranthene, phenanthrene, and 

pyrene. The highest concentrations of contaminants 

were found in the sediments of Lamb Run, a tributary to 

the Schuylkill whose headwaters are in an industrial 

corporate center that includes the Commodore 

Semiconductor Group National Priorities List Superfund 

Site. While Lamb Run contributes contaminated 

sediment to the Schuylkill River system, Myer’s Run 

contributes relatively clean sediments (Reif and Sloto 

1997).

Among many other industrial sites in eastern 

Pennsylvania, the Commodore Semiconductor Group 

National Priorities List Superfund Site, located in the 

Valley Forge Corporate Center, is a local point source for 

volatile organic contaminants. A 1996 cooperative study

(Sloto and McManus 1996) conducted by the U.S. 

Geological Survey and the NPS revealed the presence of 

a groundwater divide that forms a hydraulic barrier to 

the flow of contaminated groundwater from the site to 

Valley Forge National Historical Park. The divide stems 

from a cone of depression around six public wells. The 

cessation of pumping at these wells would allow the 

water levels to recover, forcing a hydraulic gradient 

between the site and the Schuylkill River and causing 

contaminated groundwater to flow through the park. In 

such a case, park management would need to investigate 

the implementation of multiple groundwater and soil 

sampling techniques, analyses, and treatment options 

such as granulated activated carbon, direct aeration, and 

counter-current airflow stripping (Schuller et al. 1982; 

Johnson et al. 1986; Gonclaves et al. 1988; Vitale et al. 

1991; Vitale et al. 2006). Volatile organic compounds can 

infiltrate and accumulate through building foundations 

as vapors, making their presence a potentially significant 

threat to surrounding buildings and park infrastructure 

(Symms et al. 1995). 

An impoundment desilting basin with at least six ponds is 

located in the park, north of Schuylkill River and west of 

Myer’s Run (Reif and Sloto 1997). This complex is 

comprised of approximately 10 palustrine (marsh) 

emergent wetlands surrounded by forested wetland

(Ellsworth and Noon 2009). This wetland complex has 

high functional value, providing habitat for several listed 

endangered or threatened species. Varying hydrologic 

conditions support a diverse plant assemblage with a 

variety of canopy structures, providing a range of habitat 

opportunities. The ponds are situated at the bottom of a 

watershed and have no surface water outlet, so they 

recharge the river-related aquifers (Ellsworth and Noon

2009). Another stock pond is spring-fed and impounded 

by a dam. These low-lying features contain layers of silt 

and coal debris, and also provide wetland habitat (GRI 

scoping meeting notes 2004). They may also contain 

contaminants such as those found in Schuylkill River 

sediments. The park confirmed in 2004 that the basin 

should remain unmitigated to protect its current 

biodiversity and facilitate resource management efforts 

(GRI scoping meeting notes 2004). 

Groundwater flow systems have primarily geologic

determinants. Most groundwater in the Valley Forge 

area is stored in weathered Triassic-age rock (Stockton 

Formation [see Map Unit Properties Table]) near the 

surface in a complex, heterogeneous multiaquifer system 

(Sloto and McManus 1996). This system is comprised of

layers of gently tilting rocks of high transmissivity,

separated by layers of rocks of relatively low 

transmissivity. The more permeable rock layers are 

generally arkosic sandstones and conglomerates,

whereas the siltstone and shale units tend to less 

permeable. Each layer has different hydrologic 

properties; fractures, bedding planes, and joints 

accommodate groundwater movement (Sloto and 

McManus 1996). As an aquifer, the Stockton Formation 

is characterized by the presence of discrete wateryielding zones, strongly directional transmissivities, and 

unpredictable flow boundaries (Kraus and Dunn 1983). 

Groundwater flow within this system can vary at any 

scale. A single fracture can control local groundwater 

flow, independent of the overall hydrogeologic structure 

(Schuller et al. 1982). Cambrian dolomites (Ledger 

Formation) also underlie Valley Forge National 

Historical Park. Precipitation filters through the more 

permeable sandstones above, contacts the surface of the 

dolomite. The water then flows out as springs through 

fractures and crevices along the contact between the 

sandstone and carbonate rocks. These springs supply 

and influence the hydrology of the wetlands in the park 

(fig. 3) (Ellsworth and Noon 2009).

Resource management should understand the 

hydrogeologic system at Valley Forge National Historical 

Park. Applied geophysical techniques provide valuable 

tools for the observation of hydrologic properties, 

characterization of groundwater flow systems, and 

detailed mapping of subsurface geology. Dye tracing 

techniques are particularly useful for the delineation of
8 NPS Geologic Resources Division 

groundwater flow paths and groundwater basin 

boundaries in carbonate bedrock units (William

Kochanov, Pennsylvania Bureau of Topographic and 

Geologic Survey, Senior Geologic Scientist, written 

communication, February16, 2010). Potentially useful 

methods include surface resistivity, magnetometry, 

shallow seismic reflection or refraction, electric logging, 

and cross hole seismic survey (Emrich 1984; Candon 

1986). Streams combine input from multiple sources

within watersheds, such as wind, surface runoff, 

groundwater, sewage outfalls, landfills, dredged fill dirt, 

and rainfall. Thus, they provide a cumulative measure of 

the status of the watershed’s hydrologic system. 

Consistent measurement of these parameters is crucial to 

the establishment of watershed baselines.

Several ongoing resource management projects and 

research within the park area address fluvial processes 

and water issues. These include a multi-component 

fluvial geomorphological study of the entire watershed, 

new storm-water regulations, consideration of the 

removal of the Valley Creek dam, potential sediment 

removal in silted areas, and water quality studies. The 

National Science Foundation (NSF) has funded several 

water-related projects within Valley Forge National 

Historical Park (GRI scoping meeting notes 2004). Lord 

et al. (2009) have suggested techniques for monitoring 

stream systems. Contact the NPS Water Resources 

Division for additional technical assistance regarding 

water issues.

Paleontological Resources

Fossils add to the understanding of the geologic history 

of the Piedmont province and Mesozoic rift basins in the 

Valley Forge area. Fossils can correlate geologic units 

across time and space, provide information about

depositional environments, and record post-burial 

processes such as geochemical changes, deformational 

regimes, and changes in bedding orientation. 

The NPS conducted a literature-based paleontological 

inventory of known fossil occurrences in Valley Forge 

National Historical Park (Kenworthy et al. 2006). No 

formal, field-based paleontological resource inventories 

have been completed for the park, and it lacks museum 

collections of paleontological material (Kenworthy et al. 

2006). Fossil resources include those found in caves, as 

well as invertebrate fossils and stromatolites in regional 

geologic units. 

Several geologic units in the park (e.g., the Cambrian-age 

Chickies, Ledger, and Elbrook formations) are known to 

contain fossils regionally. Some of the park’s monuments 

and building materials also contain fossil resources. 

Heavy vegetative cover and deeply weathered bedrock 

preclude extensive outcrop exposure. The Chickies 

Formation contains the trace fossil Skolithos, which is the 

fossilized tubular burrow of an unknown worm-like 

organism (fig. 4). The Ledger Formation contains some 

conodonts (teeth of extinct marine animals), trilobites 

(not locally), and stromatolites (Kenworthy et al. 2006). 

Stromatolites (fossil algae mats) are present within the 

Ledger Formation in the main visitor parking lot 

(William Kochanov, Pennsylvania Bureau of 

Topographic and Geologic Survey, Senior Geologic 

Scientist, written communication, February16, 2010). 

The Elbrook Formation contains trilobite remains. The 

Triassic Stockton Formation contains plant fossils, 

invertebrate body and trace fossils, mollusk fossils, fish 

and amphibian remains, as well as reptilian and 

dinosaurian body fossils and footprints (Kenworthy et al. 

2006). At present, Triassic-age fossil plants are not 

known from locations within the park, but occur along 

the Pennsylvania Turnpike some 8 km (5 mi) away 

(William Kochanov, Pennsylvania Bureau of 

Topographic and Geologic Survey, Senior Geologic 

Scientist, written communication, February16, 2010).

Bone Cave, located in a dolomite quarry into the 

northwest-dipping Cambrian Ledger Formation,

contains one of the most important middle Pleistocene 

fossil deposits (about 670,00-600,000 years before 

present) in North America (Baughman et al. 2006; Bell et 

al. 2004; Santucci et al. 2001). The fossils are preserved in 

water-saturated sand and mud strata that are “concaveup” below sediments derived from the Triassic Stockton 

Formation. A layer of black, organic-rich clay underlies 

the fossiliferous layer (Santucci et al. 2001). Until the 

early 2000s, the location of Bone Cave remained 

enigmatic. As described in the above “Karst Issues” 

section, the location is now known and potential 

resource management issues may arise regarding the 

preservation and/or excavation of the fossiliferous 

deposits.

Bone Cave excavations in the late 1800s revealed the 

extremely fragile condition of its fossils. Some crumbled 

to pieces when removed from the clay, and others were 

the consistency of “cream cheese” or “over-ripe pears” 

(Mercer 1899; Daeschler et al. 1993). The investigators 

thus tended to focus on the larger, more intriguing 

specimens (McCarthy 1994). Any future plans to 

investigate or excavate the cave would require rigorous 

planning and professional execution. In addition to the 

issues raised by the condition of the fossils, some of the 

cave’s artificial fill contains asbestos that deters 

excavation in the area (William Kochanov, Pennsylvania 

Bureau of Topographic and Geologic Survey, Senior 

Geologic Scientist, written communication, February16,

2010). The importance and significance of the deposit at 

Bone Cave may require security measures to be taken at 

the site if activities are undertaken there in the future 

(Bechtel et al. 2005). 

Fossil resources require science-based resource 

management, as directed by the 2009 Paleontological 

Resources Preservation Act (Public Law 111-11). The 

NPS is currently developing regulations associated with 

the Act (Julia Brunner, NPS Geologic Resources 

Division, personal communication, May 2010). Santucci 

et al. (2009) have suggested strategies for monitoring in 

situ paleontological resources.

Slope Processes

Downcutting streams, such as Valley Creek, and other 

erosive agents created valleys, ravines, and gullies on the 
VAFO Geologic Resources Inventory Report 9

landscape at Valley Forge National Historical Park. 

Mechanical and chemical weathering processes have also 

weakened the underlying geologic units that contain 

sandstone, siltstone, mudstone, clay shale, and limestone. 

For example, the heterogeneous Stockton Formation 

includes weak, clay-rich mudstones interlayered with 

resistant sandstones and conglomerates (Sloto and 

McManus 1996). Clay-rich units such as shale and 

mudstone may disintegrate or swell when they become 

water–saturated, and are prone to fail when exposed on a 

slope. This weakness may cause a slide or slump. Where 

a resistant layer such as sandstone overlies a clay-rich 

layer, preferential erosion (undercutting) of the clay layer 

causes a dangerous blockfall situation in which the upper 

resistant layer is left unsupported. Slopes throughout the

park are prone to fail, especially when saturated with 

water following seasonal rainfall events. 

Talus and colluvial deposits at slope bases such as Mount 

Joy and Mount Misery attest to mass movements in the 

past. Today, the processes of slope creep, slumping, and 

sliding may threaten roads and trails throughout the park 

(GRI scoping meeting notes 2004). Erosion caused by 

precipitation is particularly active along trails (William 

Kochanov, Pennsylvania Bureau of Topographic and 

Geologic Survey, Senior Geologic Scientist, written 

communication, February16, 2010). The Chickies 

Formation crops out directly upslope from the Upper 

Forge Site bridge, and blocks of this quartzite unit form 

extensive talus slopes to the south along the trail 

(Wiswall 1993). Blockfall in this setting may put trail 

users in danger. Likewise, blockfall is evident near the 

comfort station (fig. 5).

Three-dimensional investigations of colluvium could 

shed light on the exact nature of their formation, their

influence on groundwater flow, and the potential for 

future movements. Such studies typically involve test 

pitting and deep boring, permeability measurements, and 

observational determination of colluvial composition, 

layering, and degree of weathering (Layton et al. 2006).

Wieczorek and Snyder (2009) have suggested techniques 

for the monitoring of slope processes.

Disturbed Lands and Mineral Development

The mineral resources of rocks within and surrounding 

Valley Forge National Historical Park have long 

attracted humans. The ruins of pre-encampment copper 

mines and iron forges are present in the park and 

surrounding area (McCarthy 1994; GRI scoping notes, 

2004). Extensive quarrying of the Cambrian-age 

limestones for building material and lime left many scars 

on the landscape. A small quarry is present on Mt. Joy 

and a deep limestone quarry is located in the 

southwestern corner of the park (GRI scoping meeting 

notes 2004). Some quarries, such as that in which Bone 

Cave is located, were flooded and filled with hazardous 

(asbestos-laden) industrial fill and waste material. The 

long outcrop on the south side of the visitor’s parking lot 

is part of the wall of an abandoned dolostone quarry 

(Wiswall 1993). 

