Geology of the Chesapeake and Ohio Canal 

National Historical Park and 

Potomac River Corridor, District of Columbia, 

Maryland, West Virginia, and Virginia

U.S. Department of the Interior

U.S. Geological Survey

Professional Paper 1691
On the cover: In the background, 1:62,500-scale topographic map of the Seneca quadrangle, Md., 

(surveyed in 1907 by the U.S. Geological Survey and produced in cooperation with the State of Maryland). In the foreground, clockwise from top left, photograph of canal boat along the towpath (courtesy of National Park Service); view of the north portal of the Paw Paw Tunnel, looking south (around 

1900) (photograph courtesy of National Park Service); photograph of Devil’s Eyebrow, an anticline of 

shale, sandstone, and limestone of the Silurian Bloomsburg Formation at the site of the Round Top 

Cement Mill, west of Hancock, Md. (photograph taken in 1897 by C.D. Walcott, Director of the U.S. 

Geological Survey); and photograph (around 1876) of a barge at Lock 33, across the Potomac River 

at Harpers Ferry, W. Va. (courtesy of National Park Service).
 

 

 

Geology of the Chesapeake and Ohio Canal 

National Historical Park and Potomac River 

Corridor, District of Columbia, Maryland, 

West Virginia, and Virginia 

By Scott Southworth, David K. Brezinski, Randall C. Orndorff, John E. Repetski, 

and Danielle M. Denenny 

Professional Paper 1691 

U.S. Department of the Interior 

U.S. Geological Survey 
U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia: 2008 

For product and ordering information:
 

World Wide Web: http://www.usgs.gov/pubprod
 

Telephone: 1–888–ASK–USGS
 

For more information on the USGS—The Federal source for science about the Earth, its natural and living resources, 


natural hazards, and the environment:
 

World Wide Web: http://www.usgs.gov
 

Telephone: 1–888–ASK–USGS
 

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 


U.S. Government. 

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report. 

Suggested citation:
 

Southworth, Scott, Brezinski, D.K., Orndorff, R.C., Repetski, J.E., Denenny, D.M., 2008, Geology of the Chesapeake 


and Ohio Canal National Historical Park and Potomac River Corridor, District of Columbia, Maryland, West Virginia, 


and Virginia: U.S. Geological Survey Professional Paper 1691, 144 p., 1 pl.
 

ISBN 978–1–4113–2010–9 
iii 

Contents 

Abstract...........................................................................................................................................................1
 

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

Historical Background ..................................................................................................................................1
 

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

Physiography.........................................................................................................................................2
 

Bedrock Geology...................................................................................................................................4
 

Cenozoic Landscape and Surficial Deposits....................................................................................7
 

Introduction...................................................................................................................................7
 

River Morphology.........................................................................................................................9
 

Alluvial Terraces ........................................................................................................................10
 

Abandoned Entrenched Meanders ........................................................................................10
 

Recent Flood-Plain Alluvium....................................................................................................11
 

Colluvium.....................................................................................................................................14
 

Karst Landforms.........................................................................................................................14
 

Fossils Along the C&O Canal ............................................................................................................14
 

Introduction.................................................................................................................................14
 

Piedmont Province ....................................................................................................................15
 

Blue Ridge Province..................................................................................................................16
 

Valley and Ridge Province........................................................................................................17
 

Cambrian Fossils...............................................................................................................17
 

Ordovician Fossils.............................................................................................................17
 

Silurian Fossils...................................................................................................................22
 

Devonian and Mississippian Fossils..............................................................................22
 

Engineering Geology ..........................................................................................................................22
 

Introduction.................................................................................................................................22
 

Canal Excavation and Towpath ...............................................................................................23
 

Locks and Lock Stone ...............................................................................................................23
 

Cement.........................................................................................................................................25
 

Aqueducts, Culverts, and Waste Weirs .................................................................................30
 

Dams ............................................................................................................................................31
 

Control of Geology on Engineering ..................................................................................................31
 

Geologic Guide.............................................................................................................................................32
 

Piedmont Province—Potomac Terrane..........................................................................................34
 

Georgetown Section of Washington, D.C., to Seneca, Md. (MM 0 to MM 23)................34
 

Access Points....................................................................................................................34
 

Introduction .......................................................................................................................34
 

Georgetown to Glen Echo Park Area (MM 0 to MM 7) ..............................................36
 

Glen Echo Park Area to Swains Lock (MM 7 to MM 16.6).........................................36
 

Swains Lock to Seneca (MM 16.6 to MM 22.8) ...........................................................40
 
iv 

Piedmont Province—Culpeper Basin, Westminster Terrane, and Frederick Valley ...............45
 

Seneca, Md., to Point of Rocks, Md. (MM 22.8 to MM 48.2) ..............................................45
 

Access Points....................................................................................................................45
 

Introduction .......................................................................................................................45
 

Seneca to Edwards Ferry (MM 22.8 to MM 30.8) ........................................................47
 

Edwards Ferry to Monocacy Aqueduct (MM 30.8 to MM 42.2) ................................48
 

Monocacy Aqueduct to Point of Rocks (MM 42.2 to MM 48.2) ................................48
 

Blue Ridge Province...........................................................................................................................53
 

Point of Rocks, Md., to Fort Duncan (MM 48.2 to MM 63) ..................................................53
 

Access Points....................................................................................................................53
 

Introduction .......................................................................................................................53
 

Point of Rocks to Brunswick (MM 48.2 to MM 55)......................................................55
 

Brunswick to Fort Duncan (MM 55 to MM 62.5)..........................................................55
 

Valley and Ridge Province—Great Valley Section........................................................................65
 

Fort Duncan to McCoys Ferry (MM 62.5 to MM 110)...........................................................65
 

Access Points....................................................................................................................65
 

Introduction .......................................................................................................................65
 

Fort Duncan to Antietam Creek (MM 62.5 to MM 69) .................................................65
 

Antietam Creek to Big Slackwater (MM 69 to MM 85.5)............................................68
 

Big Slackwater to Williamsport (MM 85.5 to MM 99.5)..............................................70
 

Williamsport to McCoys Ferry (MM 99.5 to MM 110) .................................................72
 

Valley and Ridge Province—West of North Mountain Thrust Fault ..........................................76
 

McCoys Ferry to Cumberland, Md. (MM 110 to MM 184.5) ................................................76
 

Access Points....................................................................................................................76
 

Introduction .......................................................................................................................76
 

McCoys Ferry to Licking Creek Aqueduct (MM 110 to MM 116) ..............................78
 

Licking Creek Aqueduct to Round Top Cement Mill (MM 116 to MM 127.5)...........78
 

Round Top Cement Mill to Sideling Hill Aqueduct (MM 127.5 to about MM 137)...80
 

Sideling Hill Aqueduct to Paw Paw Tunnel (MM 137 to MM 156) ............................88
 

Paw Paw Tunnel to Oldtown (MM 156 to MM 167)....................................................90
 

Oldtown to Spring Gap (MM 167 to MM 173.5)............................................................91
 

Spring Gap to Cumberland (MM 173.5 to MM 184.5) ..................................................91
 

Acknowledgments.......................................................................................................................................93
 

References Cited..........................................................................................................................................93
 

Appendix 1— Sources of Data—Geologic Maps of 7.5-Minute Quadrangles .................................99
 

Plates 

1.	 Geology of the Chesapeake and Ohio Canal National Historical Park and 

Potomac River Corridor, District of Columbia, Maryland, West Virginia, and 

Virginia...........................................................................................................................[in pocket] 
2–20.	 Geologic maps of the Chesapeake and Ohio Canal National Historical Park and 


Potomac River Corridor from:
 

 2.	 Georgetown (MM 0) to near Glen Echo Park (about MM 7)....................................107
 

 3.	 Near Glen Echo Park (about MM 7) to Swains Lock (MM 17) ................................109
 

 4.	 Swains Lock (MM 17) to Seneca (MM 22.8) .............................................................111
 

 5.	 Seneca (MM 22.8) to Edwards Ferry (MM 30) ...........................................................113
 

 6.	 Edwards Ferry (MM 30) to Monocacy River Aqueduct (MM 42.2).........................115
 

 7.	 Monocacy River Aqueduct (MM 42.2) to Point of Rocks (MM 48.2)......................117
 

 8.	 Point of Rocks (MM 48.2) to Brunswick (MM 55)......................................................119
 

 9.	 Brunswick (MM 55) to Fort Duncan (MM 62.5)..........................................................121
 

 10.	 Fort Duncan (MM 62.5) to Antietam Creek (MM 69) .................................................123
 

 11.	 Antietam Creek (MM 69) to Big Slackwater (MM 85.5)............................................125
 

 12.	 Big Slackwater (MM 85.5) to Williamsport (MM 99.5)..............................................127
 

 13.	 Williamsport (MM 99.5) to McCoys Ferry (MM 110) .................................................129
 

 14.	 McCoys Ferry (MM 110) to Licking Creek Aqueduct (MM 116) ..............................131
 

 15.	 Licking Creek Aqueduct (MM 116) to Round Top Cement Company mill 


(MM 127.5) ......................................................................................................................133
 

 16.	 Round Top Cement Company mill (MM 127.5) to Little Orleans (MM 141) ............135
 

 17.	 Little Orleans (MM 141) to Paw Paw Tunnel (MM 156) ............................................137
 

 18.	 Paw Paw Tunnel (MM 156) to Oldtown (MM 167).....................................................139
 

 19.	 Oldtown (MM 167) to Spring Gap (MM 173.5)............................................................141
 

 20.	 Spring Gap (MM 173.5) to Cumberland (MM 184.5) ..................................................143
 

Figures 

1.	 Sketch map showing the geologic provinces in the mid-Atlantic region and the 


locations of the Chesapeake and Ohio (C&O) Canal, the Potomac River, and the 


Potomac River basin ....................................................................................................................2
 

2.	 Maps showing the earliest beginnings of canal transport....................................................3
 

3.	 Tectonic map of the study area..................................................................................................5
 

4.	 Diagrammatic sketches showing the geologic history of the Potomac River valley 


over the past 1 billion years ........................................................................................................6
 

5.	 Maps of alluvium and alluvial terraces of the Potomac River ..............................................8
 

6.	 Schematic cross sections of the Potomac River valley at Cumberland, Md. (A-A’), 


and the Georgetown section of Washington, D.C. (B-B’) ......................................................9
 

7.	 Sketch map showing large water-filled potholes and channels on the eastern 


half of Bear Island near MM 13 and Widewater...................................................................10
 

8.	 Potholes in migmatite of the Neoproterozoic and (or) Lower Cambrian Mather 


Gorge Formation along the bluff of Mather Gorge in Virginia.............................................11
 

9.	 Aerial photograph showing fish weirs constructed by Native Americans.......................11
 

10.	 Remnants of Quaternary alluvial terraces..............................................................................12
 

11.	 Sketch maps illustrating the evolution of the abandoned incised meanders of the 


Potomac and Little Cacapon Rivers near Paw Paw, W. Va. ................................................12
 

12.	 Schematic maps of the Great Falls of the Potomac River showing the evolution 


of the abandoned channel of Widewater and of Glade Hill ................................................13
 

v 
13.	 Rounded boulder of diabase (5 ft in diameter) deposited by the Potomac River 


on terrace at south end of Glade Hill.......................................................................................13
 

14.	 Recent alluvium along the Monocacy River ..........................................................................13
 

15.	 Schematic map showing the distribution of carbonate rocks and noncarbonate 


rocks along the C&O Canal and Potomac River corridor.....................................................14
 

16.	 The cave at Dam 4 near MM 83.5 ............................................................................................15
 

17.	 Fossils from rocks of the Frederick Valley section, western Piedmont province............16
 

18.	 Dinosaur tracks in the Upper Triassic Balls Bluff Siltstone exposed in the 


Culpeper Stone Co. quarry near Culpeper, Va. ......................................................................17
 

19.	 Brachiopods in Silurian and Devonian rocks.........................................................................17
 

20.	 Key index fossils present in the rocks found along the C&O Canal in the Valley 


and Ridge province.....................................................................................................................18
 

21.	 Key guide conodonts to the strata of the Valley and Ridge province................................20
 

22.	 Map showing locks, aqueducts, dams, and known quarries near the canal...................24
 

23.	 Stone berms constructed to prevent erosion between the towpath and the 


Potomac River .............................................................................................................................25
 

24.	 An example of modern erosion in the canal near Lock 34 (west of MM 62) ....................25
 

25.	 Evidence of excavated bedrock...............................................................................................26
 

26.	 Historical photographs of Paw Paw Tunnel, near MM 155 .................................................26
 

27.	 Examples of Lower Cretaceous Potomac Formation ...........................................................27
 

28.	 Upper Triassic Poolesville Member of the Manassas Sandstone .....................................27
 

29.	 East end of Lock 24 (Riley’s Lock at MM 23) ..........................................................................28
 

30.	 Nonfoliated Ordovician Ellicott City Granodiorite .................................................................28
 

31.	 Quartzite of the Lower Cambrian Weverton Formation........................................................28
 

32.	 Limestone of the Lower Cambrian Bolivar Heights Member of the Tomstown 


Formation .....................................................................................................................................29
 

33.	 Lock 60 at MM 149.5...................................................................................................................29
 

34.	 Round Top Cement Company, west of Hancock, Md............................................................29
 

35.	 Seneca Creek Aqueduct, just east of MM 23 ........................................................................30
 

36.	 The Monocacy River Aqueduct at MM 42..............................................................................30
 

37.	 Historical photograph (1936) of Dam 3....................................................................................31
 

38.	 Dam 6, which was located north of Great Cacapon, W. Va., near MM 134......................32
 

39.	 Historical photograph (1940) showing reconstruction of the towpath and canal 


at Widewater...............................................................................................................................33
 

40.	 Excavation of the canal through bedrock...............................................................................33
 

41.	 Generalized structure map and cross section of a portion of the C&O Canal 


National Historical Park and Potomac River corridor as it crosses the Piedmont 


province and easternmost part of the Blue Ridge province................................................35
 

42.	 Part of a National Historical Marker where the C&O Canal enters Rock Creek .............36
 

43.	 Igneous rocks of the Early to Middle Ordovician Georgetown Intrusive Suite 


near Key Bridge in Washington, D.C. ......................................................................................37
 

44.	 Sandstone cobbles along canal...............................................................................................38
 

45.	 Undeformed sedimentary mélange of the Lower Cambrian Sykesville Formation 


at MM 9.8 .....................................................................................................................................38
 

46.	 Sheared mélange of the Lower Cambrian Sykesville Formation near MM 11.5..............38
 

47.	 The Great Falls of the Potomac River......................................................................................39
 

vi 
48.	 Phyllonite of the Neoproterozoic and (or) Lower Cambrian Mather Gorge 


Formation near MM 12.5............................................................................................................39
 

49.	 Migmatite of the Neoproterozoic and (or) Lower Cambrian Mather Gorge 


Formation .....................................................................................................................................40
 

50.	 Migmatite of the Neoproterozoic and (or) Lower Cambrian Mather Gorge 


Formation .....................................................................................................................................41
 

51.	 Dark amphibolite within the Neoproterozoic and (or) Lower Cambrian Mather 


Gorge Formation intruded by white pegmatite of the Ordovician(?) Bear Island 


Granodiorite.................................................................................................................................42
 

52.	 Milky white, fractured vein quartz intruding metagraywacke of the Neoproterozoic 


and Lower Cambrian Mather Gorge Formation near Widewater near MM 13.5.............42
 

53.	 Underground gold mine north of Great Falls Tavern.............................................................43
 

54.	 Late Devonian lamprophyre dikes intruding metagraywacke of the Neoproterozoic 


and (or) Lower Cambrian Mather Gorge Formation..............................................................44
 

55.	 Folding in metagraywacke and schist of the Neoproterozoic and (or) Lower 


Cambrian Mather Gorge Formation.........................................................................................44
 

56.	 Folded metagraywacke of the Neoproterozoic and (or) Lower Cambrian Mather 


Gorge Formation .........................................................................................................................45
 

57.	 Sheared phyllite and folded white vein quartz of the Neoproterozoic(?) and Lower 


Cambrian(?) Ijamsville Phyllite that lies above the Martic thrust fault along the 


Monocacy River..........................................................................................................................47
 

58.	 Tightly folded, thin-bedded limestone within the Ijamsville Phyllite along the 


Monocacy River..........................................................................................................................47
 

59.	 Metasiltstone of the Lower and Middle Cambrian Araby Formation showing 


foliation with near-vertical cleavage along the CSXT railroad north of MM 43 ..............48
 

60.	 Examples of the varieties of limestone in the Upper Cambrian Frederick 


Formation .....................................................................................................................................49
 