Abandoned and inactive mines and quarries pose safety, 

environmental, and health problems in the Valley Forge 

area. According to the NPS Abandoned Mineral Lands 

(AML) database, there are four AML sand and gravel 

quarry sites (containing 6 features) within the park. Mine 

and quarry features interrupt the regional hydrologic 

regime and threaten the park’s soils, groundwater, and 

small streams and springs. Remediation of mine- and 

industry-affected areas in southeastern Pennsylvania is a 

cooperative effort. In addition to the NPS, agencies such 

as the U.S. Geological Survey, the Pennsylvania 

Department of Environmental Protection, and the 

Pennsylvania Environmental Council, as well as many 

local offices, contribute to ongoing remediation and 

restoration projects (Anonymous 2002). These projects 

include the removal of the Mt. Joy road and parking lot, 

restoration of encampment-era topography in areas 

obscured by quarries, trail restoration, and the relocation 

of old mining sites using charcoal horizons in the 

substrate, dips, and waste material piles (GRI scoping 

meeting notes 2004). Despite constituting potential 

hazards, many of the quarries (particularly those in park 

maintenance areas) have administrative and educational

multi-use capabilities (William Kochanov, Pennsylvania 

Bureau of Topographic and Geologic Survey, Senior 

Geologic Scientist, written communication, February16,

2010). Geologic mapping takes advantage of such quarry 

features to determine the location of geologic features. 

The outcrop near the visitor parking lot illustrates tilting 

of originally horizontal sedimentary layers during 

mountain building. This outcrop also contains the thin, 

irregular to wavy laminations of stromatolites, some of 

the earliest recorded life forms on Earth (Wiswall 1993). 

Geologic Influences on Cultural Resources

Prior to the Revolutionary War, an iron forge was 

located on Valley Creek; however, the park’s emphasis is 

primarily on the Continental Army’s sojourn. Since the 

1777-1778 encampment at Valley Forge, multiple 

landowners and landscape changes have altered or 

obscured the area’s historical cultural resources (Parrish 

and Cohen 1987). Such cultural noise can mask many 

original historical features. An integrated geophysicalbiogeochemical survey prior to archaeological 

excavation can help direct attention to areas likely to 

reveal cultural resources; this approach is superior to a 

random sampling technique in such contexts (Parrish et 

al. 1987). Geophysical methods such as cesium 

magnetometer surveys, soil-penetrating radar scans, and 

soil resistometry have contributed to the identification of 

subsurface prehistoric and historic geological and 

cultural features at Valley Forge National Historical Park 

(Schenck 1978). In 1987, a detailed magnetic gradient 

survey revealed the locations of numerous hearth sites in 

the park by identifying peaked concentrations of 

calcium, phosphorous, and potassium (Parrish and 

Cohen 1987; Parrish et al. 1987). Biogeochemical 

techniques such as the analysis of broom sedge 

(Andropogon virginicus) crops for chemical anomalies

(soils developed over former hearths often have high 

calcium, phosphorous, and potassium levels that are 

sequestered into the plants) have also revealed cultural 

remains on the encampment landscape (Parrish and 
10 NPS Geologic Resources Division 

Cohen 1987). Noninvasive gravity surveys, electrical 

resistivity transects, and magnetometers helped to 

relocate Bone Cave after 110 years (Hojdila et al. 2005).

Although most visitors are primarily interested in the 

historical context of the Valley Forge National Historical 

Park landscape, the park also provides an ideal outdoor 

classroom for Earth science students. Endeavors by local 

college professors provided lessons to teachers for 

grades 6–12 on general scientific inquiry, surficial 

geology, environmental geology, hydrology, 

geomorphology, sedimentology, and meteorology as part 

of a local school’s Earth science curriculum (Good 1998; 

Cannon 1999).

The diversity of geologic phenomena in the Valley Forge 

area can be used to educate geology students. The 

concept of geologic time is readily accessible in the 

Precambrian- to Mesozoic-age rocks within and 

surrounding the park. Rocks and their fossils indicate 

depositional environments and conditions of life in the 

distant past. The geologic structures in the area record 

several episodes of mountain building and continental 

rifting. The Triassic rocks within the park record a large 

rift basin superimposed on the Piedmont physiographic 

province, as the eastern coast of the North American 

continent was extended following the Alleghanian 

orogeny. Pleistocene periglacial conditions changed the 

courses of rivers in the area, causing floral and faunal 

migrations and leaving vast deposits that included glacial 

till and outwash. The fossil record from Bone Cave 

attests to this environment. A meaningful scientific 

contribution to the experience of park visitors could be 

developed by relating the geology to human concerns 

such as floods, slope failures, coal mining, water 

pollution, oil and gas exploration, waste management, 

and urban development (Nikitina 2003). The relation of 

the geology and geologic processes visible in the park to 

the history of the encampment and forge lends deeper 

meaning to the human history of the area. 

One of the major management goals at Valley Forge 

National Historical Park is to preserve its historical 

context by recreating log cabin and hut structures, 

preserving earthworks and redoubts, and restoring 

historic structures dating to the encampment. Some 

structures, such the Maurice Stephens House built 

around 1816 (once thought to be Huntington’s 

Quarters), postdate the encampment. An accurate 

recreation of the encampment landscape would require 

the removal of these structures and the construction of 

others. Total restoration of the cultural landscape of the 

encampment in this case is likely not possible in keeping 

with the NPS protection policies for historic structures 

defined in the Cultural Resource Management Guideline 

(DO-28) (Scott and Ofenstein 2008). A study of the 

evolution of historic land use patterns at the park could 

help prioritize interpretive and restorative efforts. 

The maintenance of any historic landscape often entails 

monitoring or slowing natural geologic processes. 

Geologic slope processes such as landsliding, slumping, 

chemical weathering, block sliding, and slope creep are 

slowly, but constantly, changing the landscape at the 

park. Runoff erodes sediments from any open areas and 

carries them down streams and gullies. Erosion naturally 

diminishes higher areas and fills in lower areas, distorting 

the historical landscape. Extensive quarrying and other 

land-use practices (e.g., urban development and farming)

have changed the regional topographic expression. 

Issues may also arise from the differing goals of cultural 

and natural resource management. For example, a 

proposal for the restoration of a historic trench or 

encampment may entail the removal of surrounding 

natural resources, installation of stabilization structures, 

and/or planting of exotic flora.
VAFO Geologic Resources Inventory Report 11

Figure 3. Wetlands are present near the contact between the Triassic sandstones (Stockton Formation; TRs) and Cambrian dolomite 

(Ledger Formation; Cl). Green line outlines land associated with Washington Memorial Chapel (non Federal land) within the park. The 

Schuylkill River is visible in the upper left of the image. National Park Service graphic utilizing GRI digital geologic data and aerial 

imagery from ESRI Arc Image Service, USA Prime Imagery. Annotated by K. Noon (NPS Water Resources Division). Graphic from 

Ellsworth and Noon (2009).
12 NPS Geologic Resources Division 

Figure 4. Cross-sections of Skolithos (worm burrow) trace fossils are visible within the building stone of the comfort station near the 

visitor center. Local Chickies Quartzite was quarried for the structure. Photograph by and courtesy of Vincent Santucci (George 

Washington Memorial Parkway).
VAFO Geologic Resources Inventory Report 13

Figure 5. Exposure of Ledger Formation in abandoned dolostone quarry near the visitor center. Fallen blocks of material accumulated at 

the base of the exposure. Photograph by and courtesy of Vincent Santucci (George Washington Memorial Parkway).

VAFO Geologic Resources Inventory Report 15

Geologic Features and Processes

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

in Valley Forge National Historical Park.

Cambrian-Triassic Unconformity

Valley Forge National Historical Park contains a unique 

geological feature called an angular unconformity that 

exists between the limestone-bearing Cambrian Ledger 

Formation and the mixed sedimentary strata of the 

Triassic Stockton Formation. The red sandstones and 

shales of the Stockton Formation dip gently to the north, 

whereas the layering within the Ledger Formation dips 

steeply to the south (Wiswall 1993). This tangible gap in 

geologic history spans more than 300 million years. This 

feature helped researchers relocate Bone Cave. The two 

rock types on either side of the unconformity are visible 

in the Port Kennedy quarry (Wiswall 1993). Early 

illustrations show red shale directly overlying the 

limestone of the quarry at the site of Bone Cave (see 

historic photograph in Daeschler et al. 2005). This red 

shale formed much of the sediment that had collapsed 

into the sinkhole uncovered during fossil excavation 

(Daeschler et al. 2005). The unconformity is mapped on 

the GRI digital geologic map derived from Berg et al. 

(1980) (Appendix A). Recent large-scale mapping by 

Kochanov (in prep.) details the nature of this feature

(William Kochanov, Pennsylvania Bureau of 

Topographic and Geologic Survey, Senior Geologic 

Scientist, written communication, February16, 2010). 

Caves and Karst

Bone Cave

Bone Cave is a vertical 12-m-deep (40-ft-deep) solution 

cavity (paleo-sinkhole) in the Cambrian Ledger 

Formation that was briefly opened to the surface,

creating a natural trap for the accumulation of remains 

(Santucci et al. 2001; Daeschler et al. 2005). More than 

1,200 fossils from the cave are held at the Academy of 

Natural Sciences in Philadelphia. The Bone Cave site is 

one of the most significant Middle Pleistocene 

(approximately 600,000 to 670,000 years ago) vertebrate 

fossil finds in North America. The fossiliferous deposit 

was not exhausted before flooding groundwater 

inundated the cave in 1896. The quarry was later filled 

with 9–12 m (30–40 ft) of industrial waste from a nearby 

factory (Daescher et al. 2005). 

Edward Drinker Cope was among the leading 

paleontologists describing fossils from the cave in the 

late 1800s. Numerous fossils were recovered, 

representing 14 plant species, nine coleopterid insects, 

five reptiles (four turtles, one snake), one bird, and 33 

mammal species (Daeschler et al. 1993; Santucci et al. 

2001; Baughman et al. 2006). Significant fossils include 

the only known representatives of a skunk (Osmotherium 

spelaeum) and a muskrat species (Ondatra hiatidens). 

The diluvian fisher (Martes diluviana) is also only known 

from the site. Other notable fossils include ground sloths, 

mammoths, the earliest record of American black bear 

(Ursus americanus), the type locality of the gracile 

sabertooth (Smilodon gracilis) (fig. 6), and the only 

population sample of large North American Pleistocene 

tapirs (fig. 7) (Baughman et al. 2006; Santucci et al. 2001). 

A description of all the fossil remains and history of 

excavation of Bone Cave is beyond the scope of this 

report. Prior to its rediscovery in 2005, Daeschler et al. 

(1993) presented a thorough review and geologic 

contextualization of the Bone Cave fauna and flora. 

Karst Processes

Bone Cave is one example of the action of karst 

processes on the landscape at Valley Forge National 

Historical Park. The lack of streams in areas underlain by 

dolostone, such as those visible from Inner Line Drive, 

County Line Road, and Gulph Road, is typical of karst 

topography (Wiswall 1993). Karstic dissolution occurs 

when acidic water reacts with carbonate rock surfaces 

along cracks and fractures. Most meteoric water is of 

relatively low pH due to the reaction between carbon 

dioxide in the atmosphere and water. The product of this 

reaction is carbonic acid (Palmer 1984). Groundwater 

may become more acidic as it flows through decaying 

plant debris and soils. The acid dissolves calcium 

carbonate, producing soluble ions (Palmer 1984). Most 

of the carbonaceous limestones and dolomites in 

southeastern Pennsylvania are inherently porous,

consisting of voids and underground drainage systems. 

Many large surface karst features, including sinkholes 

formed by mechanical processes such as collapse, were 

triggered by subsurface solution. Sinkholes are produced 

by the collapse of overlying rock and soil into a void. 

They may also form where cracks in the underlying 

limestone are widened by dissolution, so that soil and 

pieces of bedrock subside into the enlarging solutional 

openings (Palmer 1981). They are typically funnelshaped topographic depressions that are elliptical to

circular in landscape view (Palmer 1981; Drumheller 

1985). A sinkhole occurring in a streambed may capture 

the water flow to create a disappearing stream. 

Active karst processes in the Valley Forge area include 

limestone dissolution, underground cavity development, 

spring activity, sinkhole collapse, and erosion (fig. 8). 

The dominant carbonate-bearing units in the region

include the Cambrian-age Ledger and Elbrook 

formations. These units contain thick deposits of soluble 

limestone and dolostone interlayered with shales, 

siltstones, and sandstones (quartzites). The rocks record 

a longstanding marine basin on the subsiding margin of 

the continent during the Cambrian Period. Detrital sand, 

silt, and mud were deposited on this margin, followed by 

the deepening of the basin and the deposition of 
16 NPS Geologic Resources Division 

carbonate chemical sediments approximately 500 million 

years ago (Bechtel et al. 2005). 