61.	 Conical hill north of Nolands Ferry and Maryland Route 28 representing 


topographic inversion ................................................................................................................49
 

62.	 Stereoscopic pair of aerial photographs showing the change in the morphology 


of the Potomac River valley in the Piedmont province across the boundary 


between the Culpeper basin and Potomac terrane..............................................................50
 

63.	 Conglomerate of the Upper Triassic Reston Member of the Manassas Sandstone 


that forms the base of the Culpeper basin .............................................................................50
 

64.	 Limestone conglomerate interbedded with red siltstone of the Upper Triassic 


Leesburg Member of the Balls Bluff Siltstone.......................................................................51
 

65.	 Limestone conglomerate of the Upper Triassic Leesburg Member of the Bull Run 


Formation .....................................................................................................................................51
 

66.	 Lower Cambrian Tomstown Formation ...................................................................................52
 

67.	 Lower Cambrian rocks of the Blue Ridge province ..............................................................52
 

68.	 Generalized structure map and cross section of a portion of the C&O Canal 


National Historical Park and Potomac River corridor as it crosses the 


westernmost part of the Piedmont province, the Blue Ridge province, and the 


easternmost part of the Great Valley Section........................................................................54
 

69.	 Neoproterozoic Catoctin Formation ........................................................................................55
 

70.	 Stereoscopic pair of aerial photographs and sketch centered on Point of Rocks, 


Md. ................................................................................................................................................56
 

71.	 Pods of marble in the Neoproterozoic Swift Run Formation...............................................56
 

vii 
72.	 Basement rocks of the Blue Ridge province..........................................................................56
 

73.	 Weverton Cliffs at the southern end of South Mountain in Maryland...............................57
 

74.	 Polymictic conglomerate of the Lower Cambrian Loudoun Formation north of the 


Potomac River on the east flank of South Mountain ............................................................58
 

75.	 Mesoproterozoic rocks between Weverton, Md., and Harpers Ferry, W. Va. .................58
 

76.	 Stereoscopic pair of aerial photographs and sketch centered on Harpers Ferry, 

W. Va., showing the Potomac River gorge and water gap ..................................................59
 

77.	 Sketch of the bluffs of Elk Ridge north of the C&O Canal ....................................................60
 

78.	 View of eastern end of Elk Ridge .............................................................................................60
 

79.	 Historical photograph of the westernmost overturned anticline of quartzite in 


the Lower Cambrian Buzzard Knob Member of the Weverton Formation ........................61
 

81.	 View of the westernmost overturned anticline shown in figure 80....................................61
 

80.	 Historical photograph showing the easternmost overturned anticline in quartzite 


of the Lower Cambrian Buzzard Knob Member of the Weverton Formation....................61
 

82.	 Crossbedding in quartzite of the Lower Cambrian Weverton Formation at 


Maryland Heights .......................................................................................................................62
 

83.	 Geologic sketch of the bluffs from about MM 61 to the Great Valley west of 


Lock 36 (MM 62.5).......................................................................................................................62
 

84.	 Folded and cleaved metasiltstone of the Lower Cambrian Harpers Formation...............63
 

85.	 Cleavage in metasiltstone of the Lower Cambrian Harpers Formation folded as 


the result of motion along a thrust fault..................................................................................64
 

86.	 Outcrops of Lower Cambrian Harpers and Antietam Formations ......................................64
 

87.	 Generalized structure map and cross section of a portion of the C&O Canal 


National Historical Park and Potomac River corridor as it crosses the western 


part of the Blue Ridge province and the Great Valley section of the Valley and 


Ridge province ............................................................................................................................66
 

88.	 Geologic sketch from about MM 65.2 westward ..................................................................67
 

89.	 Limestone of the Lower Cambrian Bolivar Heights Member of the Tomstown 


Formation near MM 68...............................................................................................................67
 

90.	 Examples of karst features in the Fort Duncan Member of the Tomstown 


Formation near MM 68...............................................................................................................68
 

91.	 Outcrops of Lower and Middle Cambrian Waynesboro Formation in the vicinity 


of the C&O Canal.........................................................................................................................69
 

92.	 Historical photograph (around 1895) showing Botelers and Reynolds Cement 


Mill’s quarries in near-vertical limestone (in West Virginia opposite MM 71.6) 


and Botelers and Reynolds Dam for mill race for the cement mill.....................................69
 

93.	 Historical sketch (around 1867) of soldiers and citizens in and around Killiansburg 


cave near MM 75.7.....................................................................................................................70
 

94.	 Historical photograph (date unknown) showing location north of Dam 4 ........................70
 

95.	 Geologic sketch from McMahons Mill near MM 88 westward to Lock 41 depicting 


Ordovician rocks of the older Stoufferstown Member of the Stonehenge Limestone 


thrust above limestone of the younger Rockdale Run Formation.......................................71
 

96.	 Mylonitic limestone within the Lower Ordovician Stoufferstown Member of the 


Stonehenge Limestone ..............................................................................................................71
 

97.	 Depositional features in the Lower and Middle Ordovician Rockdale Run 


Formation .....................................................................................................................................72
 

98.	 Historical photograph (around 1920) of Cushwa basin at Williamsport (MM 100)..........73
 

viii 
99.	 Upper part of Middle Ordovician Martinsburg Formation along the railroad north 


of MM 101 ....................................................................................................................................73
 

100.	 Geologic sketches showing structures in Cambrian and Ordovician rocks of the 


Great Valley section of the Valley and Ridge province ........................................................74
 

101.	 Mudcracks on Upper Cambrian and Lower Ordovician Conococheague 


Limestone at MM 107.8..............................................................................................................75
 

102.	 Stromatolites in the Middle and Upper Cambrian Elbrook Limestone east of 


McCoys Ferry ..............................................................................................................................75
 

103.	 Upper Ordovician and Lower Silurian Tuscarora Quartzite.................................................76
 

104.	 Side-looking airborne radar (SLAR) image, generalized structure map, and cross 


section of a portion of the C&O Canal National Historical Park and Potomac River 


corridor as it crosses the Valley and Ridge province, including the westernmost 


part ................................................................................................................................................77
 

105.	 Fort Frederick and Big Pool at MM 112...................................................................................79
 

106.	 Historical photograph of workers and mules scraping the floor of Big Pool after 


it was drained to remove silt.....................................................................................................79
 

107.	 Upper Silurian and Lower Devonian Helderberg Limestone and Lower Devonian 


Shriver Chert................................................................................................................................80
 

108.	 Thin-bedded sandstone and shale of the Upper Devonian Brallier Shale .......................80
 

109.	 Upper Devonian Foreknobs Formation....................................................................................81
 

110.	 Middle Devonian Marcellus Shale southeast of MM 125 in West Virginia ......................82
 

111.	 Historical photograph of the Devil’s Eyebrow........................................................................82
 

112.	 Sketch of the Devil’s Eyebrow and ruins of Round Top Cement Company mill west 


of Hancock, Md...........................................................................................................................83
 

113.	 Folds in Upper Silurian Bloomsburg and Wills Creek Formations......................................83
 

114.	 Historical photograph (1897) looking down the axis of an anticline ..................................83
 

115.	 Shale of the Middle Silurian Rose Hill Formation..................................................................84
 

116.	 Historical photograph from the overlook on Cacapon Mountain in West Virginia..........84
 

117.	 Geologic sketch of the bedrock along the section of the C&O Canal from east of 


MM 133 west to Dam 6 and Tonoloway Ridge.......................................................................85
 

118.	 Middle Silurian Keefer Sandstone...........................................................................................85
 

119.	 Geologic sketch of folded and faulted rocks of the Upper Silurian Wills Creek 


Formation east of MM 134.........................................................................................................86
 

120.	 Folded and faulted strata at Fluted Rocks..............................................................................86
 

121.	 Upper Silurian and Lower Devonian Helderberg Limestone...............................................87
 

122.	 Brachiopods in Lower Devonian Oriskany Sandstone underlying Tonoloway Ridge 


at Dam 6........................................................................................................................................87
 

123.	 Outcrop of Lower and Middle Devonian Needmore Shale .................................................87
 

124.	 Historical photograph (undated) showing folded and faulted shale and sandstone 


of the Upper Devonian Brallier Shale......................................................................................87
 

125.	 Mississippian rocks of the Sideling Hill syncline..................................................................88
 

126.	 Sandstone in the Upper Devonian and Lower Mississippian Rockwell Formation 


exposed along the Sideling Hill Creek Aqueduct at MM 136.5 ...........................................88
 

127.	 Structures in the Upper Devonian Brallier Shale..................................................................89
 

128.	 Upper Devonian Brallier Shale near MM 155........................................................................90
 

129.	 Reckley Flat, north of MM 160..................................................................................................91
 

ix 
130.	 Devonian Hampshire Formation, exposed east of Town Creek on the limb of the 

Town Creek syncline ..................................................................................................................91 

131.	 Lower Devonian Oriskany Sandstone on the west limb of the Broadtop anticline.........91
 

132.	 Upper Devonian Brallier Shale near MM 172........................................................................92
 

133.	 Historical photograph showing the C&O Canal and wharf area at Cumberland.............92
 

Table 

1.	 Dimension stone used for the C&O Canal locks and aqueducts and the source 

geologic unit ................................................................................................................................25 

x 

Conversion Factors 

Multiply By To obtain 

Length 

inch (in.) 2.54 centimeter (cm) 

foot (ft) 0.3048 meter (m) 

mile (mi) 1.609 kilometer (km) 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Geology of the Chesapeake and Ohio Canal National 

Historical Park and Potomac River Corridor, District of 

Columbia, Maryland, West Virginia, and Virginia 

By Scott Southworth,1 David K. Brezinski,2 Randall C. Orndorff,1 John E. Repetski,1 and Danielle M. Denenny1 

Abstract 

The Chesapeake and Ohio Canal National Historical Park 

is 184.5 mi long and extends from Washington, D.C., to Cumberland, Md. The canal passes through three physiographic provinces including the Piedmont, Valley and Ridge, and the Blue 

Ridge; the map area also includes rocks of the Coastal Plain and 

Appalachian Plateaus provinces. Each province contains unique 

packages of rocks that influenced the character of the canal and 

towpath. The ages of the bedrock encountered along the length 

of the park range from Mesoproterozoic to Jurassic and represent a variety of tectonic and depositional environments. The 

different rock types and surficial deposits dictated the various construction methods for the canal, which was excavated 

in Quaternary flood-plain deposits as well as through bedrock. 

The ancient course of the Potomac River and the deposits it left 

behind also influenced the location of the canal and towpath. The 

engineers made good use of the many rock types to construct the 

locks, dams, aqueducts, and culverts that guided water from the 

Potomac River into the canal and maintained the water level as 

canal boats traveled between higher elevations in western Maryland to sea level in Washington, D.C. The canal and towpath provide a unique transect across the central Appalachian region for 

examining the rich geologic diversity and history. 

Introduction 

The Chesapeake and Ohio Canal National Historical Park 

(herein referred to as the C&O Canal) is unique in that it is the 

only land within the National Park Service (NPS) system that 

crosses 5 physiographic provinces along a major river. From the 

Georgetown section of Washington, D.C., to Cumberland, Md., 

the C&O Canal provides an opportunity to examine the geologic 

history of the central Appalachian region and how the canal contributed to the development of this area. This report and map 

1

U.S. Geological Survey, Reston, VA 20192.
 

2

Maryland Geological Survey, Baltimore, MD 21218.
 

cover the entire park within an area that is 184.5 mi long and 2 mi 

wide (see plate 1). The geologic guide is presented as if traveling 

the canal from east to west, from Georgetown to Cumberland, by 

provinces and sections, such as the Piedmont (fig. 1). Geologic 

features are keyed to the NPS mile markers (MM) that are found 

along the left-hand side of the towpath as one travels westward 

from Georgetown (MM 0) to Cumberland (MM 184.5). Distances 

shown are approximate and taken from Clague (1977). Included 

in the guide are references to detailed geologic information. Additional historical information is found in other guidebooks of the 

C&O Canal (Boy Scouts of America, 1983; National Park Service, 1991; Hahn, 1995; High, 1997; Davies, 1999). 

Historical Background 

The C&O Canal is located alongside the Potomac River, 

one of many tributary rivers that empty into the Chesapeake 

Bay. During colonial times, large ships could travel far up these 

rivers until they encountered the Fall Line (or Fall Zone), which 

is the boundary between the sandy, gravelly Coastal Plain and 

the rocky Piedmont province. It was at this boundary that ships 

encountered rapids, waterfalls, or shallow and narrow channels 

and could go no further upstream, and so it was there that settlements and centers of commerce were established. Cities such 

as Baltimore, Georgetown (later to become part of Washington, 

D.C.) in Maryland, and Fredericksburg and Richmond in Virginia are typical examples of communities that developed along 

the Fall Line. As the population in the Tidewater region grew 

and expanded westward, the Potomac River became one of the 

most viable means to cross the Appalachian Mountains to the 

fertile Ohio River valley and beyond. In 1785, the Patowmack 

Company, under the leadership of George Washington, began 

a series of “skirting” canals and riverbed improvements from 

Georgetown to Harpers Ferry (Brown, 1963; Garrett and Garrett, 1987). These skirting canals and sluices connected iron-ore 

prospects, furnaces, and foundaries with the armory at Harpers 

Ferry (fig. 2). The Patowmack Company’s failure to construct a 

passage around the Great Falls of the Potomac in Virginia, and 

the success of the Erie Canal in New York (built between 1817 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

79° 78° 77° 

40° 

39° 

38° 

Pennsylvania 

Maryland 

West 

Virginia 

Virginia 

Great Valley section 

Blue Ridge 

Culpeper

basin section 

Potomac River 

and C&O Canal 

Piedmont

Coastal Plainprovince 

Valley and Ridge province 

Gettysburg

basin section 

Plateaus 

province 

province

province 

Appalachian 

Cumberland 

Limit of Potomac River 

drainage basin 

Washington, D.C. 

0 50 MILES 

Figure 1. Sketch map showing the geologic provinces in 

the mid-Atlantic region and 

the locations of the Chesapeake and Ohio (C&O) Canal, 

the Potomac River, and the 

Potomac River basin. 

and 1825) provided momentum for the “Great National Project,” which was to build a canal along the Potomac River that 

would eventually reach the Ohio River at Pittsburgh, Pa. 

Construction of the canal, towpath, and associated structures began on July 4, 1828. The canal was open to Seneca, 

Md., by 1831; to Harpers Ferry, W. Va., by 1834; to near Woodmont, Md., by 1839; and was completed only to Cumberland, 

Md., in 1850. Although work on both the C&O Canal and the 

Baltimore and Ohio (B&O) Railroad began on the same day, the 

B&O Railroad had been operating for eight years along essentially the same route when the canal finally reached Cumberland, Md. The C&O Canal was used mainly to transport coal 

from the Appalachian Plateaus province, where it was mined, 

to eastern parts of Maryland, Virginia, and Washington, D.C., 

where it was used to heat homes and buildings. 

A typical canal trip took 4.5 days one way or 9 days round 

trip. Mules towed a 92-ft-long boat that, when fully loaded, 

weighed more than 120 tons. At peak activity, the canal saw 540 

boat trips per year. Beginning with the Johnstown, Pa., flood in 

1889, a series of devastating floods ruined the canal and there 

was insufficient money to rebuild it. The B&O Railroad purchased, rebuilt, and operated the canal until 1924 when it was 

again flooded and then drained. The B&O Railroad gave the 

canal property to the U.S. Government in 1938 in lieu of a $2 

million debt. Justice William Douglas successfully campaigned 

in the 1950s to prevent filling in the canal for the construction 

of a scenic highway. The C&O Canal was designated a National 

Monument by President Dwight D. Eisenhower in 1961, and a 

National Historical Park by President Richard M. Nixon in 1971. 

George Washington’s vision of an industrial corridor along the 

Potomac River did not happen. Manufacturing plants powered 

by the Potomac River were obsolete when electricity was developed, and local iron production was replaced by steel mills near 

Pittsburgh, Pa. The result is a river valley nearly restored to its 

natural state in the backyard of the Nation’s capital. 