Caves in Valley Forge National Historical Park likely 

formed by both vadose (above the water table) and 

phreatic (at or below the water table) solution. Such 

formation occurs when a nascent passage conducts water 

through the vadose zone to the phreatic zone, and 

solutional enlargement of the conduit occurs 

simultaneously in both zones. The local water table is 

highly irregular and discontinuous. Features 

characteristic of both solution types are present in a 

conduit described by Palmer (1984, 1990); these

references could be valuable for researchers attempting 

to interpret the cave features at Valley Forge National 

Historical Park.

Historic Connections with Geology

The known history of the Valley Forge area begins 

thousands of years before the army encampment, and 

was continuously influenced by the area’s geology. A 

Late Archaic–Early Woodland (more than 1,000 years 

ago) site was located on the Schuylkill River floodplain 

near Valley Forge National Historical Park. Holocene

river dynamics, such as incised meanders, infilling, 

meander migration, and small-scale channel fluctuations,

created a topographically elevated site that early 

inhabitants chose for settlement. This location had thick 

deposits of overbank sediments that may have provided 

rich soils for agriculture (Ervin 1985). 

Triassic-age mudstones, siltstones, sandstones, and 

conglomerates underlie the northern highlands near 

Schuylkill River in gently tilted and folded beds. The 

Stockton Formation locally strikes east-west and dips 15° 

to the north. This unit unconformably rests on Paleozoic

and Precambrian rocks (Sloto and McManus 1996). 

Some quarries within the park and surrounding region

mark the contact between the Cambrian carbonates and 

the Triassic clastics (William Kochanov, Pennsylvania 

Bureau of Topographic and Geologic Survey, Senior 

Geologic Scientist, written communication, February16,

2010). Weathering and erosion of these sedimentary 

rocks created the rolling hills and narrow valleys 

characteristic of the Valley Forge area (figs. 9, 10) that 

made it an ideal location for the encampment. 

George Washington chose Valley Forge as an 

encampment site because it provided enough level area 

for the construction of quarters and facilities (fig. 11). 

The encampment was underlain by resistant geologic 

units (such as quartz-rich layers of the Chickies 

Formation) that formed high ground. This location was 

also easily defensible, bordered by Schuylkill River to the 

north and adjacent to Mt. Joy and Mt. Misery.

Freshwater was provided by several streams and rivers 

within a small area. Dolostone underlies the broad, 

rolling upland along Outer Line Drive between park 

headquarters and the National Memorial Arch, whereas 

more soluble limestones underlie the valley at the base of 

a linear surface slope. This dolostone plateau at Valley 

Forge provided the Continental Army with a height 

advantage over enemy troops advancing from the south 

(Wiswall 1993). 

As is interpreted at nearby Hopewell Furnace National 

Historic Site (Thornberry-Ehrlich 2010), Pennsylvania 

contributed much to the colonial iron industry. Valley 

Forge was named after the location of an iron forge on 

Valley Creek. Such forges took advantage of the 

abundant local iron ore, limestone (for flux), wood (for 

charcoal), and water in their operations. This area has 

long attracted prospectors in search of mineral wealth. 

One of the earliest copper mines in the country was 

located within Valley Forge National Historical Park (the 

site now occupied by a parking lot). Mineral specimens 

from this mine included quartz crystals, malachite, and 

pyrite (McCarthy 1994). The area’s abundant limestonebearing rocks provided lime, produced by burning, to 

use in making cement. In 1805, Alexander Kennedy 

purchased property near Valley Forge that became Port 

Kennedy. When the Schuylkill Canal opened in 1825, 

this area was a prosperous village with up to 100 

schooners and barges transporting lime up and down the 

waterway each day (McCarthy 1994). Limestone quarries 

throughout the area (often within the Cambrian Ledger 

Formation) exposed caverns, caves, and sinkholes, some 

of which contained invaluable fossil remains. Other 

caverns may contain stalactites and stalagmites. Most of 

these caves were quarried away before conservation of 

such features became general practice. Other caves were 

destroyed during the construction of Route 422, the 

highway that runs beside Valley Forge National 

Historical Park (McCarthy 1994).
VAFO Geologic Resources Inventory Report 17

Figure 6. Fossil jaw of gracile sabretooth (Smilodon gracilis) recovered from Bone Cave. Bone Cave is the type locality for this species. This 

specimen is on display at the park, from the collections of the Academy of Natural Sciences (Philadelphia). The rear portion of the skull is not 

preserved. Photograph by and courtesy of Vincent Santucci (George Washington Memorial Parkway).

Figure 7. Fragmentary jaw of a large tapir (Tapirus copei) discovered at Bone Cave. Bone Cave contains the only population sample of large 

Pleistocene-aged tapirs from North America. Scale bar is 10 cm. Specimen is on display at the park, from the collections of the Academy of 

Natural Sciences (Philadelphia). Photograph by and courtesy of Vincent Santucci (George Washington Memorial Parkway).
18 NPS Geologic Resources Division 

Figure 8. Generalized cross-sectional view of the development of a karstic landscape through dissolution and collapse. Graphic by Trista L. 

Thornberry-Ehrlich (Colorado State University).
VAFO Geologic Resources Inventory Report 19

Figure 9. View looking west from the Statue of General Anthony Wayne at Valley Forge National Historical Park. Note the relatively steep 

slope and flat valley bottom. The break in slope marks the contact between the carbonate and non-carbonate bedrock units (William 

Kochanov, Pennsylvania Bureau of Topographic and Geologic Survey, Senior Geologic Scientist, written communication, February 16, 2010). 

Photograph is from http://www.worldfromtheweb.com/Parks/Philadelphia/P5110782.html (accessed December 23, 2008). Used with 

permission.

Figure 10. Rolling karst landscape as seen from Huntington Camp (looking west) at Valley Forge National Historical Park. The large hill in the 

background is Mt. Joy, composed of schist and phyllites of the Harpers Formation. National Park Service photograph courtesy Amy Ruhe and 

Kristina Heister (Valley Forge National Historical Park).
20 NPS Geologic Resources Division 

Figure 11. Recreation of cabins at Muhlenberg Camp to appear typical of the Colonial Army’s winter quarters. Most cabins housed 12 men. 

Note the level ground in the encampment area atop carbonate rocks of the Ledger Formation. Photograph by and courtesy of Vincent 

Santucci (George Washington Memorial Parkway).
VAFO Geologic Resources Inventory Report 21

Map Unit Properties

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

Inventory digital geologic map of Valley Forge National Historical Park. The

accompanying table is highly generalized and for background purposes only. Grounddisturbing 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 Valley Forge 

National Historical Park provided information for the 

“Geologic Issues,” “Geologic Features and Processes,” 

and “Geologic History” sections of this report. Geologic 

maps are two-dimensional representations of complex 

three-dimensional relationships; their color coding 

illustrates the distribution of rocks and unconsolidated 

deposits. Bold lines that cross or separate the color 

patterns mark structures such as faults and folds. Point 

symbols indicate features such as dipping strata, sample 

localities, mines, wells, and cave openings. 

Incorporation of geologic data into a Geographic 

Information System (GIS) increases the usefulness of 

geologic maps by revealing the spatial relationships 

among geologic features, 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 are not soil maps, and do not 

show soil types, but they do show parent material—a key 

factor in soil formation. Furthermore, resource managers 

have used geologic maps to make connections between 

geology and biology; for instance, geologic maps have 

served as tools for locating sensitive, threatened, and 

endangered plant species, which may prefer a particular 

rock unit. 

Although geologic maps do not show where earthquakes 

will occur, the presence of a fault indicates past 

movement and possible future seismic activity. Similarly, 

map units show areas that have been susceptible to 

hazards such as landslides, rockfalls, and volcanic 

eruptions. Geologic maps do not show archaeological or 

cultural resources, but past peoples may have inhabited 

or been influenced by depicted geomorphic features. For 

example, alluvial terraces may have been preferred use 

areas and formerly inhabited alcoves may occur at the 

contact between two rock units.

The geologic units listed in the following table 

correspond to the accompanying digital geologic data. 

Map units are listed in the table from youngest to oldest. 

Please refer to the geologic timescale (fig. 12) for the age 

associated with each time period. The table highlights 

characteristics of map units such as: susceptibility to 

erosion and hazards; the occurrence of paleontological 

resources (fossils), cultural resources, mineral resources, 

and caves or karst; and suitability as habitat or for 

recreational use. Some information on the table is

conjectural and meant to serve as suggestions for further 

investigation.

The GRI digital geologic maps reproduce essential 

elements of the source maps including the unit 

descriptions, legend, map notes, graphics, and report. 

The following references are the sources for the GRI 

digital geologic data for Valley Forge National Historical 

Park: 

Berg, T. M., A. D. Glover, S. I. Root, W. E. Edmunds, D. 

M. Hoskins, W. D. Sevon, A. R. Geyer, D. B. 

MacLachlan, and A. A. Socolow. 1980. Geologic map 

of Pennsylvania (scale 1:250,000). 4th ser., 

Pennsylvania Department of Conservation and 

Natural Resources (PA DCNR): Map 1. Bureau of 

Topographic and Geologic Survey, Middletown, PA.

Miles, C. E., and T. G. Whitfield (compilers). 2001. 

Bedrock geology of Pennsylvania (scale 1:250,000). 

Digital dataset extracted from Geologic map of 

Pennsylvania (scale 1:250,000). 4th ser., Pennsylvania 

Department of Conservation and Natural Resources 

(PA DCNR): Map 1. Bureau of Topographic and 

Geologic Survey, Middletown, PA.

The GRI team implements a geology-GIS data model 

that standardizes map deliverables. This data model 

dictates GIS data structure including data layer 

architecture, feature attribution, and data relationships 

within ESRI ArcGIS software, and increases the overall 

utility of the data. GRI digital geologic map products 

include data in ESRI personal geodatabase and shapefile 

GIS formats, layer files with feature symbology, Federal 

Geographic Data Committee (FGDC)-compliant 

metadata, a Windows help file that contains all of the 

ancillary map information and graphics, and an ESRI 

ArcMap map document file that easily displays the map 

and connects the help file directly to the map document.

GRI digital geologic data are included on the attached 

CD and are available through the NPS Data Store

(http://science.nature.nps.gov/nrdata/). Data will be 

available on the Natural Resource Information Portal 

when the portal goes online. As of August 2010, access is 

limited to NPS computers at http://nrinfo/Home.mvc.
VAFO Geologic Resources Inventory Report 22

Map Unit Properties Table: Valley Forge National Historical Park

Colored rows indicate geologic units mapped within Valley Forge National Historical Park

Age Map Unit

(Symbol) Features and Detailed Geologic Description Erosion Resistance

Suitability for 

Infrastructure Hazards PaleontologicalResources

Cultural 

Resources Karst Issues Mineral Resources Habitat Recreation GeologicSignificance

QUATERNARY

Trenton Gravel 

(Qt) 

Unit includes some alluvium and swamp 

deposits, contains gray or reddish-brown 

gravelly sand interbedded with sand, clay, and 

silt beds. Crossbeds present locally.

Very low

Avoid stream 

edge/riparian 

and wetland 

areas for heavy 

development, 

especially for

wastewater 

treatment 

facilities, due to 

proximity to 

water and high 

permeability.

Unit is associated 

with stream banks 

and riparian zone 

areas, and may be 

unstable if exposed 

on a slope or watersaturated.

May contain iceage and modern 

remains

May contain 

remains from 

historic events 

and 

settlements

None Sand, clay, 

silt, gravel

Unit supports 

riparian 

habitats and 

marshlands.

Unit is suitable 

for light 

recreation,

unless at a 

stream edge or 

wetland.

Unit records 

modern 

geomorphological 

changes in the park 

landscape.

TERTIARY

Pennsauken and 

Bridgeton 

formations, 

undifferentiated 

(Tpb);

Bryn Mawr 

Formation

(Tbm)

Tpb contains dark, reddish-brown 

feldspathic quartz sand beds with 

interlayered fine gravel and some clay and silt. 

Tbm consists of gravelly sand and silt in 

reddish-brown high-level terrace deposits. 

Very low

Avoid most 

terrace deposits 

for heavy 

development 

due to 

instability of 

slopes and high 

permeability.

Unconsolidated 

nature of units 

renders them 

susceptible to mass 

wasting on steep to 

moderate slopes.

May contain plant 

fragments

Terrace areas 

may contain 

traces of 

American 

Indian

campsites

None Sand, clay, 

silt, gravel

Units support 

riparian 

habitats and 

well-drained 

soils.

Units are 

suitable for most 

recreation,

unless high 

slopes are 

present

Units record 

history of erosion 

and deposition 

throughout the 

Tertiary.

CRETACEOUS

Patapsco(?) 

Formation

(Kp)

Unit is intensely colored, variegated, iron-rich 

clay with some isolated interbedded sand. Low

Variations in 

bedding, 

sediment, and 

degree of 

cementation 

may render unit 

unstable on 

slopes; generally 

suitable for 

most 

development.

Clay- rich massive 

bedded layers may 

spall in large blocks 

when unit is exposed 

on slope; susceptible 

to slumps and slides.

May contain 

Cretaceous-age 

fossils

None 

documented None Sand, clay

Unit supports 

eastern 

hardwood 

forests 

regionally.

Unit is suitable 

for most 

recreation,

unless exposed 

on slopes.

Widespread unit 

records Cretaceous 

depositional 

environment.

JURASSIC

Sedimentary strata 

at Jacksonwald and 

Aspers

(Js);

Diabase

(Jd)

Js is arkosic sandstone with some gray to 

black shale and limestone interbeds, ripplecross-laminated siltstone, and boulder 

conglomerate. Jd consists of medium- to 

coarse-grained tholeiite in dikes, sheets, and 

scant flows. Individual bodies vary in titanium 

content and presence of plagioclase 

phenocrysts. 

Moderate to 

high

Variations in 

rock type and 

degree of 

cementation 

may render unit 

unstable on 

slopes.

Jd may form resistant 

ridges that are prone 

to rockfall when 

undercut and/or 

weathered.

Js is fossiliferous

Diabase was 

historically 

popular 

building stone

Limestone 

units are 

prone to 

dissolution.

Labradorite, 

pyroxene, 

plagioclase 

phenocrysts 

(cm-size), 

diabase

Jd weathers 

to support 

iron- and 

magnesiumrich 

substrates.

Units are 

suitable for most 

recreation,

unless heavily 

altered or 

fractured.

Jd includes the 

dark-gray York 

Haven Diabase and 

the younger 

Rossville Diabase.

TRIASSIC

Brunswick 

Formation

(TRb);

Limestone 

fanglomerate

(TRfl);

Quartz 

fanglomerate 

(TRfq)

TRb is primarily reddish-brown mudstone, 

siltstone, and shale with some green and 

brown interbedded shale. Some red and darkgray argillite interlayers near the base. TRfl is 

yellowish-gray to medium-gray quartz matrix 

with limestone and dolomite pebbles, 

cobbles, and fragments. Some shale-clast beds 

are locally interlayered. TRfq contains a 

reddish-brown sandy matrix with wellrounded quartzite pebbles, cobbles, and rare 

boulders. 

Moderate

Heterogeneous 

nature of units 

may render 

them unstable 

on slopes.

Units are prone to 

rockfall where 

weathered shale 

underlies resistant 

sandstone ledges or 

where limestone has 

dissolved beneath 

overlying rock layers.

May contain 

Triassic-age fossils

None 

documented

Limestone 

cobbles in 

TRfl may 

dissolve 

completely,

creating 

voids and 

compromising unit 

integrity.

None 

documented

Units 

weather to 

create myriad 

substrates.

Units are 

suitable for most 

recreation,

unless heavily 

weathered and 

friable.

TRb spans the 

Jurassic-Triassic 

boundary.
Colored rows indicate geologic units mapped within Valley Forge National Historical Park

VAFO Geologic Resources Inventory Report 23

Age Map Unit

(Symbol) Features and Detailed Geologic Description Erosion Resistance

Suitability for 

Infrastructure Hazards PaleontologicalResources

Cultural 

Resources Karst Issues Mineral Resources Habitat Recreation GeologicSignificance

TRIASSIC

Hammer Creek 

Formation

(TRh);

Lockatong 

Formation

(TRl);

Hammer Creek 

conglomerate 

(TRhc)

TRh contains quartzose sandstone, siltstone, 

and mudstone that appear gray and pale-red 

in outcrop with fine- to coarse-grained 

textures. TRl is dark-gray to black, thickly 

bedded argillite. Lenses and layers of thinbedded black shale, impure limestone, and 

calcareous shale present locally. TRhc is 

comprised of interbedded cobble and pebble 

quartz conglomerate with red sandstone. 

Moderate

Units are 

suitable for 

most 

development 

unless highly 

fractured, 

weathered, or 

undercut on a 

slope.

Weathered 

mudstone or 

dissolved limestone 

underlying more 

resistant sandstone 

or argillite can create 

rockfall hazards.

May contain 

Triassic-age fossils

None 

documented

Impure 

limestone 

layers are 

susceptible 

to 

dissolution 

and may 

compromise 

integrity of 

units.

Sandstone None 

documented

Units are

suitable for most 

recreation,

unless heavily 

weathered on 

slopes.

Units record 

deposition within a 

Triassic rift basin of 

varying depth.

TRIASSIC

Stockton 

Formation

(TRs);

Stockton 

conglomerate 

(TRsc)

TRs contains light-gray to buff, coarse 

arkosic sandstone, with some reddish-brown 

to purple sandstone, siltstone, and mudstone 

interbeds. TRsc contains quartz cobbles in a 

sandy, poorly sorted matrix with some 

conglomeratic sandstone present locally. 

Moderate

Heterogeneous 

nature of units 

may render 

them unstable 

on slopes.

Units form 

alternating ridges 

that may be prone to 

rockfall and 

landslides, especially 

where resistant 

sandstone layers are 

underlain by 

weathered red 

shales.

Variety of body and 

trace fossils

documented 

outside of park. 

Fossils not yet 

documented from 

TRs within the 

park.

None 

documented

None 

documented Sandstone None documented

Conglomeratic 

outcrops may 

attract climbers.

Units record 

deposition and 

reworking within a 

Triassic rift basin.

Upper unit of 

Cambrian-Triassic 

unconformity 

documented within 

park.

ORDOVICIAN

Martinsburg 

Formation

(Om);

Hamburg sequence 

rocks

(Oh);

Shale and 

graywacke of 

Hamburg sequence 

(Ohsg);

Limestone of 

Hamburg sequence 

(Ohl)

Om contains dark-gray to gray shale and 

slightly metamorphosed slate. Oh includes 

greenish-gray, purple, and maroon shale, 

siltstone, and graywacke with some flysch 

beds containing some of unit Om. Ohsg

consists of shale with conspicuous zones of 

graywacke. Ohl is thick-bedded limestone of 

the Hamburg sequence.

Moderate

Avoid dissolved 

units for 

development of 

wastewater 

treatment 

facilities;

weathered units 

may fail on 

slopes.

Units are prone to 

rockfall where 

weathered shale 

underlies resistant 

sandstone ledges or 

where limestone has 

dissolved beneath 

overlying rock layers.

May contain 

Ordovician-age 

fossils

Slate may have 

been locally 

quarried

historically.

Unit Ohl is 

susceptible 

to 

dissolution 

and may 

undermine 

stability of 

overlying 

units.

Slate Cave habitat

Dissolved caves 

and cavities may 

attract attention.

Units record deepwater marine 

depositional 

environment.

ORDOVICIAN

Cocalico 

Formation

(Oco);

Jacksonburg 

Formation

(Ojk);

Annville Formation 

(Oan)

Oco includes gray phyllitic shale, maroon 

shale, and siltstone with silty siliceous 

interbeds. Some argillaceous and quartzose

sandstones present locally. Ojk contains 

dark-gray shaly limestone with slaty cleavage. 

Some medium- to thick-bedded limestone is 

present in basal sections. Oan consists of 

light-gray, massive-bedded limestone. Unit is 

calcium-rich and mottled in the basal layers. 

Moderate

Avoid heavily 

dissolved units 

for 

development 

due to weakness 

of friable 

textures.

Limestone cements 

are prone to 

dissolution and may 

compromise 

outcrops of Ojk.

May contain 

Ordovician-age 

fossils

None 

documented

Limestone 

layers and 

units are 

susceptible 

to 

dissolution,

forming 

caves, 

sinkholes, 

and cavities.

Sandstone, 

slate

Oan

weathers to 

produce 

calcium-rich 

substrates.

Cave-bearing 

units may attract 

attention.

Oco has 

allochthonous and 

autochthonous 

elements.

ORDOVICIAN

Beekmantown 

Group

(Ob);

Ontelaunee 

Formation

(Oo);

Epler Formation 

(Oe);

Rickenbach 

Formation

(Ori);

Stonehenge 

Formation

(Os)

Ob includes Oo, Oe, Ori, and Os. Oo is lightto dark-gray dolomite with very fine to 

medium crystalline textures, interlayered 

light-gray limestone and interbedded 

nodular, dark-gray chert beds at the base. Oe

includes light-gray fine-grained limestone 

interlayered with gray dolomite. Some

coarsely crystalline lenses are present locally. 

Ori contains medium- to dark-gray coarsely 

crystalline dolomite in the basal beds topped 

by medium- to light-gray finely crystalline 

dolomite. Chert lenses, interbeds, and 

nodules are present locally. Os includes 

medium-light-gray to medium-gray, finely 

crystalline massive limestone with dark 

siliceous layers and conglomerate beds.

Moderate

Avoid heavily 

dissolved units 

for heavy 

development 

due to weakness 

of friable 

textures;

dissolved units 

are inherently 

porous and do 

not act as good 

filters for 

wastewater.

Resistant chert 

nodules and beds 

pose rockfall hazard 

when surrounding 

limestone and 

dolomite dissolve 

preferentially.

Os has fossil 

fragments in 

discrete lenses

Chert nodules 

may have 

provided tool 

material to 

American 

Indians.

Interlayered 

limestone 

and 

dolomite are 

susceptible 

to 

dissolution.

Limestone, 

dolomite

Units may 

dissolve to 

form cave 

habitats and 

produce 

calcium- and

magnesiumrich 

substrates.

Dissolved caves 

and cavities may 

attract attention.

Widespread, thick 

units that show

geographic facies 

changes and record 

shifting 

depositional 

environments.
Colored rows indicate geologic units mapped within Valley Forge National Historical Park

VAFO Geologic Resources Inventory Report 24

Age Map Unit

(Symbol) Features and Detailed Geologic Description Erosion Resistance

Suitability for 

Infrastructure Hazards PaleontologicalResources

Cultural 

Resources Karst Issues Mineral Resources Habitat Recreation GeologicSignificance

ORDOVICIAN AND CAMBRIAN

Conestoga 

Formation

(OCc)

OCc contains light-gray, thinly bedded 

contorted impure limestone with shaly 

partings. Conglomeratic layers present at base 

of unit. Locally metamorphosed to phyllite. 

Moderate

Unit weathers 

readily to sand,

which may 

render it too 

permeable for 

septic systems 

or too unstable 

for heavy 

development.

Unit is associated 

with rockfalls and 

slumps when 

exposed on slopes;

alternating resistant 

and weaker layers 

are prone to fail if 

undercut. Black shale 

units may contain 

arsenic that could be 

released to 

groundwater and 

soils.

May contain 

Cambrian- to 

Ordovician-age 

fossils

Scant chert 

nodules may 

have provided 

tool material to

American 

Indians.

Impure 

limestone 

layers may 

partially 

dissolve and 

compromise 

integrity of 

unit.

Pink marble 

locally

Unit 

underlies 

deep, sandy, 

well-drained 

soils.

Unit is friable 

and may form an

unstable base for 

trails on slopes.

Unit spans 

Cambrian to 

Ordovician 

boundary.

CAMBRIAN

Richland 

Formation

(Cr);

Allentown 

Formation

(Cal);

1

Cr contains gray dolomite with some oolitic 

beds and medium-gray limestone and darkgray oolitic chert present locally. Cal is 

medium- to medium-dark-gray, thick-bedded 

interlayered dolomite and impure limestone. 

Some chert stringers and nodules present 

locally, with oolitic and stromatolitic 

calcareous siltstones at the base that weather 

to orange-brown. Cm includes pink to white 

and gray limestone and finely laminated 

crystalline dolomite. Ce is microcrystalline 

limestone with dolomite and metamorphosed 

layers of phyllite and marble. 

Millbach 

Formation

(Cm);

1

Elbrook Formation 

(Ce)

Moderate to 

moderately 

high for 

metamorphic 

layers

Units are 

suitable for 

most 

development 

unless highly 

dissolved 

and/or 

fractured.

Units may be 

associated with karst 

hazards if dissolution 

is prevalent. When 

undercut, units pose 

rockfall hazards.

Stromatolites in Cal

and Cm. Rare 

trilobite fossils in 

Ce, not yet 

documented in the 

park or 

surrounding area.

Chert nodules 

may have 

provided tool 

material to 

American

Indians.

Interlayered 

limestone 

and 

dolomite are 

susceptible 

to 

dissolution.

Marble, 

limestone

None 

documented

Cave-bearing 

units may attract 

attention.

Units record 

marine depositional 

environments.

CAMBRIAN

Leithsville 

Formation

(Clv);

Buffalo Springs 

Formation

(Cbs);

Lower (Middle?) 

Cambrian rocks, 

undivided

(Cul);

Zooks Corner 

Formation

(Czc)

Clv contains medium- to dark-gray 

crystalline dolomite that weathers to lightyellowish-brown and gray. Unit is massive 

with some oolitic, pink to gray mottled chert, 

thin shale, and dolomitic shale interbeds with 

scattered sand grains. Cbs contains lightpinkish-gray limestone and dolomite with 

fine to coarsely crystalline textures. 