Geologic Setting 

Physiography 

The C&O Canal extends from Rock Creek in the Georgetown section of Washington, D.C. (MM 0), to the confluence of 

the North Fork of the Potomac River with Wills Creek at Cumberland, Md. (MM 184.5). It is located along the northern bank 

of the Potomac River in Maryland. The easternmost 5 mi of 

the canal lies within the city limits of Washington, D.C. The 

Potomac River drainage basin (fig. 1) encompasses 14,670 mi2 of 

Virginia, Maryland, West Virginia, Pennsylvania, and the District 

of Columbia. The river valley transects five major physiographic 

provinces and at least five subprovinces, called “sections.” From 

east to west, they are as follows: (1) the Coastal Plain province, 

(2) the Piedmont province, which includes the Potomac terrane 

(eastern Piedmont), the Westminster terrane (central Piedmont), 

Culpeper basin, and Frederick Valley (western Piedmont); (3) the 

Blue Ridge province; (4) the Valley and Ridge province, which 

includes the Great Valley section; and (5) the Appalachian Plateaus province (fig. 1). The provinces and sections are unique 

because of the underlying bedrock, surficial deposits, and resultant landscape. Because the Appalachian Plateaus begin just west 

of Cumberland, Md., and the Coastal Plain begins east of Theodore Roosevelt Memorial Bridge in Washington, D.C., the C&O 

Canal traverses only three of the five provinces. 
MM 75 

West Virginia 

Maryland 

Virginia 

Limestone quarries and cement mill 

Antietam Iron Furnace 

Skirting canal 

opposite 

Weverton, Md. 

Potomac 

Iron Furnace 

Shenandoah Canal 

Harpers Ferry Armory 

Elk Run-Keep Tryst 

Iron Furnace 

Shepherdstown 

Goose Creek 

Canal 

Seneca Falls 

skirting canal 

Patowmack Canal, five locks and 

iron works at Great Falls, Va. 

Three locks at Little Falls 

Foxall Foundry, 

Duvall Foundry, and 

Godey Lime Kiln 

Lime kiln and manganese prospect 

Shenandoah Falls skirting canal 

N 

0 1 0 M IL E S 

R iver 

Potom ac 

A 

C&O Canal 

B 

A t l a n t i c

O c e a n 

C 

Alexandria 

Aqueduct 

Virginia 

Washington, D.C. 

Potomac 

River 

Rock C reek 

C&O 

Canal 

fill 

Canal 

Canal 

MM 0 

Geologic Setting 3 

Figure 2. Sketch maps showing the earliest beginnings of canal transport. A, The historic Patowmack Company’s skirting canals and 

locks, iron and manganese prospects and furnaces, limestone quarries, mills, kilns, foundries, and armory associated with early commerce along the Potomac River. B, U.S. canal system in 1850 (National Park Service, 1991). C, Canals and Potomac River near Washington, D.C., in 1771 as shown on a map by Pierre L’Enfant (Hall, 1991). The C&O Canal in the Georgetown section of Washington, D.C. 

(where MM 0 is the mile marker for the beginning of the canal), the Alexandria Aqueduct, and the Alexandria Canal were built by 1843. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

4 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Coastal Plain deposits are scattered in this region due to 

erosion. The Fall Line (or Fall Zone) is the boundary between 

the Piedmont and Coastal Plain. The boundary is not sharp, 

but is more of a broad area characterized by waterfalls in the 

Potomac River from Washington, D.C. (Little Falls) west to 

Seneca, Md. (Seneca Falls), which is a distance of about 17 mi. 

The Piedmont province is the relatively low-relief area east of 

the Blue Ridge that extends from Washington, D.C., to Point of 

Rocks, Md. In general, the Culpeper basin and Frederick valley 

sections form a lowland and the Westminster and Potomac terranes form dissected uplands. The boundary between the Piedmont and Blue Ridge provinces is at the foot of Catoctin and 

Furnace Mountains. The Blue Ridge province extends from 

Catoctin and Furnace Mountains west to Blue Ridge and Elk 

Ridge. The Valley and Ridge province extends from Blue Ridge 

and Elk Ridge to Cumberland, Md.; however, the Great Valley 

section is contained between Blue Ridge and Elk Ridge on the 

east and North Mountain on the west. The Great Valley section 

contains the Shenandoah Valley in Virginia and the Hagerstown 

Valley in Maryland. The Appalachian Plateaus extend westward 

from Cumberland into western Maryland. 

The Potomac River eroded the rocks of these distinct 

provinces and sections and therefore had a direct bearing on 

the landscape evolution of the river valley, and thus also upon 

the engineering of the C&O Canal. The majority of the canal 

was excavated in Quaternary alluvium adjacent to the Potomac 

River, so good exposures of bedrock are not always evident. 

Elsewhere, alluvium was thin or absent, and manual drilling 

and blasting using black powder was necessary to excavate the 

canal and its towpath through bluffs of bedrock. 

Bedrock Geology 

For over 100 years, the bedrock exposures along the 

Potomac River valley have been studied in order to help 

unravel the geological history of the central Appalachian 

region. There are more than 100 bedrock formations identified along the Potomac River. Of these, there are 27 type localities of rock formations along the river, 21 type localities near 

the river, and 24 type localities elsewhere within the drainage basin. The type localities are areas where the rocks were 

named and described because of good exposure. These rocks 

record a complex developmental history of ocean-basin creation and destruction in response to plate-tectonic processes. 

The geology of the C&O Canal originally was mapped at 

1:24,000 scale by many workers (see Appendix 1). The geologic units, faults, folds, and structural point data for this report 

were assembled from those larger scale maps in order to prepare a continuous geologic map for this report. On the detailed 

maps in this report (plates 2 through 20), fold axes and structural 

symbols for strike and dip of bedding, schistosity, and cleavage are not shown in order to clearly portray the geology; however, in order to understand the regional structure of the various provinces through which the canal passes, a summary map 

that shows just faults and folds is provided (fig. 3). Note that the 

density of fault and fold symbols varies from province to province and is based only on what has been mapped at the surface. 

Larger scale representations of structures are shown in some of 

the illustrations that accompany the Geologic Guide section of 

the report. Tectonic events of this part of the central Appalachian 

region are illustrated in figure 4 and are described from oldest to 

youngest in the following paragraphs. 

The Blue Ridge province exposes some of the oldest rocks 

known from this region. These granitic gneisses were formed 

more than 1 billion years ago during the Mesoproterozoic Grenville orogeny (fig. 4A). These plutonic rocks were intruded in 

several stages over a period of 100 million years (m.y.) (Aleinikoff and others, 2000) to form a basement upon which all other 

rocks of the Appalachian region were deposited. Metadiabase 

and metarhyolite dikes that intrude these basement rocks and the 

extrusive flows that overlie them are the result of Neoproterozoic (700-545 Ma) continental rifting that produced the Iapetus 

Ocean. These volcanic rocks were intruded through cracks in 

the granitic gneisses and extruded onto the land surface during 

the breakup of the continental land mass (fig. 4B). 

Fluvial and shallow-marine sediments were deposited on 

the newly formed margin of the continent (fig. 4C). Today, these 

sedimentary rocks are exposed on (from east to west) Catoctin Mountain, Short Hill, South Mountain, Blue Ridge, and Elk 

Ridge. They also occur in the western Piedmont (in the Sugarloaf Mountain anticlinorium and Frederick Valley synclinorium) and Great Valley section of the Valley and Ridge province. The Cambrian and Ordovician (545-480 Ma) carbonate 

rocks that make up much of the Great Valley section represent 

a grand platform in a shallow sea that deepened to the east (fig. 

4D). These shelly carbonate rocks are overlain by Ordovician 

(450 Ma) shale. The shale was deposited by the erosion of a rising highland to the east and marks the beginning of the Taconian orogeny (fig. 4E). This highland became one boundary 

of the Appalachian basin, which was centered in what is now 

West Virginia. During the Late Ordovician, oceanic sedimentary 

rocks of the Iapetus Ocean (found in the Potomac terrane of the 

eastern Piedmont) were thrust westward onto other deepwater 

sedimentary rocks of the western Piedmont (found in the Westminster terrane) along the Pleasant Grove thrust fault. Moreover, 

rocks of the Westminster terrane were thrust concurrently onto 

the continental-margin rocks of the Sugarloaf Mountain anticlinorium and Frederick Valley synclinorium along the Martic 

thrust fault. Sandstone, shale, siltstone, quartzite, and limestone 

were then deposited in the shallow-marine to deltaic environment of the Appalachian basin. These rocks currently underlie 

the Valley and Ridge province. Such shallow-marine to fluvial 

sedimentation continued for a period of about 200 m.y. during 

the Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian Periods (fig. 4F). Many of these rocks consist 

of sediments shed from highlands that were rising to the east as 

the result of tectonic events in the Ordovician (Taconian orogeny) and Devonian (Acadian orogeny). 

The Iapetus Ocean narrowed and closed up during the late 

Paleozoic mountain-building event known as the Alleghanian 

orogeny, during which the North American continental plate 
VALLEY AND RIDGE PROVINCE APPALACHIAN Ordovician to Mississippian GREAT VALLEY PLATEAUS clastic marine and fluvial SECTION PROVINCE deposits of the

BLUE RIDGE PROVINCE

Mesoproterozoic granitic gneiss

overlain by Neoproterozoic and

Early Cambrian sedimentary and

volcanic rock

Triassic and Jurassic

clastic sedimentary rocks

with volcanic and intrusive

diabase rocks of rift basin 

Physiographic boundaryThrust faultOverturned thrust fault—Sawteeth show dipNormal fault—Sawteeth on upper plateFold axisC&O Canal 

0 5 10 MILES 

CULPEPER BASIN 

EXPLANATION 

Ordovician clastic turbidite deposits 

FREDERICK VALLEY SECTION

Cambrian and Ordovician carbonatecontinental platform and shelf deposits

WESTMINSTER SECTION

Neoproterozoic and Cambrian slope deposits

POTOMAC TERRANE

Neoproterozoic and Cambrian

accreted sedimentary rocks

intruded by Ordovician igneous

rocks

PIEDMONT PROVINCE 

Appalachian basin Cambrian and Ordovician carbonate continental shelf

and platform deposits 

North Mountain fault

Keedysville detachment fault

Bull Run Mountain fault 

Closely spaced folds and faultsWidely spaced folds and faultsNo folds or faults 

COASTAL PLAIN

PROVINCE

Cretaceous sediment 

Geologic Setting 5 Tectonic map of the study area. Shaded areas represent regions of abundant folds and faults. Figure 3. 
WEST 

Mesoproterozoic A 1.1 Ga to 950 Ma Granitic 

gneiss 

Grenville orogeny 

Neoproterozoic B 770 to 575 Ma Catoctin Formation 

(volcanic rocks) 

Continental rifting to form Iapetus Ocean 

Cambrian C 545 Ma 

Fossils appear 

Continental margin and shelf develop 

Cambrian and D Ordovician 

545 to 480 Ma 

Carbonate shelf platform, platform edge, and oceanic basin 

develop on passive margin of continent Pleasant Grove 

thrust fault 

Martic thrust fault 

Ordovician
 

480 to
 E 

450 Ma 

EAST 

Turbidites of Piedmont 

province deposited 

Lithified turbidites thrust 

onto deepwater sediments 

Plummers Island 

thrust fault 

Carbonate shelf founders, flysch of Martinsburg Formation deposited. Piedmont rocks transported onto continental margin rocks, 

plutonic rocks intrude eastern Piedmont.
 

Rivers and swamps 


F 

G 

H 

Mississippian 

Devonian
 

370 Ma
 

Silurian 

Permian 

280 Ma 

Triassic and 

Jurassic 

220 to 200 Ma 

Cretaceous 

and 

Tertiary 

220 to 200 Ma 

J Present 

coal 

Sedimentation into Appalachian basin 

Continental collision between North American and African tectonic plates and Alleghanian orogeny 

Diabase dikes and flows 

Culpeper basin 

Continental rifting creates basins and results in opening of Atlantic Ocean. Erosion from highlands 

provides sediment for deposition on Coastal Plain. 

PIEDMONT 

VALLEY AND RIDGE Great 

Valley 

BLUE 

RIDGE 

Frederick 

Valley 

Culpeper 

basin 

Present erosion level 

Devonian lamprophyre 


dikes intrude rocks
 

in Mather Gorge
 

COASTAL 

PLAIN 

Potomac COASTAL 

terrane PLAIN 

I 

6 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
 

 

Geologic Setting 7 

Figure 4 (facing page). Diagrammatic sketches showing the geologic history of the Potomac River valley over the past 1 billion years. 

A, Granitic gneiss intrusions, metamorphism, and deformation related to the Grenvillian orogeny lasted 60 m.y., from 1.1 Ga to 950 Ma. 

These rocks are found in the Blue Ridge province. 

B, Continental rifting and volcanic activity in the Grenville terrane (the current Blue Ridge province) and turbidite deposition in a deepwater basin to the east (the current Piedmont province) lasted about 200 m.y., from about 770 to 575 Ma. 

C, The margin of the continent became stable with carbonate rocks deposited in quiet water (rocks of the current Great Valley and 

Frederick Valley sections). To the east (the current Piedmont province), thrust sheets of the turbidite deposits created a sedimentary 

mélange. Shelly fossils appeared at about 545 Ma. 

D, Deepwater rocks were deposited into a basin east of the shelf margin for about 65 m.y. (the current western part of the Piedmont 

province). 

E, The stable shelf foundered as the Taconian orogeny (480 to 450 Ma) elevated the rocks to the east and provided a source for the clastic material that makes up the shale of the Middle and Upper Ordovician Martinsburg Formation (the current center of the Great Valley 

section). Rocks in the Piedmont province were intruded by plutonic rocks (near the Georgetown section of Washington, D.C.) and were 

transported westward along the Pleasant Grove and Martic thrust faults. 

F, A thick sequence of sedimentary rocks was deposited in a deepening Appalachian basin for 120 m.y. Most of these rocks are now 

found in the Valley and Ridge province. At about 370 Ma, igneous rocks were intruded in older rocks near Great Falls. 

G, At about 280 Ma, the continental tectonic plates of North America and Africa collided, resulting in the Alleghanian orogeny. Many of 

the folds and faults in rocks west of the Piedmont province are related to this event. 

H, About 60 m.y. later, continental rifting began and lasted for about 20 m.y. (220 to 200 Ma). Thick sequences of sedimentary rock were 

deposited in fault-bounded basins, and there was volcanic activity. The end result was the creation of the Atlantic Ocean. The Culpeper 

and Gettysburg basins in the western Piedmont province also are the result of this event. 

I, For the last 200 m.y., the landscape has eroded and rivers have carried the sediment eastward to deposit the thick strata of the Atlantic Coastal Plain. 

J, Further erosion has removed much of these extensive Coastal Plain deposits and sculpted the bedrock to create the modern landscape. Some patches of Coastal Plain deposits remain near the Fall Line of the Potomac River. 

collided with the African continental plate to form the Appalachian Mountain belt (fig. 4G). The rocks were deformed by folds 

and faults to produce the Sugarloaf Mountain anticlinorium and 

the Frederick Valley synclinorium in the western Piedmont, the 

Blue Ridge-South Mountain anticlinorium, the Massanutten synclinorium in the Great Valley section, and the numerous anticlinoria and synclinoria of the rest of the Valley and Ridge province. During this orogeny, rocks of the Great Valley section, Blue 

Ridge, and Piedmont provinces were transported westward onto 

younger rocks of the Valley and Ridge province along the North 

Mountain fault (fig. 4G). Rocks that were already deformed in 

the eastern Piedmont also were folded and faulted and existing 

thrust faults were reactivated as both strike-slip and thrust faults. 

After the Alleghanian orogeny, the deformed rocks of 

the joined continents began to break apart, a process that took 

place between 220 and 200 Ma in the Mesozoic (fig. 4H). 

This episode of rifting or crustal fracturing initiated the formation of the current Atlantic Ocean (fig. 4I). Large alluvial fans 

and streams carried debris shed from the earlier uplifted Blue 

Ridge and Piedmont provinces and deposited it in fault-created 

troughs, such as the Culpeper basin in the western Piedmont. 

The large faults that formed the western boundary of the Culpeper basin provided an escarpment that quickly became covered with such eroded debris. Igneous rocks were intruded into 

these strata as subhorizontal sheets (sills) and near-vertical 

dikes that extend beyond the basin into adjacent rocks. After 

these molten igneous rocks were emplaced at 200 Ma, the 

region underwent a period of slow uplift and erosion. Thick 

deposits of unconsolidated gravel, sand, and silt shed from 

the eroded mountains were deposited as part of the Atlantic 

Coastal Plain (fig. 4I). The process continues today: the mountains are eroded, sediment is deposited on the Coastal Plain, 

and the landscape is dissected by rivers bordered by alluvial 

terraces, thereby creating the landscape of the present valley 

(fig. 4J). For additional technical information and abundant 

references, see the summaries in Hatcher and others (1989). 

Cenozoic Landscape and Surficial Deposits

Introduction

The landscape and geomorphology of the Potomac River 

valley are the result of erosion and deposition from about the 

middle part of the Cenozoic Era to the present (about the past 5 

m.y.). The distribution of flood-plain alluvium and ancient fluvial terraces of the Potomac River and adjacent tributaries record 

the historical development of the drainage system (fig. 5). There 
8 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Potomac 

P otomac 

Potomac 

Potomac 

River 

River 

River 

River 

A Western Section Hancock, Md. 

VALLEY AND RIDGE 


PROVINCE
 

Cumberland, Md. 

Oldtown, Md. 

Paw Paw, W. Va. 

A 

B Middle Section C Eastern Section B Maryland 

Point of Rocks, Md. Williamsport, Md. West

Virginia Culpeper 

C basin

Virginia Great Valley

section 

INDEX MAP 

Shepherdstown,

 Va. W.

Washington, D.C. 

PIEDMONT
PROVINCE


Seneca, Md. 

PROVINCE

Harpers Ferry, 

Potomac terrane BLUE RIDGE crystalline rocks

Great Falls, Md. Va. W.

0 3 6 12 18 24 MILES 

Washington,

D.C. COASTAL

PLAINPROVINCE 

alley , Western section, including the V

, Eastern section, including the Pied­ C

A . The alluvium defines the modern flood plain. 

alley and Ridge province and the Blue Ridge province. 

Maps of alluvium (yellow) and alluvial terraces (orange) of the Potomac River

alley section of the V , Middle section, including the Great V

B

Figure 5. and Ridge province. mont province and its Culpeper basin and Potomac terrane crystalline rocks, and the Coastal Plain province. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

Potomac River 

PotomacRiver

thin thin 

Qa Qa 

A
 thin 

PotomacRiver

thin A'


Qa Qa QTt 600 FEET 

Bedrock 

QTt QTt 

SEA LEVEL 

Cumberland, Md. Theodore 

A 

A' 

Georgetown 

section of 

Washington, D.C. 

B' 

B INDEX MAP 

Roosevelt 

PotomacRiver Island 

PotomacRiver

B
 B'
 QTt Qa

SEA LEVEL 

Bedrock 

200 FEET 0 3000 FEET 

Geologic Setting 9 

Figure 6. Schematic cross sections of the Potomac River valley at 

Cumberland, Md. (A-A’), and the Georgetown section of Washington, D.C. (B-B’), showing the change in terrace morphology, topographic elevation, and surficial deposits along its course. Evidence 

that the Potomac River migrated across the valley is best preserved 

at Cumberland, Md.; Paw Paw Bends (river meanders near Paw 

Paw, W. Va.); Williamsport, Md.; and near Seneca, Md., where 

terrace gravels are preserved overlying shale bedrock. The modern 

flood plain is the most extensive at Cumberland and Seneca. QTt, 

Tertiary to Holocene terrace deposits; Qa, Holocene alluvium. 

is no evidence that the river migrated laterally across a broad 

region; rather, it has cut downward, mainly using its early course. 

The distribution, thickness, and height above the present river level of terraces and sediments deposited on the flood 

plain varies from province to province and from rock type to 

rock type (fig. 6). Elevations of terraces along the river show that 

the slope angles of the ancient and modern river valley are similar; this in turn suggests that the terraces formed as the result of 

either eustatic sea-level drop or uplift (Zen, 1997a,b). The Pliocene (5.3 Ma) fluvial deposits at Tysons Corner, Va., occur over 

330 ft above the Potomac River (470 to 500 ft elevation), and are 

about 5 mi away, south of Great Falls. Mt. Sterling, near Sterling, 

Va., is at about 270 ft elevation and is located about 3 mi south 

of the Potomac River in the Culpeper basin. By analogy, some 

of the high terraces along the Potomac River could be as old as 

Miocene or Pliocene. In the absence of supportive data, the landforms and deposits probably formed during late Tertiary to Quaternary time, when a wetter climate, sparse vegetation, and frozen 

ground caused increased precipitation to run into the ancestral 

river, enhancing downcutting and erosion (Zen, 1997a,b). 

River Morphology 

The morphology of the Potomac River changes as it 

drops about 610 ft in elevation as it flows more than 190 mi 

from the western part of the Valley and Ridge province to the 

Coastal Plain in the District of Columbia (figs. 5 and 6). From 

the tidal part of the Potomac near Theodore Roosevelt Memorial Bridge to the Culpeper basin at Seneca Falls, the Potomac 

River has cut a gorge into bedrock. From Great Falls west to 

Seneca Falls, the gorge has numerous islands and a considerable amount of alluvium is preserved. Numerous flat-topped 

islands are located around Great Falls Tavern along the C&O 

Canal in Maryland and near Great Falls Park, Va. The islands 

are bedrock strath terraces (remnants of bedrock in valley 

floors) that formed when the Potomac River cut across the 

ancient valley floor and incised channels into it. The riverbed 

is rock with channels and depressions as much as 80 ft deep 

(Reed, 1981). Some alluvium was deposited on the bedrock 

terraces and radiocarbon dates on organic matter suggest that 

they have been vegetated for more than 10,000 years (Reed, 

1981). At Great Falls, the Potomac River drops 70 ft from 

Olmstead Island, which is a strath terrace at 140 ft elevation. 

The Potomac River then drops another 70 ft to sea level. 

From Seneca Falls to Point of Rocks, Md., the Potomac 

River drops about 300 ft in elevation; the channel is about 5 ft 

deep and flows on bedrock with scattered cobbles and boulders in the riverbed. The deepest part of the channel (the thalweg) is between the northern shore and the islands in the center of the river. The modern flood plain is broad. Bedrock is 

exposed in tributaries, which suggests that the alluvium is 2 to 

20 ft thick at most. Terraces also are broad and can be as much 

as 270 ft above and as far as 3 mi away from the present river. 

From Point of Rocks, Md., to Harpers Ferry, W. Va., the 

Potomac River flows across resistant bedrock ledges of the 

Blue Ridge province. There are islands and flood plains consisting of alluvium, but the few preserved terraces are located 

along the north shore and slope to the flood plain. The river 

drops about 40 ft, from 250 to 210 ft elevation. 

From Harpers Ferry west to McCoys Ferry, Md., the 

Potomac River drops about 130 ft in elevation. Entrenched 

meanders cut into a plateau with near-vertical bluffs of carbonate rock of the Great Valley section. The largest meander occurs 

in the shale of the Martinsburg Formation at Williamsport, Md., 

where extensive gravel terraces are preserved as much as 220 ft 

above the river. Along this section, there are no falls and there is 

a lack of coarse alluvium because the river cuts into soft shale. 

The section of the river with the most abandoned incised 

meanders, entrenched meanders, and broadest terraces adjacent to the modern flood plain of the Potomac River is from 

McCoys Ferry west to Cumberland, Md., in the Valley and 

Ridge province. Along this section, the river is about 5 ft deep 

and flows over bedrock with large amounts of coarse alluvium 

in the riverbed. The river drops about 230 ft, from 610 to 380 

ft elevation, in this region. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

10 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Alluvial Terraces 

On the geologic maps (plates 2 to 20), terraces that were 

mapped at different elevation levels are shown as one group 

because the four distinct terrace elevation levels are difficult to 

correlate. Terrace formation can be understood by study of the 

modern river system. The channel bottom of the Potomac River 

is a nearly flat surface where the bedrock has been eroded by 

running water. The bedrock surface has local irregularities—such 

as ridges, swales, and potholes (figs. 7 and 8)—that formed by 

differential erosion of various rock types. A veneer of boulders, 

cobbles, gravel, sand, and some silt was deposited on the channel 

bottom. In areas of low hydraulic energy (slack water), there are 

thick deposits of fine material, whereas only a little coarse material remains on the riverbed where the current is torrential. Fish 

weirs constructed in the 18th century (Guzy, 1999) or perhaps 

earlier by Native Americans (fig. 9) near Shepherdstown, W. Va.; 

Sandy Hook and Brunswick, Md.; and near the Monocacy River 

in Maryland indicate that the riverbed is shallow and that erosion 

of and deposition in the bed did not affect their construction. 

The river migrated over time across the alluvial plain and 

cut down into the bedrock in response to changing climatic or 

tectonic conditions. As a new channel was cut into bedrock, 

the elevated former riverbed was exposed to weathering and 

became vegetated. Subsequent erosion left an irregular patchwork of terraces that represents stages in the river’s history. 

One of the more common types of terraces of the Potomac 

River is an inclined surface created as the river migrated from 

a higher to a lower elevation along a continuous slope. Allu77°14' 77°13'30” 

38°59' 38°59' 

0 1 MILE 

Sherwin 

Island 

C&O Canal 

MM 13 

Bear Island 

Potomac River 

MM 12 

Bedrock terrace 

Bedrock island 

Erosional channel in bedrock 

Water in erosional channel 

or pothole 

EXPLANATION 

Widewater 

C&O Canal towpath 

Maryland 

Virginia 

vial material commonly is not preserved on these slopes. Good 

examples may be seen along the South Branch of the Potomac 

River, the Cacapon River, Little Cacapon River; and on the 

meanders of the Potomac River north of Paw Paw, W. Va., and 

east of Four Locks, Md. in the Great Valley section. 

In general, alluvial terraces and deposits of mainly quartz 

gravel, cobbles, and boulders are best developed and preserved 

on areas underlain by siltstone and shale bedrock (fig. 10A). This 

relationship holds true in the Great Valley and Culpeper basin 

sections. Large alluvial boulders may have been transported by 

ice rafting or by interference from tree roots (fig. 10B). The most 

extensive deposits are found upstream of ridges capped by resistant sandstone in the Valley and Ridge province. Their presence suggests that the former river may have been impounded 

by the water gaps through the ridges. Such examples can be seen 

around Cumberland, Md., just west of the Patterson Creek anticlinorium (MM 175). In this area, broad terraces underlie the 

town, Mexico Farm Airport, and Death Valley (fig. 5). 

Abandoned Entrenched Meanders 

Abandoned entrenched meanders of the ancestral Potomac 

River demonstrate how the river locally migrated into its present position. An outstanding example of a former riverbed can be 

seen near Paw Paw, W. Va., where West Virginia Route 51 follows Purslane Run. The distribution of strath terraces and material deposited on them provide the chronologic development of 

the landscape in this area (Braun, 1976; Fitzpatrick, 1987); the 

Figure 7. Sketch map showing large water-filled potholes 

and channels on the eastern 

half of Bear Island near MM 

13 and Widewater. Interpreted 

from 5-ft-contour topographic 

map compiled by the National 

Park Service (NPS, unpub. 

data).
 

 

 

Geologic Setting 11 

A B
 

Figure 8. Potholes in migmatite of the Neoproterozoic and (or) Lower Cambrian Mather Gorge Formation along the bluff of Mather 

Gorge in Virginia, opposite MM 13 and Bear Island. A, View from across river channel; 16-ft-long canoe for scale; arrow points to area 

shown in B. B, Close-up view of pothole from part A. Photographs by David F. Usher (U.S. Geological Survey). 

Figure 9. Aerial photograph showing fish weirs constructed by 

Native Americans; arrow points to a weir segment. The weirs are 

constructed from alluvial boulders piled about 3 ft high on the bed 

of the Potomac River. 

meander at Reckley Flat and Purslane Run was abandoned when 

the ancestral Potomac River eroded into the ancestral valley of 

the Little Cacapon River (fig. 11A). The meander at Paw Paw was 

abandoned as the meander neck was narrowed and eroded (fig. 

11B). In addition, the canal at Four Locks (MM 109) occupies an 

abandoned fluvial channel that bypassed a meander loop. To the 

east, there is an “island” within an abandoned river meander adjacent to Conococheague Creek at Williamsport (MM 100). 

A well-developed, abandoned channel may be seen at 

Widewater (MM 13), which is a large body of water incorporated within the C&O Canal east of Great Falls. In Great 

Falls Park, Va., the Potomac River flowed around an island 

locally known as Glade Hill (fig. 12). The boulder deposit on 

the crest of the hill indicates that the river previously flowed at 

that higher level (fig. 13) (Reed and others, 1980; Zen, 1997a; 

Southworth and others, 2000). 

Recent Flood-Plain Alluvium 

The areal distribution of the modern flood plain (Qa) was 

determined by mapping the debris deposited by the floods of 

January and September 1996. The flood plain of the Potomac 

River is relatively broad (fig. 5) upstream from constrictions 

such as (1) west of the Patterson Creek anticlinorium at Spring 
 

 

 

 

 

Terrace

deposit

Road

level

Devonian

shale

A B 

Figure 10. Remnants of Quaternary alluvial terraces. A, Alluvial terrace deposit of the Potomac River in the Valley and Ridge province 

near Cumberland, Md., near MM 180. The large samdstone boulders are about 3 ft across. B, A large quartzite boulder of quartzite (possibly from the Lower Cambrian Weverton Formation of the Blue Ridge province) that was deposited on a terrace about 65 ft above and 1 

mi away from the Potomac River near Sterling, Va. 

 

Tertiary Timeline Present 

Qa 

MARYLAND 

Potom ac River

Paw Paw Purslane Run 

Potom ac River 

Paw Paw Purslane Run 

Paw Paw Purslane Run 

Paw Paw 

QTt Purslane Run 

Reckley Flat 

Reckley Flat 

Reckley Flat 

Reckley 

Flat 

QTt 

QTt 

ittle Cacapon R iv e r ittle Cacapon R iv e r 

P otom ac River 

P o t om ac Rive r 

Qa 

L

L

Qa WEST VIRGINIA

A B C D QTt 

0 1 MILE N 

12 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Figure 11. Sketch maps illustrating the evolution of the abandoned incised meanders of the Potomac and Little Cacapon Rivers near 

Paw Paw, W. Va. (centered on MM 157), a process that occurred probably within the past 1 million years. A-C, Planimetric sketches. 

D, Generalized surficial geologic map. Map-unit abbreviations are as follows: QTt, Tertiary to Holocene terrace deposits; Qa, Holocene 

alluvium. 

Gap near Cumberland, Md.; (2) west of Broadtop Mountain at 

Oldtown, Md.; (3) west of North Mountain at McCoys Ferry, 

Md.; (4) where the river meanders 90° to the south across 

rocks of the Martinsburg Formation at Williamsport, Md.; (5) 

where the river crosses rocks of the Culpeper basin west of 

Seneca Falls; (6) upstream of where the river turns south 90° 

near Swains Lock, Md. (MM 16.5); and (7) at the Fall Line, 

where the Potomac River meets the Coastal Plain. 

The thickness and size of the alluvial material also varies. Along the shores and islands, as much as 20 ft of silt overlies several feet of gravel (fig. 14); drill holes on the flood plain 

north of Whites Ferry, Md. (MM 35), show depths of 21 and 22 

ft (Froelich, 1975). Radiocarbon dates indicate that this material probably was deposited about 10,000 years ago as the climate warmed at the end of continental glaciation (Reed, 1981; 

Froelich and others, 1992). Modern floods, such as those during 1996, tend to scour existing deposits and then redeposit the 

material elsewhere. During such floods, the surface of the towpath was locally scoured and sometimes breached through to 

the canal. Thick deposits of silt and mud in the canal have been 

accumulating since the canal was drained in 1928. Along the 

river, coarse gravel and cobbles tend to be deposited in areas 

where there is high-energy flow. Fine silt and sand tends to be 

deposited in areas of low energy flow. 
 

A B C 

Widewater 

(abandoned

former 

channel) 

Widewater 

(deeply incised

channel) 

Current position 

of a deeply 

incised channel 

of Mather Gorge 

Upland 

Upland 

Upland Upland 

Upland 

A 

A' 

C&O Canal 

Potomac 

River 

Great 

Falls 

Glade 

Hill 

Glade 

Hill 

Boulder 

deposit on 

Glade Hill 

Island 

Bedrock 

strath 

terrace 

Bedrock 

Ancestral 

Potomac 

River 

Bedrock 

strath 

terraces 

Bear 

Island 

Boulder deposit 0 1 MILE Mather Widewater on Glade Hill N A Gorge A' 

D 

Geologic Setting 13 

Figure 12. Schematic maps of the Great Falls of the Potomac River showing the evolution of the abandoned channel of Widewater 

and of Glade Hill. A, The area as it may have looked before the Pleistocene. The ancestral Potomac River was wide and Glade Hill was 

merely a boulder-covered island in the channel. B, Narrower ancestral Potomac River, the deeply incised channel where Widewater 

would eventually be located, and the beginnings of the bedrock strath terraces. C, Present-day features. The Potomac River channel is 

currently incised into Mather Gorge and Widewater is separated from the Potomac River by a terrace (modified from Reed and others, 

1980); the canal enginers incorporated Widewater into the canal construction. D, Schematic cross section shown in C. 

Figure 13. Rounded boulder of diabase (5 ft in diameter) deposited by the Potomac River on terrace at south end of Glade Hill, 

approximately 135 ft above the present river level (south of MM 14 

in Virginia). Photograph by David F. Usher (U.S. Geological Survey). 