Numerous siliceous and clay-rich laminae are 

present. Cul includes tectonic slices of Czc, 

Cl, Ck, Cv, Ca, Cah, and Ch. Czc includes 

medium-gray finely crystalline dolomite with 

interlayered siliceous to argillaceous stingers. 

Moderate

Heterogeneous 

nature of units 

may render 

them unstable 

on slopes, but 

suitable for 

most light 

development.

Limestone units may 

be associated with 

karst dissolution and 

processes, creating 

voids that may cause 

failure.

Stromatolites in 

Cbs near the top of 

the unit

Chert nodules 

may have 

provided tool 

material to 

American 

Indians.

Interlayered 

limestone 

and 

dolomite are 

susceptible 

to 

dissolution,

forming 

caves, 

sinkholes,

and smaller 

cavities.

Dolomite, 

limestone

Units 

weather to 

form myriad 

substrates 

that support 

many forest

types.

Units are 

suitable for most 

recreation,

unless heavily 

dissolved or 

fractured; cave 

habitats for bats 

and other fauna.

Cul is arranged in 

tectonic slices, 

recording the 

depositional 

environments and 

tectonic history of 

the area.

CAMBRIAN

Ledger Formation 

(Cl);

Kinzers Formation 

(Ck);

Vintage Formation 

(Cv)

Cl contains massive pure dolomite with some 

siliceous grading near the bed centers. Unit 

appears light-gray and mottled in outcrop. Ck

consists of dark-brown shale, gray to white 

spotted limestone, and marble in the lower 

beds and sandy, friable limestone in the upper 

beds. Cv is dark-gray argillaceous dolomite 

with knotty textures and light-gray marble in 

the basal layers locally.

Moderate

Units are 

suitable for 

most 

development 

unless highly 

dissolved 

and/or 

fractured.

Units weather to 

form friable, porous 

rocks that are 

unstable and may fail 

easily.

Stromatolites are 

preserved within Cl

exposures in the 

park.

None 

documented

Limestone, 

and to a 

lesser extent, 

dolomite are 

susceptible 

to 

dissolution.

Cl hosts 

karst 

features 

within the 

park.

Marble

Units 

weather to 

well-drained 

sandy soils 

with 

abundant 

clay clasts.

Units are 

suitable for most 

recreation,

unless steep 

slopes are 

present; caves 

may attract 

attention

Units record 

oscillating 

Cambrian marine 

and nearshore 

depositional 

environments.

Bone Cave 

developed in Cl.

Cl is lower unit of 

Cambrian-Triassic 

unconformity 

documented within 

park.

 1 Unit is of correct stratigraphic age and may be correlative, but not relevant to the Valley Forge area (William Kochanov, Pennsylvania Bureau of Topographic and Geologic Survey, Senior Geologic Scientist, written communication 2010).
Colored rows indicate geologic units mapped within Valley Forge National Historical Park

VAFO Geologic Resources Inventory Report 25

Age Map Unit

(Symbol) Features and Detailed Geologic Description Erosion Resistance

Suitability for 

Infrastructure Hazards PaleontologicalResources

Cultural 

Resources Karst Issues Mineral Resources Habitat Recreation GeologicSignificance

CAMBRIAN

Antietam and 

Harpers 

formations, 

undivided

(Cah);

Antietam 

Formation

(Ca);

2

Cah includes Ca and Ch. Ca consists of gray 

quartzite that appears buff-colored in 

weathered outcrops. Cha includes light-gray, 

fine- to medium-grained quartzite and 

feldspathic sandstone in massive beds. Some 

quartz pebble conglomerate in basal beds. Ch

contains dark-greenish-gray phyllite and 

schist with thin quartzite interbeds. Cch

consists of light-gray, massive quartzite and 

quartz schist with some thin, interlayered 

slate in the upper beds and conglomeratic 

beds at base. 

Hardyston 

Formation

(Cha);

Harpers Formation 

(Ch);

Chickies Formation 

(Cch)

Moderate to 

very high for 

quartzite

Quartzite units 

cap ridges and 

are suitable for 

most 

development 

unless highly 

fractured 

and/or exposed 

on steep slopes.

Units are associated 

with steep slopes and 

rockfall.

Skolithos in Cha

and Cch. Skolithos

fossils within park 

exposures of Cch.

Quartzite 

historically 

provided 

building 

material 

regionally, 

including the 

restroom 

facilities near 

the visitor 

center and 

other 

structures 

within the 

park.

None 

documented Quartzite

Units 

underlie 

ridgetop and

well-drained 

habitat.

Resistant 

quartzite layers 

may attract rock 

climbers.

Erosion resistant 

units underlie 

topographic highs 

of Mount Joy and 

Mount Misery.

PROBABLY LOWER PALEOZOIC

Pegmatite

(Xpg);

Granitic gneiss and 

granite

(Xgr);

Mafic gneiss 

(Xmgh);

Ultramafic gneiss 

(Xu);

Octoraro 

Formation

(Xo);

“Glenarm 

Wissahickon” 

Formation

(Xgw);

Wissahickon 

Formation

(Xw)

Xpg contains coarse- to medium-grained

textures and granitic compositions. Xgr is 

metamorphosed granodiorite. Xmgh is darkcolored, medium-grained gneiss of 

metamorphosed sedimentary rocks. Xu

includes serpentinite, steatite, and other 

altered peridotites and pyroxenites. Xo

consists of albite-chlorite schist, phyllite, and 

hornblende gneiss. Granitized members are 

present locally. Xgw contains oligoclase-mica 

schist and lenticular amphibolite bodies of 

ocean floor basalt origin. Xw consists of 

oligoclase-mica schist, hornblende gneiss, 

augen gneiss, and some granitized quartz and 

feldspar- rich members locally. 

Moderately 

high to high

Avoid areas of 

intense 

preferential 

compositional 

weathering 

(along foliation 

and between 

heterogeneous 

lenses). Suitable 

for most 

development 

unless highly 

weathered 

and/or 

fractured.

Rockfall hazard 

when unit is exposed 

on slope, especially 

where slope and 

dominant foliation 

planes are parallel.

None

Xpg crystals 

may have 

provided 

prehistoric 

trade material.

None Pegmatite, 

augen gneiss

None 

documented

Units are 

suitable for most 

recreation,

unless highly 

altered, cleaved, 

and/or 

fractured.

Units record early 

metamorphic 

history and 

depositional 

environments.

PRECAMBRIAN

Metadiabase

(md);

Anorthosite

(a);

Graphitic felsic 

gneiss

(gqm);

Felsic and 

intermediate gneiss 

(ggd);

Banded mafic 

gneiss

(gga);

Graphitic felsic 

gneiss

(gg);

Franklin Marble 

(fm)

Unit md is dark-gray, fine-grained greenish 

altered diabase intrusions. Unit a contains 

medium- to coarse-grained, bluish-gray 

plagioclase-rich rock with alteration minerals 

present locally. Unit gqm is medium-grained, 

dark-gray, gneissic feldspar and quartz with 

some altered areas. Unit ggd contains 

medium-grained, pink to greenish-gray gneiss 

of quartz, feldspar, and mica. Unit gga

interfingers with ggd and contains dark, fineto medium-grained banded gneiss of 

sedimentary origin. Unit gg contains 

medium-grained gray gneiss and marble with 

quartz, feldspar, graphite, and metamorphic 

minerals. Unit fm is coarse-grained, white, 

crystalline marble with disseminated graphite 

flakes.

High

Units are

suitable for 

most 

development 

unless heavily 

altered and/or 

fractured.

Rockfall hazard 

when unit is exposed 

on slope, especially 

where slope and 

dominant cleavage 

direction are parallel.

None

Marble may 

have provided 

quarry material

historically.

Some 

dissolution 

of marble 

beds is 

possible.

Gneiss, 

marble, 

graphite

None 

documented

Units are 

suitable for most 

recreation,

unless highly 

altered, cleaved, 

and/or 

fractured.

Units record 

accretion of distinct 

crustal blocks onto 

the North 

American 

continent.

 2 Although the Antietam Formation is shown to occur on some geologic maps, field evidence suggests that it may be the Chickies and/or Harpers formations (William Kochanov, Pennsylvania Bureau of Topographic and Geologic Survey, Senior Geologic Scientist, written 

communication 2010). 
Colored rows indicate geologic units mapped within Valley Forge National Historical Park

VAFO Geologic Resources Inventory Report 26

Age Map Unit

(Symbol) Features and Detailed Geologic Description Erosion Resistance

Suitability for 

Infrastructure Hazards PaleontologicalResources

Cultural 

Resources Karst Issues Mineral Resources Habitat Recreation GeologicSignificance

PRECAMBRIAN

Felsic and 

intermediate gneiss 

(fgh);

Felsic gneiss

(fgp);

Hornblende gneiss 

(hg);

Mafic gneiss

(mgh);

Mafic gneiss

(mgp);

Felsic to mafic 

gneiss

(gn)

Unit fgh contains light-colored, mediumgrained gneiss of probable sedimentary 

origin. Unit fgp consists of light-colored, 

medium-grained silicic gneissic rocks. Unit 

hg is dark-colored, medium-grained gneiss of 

probable sedimentary origin. Units mgh and 

mgp are similar, consisting of dark, mediumgrained gneiss of probable sedimentary 

origin. Unit gn is quartz- and feldspar-bearing 

gneiss with light color and medium-grained 

texture of probable igneous origin.

High unless 

highly 

weathered

Avoid areas of 

intense 

preferential 

compositional 

weathering 

(along foliation 

and between 

heterogeneous 

lenses). Suitable 

for most 

development 

unless highly 

weathered 

and/or 

fractured.

Unit may pose 

rockfall hazard if 

undercut or exposed 

on a slope.

None None 

documented None Gneiss 

Mafic gneiss 

develops into 

iron-, 

magnesium-, 

and calciumrich

substrates 

that support 

myriad forest 

types. 

Units are 

suitable for most 

recreation,

unless highly 

altered, cleaved, 

and/or 

fractured.

Metamorphosed 

rocks retain 

features and 

compositions 

characteristic of 

sedimentary and/or 

igneous origin.
VAFO Geologic Resources Inventory Report 27

Geologic History

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

geologic map of Valley Forge National Historical Park, the environment in which those 

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

Valley Forge National Historical Park is near the eastern 

edge of the Piedmont physiographic province. The 

Atlantic Coastal Plain province lies to the east, underlain 

by unconsolidated sediments. The landscape of the 

Valley Forge area contains features that are intimately 

tied with the long geologic history of the Appalachian 

Mountains and the evolution of the North American 

continent’s eastern coast. A regional perspective is 

presented here to connect the landscape and geology of 

the park to its surroundings. 

Precambrian and Early Paleozoic Era: Ancient Mountain 

Building and Iapetus Ocean Formation

The recorded history (reflected in exposed rocks) of the 

Appalachian Mountains begins in the Proterozoic

(fig. 12). A supercontinent formed during the midProterozoic Grenville orogeny (mountain-building 

event) that consisted of most of the continental crust in 

existence at that time. This included the crust of North 

America and Africa. Sedimentation, deformation, 

plutonism (intrusion of igneous rocks), and volcanism 

associated with this event are apparent in the 

metamorphic granites and gneisses in the core of the 

modern Blue Ridge Mountains, west of Valley Forge 

(Harris et al. 1997). These rocks formed over a 100-

million–year period and are more than a billion years old, 

placing them among the oldest rocks known from this 

region. They were later uplifted and thereby exposed to 

erosion for hundreds of millions of years. Their leveled 

surface forms a basement upon which all other rocks of 

the Appalachian Mountains were deposited (Southworth 

et al. 2001). Rocks of this age in the vicinity of Valley 

Forge include mafic to felsic gneisses, marbles, and 

metadiabases (Berg et al. 1980). 

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. 13A). This

volcanism lasted tens of millions of years and alternated 

between flood basalt flows and ash falls. The volcanic 

rocks covered the Precambrian granitic/gneissic 

basement in southern Pennsylvania. Metamorphosed 

into greenstones, these rocks are exposed as the Catoctin 

Formation in central Maryland.

The tectonic tension caused the supercontinent to break 

up, and a sea basin formed that later became the Iapetus 

Ocean (approximately 600–400 million years ago). This 

basin subsided and collected many of the sediments that 

would eventually form the rock units of the Appalachian 

Mountains (fig. 13B). Some of these sediments were 

deposited as alluvial fans, large submarine landslides, and 

turbidity flows (Southworth et al. 2001). Subsequent

deposits of sands, silts, and muds covered the earlier 

sediments in nearshore, deltaic, barrier island, and tidal 

flat areas along the eastern margin of the continent 

(Schwab 1970; Kauffman and Frey 1979; Simpson 1991). 

Some of these sediments are present in the detrital sand, 

mud, and silt of the Cambrian-age Chickies and Harpers 

formations in the Valley Forge area (e.g., the western part 

of the park) (Berg et al. 1980; Wiswall 1993). 