WEST EAST 

Silt, clay, 

and gravel

Silt and sand

Gravel

Figure 14. Recent alluvium along the Monocacy River, near 

its confluence with the Potomac River near MM 42, consists of 

gravel, sand, and silt as much as 20 ft thick. 
 

 

 

 

 

 

 

 

 

 

14 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Colluvium 

Colluvium is abundant on all slopes adjacent to ridges 

that are underlain by sandstone, quartzite, and other resistant rocks. The slopes have thin to thick veneers of colluvial 

boulders and blocks that have been transported by gravity or 

by freezing and thawing. Large rock streams and block fields 

locally are shown on the geologic map, but none are found 

near the canal. Near Rosslyn, Va., and the Georgetown section 

of Washington, D.C., gravel derived from the Cretaceous and 

Tertiary deposits of the Coastal Plain have been transported 

down slope as colluvium. 

On the west limb of the Patterson Creek anticlinorium, 

east of Cumberland, Md. (MM 180), are fan-shaped deposits of coarse sandstone colluvial boulders that are intermixed 

with alluvial deposits. Extensive alluvial fans along Mill Run, 

northwest of Oldtown, Md. (MM 167), are composed of fine 

colluvial sandstone cobbles that were eroded from Warrior 

Mountain. Similar deposits and landforms are found from 

Licking Creek to Fort Frederick in Maryland. 

Karst Landforms 

Karst landforms (sinkholes and caves) occur in areas 

underlain by limestone, dolomite, and marble in the Great Valley section of the Valley and Ridge province, and in the Frederick Valley, Culpeper basin, and Westminster terrane in the 

western Piedmont province (fig. 15). In the Great Valley, sinkholes are developed throughout most of the carbonate rock 

formations, but are mostly concentrated in the Elbrook, Conococheague, and Stonehenge Limestones, Rockdale Run Formation, and Chambersburg Limestone. There are caves along 

the canal and some have emergent springs (fig. 16). The Great 

Valley also has travertine and tufa deposits. Karst landforms 

are rare in the Blue Ridge province because marble occurs as 

small bodies. 

Kanawha Spring, which is in the Piedmont province east 

of Point of Rocks, Md., flows from limestone of the Frederick 

Formation, which underlies the flood plain. Limestone cobbles 

within the conglomerate of the Leesburg Member of the Bull 

Run Formation southeast of Point of Rocks dissolved to form 

hummocky topography with abundant sinkholes and springs; 

travertine and tufa deposits also may be seen here. These features evolved as springs and streams that were supersaturated 

with calcite discharged and flowed over rough channels where 

the calcite was deposited and built up over time (White, 1988). 

In addition, marble and limestone of the Westminster terrane 

(exposed along Monocacy River north of Indian Flats) underlie linear valleys and contain abundant sinkholes to the north. 

Fossils Along the C&O Canal 

Introduction 

The rocks that are exposed along the C&O Canal contain a variety of preserved remains, traces, tracks, or imprints 

of ancient plants and animals. Although the Triassic and 

Jurassic rocks of the Culpeper basin in the western Piedmont 

province contain dinosaur footprints that are as old as 210 Ma, 

many of the rocks farther to the west in the Valley and Ridge 

province contain fossils that are much older, ranging in age 

from 530 to 340 Ma. Along the C&O Canal, the most common 

types of fossils are shells of creatures that inhabited the ancient 

seas that covered this region hundreds of millions of years ago. 

Less commonly, remains of animals and plants that lived on 

land are preserved in some of the younger exposed rocks. 

For an animal or plant to become a fossil, several things 

must occur. First, the animal or plant must inhabit an environValley and Ridge 

province Great Valley 

section 

Blue Ridge 

province 

Culpeper 

basin 

Piedmont 

province 

Potomac terrane 

0 10 MILES N 

Figure 15. Schematic map 

showing the distribution of 

carbonate rocks (dark gray) 

and noncarbonate rocks (light 

gray) along the C&O Canal and 

Potomac River corridor. Carbonate rocks are susceptible 

to the development of karst 

landforms. Many caves and 

sinkholes are known to exist in 

the carbonate rocks along the 

canal. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Geologic Setting 15 

A B
 

Figure 16. The cave at Dam 4 near MM 83.5. The cave and emergent spring are in the Conococheague Limestone. A, View looking into 

the cave. B, View looking out of the cave. 

ment that is conducive to preservation. As a general rule, organicrich, dark-gray, fine-grained rocks (such as shale and limestone) 

preserve fossils better than light-colored, coarse-grained rocks 

(such as sandstone and conglomerate). A second prerequisite for 

fossilization is rapid burial. Some of the best-preserved fossils 

are of animals and plants that were buried alive by storms and 

landslides, or that fell into sinkholes or tar pits. Once entombed 

in sediment, the animal, plant, or imprint must remain undisturbed while the sediment becomes lithified into rock; this process commonly takes place over millions of years. 

Piedmont Province 

Fossiliferous rocks are present in the Frederick Valley 

and Culpeper basin in the western Piedmont. No fossils are 

known within the rocks of the Westminster and Potomac terranes to the east; either these rocks were deposited prior to the 

development of shelly organisms, or extreme deformation and 

metamorphism destroyed any fossils that were present. 

Fossils are present, but uncommon, in Frederick Valley 

rocks. The Upper Cambrian Frederick Formation and Upper 

Cambrian and Lower Ordovician Grove Formation were deposited on the continental slope and edge, respectively, of a continental shelf. Well-preserved fossils in the limestone are mainly 

parts of trilobites (fig. 17); however, because these rocks were 

deposited in deep water, the abundance of fossils is low. 

The Frederick and Grove Formations both contain conodonts, which are microscopic (0.1 to 1.0 mm) fossils that are 

thought to be the teeth of an extinct group of marine animals 

closely related to early vertebrates (fig. 17). Conodonts consist 

of phosphatic material much like human teeth. 

In the Culpeper basin in Virginia and Maryland, the red 

sandstone and shale exposed from Seneca west to the Monocacy 

River contain abundant fossils (Kranz, 1989). The Upper Triassic Poolesville Member of the Manassas Sandstone in the eastern part of the basin in Fairfax County, Va., has yielded footprints of crocodiles and a small bird-like animal (Weems and 

Kimmel, 1993). Sandstone and siltstone of the Upper Triassic Balls Bluff Member of the Bull Run Formation near Dulles 

International Airport in Virginia have yielded bones and teeth 

of a crocodile-like parasuchian (phytosaur) (Weems, 1979) and 

a large coelacanth fish (Weems and Kimmel, 1993). Shale beds 

that are interlayered with the Balls Bluff Siltstone to the south 

were deposited in a lake and preserve the remains of arthropods and fish, as well as probable lizard footprints (Gore, 1988). 

Rocks exposed to the south in the Culpeper quarry have yielded 

parasuchian (phytosaur) teeth and footprints of a medium-size 

carnivorous dinosaur (Kayentapus minor) (fig. 18), two small 

carnivorous dinosaurs, a primitive sauropod dinosaur, a small 

ornithischian dinosaur, a prosauropod dinosaur, and a large 

armadillo-like aetosaur (Weems, 1987, 1992). 

Lower Jurassic strata that overlie the Balls Bluff Member to the south in Virginia also contain fossils. In Fauquier 

County, Va., siltstone of the Midland Formation contains abun­
16 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

1

2

3

4

5

6

7

8

9

Figure 17. Fossils from rocks of the Frederick Valley section, 

western Piedmont province. Fossils are catalogued and reposited 

in the Carnegie Museum (CM) in Pittsburgh, Pa., or in the U.S. 

National Museum (USNM) in Washington, D.C. Fossils 1 through 

5 are trilobites and 6 through 9 are conodonts. 1, Olenellus sp., 

Middle and Lower Cambrian Araby Formation (CM 53211). 2, 

Plethopeltis armatus (Billings), Upper Cambrian Grove Formation 

(CM 35116). 3, Rasettia capax (Billings), Upper Cambrian Grove 

Formation (CM 35110). 4, Westonaspis laevifrons Rassetti, Upper 

Cambrian Frederick Formation (CM 35122). 5, Acmarachis, lower 

part of the Upper Cambrian Frederick Formation (CM 53212). 6, 

Proconodontus muelleri Miller, Upper Cambrian Grove Formation 

(USNM 488831). 7, Clavohamulus hintzei Miller, Upper Cambrian 

Grove Formation (USNM 488826). 8, Cordylodus intermedius 

Furnish, Lower Ordovician Grove Formation (USNM 521104). 

9, Rossodus manitouensis, Lower Ordovician Grove Formation 

(USNM 488819). 

dant fish remains (Olsen and others, 1982). Shale of the Turkey Run Formation has yielded footprints of a small carnivorous dinosaur, large prosauropod dinosaurs, and a crocodile 

(Weems, 1992, 1993). Siltstones and shales represent sediments of ephemeral lakes and riverbanks where some reptiles 

and dinosaurs typically lived. Rocks in the Culpeper basin also 

contain plant impressions, pollen, and spores. 

Blue Ridge Province 

Rocks of the Blue Ridge province contain few known 

fossils. Most shelly fossils did not appear until about 560 Ma. 

The Neoproterozoic Catoctin Formation was dated at about 

565 Ma (Badger and Sinha, 1988), but preserved fossils have 

not been recognized either in this formation, which represents 

ancient lava flows, or in the overlying quartzites of the Lower 

Cambrian Weverton Formation. 

The interval of time during which the rocks of the Weverton Formation and overlying rocks of the Harpers and Antietam Formations were deposited represents a major biologic 

evolutionary episode. Although no fossil shells have been 

recovered from rocks of either the Weverton or Harpers Formations, some of the oldest trilobites have been found within 

the Antietam Formation. In places, metasiltstone of the Harpers Formation contains abundant burrows of ancient soft-bodied animals. The most common burrow trace, Skolithos linearis, is oriented perpendicular to the stratification, which 
 

 

 

 

Geologic Setting 17 

Figure 18. Dinosaur tracks in the Upper Triassic Balls Bluff 

Siltstone exposed in the Culpeper Stone Co. quarry near Culpeper, 

Va., are of three-toed Kayentapus minor, a carnivorous dinosaur. 

Quarter for scale. Although no dinosaur footprints are found in 

the vicinity of the C&O Canal, the rocks are similar to those near 

Culpeper. 

suggests that the burrows represent sites where the animal 

once lived. 

Valley and Ridge Province 

From the Great Valley section westward through the rest 

of the Valley and Ridge province, the rocks generally become 

younger, which is reflected by the abundance and diversity of fossils (fig. 19, 20, and 21). For instance, fossils are extremely rare in 

rocks of the Lower Cambrian Tomstown and Waynesboro Formations at the eastern margin of the Great Valley because not many 

types of shelly animals lived during this period of time. Progressing farther to the west, younger rocks are encountered and the 

fossils are more abundant and diverse. This general trend continues through the Ordovician and into the Silurian and Devonian. 

Cambrian Fossils 

The oldest rocks of the Valley and Ridge province are 

Cambrian and are restricted to the Great Valley section. Deformation associated with the formation of the Blue Ridge-South 

Mountain anticlinorium destroyed most of the fossils. As a 

result, the best Cambrian fossils have been recovered from the 

Elbrook and Conococheague Limestones on the western side 

of the Great Valley (fig. 20). 

The most common types of Cambrian fossils are trilobites and stromatolites (algal colonies). Many of the trilobite 

species found in the Conococheague Limestone also are found 

in the Frederick Formation of the Frederick Valley in the western Piedmont province. The similar trilobite remains in both 

formations indicate that these two different geologic units are 

equivalent in time (that is, they are correlative). Even though 

trilobites are the main biostratigraphic tools for correlating 

Cambrian rocks, conodonts appear in the Late Cambrian and 

supplant the trilobite as the most useful fossil for correlation. 

Ordovician Fossils 

Lower Ordovician rocks contain fossils that are very similar to those found in the Upper Cambrian rocks (fig. 20), but 

younger Ordovician rocks contain fewer trilobites and more 

snails, brachiopods, and cephalopods. For instance, the Lower 

Ordovician Stonehenge Limestone contains mainly trilobites 

and conodonts. The transition to other fossil types is evident 

in the overlying Lower and Middle Ordovician Rockdale Run 

A B C
 

Figure 19. Brachiopods in Silurian and Devonian rocks. A, Brachiopod shell fragments from the Middle to Upper Silurian McKenzie 

Formation. B, Internal molds of brachiopods in the Upper Devonian Foreknobs Formation. C, Internal molds of brachiopods in the Lower 

Devonian Oriskany Sandstone. 
 

SERIES FORMATION 

Rockwell Fm. 

Hampshire Fm. 

Greenland Gap 

Group 

Mahantango Fm. 

Marcellus Sh. 

Needmore Sh. 

Oriskany Ss. 

Shriver Fm. 

New Creek Ls. 

Keyser Ls. 

Wills Creek and 


Tonoloway Fms., undiv.
 

Bloomsburg Fm.
 

McKenzie Ls. 

Rose Hill Fm. 

Tuscarora Ss. 

Juniata Fm. 

Martinsburg 

Formation 

Chambersburg Fm. 

St. Paul Group 

Pinesburg Station 

Dolomite 

Rockdale Run 

Formation 

Stonehenge Ls. 

Conococheague 

Formation 

Elbrook Formation 

Waynesboro Fm. 

Tomstown Fm. 

Antietam Ss. 

CAMBRIAN ORDOVICIAN SILURIAN DEVONIAN MISS.

Lower Lower Middle Upper Lower Middle Upper Lower Upper Lower Middle Upper

1 2 

3 

4 

5 

6 

7 

8 

9 10 

11 

12 13 

14 

15 

16 

17 18 

19 

20 

21 

22 

23 

24 26 

27 

28 

29 

30 

31 

32 33 

34 

35 

36 

37 

38 

39 

40 

41 

42 43 

44 

45 

46 

47 

48 

49 50 

51 52 

53 

54 55 

56 57 

25 

18 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Geologic Setting 19 

Figure 20 (facing page). Key index fossils present in the rocks 

found along the C&O Canal in the Valley and Ridge province. 

Some of the individual specimens illustrated here are from other 

regions in the Appalachians, and have been noted as such, but 

all of these species have been recovered from formations in the 

immediate vicinity of the C&O Canal or from formations correlative 

with those along the canal. Abbreviations are as follows: Miss., 

Mississippian; L., Lower; Dol., Dolomite; Fm., Formation; Ls., Limestone; Sh., Shale; Ss., Sandstone. 

1. Glossopleura sp., Middle Cambrian, Elbrook Formation, X5. 

2. Elrathina sp., Middle Cambrian, Elbrook Formation, X4. 

3. Modocia sp., Upper Cambrian, Elbrook Formation, X4. 

4. Genevievella sp., Upper Cambrian, Elbrook Formation, X5. 

5. Crepicephalus sp., Upper Cambrian, Elbrook Formation, X5. 

6. Ptychaspis sp., Upper Cambrian, Conococheague Limestone, X4. 

7. Calvinella sp., Upper Cambrian, Conococheague Limestone, X4. 

8. Pseudosaratogia sp., Upper Cambrian, Conococheague Limestone, X3. 

9. Bellefontia sp., Lower Ordovician, Stonehenge Limestone, X4. 

10. Homagnostus sp., Lower Ordovician, Stonehenge Limestone, X7. 

11. Finkelburgia stonehengensis Sando, Lower Ordovician, Rockdale Run 


Formation (from Sando, 1957, pl. 12, fig. 26), X2.
 

12. Diaphelasma pennsylvanicum Ulrich and Cooper, Lower Ordovician, Rock-


dale Run Formation (from Sando, 1957 pl. 15, fig. 1), X2.
 

13. Ophileta sp., Lower Ordovician, Rockdale Run Formation (from Bassler, 


1919, pl. 33, fig. 1.), X?.
 

14. Diparelasma marylandicum Sando, Lower Ordovician, Rockdale Run Formation (from Sando, 1957, pl. 13, fig. 15), X2.
 

15. Calliops callicephala (Hall), Middle Ordovician, Chambersburg Limestone, 


X4.
 

16. Echinospherites sp., Middle Ordovician, Chambersburg Limestone, X2.
 

17. Rhinidictya sp., Middle Ordovician, Chambersburg Limestone (from Bassler, 


1919, pl. 44, fig. 11), X18.
 

18. Strophomena hallie (Miller), Middle Ordovician, Chambersburg Limestone 


(from Bassler, 1919, pl. 54, fig. 8), X2.
 

19. Hebertella chaziensis Ruedemann, Middle Ordovician, Chambersburg 


Limestone (from Bassler, 1919, pl. 38, fig. 5), X2.
 

20. Orthorhyncula, Middle Ordovician, Maysville Group, Cincinnati, Ohio (from 


Bassler, 1919, pl. 57, fig. 10), X10.
 

21. Diplograptus, Middle Ordovician, Normanskill Shale, New York (from 


Bassler, 1919, pl. 53, fig. 10), X3.
 

22. Rafinesquina alternata (Emmons), Upper Ordovician, Maysvile Formation, 


Cincinnati, Ohio (from Bassler, 1919, pl. 57, fig. 18), X2.
 

23. Byssonychia praecursa Ulrich, Upper Ordovician, Lorraine Shale, New York 


(from Bassler, 1919, pl. 57, fig. 28).
 

24. Schuchertella tenuis Hall, Silurian, Rochester Shale (from Swartz and others, 1923, pl. 18, fig. 8), X10.
 

25. Uncnulus obtusiplicata (Hall), Silurian, McKenzie Formation (from Swartz 


and others, 1923, pl. 21, fig. 8), X1.
 

26. Atrypa reticularis (Linne), Silurian, Rochester Shale (from Swartz and others, 1923, pl. 21, fig. 8), X1.
 

27. Hormatoma hopkinsi Prouty, Silurian, McKenzie Formation (from Swartz 


and others, 1923, pl. 29, fig. 23), X2.
 

28, Hormatoma rowei Swartz, Silurian, Tonoloway Limestone (from Swartz and 


others, 1923, pl. 29, fig. 12), X2.
 

29. Cyathophyllum sp., Silurian, Keyser Limestone, X2.
 

30. Dolichopterus cumberlandensis Swartz, Silurian, Wills Creek Formation 

(from Swartz and others, 1923, pl. 47, fig. 1), X1. 

31. Gypidula sp., Silurian, Keyser Formation, X2. 

32. Stropheodonta varistriata (Conrad), Silurian, Tonoloway Limestone (from 

Swartz and others, 1923, pl. 22, fig. 19), X2. 

33. Camarotoechia tonolowayensis Swartz, Silurian, Tonoloway Limestone 

(from Swartz and others, 1923, pl. 22, fig. 19), X2. 

34. Calymene sp., Silurian, Keyser Limestone, X3. 

35. Leperditia scalaris Swartz, Silurian, Keyser Limestone, X3. 

36. Leptaena rhomboidalis (Wilckens), Lower Devonian, Helderberg Formation 

(Swartz and others, 1913, pl. 16, fig. 16), X2. 

37. Favosites sp., Silurian, Keyser Limestone, X4. 

38. Schuchertella woolworthana (Hall), Lower Devonian, New Scotland Limestone (from Swartz and others, 1913, pl. 51, fig. 7), X2. 

39. Meristella arcuata (Hall), Devonian, New Scotland Formation (from Swartz 

and others, 1913, pl. 74, fig. 5), X?. 

40. Stropheodonta becki (Hall), Lower Devonian, New Scotland Limestone 

(from Swartz and others, 1913, pl. 57, fig. 12), X3. 

41. Tentaculites aculus Hall, Lower Devonian, Helderberg Formation (from 

Swartz and others, 1913, pl. 86, fig. 7), X4. 

42. Spirifer intermedus Hall, Lower Devonian, Oriskany Sandstone (from 

Swartz and others, 1913, pl. 69, fig. 18), X?. 

43. Cypricardinia indenta (Conrad), Middle Devonian, Hamilton Formation (from 

Swartz and others, 1913b, pl. 34, fig. 6), X2. 

44. Loxonema hamiltoniae Hall, Middle Devonian, Hamilton Formation (from 

Swartz and others, 1913b, pl. 36, fig. 16), X1.25. 

45. Polypora compacta (Hall), Lower Devonian, New Scotland Formation (from 

Swartz and others, 1913, pl. 51, fig. 11), X2. 

46. Cyclonema liratum Prosser, Middle Devonian, Hamilton Formation (from 

Swartz and others, 1913b, pl. 36, fig. 8), X1.5. 

47. Palaeoneilo fecunda Hall, Middle Devonian, Hamilton Formation (from 

Swartz and others, 1913b, pl. 36, fig. 19), X1.25. 

48. Mucrospirifer mucronatus (Conrad), Middle Devonian, Hamilton Formation 

(from Swartz and others, 1913b, pl. 41, fig. 12), X2. 

49. Cyclonema concinnum Hall, Upper Devonian, Foreknobs Formation (from 

Swartz and others, 1913b, pl. 67, fig. 27), X?. 

50. Pterinea chemungensis (Conrad), Upper Devonian, Foreknobs Formation 

(from Swartz and others, 1913b, pl. 61, fig. 22), X1. 

51. Productella lachryma (Conrad), Upper Devonian, Foreknobs Formation 

(from Swartz and others, 1913b, pl. 50, fig. 22), X?. 

52. Mucrospirifer mucronatus (Conrad), Upper Devonian, Foreknobs Formation 

(from Swartz and others, 1913b, pl. 68, fig. 2), X1. 

53. Loxonema terebrum Hall, Upper Devonian, Foreknobs Formation (from 

Swartz and others, 1913b, pl. 69, fig. 3), X2. 

54. Leptodesma medon Hall, Upper Devonian, Foreknobs Formation (from 

Swartz and others, 1913b, pl. 63, fig. 15), X?. 

55. Dalmanella sp., Upper Devonian, Foreknobs Formation (from Swartz and 

others, 1913b, pl. 52, fig. 6), X3. 

56. Rugosochonetes sp., Lower Mississippian, Riddlesburg Shale Member of 

Purslane Formation (from Brezinski, 1988, pl. 1, fig. 1), Carnegie Museum 35565, 

X3. 

57. Macropotamorhyncus sp., Lower Mississippian, Riddlesburg Shale 

Member of the Purslane Formation (from Brezinski, 1988, pl. 1, fig. 3), Carnegie 

Museum 35569, X2. 
 

1 2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 14 

15 

16 

17 18 19 

20 

21 22 

23 24 

25 

26 

27 

28 

29 

30 

31 

32 

33 

34 

35 

36 

37 

Rockwell Fm. 

Hampshire Fm. 

Greenland Gap 

Group 

Mahantango Fm. 

Marcellus Sh. 

Needmore Sh. 

Oriskany Ss. 

Shriver Fm. 

Wills Creek and 

Tonoloway Fms., undiv. 

New Creek Ls. 

Keyser Ls. 

Bloomsburg Fm. 

McKenzie Ls. 

Rose Hill Fm. 

Juniata Fm. 

Tuscarora Ss. 

St. Paul Group 

Rockdale Run 

Formation 

Pinesburg Station 

Dolomite 

Stonehenge Ls. 

Conococheague 

Formation 

Antietam Ss. 

Elbrook Formation 

Waynesboro Fm. 

Tomstown Fm. 

Martinsburg 

Formation 

Chambersburg Fm. 

FORMATION 

MISS. 

SERIES CAMBRIAN ORDOVICIAN SILURIAN DEVONIAN Lower Middle Upper Lower Middle Upper Lower Middle Upper Lower Upper Lower 

20 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Geologic Setting 21 

Figure 21 (facing page). Key guide conodonts to the strata of 

the Valley and Ridge province. Some of the individual specimens 

illustrated here are from other regions in the Appalachians and 

are noted as such, but all of these species have been recovered 

from formations in the immediate vicinity of the C&O Canal or from 

formations correlative with those along the canal. Abbreviations 

are as follows: Miss., Mississippian; L., Lower; Dol., Dolomite; Fm., 

Formation; Ls., Limestone; Sh., Shale; Ss., Sandstone. 

1. Phakelodus elongatus (An), Upper Cambrian, Nolichucky Shale, eastern 

Tennessee, X55 (USNM 521105). 

2. Westergaardodina matsushitai Nogami, Upper Cambrian, Nolichucky Shale, 

eastern Tennessee, X40 (USNM 521106). 

3. Proconodontus serratus Miller, Upper Cambrian, Lime Kiln Member of the 

Frederick Formation, quarry at Lime Kiln, Md., X30 (USNM 521107). 

4. Cambrooistodus minutus (Miller), Upper Cambrian, Lime Kiln Member of the 

Frederick Formation, quarry at Lime Kiln, Md., X55 (USNM 521108). 

5. Hirsutodontus simplex (Druce and Jones), Upper Cambrian, Grove Formation, Stup Farm section of Taylor and others (1996), Frederick, Md., X35 (USNM 

521109). 

6. Cordylodus angulatus Pander, Lower Ordovician, Stonehenge Limestone 

(from Brezinski and others, 1999, fig. 3-T), X25 (USNM 507012). 

7. Loxodus bransoni Furnish, Lower Ordovician, basal foot of Rockdale Run 

Formation (from Brezinski and others, 1999, fig. 3-V), X40 (USNM 507014). 

8. Rossodus manitouensis Repetski and Ethington, oistodontiform element, 

Lower Ordovician, top foot of Stonehenge Limestone (from Brezinski and others, 1999, fig. 3-W), X25 (USNM 507015). 

9. Rossodus manitouensis Repetski and Ethington, coniform element, Lower 

Ordovician, basal foot of Rockdale Run Formation (from Brezinski and others, 

1999, fig. 3-X), X40 (USNM 507016). 

10. Scolopodus sulcatus Furnish, Lower Ordovician, one foot above base of 

Rockdale Run Formation (from Brezinski and others, 1999, fig. 3-Y), X25 (USNM 

507017). 

11. Eucharodus toomeyi (Ethington and Clark), Lower Ordovician, Rockdale Run 

Formation, C&O Canal section (from Brezinski and others, 1999, fig. 3-O), X20 

(USNM 507007). 

12. Oepikodus communis (Ethington and Clark), Lower Ordovician, Rockdale 

Run Formation, C&O Canal section (from Brezinski and others, 1999, fig. 3-M), 

X35 (USNM 507005). 

13. Pteracontiodus cf. Pt. gracilis Ethington and Clark, Middle Ordovician, 

upper part of Rockdale Run Formation, C&O Canal section (from Brezinski and 

others, 1999, fig. 3-E), X40 (USNM 506997). 

14. Tropodus comptus (Branson and Mehl), Lower Ordovician, Rockdale Run 

Formation, C&O Canal section (from Brezinski and others, 1999, fig. 3-P), X20 

(USNM 507008). 

15. Colaptoconus quadraplicatus (Branson and Mehl), Lower Ordovician, 

Rockdale Run Formation, C&O Canal section (from Brezinski and others, 1999, 

fig. 3-R), X35 (USNM 507010). 

16. Histiodella altifrons Harris, Middle Ordovician, uppermost one foot of 

Rockdale Run Formation, C&O Canal section (from Brezinski and others, 1999, 

fig. 3-F), X55 (USNM 506998). 

17. Cahabagnathus friendsvillensis (Bergström), Middle Ordovician, 39 m 

above base of St. Paul near Worleytown, Franklin Co., Pa., USGS fossil locality 

9306-CO, X25 (USNM 521110). 

18. Leptochirognathus quadratus Branson and Mehl, Middle Ordovician, 48 ft 

below top of Pinesburg Station Dolomite near Marion, Franklin Co., Pa. (from 

Brezinski and others, 1999, fig. 3-C), X30 (USNM 506995). 

19. Appalachignathus delicatulus Bergström and others, Middle Ordovician, 

Lincolnshire Limestone, Tumbling Run section, Shenandoah Co., Va., X40 

(USNM 521111). 

20. Amorphognathus tvaerensis Bergström, Upper Ordovician, Nealmont 

Formation, Tyrone, Pa., X5 (USNM 521112). 

21. Plectodina n. sp., Middle Ordovician, 260 ft below top of Rockdale Run 

Formation, C&O Canal section, X30 (USNM 507002). 

22. Phragmodus undatus Branson and Mehl, Upper Ordovician, Jacksonburg 

Limestone, Northampton Co., Pa., X30 (USNM 521113). 

23. Panderodus gracilis (Branson and Mehl), Upper Ordovician, Jacksonburg 

Limestone, Northampton Co., Pa., X5 (USNM 521114). 

24. Oulodus sp., Upper Silurian, Tonoloway Limestone, 34 ft above base, Giles 

Co., Va., USGS fossil locality 11569-SD of K. Denkler and A.G. Harris, X25 

(USNM 521115). 

25. Oulodus elegans (Walliser), Upper Silurian, upper part of the Keyser 

Limestone, Lambert Gap, W. Va. (from Denkler and Harris, 1988, pl. 1, fig. U), 

X40 (USNM 418600). 

26. Ozarkodina remscheidensis eosteinhornensis (Walliser), Silurian, lower 

limestone member of the Keyser Limestone, Lambert Gap, W. Va., USGS fossil 

locality 10700-SD of K. Denkler and A.G. Harris, X30 (USNM 521116). 

27. Ozarkodina remscheidensis remscheidensis (Zeigler), Tonoloway Limestone, 39.5 ft above base, Giles Co., Va., USGS fossil locality 11572-SD of K. 

Denkler and A.G. Harris, X35 (USNM 521117). 

28. Icriodus claudiae Klapper, Lower Devonian, Licking Creek Limestone, 

Frederick Co., Va. (from Harris and others, 1994), USGS fossil locality 12151-SD, 

X20 (USNM 521118). 

29. Icriodus difficilis Ziegler and Klapper, Upper Devonian, Harrell Shale, Mineral Co., W. Va. (from Weary and Harris, 1994), USGS fossil locality 12137-SD, 

X30 (USNM 521119). 

30. Polygnathus alatus Huddle, Upper Devonian, Harrell Shale, Mineral Co., W. 

Va. (from Weary and Harris, 1994), USGS fossil locality 12137-SD, X25 (USNM 

521120). 

31. Mesognathus dengleri (Bischoff and Ziegler), Upper Devonian, Harrell 

Shale, Grant Co., W. Va. (from Weary and Harris, 1994), USGS fossil locality 

12137-SD, X20, (USNM 521123). 

32, 35. Ancyrodella alata Glenister and Klapper, lower and upper views of 

immature specimen (fig. 32) and upper view of mature specimen (fig. 35), 

Upper Devonian, Harrell Shale, Allegany Co., Md. (from Weary and Harris, 

1994), USGS fossil locality 12293-SD, pl. 2, figs. 11, 12, X20 (fig. 32) and X25 (fig. 

35), (USNM 481789 and 521124, respectively). 

34. Polygnathus linguiformis linguiformis Hinde, Upper Devonian, Scherr Formation, Greenland Gap Group, Randolph Co., W. Va., X20 (USNM 521121). 

37. Polygnathus sp., Upper Devonian, Scherr Formation, Greenland Gap Group, 

Randolph Co., W. Va., X25 (USNM 521122). 

33. Palmatolepis sp., Upper Devonian, Scherr Formation, Greenland Gap Group, 

Randolph Co., W. Va., X30, (USNM 521125). 

36. Delotaxis sp., Upper Devonian, Scherr Formation, Greenland Gap Group, 

Randolph Co., W. Va., X20, (USNM 521126). 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

22 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Formation that contains many snails and only a few trilobites. 

The change is most obvious in the diverse fauna present in the 

Echinosphaerites beds of the Middle Ordovician Chambersburg Limestone. These beds, which were named for an echinoderm Echinosphaerites, contain abundant brachiopods, bryozoans, and only a few trilobites. The changes observable in the 

macrofossils are further illustrated by rapid changes in conodonts through the Ordovician, as illustrated in figure 21. 

The diverse numbers of organisms that had inhabited the 

clear waters during limestone deposition were replaced by a 

group of organisms that could live in the muddy sea bottoms that 

existed during deposition of the Martinsburg Formation in the 

Middle and Late Ordovician. These include snails and clams; 

brachiopods and bryozoans are absent. To exist on the muddy 

sea floor, animals had to be able to filter out clay and silt. 

Silurian Fossils 

The Silurian was a time of great diversity in brachiopods; many Silurian units contain abundant brachiopod 

remains. The units that best exemplify species diversity are 

the Middle Silurian Rose Hill Formation, Middle and Upper 

Silurian McKenzie Formation, and Upper Silurian and Lower 

Devonian Keyser Limestone, all of which were deposited in 

very shallow marine waters. Some of the thin beds of limestone within the McKenzie Formation are made up completely of brachiopod or snail shells (see figure 19). This 

highly fossiliferous limestone can be seen along the abandoned Western Maryland railroad bed near MM 133. The 

limestone is interbedded with units such as the Upper Silurian Bloomsburg and Wills Creek Formations, which were 

deposited mostly above high tide and, as a result, contain 

almost no fossil remains. The exception is the large ostracode Leperditia that is found in the Wills Creek Formation. 

The Upper Silurian Tonoloway Limestone is known to contain the rare remains of extinct arthropods known as eurypterids. These arthropods lived in shallow marine water and 

looked like scorpions, so they are commonly called sea scorpions. Eurypterids flourished in the Late Silurian. 