Approximately 500 million years ago, the ocean basin 

deepened and the shoreline transgressed onto the 

continental margin, forming the depositional setting for 

chemical sediments (Bechtel et al. 2005).

A carbonate platform, thicker to the east, provided the 

depositional setting for huge masses of carbonate, 

sandstone, and shale rocks during the Cambrian and

Ordovician periods (545-480 million years ago) (Means 

1995). Carbonate units such as the Elbrook and Ledger 

formations (underlying the area east and south of Mount 

Joy) and the Vintage Formation formed in this 

depositional environment (Means 1995; Bechtel et al. 

2005). Cambrian to Ordovician units in the Valley Forge

area, such as the Conestoga Formation (underlying the 

valley south of the park), record the locally intermittent 

transition from the carbonate platform to more 

nearshore terrestrial deposition associated with nascent 

uplift during the Taconic orogeny. 

Paleozoic Era: Appalachian Mountain Building

Taconic Orogeny

After 50 million years spanning the Early Cambrian 

through Early Ordovician periods, orogenic activity 

along the eastern margin of the continent began again 

(Means 1995; Bechtel et al. 2005). The Taconic orogeny 

(approximately 440–420 million years ago in the central 

Appalachians) was a volcanic arc to continent 

convergence. Oceanic crust and the volcanic arc from 

the Iapetus Basin were thrust onto the eastern edge of the 

North American continent, resulting in the closing of the 

ocean, subduction of the oceanic crust, creation of 

volcanic arcs, and uplift of the continental crust (Means 

1995). Initial metamorphism of the deeply buried 

igneous and nearshore sediments into metabasalts, 

quartzites, and phyllites occurred during this orogenic 

event. The collision compressed originally horizontal 

layers of sedimentary rocks into folds and thrust them

inland along large faults (Wiswall 1993).

The crust bowed downward to the west of the rising 

mountains in response to the overriding plate thrusting 

westward onto the continental margin of North America.

This process created a deep foreland basin that filled 

with mud and sand eroded from the highlands to the east 
28 NPS Geologic Resources Division 

(fig. 13C) (Harris et al. 1997). This Appalachian Basin was 

centered on what is now West Virginia. 

The oceanic sediments of the shrinking Iapetus Ocean 

were thrust westward onto other deepwater sediments of 

the western Piedmont during the Late Ordovician. Sand, 

mud, silt, and carbonate sediment were then deposited in 

the shallow marine to deltaic environment of the 

Appalachian foreland basin. These rocks, now 

metamorphosed, currently underlie the Valley and Ridge 

physiographic province west of Valley Forge (Fisher 

1976).

Acadian Orogeny

The Acadian orogeny (approximately 360 million years 

ago) continued the mountain building of the Taconic 

orogeny as the African continent drifted toward North 

America (Harris et al. 1997). Like the preceding orogeny, 

the Acadian event involved the collision of land masses, 

mountain building, and regional metamorphism (Means 

1995). This event was focused north of eastern 

Pennsylvania.

Alleghanian Orogeny

During the Late Paleozoic and following the Acadian 

orogeny, the proto-Atlantic Iapetus Ocean closed as the 

North American and African continents collided. This 

event formed the Pangaea supercontinent and the 

Appalachian mountain belt we see today. This mountainbuilding episode is called the Alleghanian orogeny (325–

265 million years ago), and was the last major orogeny to 

affect the Appalachians (fig. 13D) (Means 1995). The 

rocks deformed during as many as seven phases of 

folding and faulting, producing the numerous folds of 

the Valley and Ridge province west of Valley Forge 

(Nickelsen 1983; Southworth et al. 2001). The strata of 

the Appalachian Basin (now the Appalachian Plateau 

province) remained relatively undeformed. 

During this orogeny, rocks of the Great Valley, Blue 

Ridge, and Piedmont provinces were thrust inland along 

large faults and transported westward onto younger 

rocks of the Valley and Ridge province (Wiswall 1993). 

An estimated 20–50% (125–350 km, 80–220 mi) of crustal 

shortening took place (Harris et al. 1997).

Paleoelevations of the Alleghanian Mountains are 

estimated to have been approximately 6,100 m (20,000 ft)

above sea level, analogous to the modern Himalaya 

Range in Asia. These mountains have been beveled by 

erosion to elevations less than 730 m (2,400 ft) above sea 

level west of Valley Forge (Means 1995). Erosion 

exposed the early Paleozoic formations, including the 

carbonates, following the Alleghanian orogeny (Bechtel 

et al. 2005). 

Mesozoic Era: Rifting, Atlantic Ocean Formation, and 

Mountain Erosion

A period of rifting began during the late Triassic (230–

200 million years ago), 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 rifting episode 

initiated the formation of the current Atlantic Ocean and

caused many block-fault basins with accompanying 

volcanism to develop (fig. 13E) (Harris et al. 1997; 

Southworth et al. 2001). At this time, the uplifted 

Paleozoic units in the Valley Forge area, including the 

carbonate rocks, were lowered in the GettysburgNewark Rift Basin and reburied (Bechtel et al. 2005). 

Valley Forge National Historical Park is located near the 

southern edge of the Newark Basin (Wiswall 1993). 

Extension and deposition were the dominant regional 

geologic processes during the Mesozoic Era. Thick 

deposits of unconsolidated gravel, sand, and silt were 

shed from the eroding Alleghanian Mountains. These 

sediments were deposited atop the much older Cambrian 

rocks at the base of the mountains as alluvial fans, in

thicknesses that occasionally exceeded 5 km (3 mi). 

Throughout the basin, Triassic sandstone and red shale 

beds rest unconformably on the Cambrian-age units 

(Bechtel et al. 2005). The Triassic-age Stockton 

Formation (underlying the northern and eastern parts of 

the park), Hammer Creek and Brunswick formations, 

and Jurassic sediments record this depositional 

environment in the Valley Forge area. The rapid 

lithologic changes within the Triassic rocks indicate 

shifting depositional regimes that included nearshore, 

fluvial, lacustrine, and alluvial fan depositional settings

(Sloto and McManus 1996). Deposition spread eastward 

to form part of the Atlantic Coastal Plain province. Thick 

layers of Cretaceous units such as the Patapsco 

Formation and Tertiary units such as the Pensauken, 

Bridgeton, and Bryn Mawr formations record this 

environment (fig. 13F) (Duffy and Whittecar 1991; 

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

rare, scattered remnants of these units are present in the 

Valley Forge area. Exposed metamorphic rocks in the 

Blue Ridge province suggest that an immense amount of 

material was deposited. Many of the rocks exposed at the 

surface must have been at least 20 km (10 mi) below the 

surface prior to regional uplift and erosion.

Cenozoic Era: Karst Development and Ice Ages

The North American plate has continued to move

westward 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 Era 

(Harris et al. 1997). Over these millions of years, the stillburied carbonate rocks in the Valley Forge area 

underwent chemical weathering or dissolution along 

joints, cracks, fractures, or bedding planes. In some 

areas, these processes produced a subsurface network of 

interconnecting conduits and voids within the carbonate 

bedrock (Bechtel et al. 2005; William Kochanov, 

Pennsylvania Bureau of Topographic and Geologic 

Survey, Senior Geologic Scientist, written 

communication, February16, 2010). The network of 

solution cavities ranges in size from microscopic 

conduits to large, scenic caverns. Where dissolution 

approaches the surface, collapse of the overlying material 

may form sinkhole features such as Bone Cave (Port 

Kennedy Cave) (Bechtel et al. 2005).
VAFO Geologic Resources Inventory Report 29

Quaternary-period erosion continues to create the 

present landscape. The Schuylkill River and its 

tributaries, such as Valley Creek, erode sediments from 

the lowering hillslopes and deposit alluvial terraces along 

the rivers (fig. 13F).

The Pleistocene masses of continent-scale ice sheets 

never reached the Valley Forge area, but terminated at 

365 to 610 m (1,200–2,000 ft) above sea level in 

northwestern and northeastern Pennsylvania. Upland 

surfaces have been glaciated to rounded ridges and sandand gravel-filled valleys in northern Pennsylvania. 

However, the colder ice-age climates affected the 

formation of the landscape at Valley Forge National

Historical Park. The periglacial conditions that must 

have existed in close proximity to the ice sheets 

intensified weathering and other erosional processes 

(Harris et al. 1997). Landforms and deposits such as the 

Trenton Gravel are probably late Tertiary to Quaternary 

in age. These features were formed when a wetter 

climate, sparse vegetation, and frozen ground caused 

increased precipitation to run into the ancestral river 

channels. This water flow enhanced downcutting and 

erosion by waterways and caused subsequent increases 

in downstream deposition (Means 1995; Zen 1997a,

1997b). Pleistocene fossil deposits, such as that in Bone 

Cave, record the climatic conditions and the impacts on 

local flora and fauna around 750,000 years ago (Bechtel 

et al. 2005). 

Since the Pleistocene, the rivers and tributaries in the 

Valley Forge area continue to cut through thick layers of 

sedimentary deposits. Surface runoff erodes local slopes 

and is supplemented by mass wasting processes. Streams 

redistribute these sediments in riparian zones along the 

waterways and intermittently transport them toward the 

Atlantic Ocean. Natural erosion is augmented by human 

land disturbance activities such as agriculture, 

industrialization, drilling, quarrying, and widespread 

urban development throughout the area.
30 NPS Geologic Resources Division 

Figure 12. Geologic timescale. Included are major life history and tectonic events occurring on the North American continent. Red lines 

indicate major unconformities between eras. Isotopic ages shown are in millions of years (Ma). Compass directions in parentheses indicate 

the regional location of individual geologic events. Adapted from the U.S. Geological Survey, http://pubs.usgs.gov/fs/2007/3015/ with 

additional information from the International Commission on Stratigraphy. http://www.stratigraphy.org/view.php?id=25.
VAFO Geologic Resources Inventory Report 31

Figure 13. Evolution of the landscape in the area of the Valley Forge National Historical Park from the Precambrian through the present. Ma, 

millions of years (mega-annum). Drawings not to scale. Graphic adapted from Means (1995) by Trista Thornberry-Ehrlich (Colorado State 

University). 
32 NPS Geologic Resources Division 

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://geomaps.wr.usgs.gov/parks/misc/glossarya.html. Definitions are 

based on those in the American Geological Institute Glossary of Geology (fifth edition; 2005).

absolute age. The geologic age of a fossil, rock, feature, 

or event in years; commonly refers to radiometrically 

determined ages.

abyssal plain. A flat region of the deep ocean floor, 

usually at the base of the continental rise.

active margin. A tectonically active margin where 

lithospheric plates come together (convergent 

boundary), pull apart (divergent boundary) or slide 

past one another (transform boundary). Typically 

associated with earthquakes and, in the case of 

convergent and divergent boundaries, volcanism. 

Compare to “passive margin.”

alluvial fan. A fan-shaped deposit of sediment that 

accumulates where a hydraulically confined stream 

flows to a hydraulically unconfined area. Commonly 

out of a mountainous area into an area such as a valley 

or plain.

alluvium. Stream-deposited sediment.

angular unconformity. An unconformity where the rock

layers above and below are oriented differently. Also 

see “unconformity.”

anticline. A convex-upward (“A” shaped) fold. Older 

rocks are found in the center.

aquifer. A rock or sedimentary unit that is sufficiently 

porous that it has a capacity to hold water, sufficiently 

permeable to allow water to move through it, and 

currently saturated to some level.

ash (volcanic). Fine material ejected from a volcano (also 

see “tuff”).

asthenosphere. Earth’s relatively weak layer or shell 

below the rigid lithosphere.

axis (fold). A straight line approximation of the trend of a 

fold which divides the two limbs of the fold. “Hinge 

line” is a preferred term.

barrier island. A long, low, narrow island formed by a 

ridge of sand that parallels the coast.

base flow. Stream flow supported by groundwater; flow 

not attributed to direct runoff from precipitation or 

snow melt.

base level. The lowest level to which a stream can erode 

its channel. The ultimate base level for the land surface 

is sea level, but temporary base levels may exist locally. 

basement. The undifferentiated rocks, commonly 

igneous and metamorphic, that underlie rocks 

exposed at the surface.

basin (structural). A doubly plunging syncline in which 

rocks dip inward from all sides.

basin (sedimentary). Any depression, from continental to 

local scales, into which sediments are deposited.

basinite. A very fine-grained basalt.

batholith. A massive, discordant pluton, larger than 100 

km2 (40 mi2

), and often formed from multiple 

intrusions of magma.

beach. A gently sloping shoreline covered with sediment, 

commonly formed by the action of waves and tides.

bed. The smallest sedimentary strata unit, commonly 

ranging in thickness from one centimeter to a meter or 

two and distinguishable from beds above and below.

bedding. Depositional layering or stratification of 

sediments.

bedrock. A general term for the rock that underlies soil 

or other unconsolidated, surficial material.