The Keyser Limestone was deposited during the latest 

Silurian and earliest Devonian. This unit contains a diverse 

fossil assemblage of brachiopods, bryozoans, corals, trilobites, and stromatoporoids (sponge-like colonial animals). 

During the Late Silurian, stromatoporoids formed small reefs 

similar to the modern coral reefs off the coast of Florida. 

Although no fossil reefs have been identified along the C&O 

Canal, Brezinski (1996b) described a reef north of it near 

the village of Flintstone, Md. Small stromatoporoids can be 

found along the abandoned Western Maryland railroad grade 

at Dam 6. 

Devonian and Mississippian Fossils 

Perhaps the most fossiliferous Lower Devonian formation along the C&O Canal is the Oriskany Sandstone. This 

white sandstone contains abundant molds of brachiopod shells. 

Because this sandstone is so porous, water dissolved the shell 

material from these fossil fragments, leaving molds where the 

shells once were (fig. 19C). Many of the younger Devonian 

shales (Needmore, Marcellus, and Brallier Shales) as well as 

sandstones of the Mahantango and Foreknobs Formations, contain abundant clams, snails, brachiopods, and cephalopods, 

most of which are preserved as molds (fig. 19). Conodonts are 

common, but trilobites are uncommon in these Devonian rocks. 

Mississippian rocks occur at only one location along the 

C&O Canal. Near the Sideling Hill Creek Aqueduct, sandstone within the Lower Mississippian Riddlesburg Shale 

Member, a marine unit of the Upper Devonian and Lower Mississippian Rockwell Formation, is exposed. This earliest Mississippian sandstone was deposited as beach sand and as such 

contains no fossils. However, the black shale that is present 

immediately above this sandstone represents mud deposited 

in a nearshore lagoon and it contains a few brachiopods. Conodonts have been found in some of the rare carbonate horizons 

in this shale unit. 

Engineering Geology 

Introduction 

To fully understand the engineering achievement of the 

C&O Canal, walk along the shore of the Potomac River in Virginia or West Virginia and envision what would be necessary 

in order to transport 120 tons of cargo in a 92-ft-long boat in a 

waterway that parallels the river. A 6-ft-deep canal that was 40 

to 60 ft wide at the top had to be excavated. Small tributaries 

of the river had to be crossed, using culverts to carry the tributaries beneath the canal. Large tributaries required aqueducts to 

carry the boats over the water (fig. 22; table 1). Where bedrock 

bluffs adjoined the river, breaking rock with sledgehammers, 

drilling with special bits, and blasting with black powder were 

required to excavate a canal. Parallel to the canal and adjacent 

to the river, an elevated towpath needed to be constructed. Dams 

were required at strategic locations to divert river water into the 

canal. Locks needed to be constructed at specific locations to 

lift and lower the boats as the canal changed in elevation along 

its length. Lock keepers needed lock houses to live in because 

the canal would operate 24 hours a day. Necessary construction materials needed to be brought in, such as dimension stone 

for locks, lock houses, culverts, and aqueducts; stone rubble for 

towpath fill; clay to line the canal; cement for mortar; iron and 

manganese for the production of iron for lock fittings; and lumber for lock gates and forms. 

The 184.5-mi-long canal and towpath eventually 

required an infrastructure of over 74 lift locks and lock 

houses, 11 aqueducts, more than 182 culverts and waste 

weirs, 7 dams, and 1 tunnel through bedrock. As a result, 

building stone was in great demand for the project. The facing of all locks, aqueducts, and culverts required cut or hammer-dressed stone. Sandstone and limestone were highly 

desired because the rocks could be easily split along bedding 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Geologic Setting 23 

surfaces, providing two smooth sides for a typical block. The 

locations of locks, aqueducts, dams, and the known quarry 

sites for building stone are shown on figure 22 (1828 to 

1850). During construction, the dimension stone and cement 

were transported to the canal by river barge, wagon, and rail. 

Subsequent repairs to these structures have used stone, brick, 

and concrete in modern time. 

Construction of the canal, towpath, and other structures 

between the Georgetown section of Washington, D.C., and 

Cumberland, Md., took 22 years. Discussions and illustrations of the engineering and construction of the canal are provided in the Chesapeake and Ohio Canal Official National 

Park Handbook (National Park Service, 1991, p. 32–37) and in 

Davies (1971, 1999). 

Canal Excavation and Towpath 

The dimensions of the canal are 6 ft deep, 48 ft wide at 

the base, and 60 ft wide at the surface; upriver of Harpers Ferry, 

the dimensions are slightly smaller. The canal was excavated 

mostly by pick and shovel in unconsolidated Quaternary alluvial 

gravel, sand, silt, and clay. These materials were dug out and 

then placed on the riverside to form the towpath. Flood scour in 

1996 revealed that the towpath locally is a stone crib of vertical 

blocks filled with soil and capped with gravel. Stone embankments and berms were constructed locally along the river, adjacent to the towpath (fig. 23), and sometimes even on the far side 

of the canal to keep floodwaters out; examples may be seen at 

the McKee-Besher Wildlife Management Area west of Seneca, 

Md., and at Big Pool. The canal prism was lined with impervious clay to hold water (fig. 24). The source of the clay is not 

known. Karst in limestone must have caused local water loss 

because today there are caves exposed in the dry canal. 

Excavating the C&O Canal could not be accomplished 

in the same exact manner as the 363-mi-long Erie Canal in 

New York, which was under construction from 1817 to 1825 

and is located almost entirely in thick unconsolidated glacial 

till, outwash, and alluvium. About 34 mi of the C&O Canal 

and towpath had to be drilled and blasted through bedrock 

using sledgehammers, star-bit drill rods, and black powder 

(fig. 25A). Many of the drillholes may still be seen along the 

bluffs and in the canal bed. Men roped themselves to trees or 

hung over cliffs and used sledgehammers to pound steel rods 

(fig. 25B) held by other trustworthy men. Bedrock excavated 

for the adjacent railroad and canal berm was used as fill to create the towpath (fig. 25C). 

A 3,118-ft-long tunnel (fig. 26) was excavated through 

folded siltstone and shale of the Upper Devonian Brallier 

Shale north of Paw Paw, W. Va., from 1836 to 1841 and from 

1847 to 1850. The tunnel was the alternative to constructing a canal along 6 mi of river meanders bordered by bedrock cliffs. As many as 44 men excavated 10 to 12 ft of tunnel each week using hand drills, picks, sledge hammers, and 

black powder. There were two vertical shafts for access so 

that the tunnel could be excavated from six positions. Debris 

hauled up the shafts was dumped in spoil piles above the 

north portal. 

Locks and Lock Stone 

The water in the canal did not flow unimpeded from 

Cumberland to Georgetown, like the water flows down the 

Potomac River; instead, water in the canal was impounded at 

a constant level and released 74 times as a boat made its trip. 

There were 74 locks that raised westbound boats and lowered 

eastbound boats an average of 8 ft to accommodate the change 

in elevation of 605 ft along the length of the canal. The locks 

are 100-ft-long, 15-ft-wide chambers made of stone on timber 

foundations with watertight wooden gates at both ends. 

There also were 12 river feeder locks and guard locks 

that regulated the flow of water from the Potomac River into 

the canal. Several of these river locks, such as at Goose Creek, 

Va.; Shenandoah River in Harpers Ferry, W. Va.; and Shepherdstown, W. Va., were requested by the Virginia Legislature in 1833 to provide a means to transport Virginia products to market. The Goose Creek and Little River Navigation 

provided access from the canal to as far west as Middleburg, Va. (Trout, 1991). The Shenandoah River lock at Harpers Ferry was built by the Patowmack Company for access as 

well. Skirting canals associated with the Patowmack Canal 

remain on the Virginia shore opposite Seneca and Weverton, 

Md. River locks also were used to take boats into the Potomac 

River where bluffs prevented construction of a canal, such as 

at Big Slackwater. 

The dominant building material required for the C&O 

Canal was dimension stone. Sandstone of the Lower Cretaceous Potomac Formation (informally called “Aquia Sandstone”) was quarried from Aquia Creek in Stafford County, 

Va., and transported by barge over 35 mi up the Potomac River 

for use in Locks 1 to 7 (fig. 27). Sandstone of the Upper Triassic Poolesville Member of the Manassas Sandstone (informally called “Seneca Red Sandstone”) was quarried mostly at 

Seneca, Md., (fig. 28) with smaller amounts quarried north of 

MM 46 east of Point of Rocks, Md. This sandstone was used 

in Locks 7 to 13, 15 to 24 (fig. 29), 25 to 30, 33, and 34. Ordovician Ellicott City Granodiorite from the Patapsco River valley in eastern Maryland was used in Locks 14, 28 (fig. 30) 

to 30, 32, and 33. Lower Cambrian(?) Sugarloaf Mountain 

Quartzite quarried from the southern base of Sugarloaf Mountain was used in Lock 28. 

Locks 31 and 33 used a combination of local Lower 

Cambrian stones that included quartzite of the Weverton Formation (fig. 31), limestone, metasiltstone of the Harpers Formation, as well as Upper Triassic Manassas Sandstone. In the 

Great Valley section, locally quarried limestone (fig. 32) was 

used extensively from Lock 32 to Lock 53, as well as in Locks 

56 and 57. Locks 53 to 57 used sandstone and limestone that 

were excavated along the canal. Locks 58 to 67 used Devonian sandstone and shale that also were locally excavated 

along the canal. Some of these locks were lined with wood 
24 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor , and near Dam 5, so no canal was constructed in Map showing locks, aqueducts, dams, and known quarries near the canal. The map also shows whether the canal, towpath, and tunnel were excavated through Figure 22. bedrock or alluvium. The Potomac River was used for transport instead of a canal from near MM 86 to MM 88 (Big Slackwater) either of those two areas. L74 L52 L46 78°07'30" 6.25 MILES L73L72 Oldtown, Md. L69 to L71 L68 L75 Paw Paw Tunnel L67

L62 to

L66 L60

L61

L59 L58 L57 L56 L55L54

Dam 6

L53

L51

Licking Creek

Aqueduct

L47 to L50

Ls

Dam 5

L45

Ss

Ls

Ls

Ls

WEST

EAST

Conococheague Creek Aqueduct

L44

L43 L42

L41

Dam 4 L40

L39 Ferry Hill

L38 Antietam Creek Aqueduct

Ls, M

L32

to

L36

Harpers Ferry,

W. Va.

Dam 3

L31

L30 L29

L28

Catoctin Creek 

Aqueduct

Monocacy River

Aqueduct

L27

L26

Broad Run Trunk

(Aqueduct)

L25 Seneca Creek Aqueduct

L24 L22 and L23

L21

L15 to L20

Dam 2

L8 to L14

L7

L6

Dam 1

Georgetown, 

Washington, D.C.

L1

L2 to L5

Qtz

Ss

Qtz

N

Hancock, Md.

Tonoloway Creek

Aqueduct

Fifteenmile Creek 

Aqueduct

Sideling Hill Creek

Aqueduct

Town Creek

Aqueduct

Evitts Creek Aqueduct

Cumberland, Md.

0

39°22'30"39°00'00"

76°50'30"

Trace of canalExcavation in bedrock

Potomac River

Potomac River
Table 1. Dimension stone used for the C&O Canal locks and aqueducts and the source geologic unit. 

[See figure 22 for locations of locks and aqueducts. Names in parentheses are informal names] 

Lock number or aqueduct name Dimension stone Geologic unit 

1–7 

7–13, 15–27, 28–30, 33–34 

14, 28–30, 32–33 

28 

31, 33, 53–67 

32–53, 56–57 

68, 70–71 

Seneca Creek 

Sideling Hill Creek, Town Creek 

Catoctin Creek 

Monocacy River 

Antietam Creek, Conococheague Creek, Licking 

Creek, Tonoloway Creek, Sideling Hill Creek, 

Fifteen Mile Creek, Town Creek, and Evitts Creek 

Sandstone 

Sandstone 

Granodiorite 

Quartzite 

Mixture 

Limestone 

Sandstone 

Sandstone 

Sandstone 

Granodiorite 

Quartzite 

Limestone 

Potomac Group (Aquia Sandstone) 

Poolesville Member of Manassas Sandstone (Seneca Red Sandstone) 

Ellicott City Granodiorite 

Sugarloaf Mountain Quartzite 

Local stones quarried nearby 

Variable. Limestone unit usually quarried nearby 

Oriskany Sandstone 

Poolesville Member of Manassas Sandstone (Seneca Red Sandstone) 

Variable. Sandstone unit usually quarried nearby 

Ellicott City Granodiorite 

Sugarloaf Mountain Quartzite 

Variable. Limestone unit usually quarried nearby 

Geologic Setting 25 

A B C 

Figure 23. Stone berms constructed to prevent erosion between the towpath and the Potomac River. A, Mary’s Wall (MM 14). Towpath 

is above the wall. B, Near Blockhouse Point (MM 21). C, At Devil’s Eyebrow, west of Hancock (MM 127.5). Historical photograph by 

Charles D. Walcott (U.S. Geological Survey, 1897). 

planks (fig. 33) to help seal water that leaked from between 

the masonry stones. Sandstone of the Lower Devonian Oriskany Sandstone was used in Locks 68, 70, and 71 because it 

had been excavated to build the canal. 

Cement 

More than 500,000 barrels of cement were used to bond 

the stone in the locks and aqueducts. From 1828 to 1837, 

cement plants were built along or near the Potomac River 

at Tuscarora, Md.; Shepherdstown, W. Va.; Hooks Mill and 

Roundtop Hill near Hancock, Md.; Leopards Mill near Dam 

6; and at Cumberland, Md. (Hahn and Kemp, 1994). Beginning in 1837, limestone from the Upper Silurian Wills Creek 

Formation was quarried or mined and used for cement by the 

Round Top Cement Company, located west of Hancock along 

the C&O Canal (fig. 34). This company provided cement not 

only for the canal masonry, but also for the construction of the 

Figure 24. An example of modern erosion in the canal near Lock 

34 (west of MM 62) that has exposed the clay liner (above the 

rock hammer) that was placed above rubble fill (below the rock 

hammer) to hold water. This clay was probably taken from the 

residuum that developed on the carbonate rocks of the Tomstown 

Formation near Fort Duncan to the immediate west. 
26 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Drill rod

A
 

Drill holes

B
 

C 

Figure 25 . Evidence of excavated bedrock. A, Drill holes and 

drill rod (indicated by arrow) stuck in a limestone cliff about 50 ft 

above Big Slackwater (MM 87.5). B, Drill holes in metagraywacke 

over 50 ft above the canal at Blockhouse Point (MM 21). C, Historical photograph (around 1904–1906) showing that bedrock excavated for the Western Maryland Railroad (on the left) was used to 

fill the towpath of the C&O Canal east of Oldtown, Md., near MM 

164. Photograph courtesy of the National Park Service. 

A
 

B
 

C
 

Figure 26. Historical photographs of Paw Paw Tunnel, near 

MM 155. A, View of the south portal, looking north (1904). Photograph courtesy of National Park Service (NPS). B, View of the 

north portal, looking south (around 1900). The tunnel penetrates 

the southeast limb of an anticline of shale and thin sandstone of 

Upper Devonian Brallier Shale. The anticline’s crest is exposed 

above the upper-right stonework of the north portal. Photograph 

courtesy of NPS. C, Brick lining of the 3,118-ft-long tunnel. Photograph courtesy of NPS. 
C 

B
 

A 

Figure 27. Examples of Lower Cretaceous Potomac Formation 

(informally called “Aquia Sandstone”). A, Cut blocks of Potomac 

Formation used in construction of Lock 6. B, Closeup of A showing 

crossbedding and rounded quartz clasts. C, Grooves in the friable 

Potomac Formation made by the rope that mules used to pull 

canal boats (penny for scale). 

Geologic Setting 27 

A B
 

Figure 28. Upper Triassic Poolesville Member of the Manassas Sandstone (informally called “Seneca Red Sandstone”) was drilled 

and quarried for dimension stone along the bluffs west of Seneca, Md. A, View of quarry wall showing three vertical drill holes visible 

in the quarry face. B, Dressed blocks (showing hammer and chisel marks) in the ruins of the Seneca Stone Cutting Mill, located 300 ft 

north of MM 23. 
28 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Smooth-cut 

blocks 

Rough-cut 

blocks 

A B 

Figure 29. East end of Lock 24 (Rileys Lock at MM 23). A, Smooth-cut (wire saw) and rough-cut (hammer dressed) blocks of Upper Triassic Poolesville Member of Manassas Sandstone (informally called “Seneca Red Sandstone”). B, Closeup of rough-cut block showing 

beds of finer grained siltstone interbedded with sandstone. 