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

completely or in part.

calcareous. Describes rock or sediment that contains the 

mineral calcium carbonate (CaCO3).

carbonaceous. Describes a rock or sediment with 

considerable carbon content, especially organics, 

hydrocarbons, or coal.

carbonate. A mineral that has CO3

-2 as its essential 

component (e.g., calcite and aragonite).

carbonate rock. A rock consisting chiefly of carbonate 

minerals (e.g., limestone, dolomite, or carbonatite).

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. Chemical breakdown of minerals 

at Earth’s surface via reaction with water, air, or 

dissolved substances; commonly results in a change in 

chemical composition more stable in the current 

environment.

clast. An individual grain or rock fragment in a 

sedimentary rock, produced by the physical 

disintegration of a larger rock mass.

clastic. Describes rock or sediment made of fragments of 

pre-existing rocks (clasts).

clay. Can be used to refer to clay minerals or as a 

sedimentary fragment size classification (less than 

1/256 mm [0.00015 in]).

concordant. Strata with contacts parallel to the 

orientation of adjacent strata.

concretion. A hard, compact aggregate of mineral matter, 

subspherical to irregular in shape; formed by 

precipitation from water solution around a nucleus 

such as shell or bone in a sedimentary or pyroclastic 

rock. Concretions are generally different in 

composition from the rocks in which they occur.

conglomerate. A coarse-grained, generally unsorted, 

sedimentary rock consisting of cemented, rounded 

clasts larger than 2 mm (0.08 in).

continental crust. Crustal rocks rich in silica and alumina 

that underlie the continents; ranging in thickness from

35 km (22 mi) to 60 km (37 mi) under mountain ranges.

continental rise. Gently sloping region from the foot of 

the continental slope to the deep ocean abyssal plain; it 
VAFO Geologic Resources Inventory Report 33

generally has smooth topography but may have 

submarine canyons.

continental shelf. The shallowly submerged part of a 

continental margin extending from the shoreline to the 

continental slope with water depths less than 200 m 

(660 ft).

continental shield. A continental block of Earth’s crust 

that has remained relatively stable over a long period 

of time and has undergone minor 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 colliding.

core. The central part of Earth, beginning at a depth of 

about 2,900 km (1,800 mi), probably consisting of ironnickel alloy.

country rock. The rock surrounding an igneous intrusion 

or pluton. Also, the rock enclosing or traversed by a 

mineral deposit.

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 flow conditions such as water flow direction 

and depth.

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 a vertically oriented 

plane.

crust. Earth’s outermost compositional shell, 10 to 40 km 

(6 to 25 mi) thick, consisting predominantly of 

relatively low-density silicate minerals (also see 

“oceanic crust” and “continental crust”).

crystalline. Describes a regular, orderly, repeating 

geometric structural arrangement of atoms.

crystal structure. The orderly and repeated arrangement 

of atoms in a crystal.

debris flow. A moving mass of rock fragments, soil, and 

mud, in which more than half the particles of which 

are larger than sand size. 

deformation. A general term for the processes of faulting, 

folding, and shearing of rocks as a result of various 

Earth forces such as compression (pushing together) 

and extension (pulling apart).

delta. A sediment wedge deposited where a stream flows 

into a lake or sea.

dike. A narrow igneous intrusion that cuts across 

bedding planes or other geologic structures.

dip. The angle between a bed or other geologic surface 

and horizontal.

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

relative movement is parallel to the dip of the fault.

disconformity. An unconformity where the bedding of 

the strata above and below are parallel.

discordant. Describes contacts between strata that cut 

across or are set at an angle to the orientation of 

adjacent rocks.

divergent boundary. An active boundary where tectonic 

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.

ductile. Describes a rock that is able to sustain 

deformation (folding, bending, or shearing) before 

fracturing.

eolian. Describes materials formed, eroded, or deposited 

by or related to the action of the wind. Also spelled 

“Aeolian.”

eustatic. Relates to simultaneous worldwide rise or fall of 

sea level.

evaporite. A sedimentary rock composed primarily of 

minerals produced from a saline solution as a result of 

extensive or total evaporation of the solvent (usually 

water).

exfoliation. The breakup, spalling, peeling, or flaking of 

layers or concentric sheets from an exposed rock mass 

caused by differential stresses due to thermal changes 

or a reduction in pressure when overlying rocks erode 

away.

extrusive. Describes molten (igneous) material that has 

erupted onto Earth’s surface.

facies (metamorphic). The pressure and temperature 

conditions that result in a particular, distinctive suite 

of metamorphic minerals.

facies (sedimentary). The depositional or environmental 

conditions reflected in the sedimentary structures, 

textures, mineralogy, fossils, etc. of a sedimentary 

rock.

fan delta. An alluvial fan that builds into a standing body 

of water. This landform differs from a delta in that a 

fan delta is next to a highland and typically forms at an 

active margin.

fault. A break in rock along which relative movement has 

occurred between the two sides.

felsic. Describes an igneous rock having abundant lightcolored minerals such as quartz, feldspars, or 

muscovite. Compare to “mafic.”

footwall. The mass of rock beneath a fault surface (also 

see “hanging wall”).

formation. Fundamental rock-stratigraphic unit that is 

mappable, 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, or fault).

frost wedging. The breakup of rock due to the 

expansion of water freezing in fractures.

graben. A down-dropped structural block bounded by 

steeply dipping, normal faults (also see “horst”).

hanging wall. The mass of rock above a fault surface 

(also see “footwall”).

horst. Areas of relative “up” between grabens, 

representing the geologic surface left behind as 

grabens drop. The best example is the Basin-andRange province of Nevada. The basins are grabens and 

the ranges are weathered horsts. Grabens become a 

locus for sedimentary deposition (also see “graben”).

igneous. Refers to a rock or mineral that originated from 

molten material; one of the three main classes of 

rocks—igneous, metamorphic, and sedimentary.
34 NPS Geologic Resources Division 

intrusion. A body of igneous rock that invades (pushes 

into) older rock. The invading rock may be a plastic 

solid or magma.

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

and parallel to a subduction zone.

isostacy. The condition of equilibrium, comparable to 

floating, of the units of the lithosphere above the 

asthenosphere. Crustal loading (commonly by large ice 

sheets) leads to isostatic depression or downwarping; 

removal of the load leads to isostatic uplift or 

upwarping.

joint. A 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. Very thin, parallel layers.

landslide. Any process or landform resulting from rapid, 

gravity-driven mass movement.

lava. Still-molten or solidified magma that has been 

extruded onto Earth’s surface though a volcano or 

fissure.

levee. Raised ridge lining the banks of a stream. May be 

natural or artificial.

lineament. Any relatively straight surface feature that can 

be identified via observation, mapping, or remote 

sensing, often reflects crustal structure.

lithification. The conversion of sediment into solid rock.

lithology. The physical description or classification of a 

rock or rock unit based on characters such as its color, 

mineral composition, and grain size.

lithosphere. The relatively rigid outmost shell of Earth’s 

structure, 50 to 100 km (31 to 62 miles) thick, that 

encompasses the crust and uppermost mantle.

loess. Windblown silt-sized sediment, generally of glacial 

origin.

mafic. Describes dark-colored rock, magma, or minerals 

rich in magnesium and iron. Compare to “felsic.”

magma. Molten rock beneath Earth’s surface capable of 

intrusion and extrusion. 

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

and core.

marine terrace. A narrow coastal strip of deposited 

material, sloping gently seaward.

mass wasting. A general term for the downslope 

movement of soil and rock material under the direct 

influence of gravity.

matrix. The fine grained material between coarse (larger) 

grains in igneous rocks or poorly sorted clastic 

sediments or rocks. Also refers to rock or sediment in 

which a fossil is embedded.

meander. Sinuous lateral curve or bend in a stream 

channel. An entrenched meander is incised, or carved 

downward into the surface of the valley in which a 

meander originally formed. The meander preserves its 

original pattern with little modification.

mechanical weathering. The physical breakup of rocks 

without change in composition. Synonymous with 

“physical weathering.”

member. A lithostratigraphic unit with definable 

contacts; a member subdivides a formation.

meta–. A prefix used with the name of a sedimentary or 

igneous rock, indicating that the rock has been 

metamorphosed.

metamorphic. Describes the process of metamorphism 

or its results. One of the three main classes of rocks—

igneous, metamorphic, and sedimentary.

metamorphism. Literally, a change in form.

Metamorphism occurs in rocks through mineral 

alteration, formation, and/or recrystallization from 

increased heat and/or pressure.

meteoric water. Pertaining to water of recent 

atmospheric origin.

mid-ocean ridge. The continuous, generally submarine 

and volcanically active mountain range that marks the 

divergent tectonic margin(s) in Earth’s oceans.

migmatite. Literally, “mixed rock” with both igneous 

and metamorphic characteristics due to partial melting 

during metamorphism.

mineral. A naturally occurring, inorganic crystalline solid 

with a definite chemical composition or compositional 

range.

monocline. A one-limbed fold in strata that is otherwise 

flat-lying.

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 to 7 

km (3 to 4 miles) thick and generally of basaltic 

composition.

orogeny. A mountain-building event.

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

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

outwash. Glacial sediment transported and deposited by 

meltwater streams.

overbank deposit. Alluvium deposited outside a stream 

channel during flooding.

overburden. Rock and sediment, not of economic value, 

and often unconsolidated, that overlies an ore, fuel, or 

sedimentary deposit.

paleontology. The study of the life and chronology of 

Earth’s geologic past based on the fossil record.

palustrine. Pertaining to material growing or deposited in 

marsh or marsh-like environment.

Pangaea. A theoretical, single supercontinent that 

existed during the Permian and Triassic periods.

parent material. Geologic material from which soils 

form.

parent rock. Rock from which soil, sediments, or other 

rocks are derived.

passive margin. A margin where no plate-scale tectonism 

is taking place; plates are not converging, diverging, or 

sliding past one another. An example is the east coast 

of North America (compare to “active margin”).

pediment. A gently sloping, erosional bedrock surface at 

the foot of mountains or plateau escarpments.

peneplain. A geomorphic term for a broad area of low 

topographic relief resulting from long-term, extensive 

erosion.
VAFO Geologic Resources Inventory Report 35

permeability. A measure of the relative ease with which 

fluids move through the pore spaces of rocks or 

sediments.

plastic. Capable of being deformed permanently without 

rupture.

plate tectonics. The concept that the lithosphere is 

broken up into a series of rigid plates that move over 

Earth’s surface above a more fluid asthenosphere.

pluton (plutonic). A body of intrusive igneous rock that 

crystallized at some depth beneath Earth’s surface.

point bar. A low ridge of sand and gravel deposited in a 

stream channel on the inside of a meander where flow 

velocity slows.

porosity. The proportion of void space (e.g., pores or 

voids) in a volume of rock or sediment deposit.

prodelta. The part of a delta below the level of wave 

erosion.

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.

pyroclastic. Describes clastic rock material formed by 

volcanic explosion or aerial expulsion from a volcanic 

vent; also pertaining to rock texture of explosive 

origin. In the plural, the term is used as a noun.

radioactivity. The spontaneous decay or breakdown of 

unstable atomic nuclei.

radiometric age. An age expressed in years and 

calculated from the quantitative determination of 

radioactive elements and their decay products. 

recharge. Infiltration processes that replenish 

groundwater.

red beds. Sedimentary strata composed largely of 

sandstone, siltstone, and shale that are predominantly 

red due to the presence of ferric iron 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, or fossils with respect to 

the geologic time scale and without reference to their 

numerical age.

reverse fault. A contractional high-angle (greater than 

45°

) dip-slip fault in which the hanging wall moves up 

relative to the footwall (also see “thrust fault”).

rift. A region of crust where extension results in 

formation of many related normal faults, often 

associated with volcanic activity.

rift valley. A depression formed by grabens along the 

crest of an oceanic spreading ridge or in a continental 

rift zone.

sandstone. Clastic sedimentary rock of predominantly 

sand-sized grains.

sapping. The undercutting of a cliff by erosion of softer 

underlying rock layers.

scarp. A steep cliff or topographic step resulting from 

displacement on a fault, or by mass movement, or 

erosion. Also called an “escarpment.”

seafloor spreading. The process by which tectonic plates 

pull apart and new lithosphere is created at oceanic 

ridges.

sediment. An eroded and deposited, unconsolidated 

accumulation of rock and mineral fragments.

sedimentary rock. A consolidated and lithified rock 

consisting of clastic and/or chemical sediment(s). One 

of the three main classes of rocks—igneous, 

metamorphic, and sedimentary.

sequence. A major informal rock-stratigraphic unit that 

is traceable over large areas and defined by a 

sediments associated with a major sea level 

transgression-regression.

shale. A clastic sedimentary rock made of clay-sized 

particles that exhibit parallel splitting properties.

sill. An igneous intrusion that is of the same orientation 

as the surrounding rock.

silt. Clastic sedimentary material intermediate in size 

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

1/16 mm [0.00015 to 0.002 in]).

siltstone. A variably lithified sedimentary rock composed 

of silt-sized grains.

sinkhole. A circular depression in a karst area with 

subterranean drainage and is commonly funnelshaped.

slickenside. A smoothly polished and commonly striated 

surface representing a fault plane.

slope. The inclined surface of any geomorphic feature or 

measurement thereof. Synonymous with “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 

often overlying the parent material from which it 

formed.

spring. A site where water issues from the surface due to 

the intersection of the water table with the ground 

surface.

strata. Tabular or sheet-like masses or distinct layers of 

rock.

stratigraphy. The geologic study of the origin, 

occurrence, distribution, classification, correlation, 

and age of rock layers, especially sedimentary rocks.

stream. Any body of water moving under gravity flow in 

a clearly confined channel.

stream channel. A long, narrow depression shaped by the 

concentrated flow of a stream and covered 

continuously or periodically by water.

stream terrace. 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).

strike. The compass direction of the line of intersection 

of an inclined surface with a horizontal plane.

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

relative movement is parallel to the strike of the fault. 

Said to be “sinistral” (left-lateral) if relative motion of 

the block opposite the observer appears to be to the 

left. “Dextral” (right-lateral) describes relative motion 

to the right.

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.
36 NPS Geologic Resources Division 

suture. The linear zone where two continental 

landmasses become joined via obduction.

syncline. A downward curving (concave up) fold with 

layers that dip inward; the core of the syncline 

contains the stratigraphically-younger rocks.

system (stratigraphy). The group of rocks formed during 

a period of geologic time.

tectonic. Relating to large-scale movement and 

deformation of Earth’s crust.

terrane. A large region or group of rocks with similar 

geology, age, or structural style.

terrestrial. Relating to land, Earth, or its inhabitants.

thrust fault. A contractional dip-slip fault with a 

shallowly dipping fault surface (less than 45°

) where 

the hanging wall moves up and over relative to the 

footwall.

topography. The general morphology of Earth’s surface, 

including relief and locations of natural and 

anthropogenic features.

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

Earth’s surface.

trace fossil. Tracks, trails, burrows, coprolites (dung), 

etc., that preserve evidence of organisms’ life activities, 

rather than the organisms themselves.

transgression. Landward migration of the sea as a result 

of a relative rise in sea level.

transmissivity. A measure of the rate at which water 

travels through an aquifer.

trend. The direction or azimuth of elongation of a linear 

geologic feature.

tuff. Generally fine-grained, igneous rock formed of 

consolidated volcanic ash.

type locality. The geographic location where a 

stratigraphic unit (or fossil) is well displayed, formally 

defined, and derives its name. The place of original 

description.

unconformity. An erosional or non-depositional surface 

bounded on one or both sides by sedimentary strata.

An unconformity marks a period of missing time.

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

movement that raises the rocks.

volcanic. Describes anything related to volcanoes. Can 

refer to igneous rock crystallized at or near Earth’s 

surface (e.g., lava).

volcanic arc. A commonly curved, linear, zone of 

volcanoes above a subduction zone. 

water table. The upper surface of the saturated zone; the 

zone of rock in an aquifer saturated with water.

weathering. The physical, chemical, and biological 

processes by which rock is broken down.
VAFO Geologic Resources Inventory Report 37

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foundation in difficult ground through detailed site 

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Rader, H. L. 1951. Geology of Valley Forge Park and 

vicinity, Valley Forge, Pennsylvania. Master’s thesis, 

Lehigh University, Bethlehem, Pennsylvania, USA. 

Reif, A. G., and R. A. Sloto. 1997. Metals, pesticides, and 

semivolatile organic compounds in sediment in Valley 

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Santucci, V. L., J. Kenworthy, and R. Kerbo. 2001. An 

inventory of paleontological resources associated with 

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Santucci, V. L., J. P. Kenworthy, and A. L. Mims. 2009. 

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

Chesapeake and Ohio Canal National Historical Park 

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Maryland, West Virginia, and Virginia. A, Geologic 

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

figures (disc 2). Open-File Report: OF 01-0188. U. S. 

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February 2009.

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Vitale. 1995. Modeling VOC migration and vapor 
40 NPS Geologic Resources Division 

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Thornberry-Ehrlich, T. 2010. Hopewell Furnace 

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and associated landscapes. Pages 27–46 in R. Young 

and L. Norby, editors. Geological Monitoring. 

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water sample analysis. Pages 501–539 in D. M. Nielson, 

editor. Practical handbook of ground-water 

monitoring. Lewis Publishing, Chelsea, Michigan, 

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Vitale, R. J., O. C. Braids, and R. Schuller. 2006. Groundwater sample analysis. Pages 1113–1134 in D. M. 

Nielson, editor. Practical handbook of environmental 

site characterization and ground-water monitoring. 

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and stratigraphy of late Cenozoic alluvial fans, Augusta 

County, Virginia, U.S.A. Pages 259–279 in G. M. Clark, 

H. H. Mills, and J. S. Kite, editors. Regolith in the 

Central and Southern Appalachians. Southeastern 

Geology 39(3-4).

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slope movements. Pages 245–272 in R. Young and L. 

Norby, editors. Geological Monitoring. Geological 

Society of America, Boulder, Colorado, USA.

Wiswall, C. G. 1993. Valley Forge National Historical 

Park, Montgomery and Chester Counties–The 

geologic history. Park Guide 8, 4th

series. Bureau of 

Topographic and Geologic Survey, Middletown, 

Pennsylvania, USA. (www.dcnr.state.pa.us/

topogeo/parkguides/pg08.pdf). Accessed 28 February 

2010.

Zen, E-an. 1997a. The seven-storey river:

Geomorphology of the Potomac River channel 

between Blockhouse Point, Maryland, and 

Georgetown, District of Columbia, with emphasis on 

The Gorge complex below Great Falls. Open-File 

Report OF 97-60. U.S. Geological Survey, Reston, 

Virginia, USA. (http://onlinepubs.er.usgs.gov/djvu/

OFR/1997/ofr_97_60.djvu). Accessed February 2009.

Zen, E-an. 1997b. Channel geometry and strath levels of 

the Potomac River between Great Falls, Maryland, and 

Hampshire, West Virginia. Open-File Report OF 97-

480. U.S. Geological Survey, Reston, Virginia, USA.

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480.djvu). Accessed February 2009.
VAFO Geologic Resources Inventory Report 41

Additional References

This section lists additional references, resources, and web sites that may be of use to 

resource managers. Web addresses are current as of May 2010

Pennsylvania Karst

Berg, T. M., and C. M. Dodge, compilers. 1981. Atlas of 

preliminary geologic quadrangle maps of 

Pennsylvania. Map 61, 4th Series. Bureau of 

Topographic and Geologic Survey, Middletown, 

Pennsylvania, USA. 

(http://www.dcnr.state.pa.us/topogeo/map61/61intro.

aspx).

Bosbyshell, H. 2006. Bedrock geologic map of the 

Chester Valley and Piedmont portion of the 

Germantown, Malvern, Norristown, and Valley Forge 

quadrangles, Chester, Delaware, Montgomery, and 

Philadelphia counties, Pennsylvania. Open-File Report 

OFBM 06-04.0, 4th Series. Bureau of Topographic and 

Geologic Survey, Middletown, Pennsylvania, USA. 

(http://www.dcnr.state.pa.us/topogeo/openfile/

chester.aspx).

Ford, D., and Williams, P. 1992. Karst geomorphology 

and hydrology. Chapman & Hall, London, England, 

UK.

MacLachlan, D. B. 1994. Some aspects of the Lower 

Paleozoic Laurentian margin and slope in southeastern 

Pennsylvania. Pages 3-23 in R. T. Faill and W. D. 

Sevon, editors. Various aspects of Piedmont geology in 

Lancaster and Chester Counties, Pennsylvania. 

Guidebook 59. Annual Field Conference of 

Pennsylvania Geologists, Lancaster, Pennsylvania, 

USA.

White, W. B. 1988. Geomorphology and hydrology of 

karst terrains. Oxford University Press, New York, 

New York USA.

Geology of National Park Service Areas

National Park Service Geologic Resources Division 

(Lakewood, Colorado). http://nature.nps.gov/geology/

NPS Geologic Resources Inventory. 

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

blications.cfm

U.S. Geological Survey Geology of National Parks 

(includes 3D photographs). 

http://3dparks.wr.usgs.gov/

Harris, A. G., E. Tuttle, and S. D. Tuttle. 2003. Geology of 

National Parks. Sixth edition. Kendall/Hunt 

Publishing Co., Dubuque, Iowa, USA.

Kiver, E. P., and D. V. Harris. 1999. Geology of U.S. 

parklands. John Wiley and Sons, Inc., New York, New 

York, USA.

Lillie, R. J. 2005. Parks and plates: The geology of our 

national parks, monuments, and seashores. W.W. 

Norton and Co., New York, New York, USA.

[Geared for interpreters].

NPS Geoscientist-in-the-parks (GIP) internship and 

guest scientist program. 

http://www.nature.nps.gov/geology/gip/index.cfm

Resource Management/Legislation Documents

NPS 2006 Management Policies (Chapter 4; Natural 

Resource Management): 

http://www.nps.gov/policy/mp/policies.html#_Toc157

232681

NPS-75: Natural Resource Inventory and Monitoring 

Guideline: 

http://www.nature.nps.gov/nps75/nps75.pdf.

NPS Natural Resource Management Reference Manual 

#77: http://www.nature.nps.gov/Rm77/

Geologic Monitoring Manual

R. Young and L. Norby, editors. 2009. Geological 

Monitoring. Geological Society of America, Boulder, 

Colorado, USA.

[Website under development]. Contact the Geologic 

Resources Division to obtain a copy.

NPS Technical Information Center (Denver, repository 

for technical (TIC) documents): http://etic.nps.gov/

Geological Survey Websites

Pennsylvania Geological Survey: 

http://www.dcnr.state.pa.us/topogeo/

U.S. Geological Survey: http://www.usgs.gov/

Geological Society of America: 

http://www.geosociety.org/

American Geological Institute: http://www.agiweb.org/

Association of American State Geologists: 

http://www.stategeologists.org/

Other Geology/Resource Management Tools

Bates, R. L., and J. A. Jackson, editors. 1984. American 

Geological Institute dictionary of geological terms. 3rd 

edition. Bantam Doubleday Dell Publishing Group, 

New York, New York, USA.
42 NPS Geologic Resources Division 

U.S. Geological Survey National Geologic Map Database 

(NGMDB): http://ngmdb.usgs.gov/

U.S. Geological Survey Geologic Names Lexicon 

(GEOLEX; geologic unit nomenclature and summary): 

http://ngmdb.usgs.gov/Geolex/geolex_home.html

U.S. Geological Survey Geographic Names Information 

System (GNIS; search for place names and geographic 

features, and plot them on topographic maps or aerial 

photos): http://gnis.usgs.gov/

U.S. Geological Survey GeoPDFs (download searchable 

PDFs of any topographic map in the United States): 

http://store.usgs.gov (click on “Map Locator”).

U.S. Geological Survey Publications Warehouse (many 

USGS publications are available online): 

http://pubs.usgs.gov

U.S. Geological Survey, description of physiographic 

provinces: http://tapestry.usgs.gov/Default.html
VAFO Geologic Resources Inventory Report 43

Appendix A: Overview of Digital Geologic Data

The following page is an overview of the digital geologic data for Valley Forge National 

Historical Park. For a poster-size PDF of this overview and complete digital data, please 

see the included CD or visit the Geologic Resources Inventory publications web site:

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

SAGU Geologic Resources Inventory Report 45

Appendix B: Scoping Meeting Participants

The following is a list of participants from the GRI scoping session for Valley Forge 

National Historical Park, held on June 22–24, 2004. The contact information and email 

addresses in this appendix may be outdated; please contact the Geologic Resources 

Division for current information. The scoping meeting was the foundation for this GRI 

report.

Name Affiliation Position Phone E-Mail

Steve Ambrose NPS Hopewell Furnace NHS Chief Ranger 610-582-8773 steven_ambrose@nps.gov

Meghan Carfioli NPS Valley Forge NHP Resource Manager 610-783-1041 margaret_carfioli@nps.gov

Tim Connors NPS Geologic Resources 

Division Geologist 303-969-2093 tim_connors@nps.gov

Bruce Heise NPS Geologic Resources 

Division Geologist 303-969-2017 bruce_heise@nps.gov

John Inners Pennsylvania Geological 

Survey Geologist 717-702-2034 jinners@state.pa.us

Bill Kochanov Pennsylvania Geological 

Survey Geologist 717-702-2033 wkochanov@state.pa.us

Liza Rupp NPS Valley Forge NHP GIS 610-783-1089 liza_rupp@nps.gov

Vincent Santucci NPS Geologic Resources 

Division/Fossil Butte NM Paleontologist 307-877-4455 vincent_santucci@nps.gov

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Communities. 

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