A B 

Figure 30. Nonfoliated 

Ordovician Ellicott City 

Granodiorite. A, Blocks 

of the granodiorite were 

used in the construction 

of Lock 28, near MM 49. 

B, Closeup of A showing 

a crystalline texture 

exhibited by amphibole 

(dark) and feldspar 

(light) crystals.

A B
 

Figure 31. Quartzite of the Lower Cambrian Weverton Formation. A, Blocks of the quartzite were used to construct Lock 31 at MM 58. 


B, Closeup of A showing the vitreous orthoquartzite variety of the Weverton Formation; elsewhere, the Weverton is a granular quartzite.
 
Geologic Setting 29 

Figure 32. Limestone of the 

Lower Cambrian Bolivar Heights 

Member of the Tomstown Formation. A, Blocks of limestone 

were used in the construction 

of Lock 37 (Mountain Lock) just 

south of MM 67. B, Closeup of A 

showing beds (vertical) and burrows accentuated by cleavage 

(horizontal). Elsewhere in the 

Valley and Ridge province, the 

Bolivar Heights is a thin-bedded, laminated limestone and 

was a popular choice for dimension stone. A B 

Location of part B

A B
 

Figure 33. Lock 60 at MM 

149.5, which was constructed 

from locally quarried, roughcut sandstone of the Upper 

Devonian Foreknobs Formation. 

Wood planks lined the walls to 

help prevent water from leaking out of the canal through 

the stone blocks. Photograph 

courtesy of the National Park 

Service. 

Figure 34. Round Top Cement Company, west of Hancock, Md., where limestone was quarried, mined, and processed. A, Historical 

photograph (not dated) provides view of the plant and its surroundings from downstream along the canal. B, Historical photograph 

(1907) showing the plant buildings. Both photographs courtesy of National Park Service. 
 

 

 

 

 

 

 

30 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Washington Monument in Washington, D.C., and the Cabin 

John Bridge, which carries MacArthur Boulevard over the 

Cabin John Parkway in Montgomery County, Maryland. 

Aqueducts, Culverts, and Waste Weirs 

With the exception of the Broad Run Trunk (Aqueduct) 

that was made of wood planks, the other 10 aqueducts were 

impressive structures made of stone that did not leak. The Alexandria Aqueduct carried boats over the Potomac River to the 

Alexandria wharf in Virginia. The Seneca Creek Aqueduct (fig. 

35) was constructed from Upper Triassic Manassas Sandstone 

that was quarried to the immediate west. The Monocacy River 

Aqueduct (fig. 36) used quartzite of the Lower Cambrian(?) 

Sugarloaf Mountain Quartzite. This granular white rock was 

quarried at the base of Sugarloaf Mountain near the FrederickMontgomery County line and hauled to the site by horse-drawn 

A
 

B
 

Figure 35. Seneca Creek Aqueduct, just east of MM 23. A, 

Historical photograph (undated) showing view of water-filled 

aqueduct. Photograph courtesy of the National Park Service. B, 

The aqueduct is constructed of smooth-cut Upper Triassic Manassas Sandstone. 

wagon and light rail. The Catoctin Creek Aqueduct, now in 

ruins, used Ordovician Ellicott City Granodiorite from the Patapsco River valley (in northern Maryland) and Manassas Sandstone from the Seneca Quarry. Parts or all of the aqueducts at 

Antietam Creek, Conococheague Creek, Licking Creek, Tonoloway Creek, Sideling Hill Creek, Fifteenmile Creek, Town 

Creek, and Evitts Creek were constructed of limestone quarried 

nearby. The aqueducts at Sideling Hill Creek and Town Creek 

also used locally quarried sandstone. 

More than 200 culverts mostly were constructed from 

local stone, lined with brick, and dressed with cut stone. The 

culverts carried water from Potomac River tributaries beneath 

A
 

B
 

Figure 36. The Monocacy River Aqueduct at MM 42. A, Historical photograph (undated) looking upstream at the Monocacy 

River. Photograph courtesy of the National Park Service. B, The 

aqueduct is constructed from smooth-cut blocks of Early Cambrian(?) Sugarloaf Mountain Quartzite. 
Geologic Setting 31 

the canal and towpath. Waste weirs were ditches and flumes 

lined with stone that were used to control the water level in the 

canal by diverting excess water around the locks. 

Dams 

Dams were constructed near the top of natural steep gradients in the river in order to raise the level of water to feed the 

canal. The seven dams have foundations that rest on bedrock 

and all of the dams were constructed where resistant bedrock 

(metagraywacke and sandstone) crops out as ledges to create 

shallow falls. 

Dam 1 was built in the 1750s at Little Falls in the eastern Piedmont. Dam 2 was built in 1828 at Seneca Falls, which 

is located on metagraywacke of the Mather Gorge Formation, 

at the eastern margin of the Culpeper basin in the eastern Piedmont. Both are rubble dams constructed of blocks and boulders of local rock. Dam 3 (built in 1879 near Harpers Ferry), 

which is located on sandstone of the Antietam Formation, 

was a crib timber dam (fig. 37) that later was replaced with 

masonry. Dam 4 (built between 1832 and 1834), Dam 5 (built 

between 1833 and 1835), and Dam 6 (built in 1839 east of 

Woodmont, Md., on the Oriskany Sandstone) (fig. 38) were 

timber-framed dams that were filled with stone and covered 

with wooden planks. Dam 8 at Cumberland was a masonry 

dam that was removed in 1958 for flood control. Dam 7 was 

planned for near Paw Paw, W. Va., but was not constructed for 

unknown reasons. 

Control of Geology on Engineering 

Several engineering features are worth mentioning here 

because they illustrate the challenge posed by the geology in 

the construction of the C&O Canal. 

(1) From near MM 2 in Georgetown to Great Falls Tavern 

(MM 14), the canal and towpath were built on an incised channel on a bedrock strath terrace of the Potomac River. From 

near Cropley (MM 12) to MM 14, the canal used an abandoned channel of the Potomac River known as Widewater 

(fig. 39). This channel was cut into the bedrock terrace by the 

river, which later abandoned this course and migrated south 

to its present course. Because the channel was wider than 

the locks, timber abutments were used to fill the gaps. Channels from Widewater to the present river have wooden dams to 

keep floodwaters away from the towpath and canal. The towpath was constructed along a rocky stretch of ground adjoining Widewater. 

(2) At four locations, bedrock bluffs prevented construction of a canal and towpath. From MM 85.5 to MM 88.8 south 

of Hagerstown, Md., the north shore of the Potomac River 

is lined with high bluffs of Stonehenge Limestone and has 

no flood plain. This problem was solved, at Dam 4 and Big 

Slackwater, by having boats enter the Potomac River at Guard 

Lock 4 and rejoin the canal 4 mi upriver at Lock 41. The towpath along this stretch is a narrow bench that was drilled and 

blasted into the limestone bluff (fig. 40A). In order to avoid 

river bluffs beyond MM 154, the canal and towpath were constructed in a ravine that is a tributary to the Potomac River. 

This section then leads to the 3,118-ft-long tunnel through 

shale and siltstone near Paw Paw, W. Va. The canal and towpath use the valley of Mill Run near Oldtown, Md., to bypass 

bedrock cliffs; here, both the canal and towpath were excavated through shale (fig. 40B). 

(3) Four Locks (MM 108.5) routed the canal through an 

abandoned channel that probably once was a tributary of the 

Potomac River; the rerouting was necessary in order to cut off 

the 7-mi-long “neck.” 

(4) Big Pool (MM 113 to MM 114) and Little Pool (MM 

120) are lakes that were created by building dikes, levees, 

and berms on a broad low-lying flood plain, and then filling 

the resulting structure with water diverted from the canal and 

river. The towpath was located on a higher ridge or intervening 

island; therefore, a canal prism did not have to be excavated 

and constructed. 

Figure 37. Historical photograph (1936) of Dam 3, which 

was a timber crib filled with 

stone located north of Harpers 

Ferry, W. Va., near MM 62.5. 

View is to the south. Photograph courtesy of the National 

Park Service. 
32 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor

A

B

Figure 38. Dam 6, which was 

located north of Great Cacapon, 

W. Va., near MM 134. A, Historical photograph (around 1900), 

looking south, of the woodplanked, stone-filled crib of 

Dam 6. B, Historical photograph 

(1938), looking north, of Dam 6 

after being breached in the 1936 

flood. In the upper-right background are west-dipping beds 

of Lower Devonian Oriskany 

Sandstone that provided rubble 

for the fill. Both photographs are 

courtesy of the National Park 

Service.
 

 

 

 

 

 

 

Geologic Setting 33 

A
 

B 

Figure 39. Historical photograph (1940) showing reconstruction of the towpath and 

canal at Widewater. The canal 

occupied an abandoned channel of the river. Photograph 

courtesy of National Park 

Service 

Figure 40. Excavation of the 

canal through bedrock. A, 

View of the towpath adjacent 

to the Potomac River at Big 

Slackwater (MM 87.5), where 

canal boats used the river. 

Limestone cliffs adjacent to 

the river prevented construc­

tion of the canal. B, West of 

Oldtown, Md. (MM 168), both 

the towpath and canal were 

excavated through thin-bed­

ded fissile shale of the Devo-

nian Mahantango Formation. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

34 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Geologic Guide 

For the following Geologic Guide, please refer to 

plates 2 through 20 for detailed geologic maps of the entire 

C&O Canal and the Potomac River corridor. An index to 

plates 2 through 20 is found on plate 1. Map-unit symbols 

shown in this guide correspond to those used on plates 2 

through 20. 

Piedmont Province—Potomac Terrane 

Georgetown Section of Washington, D.C., to 

Seneca, Md. (MM 0 to MM 23) 

Access Points 

Georgetown section of Washington, D.C., south of M 

Street, N.W. 

Clara Barton Memorial Parkway 

Carderock 

MacArthur Boulevard 

Great Falls Tavern Visitor Center 

Swains Lock, off River Road 

Watts Branch Park 

Pennyfield Lock, off River Road 

Violettes Lock, off River Road 

Rileys Lock, off River Road 

Introduction 

The Potomac River has cut a beautiful gorge through the 

complexly deformed crystalline metamorphic rocks of the 

Potomac terrane (fig. 41). These rocks formed when sediments 

and extrusive volcanic material were deposited in Neoproterozoic and Early Cambrian time (about 550 Ma); the rocks subsequently were transported and deformed along thrust faults. 

Debris shed from the thrust sheets mixed with unconsolidated 

sediments to form a tectonic mélange (mixture of rocks). A 

modern analog would be the rock debris that calves off of the 

front of an advancing glacier. 

Along the Potomac River, Drake (1989) mapped the 

Mather Gorge-Sykesville tectonic motif, which is a geologic 

structure that formed when rocks of the older Mather Gorge 

Formation overrode the younger Sykesville Formation along 

the Plummers Island thrust fault. The Neoproterozoic and (or) 

Lower Cambrian Mather Gorge Formation consists of quartzrich schist and metagraywacke (_Zmg) that has been sheared 

locally to form phyllonite (_Zmp). These rocks originally 

formed as turbidites deposited in a submarine fan (Drake and 

Morgan, 1981). The complexly deformed and polymetamorphosed Mather Gorge Formation locally contains migmatite 

(_Zmm), which is rock that was partially melted due to heat and 

pressure. The Mather Gorge Formation also contains mappable 

bodies of ultramafic rock (such as serpentinite, talc, actinolite 

schist mapped as _Zu), and amphibolite (_Za) that were derived 

from oceanic crust. The Lower Cambrian Sykesville Formation 

(_s) is a mélange that consists of quartz- and feldspar-rich sedimentary rock that contains fragments and blocks of phyllonite 

derived from the Mather Gorge Formation. The Sykesville Formation also contains exotic blocks and cobbles of granitoids, 

schist, and vein quartz that were incorporated into the giant submarine-slide deposit (Drake and Morgan, 1981). 

The Lower Cambrian Laurel Formation (_l, shown only 

on plate 1) of the Loch Raven-Laurel tectonic motif is exposed 

only at the ramp of Theodore Roosevelt Bridge in Washington, 

D.C. The Laurel Formation resembles the Sykesville Formation, but differs in that it contains more numerous and a wider 

variety of clasts. 

East of the Plummers Island thrust fault, near the American Legion Bridge, four groups of igneous plutonic rocks 

intrude the Sykesville Formation. They are, from oldest to 

youngest based on isotopic dating (Aleinikoff and others, 

2002), the Ordovician(?) Bear Island Granodiorite (Ob), and 

Early to Middle Ordovician Dalecarlia Intrusive Suite (Odm, 

Odt), Georgetown Intrusive Suite (Ogh, Ogb, Ogg, Ogu), and 

Kensington Tonalite (Ok). 

Regional metamorphism of the Mather Gorge Formation occurred at 490 Ma (Becker and others, 1993), which was 

before the Taconian orogeny. Metamorphic mineral assemblages in rocks of the Potomac terrane range from chlorite grade 

in the west to sillimanite grade in the east. Rocks of the Sykesville Formation are at biotite±garnet grade and rocks of the Laurel Formation are at biotite±garnet grade with local overgrowths 

of staurolite. The deformation of these rocks started in Cambrian time during eastward subduction (Drake, 1989). The rocks 

in the composite thrust sheets were then thrust onto the continental slope and rise deposits of the Westminster terrane during 

subduction related to the Ordovician Taconian orogeny (fig. 4). 

The spectacular scenery of the Potomac Gorge is a result 

of the fairly recent incision by the Potomac River. Several different levels of strath terraces are cut into the bedrock. They 

are mostly confined to the north shore of the Potomac River, 

but some may be seen on the islands. These terraces are nearly 

flat benches of bedrock with abundant potholes (see figures 8 

and 9) and channels (Zen, 1997a). Upland areas, such as Carderock and the Palisades, are built upon such terraces. Upstream 

of Great Falls, the river valley has a wide alluvial flood plain, 

except for the area around Blockhouse Point, which forms a 

promontory into the river. 

Lower Cretaceous sand and gravel of the Potomac Formation (Kp) occur as erosional patches in Arlington, Va., and 

Washington, D.C. Marine clay, silt, and sand of the lower and 

middle Miocene Calvert Formation overlie the Potomac Formation. Elsewhere, gravel, sand, silt, and clay deposits range 

in age from late Miocene to late Pliocene. These older deposits 

are more weathered than the latest Tertiary(?) to late Pleistocene 

alluvial terrace deposits (QTt) that occur at similar elevations. 
Piedmont Province—Potomac Terrane 35 

A 

A Piedmont province EXPLANATION Blue Ridge province

(for parts 

A and 

B) Frederick Valley Point of Rocks

Balls Bluff Member of the Bull Run Formation and the Manassas Formation, 

undivided (Upper Triassic)—Form lines indicate bedding attitude Westminster terrane MARYLAND

Felsic and mafic igneous intrusive rocks, undifferentiated (Ordovician) Bull Run Monocacy River Aqueduct Mountain FaultFrederick Formation (Upper Cambrian)Araby Formation (Middle and Lower Cambrian) N 

0 5 miles Chilhowee Group, undivided (Lower Cambrian)—Includes rocks of the

Loudoun, Weverton, Harpers, and Antietam Formations and Culpeper basin carbonaceous phyllite

Sykesville Formation (Lower Cambrian)—Diamictite

Seneca Mather Gorge Formation (Lower Cambrian and (or) Neoproterozoic)— 

Metagraywacke and schist. Form lines delineate boundaries between Potomac terraneschist and graywacke, and between migmatite and phyllonite VIRGINIA 

Ijamsville Phillite (Lower Cambrian? and Neoproterozoic?)Catoctin Formation (Neoproterozoic) 

Great Falls

Contact—Dashed where inferred Tavern Visitors

Center Fault—Dashed where inferred. Bar and ball on downthrown side. 

Sawteeth on upper plate. Arrows show relative motion 

Fold axis—Showing trace onlyProvince or section boundary 

FEET

10,000 

A
 

Point of Rocks Culpeper basin 

5,000

SEA

LEVEL

5,000

10,000 B 

Frederick Valley 

Westminster terrane 

Monocacy River (Aqueduct) Culpeper basin

Seneca 

Creek 

Aquaduct 

Great Falls 

Tavern 

Visitor Center 

A'
 

Potomac terrane 

Little Falls Dam 

Georgetown
 A'


Visitor Center
 

plate 7 plate 5 plate 4 plate 3 plate 2 

Figure 41. Generalized structure map and cross section of a portion of the C&O Canal National Historical Park and Potomac River corridor as it crosses the Piedmont province and easternmost part of the Blue Ridge province. 

A, Structure map showing fold axes and major faults. 

B, Cross section showing folds, faults, and other features from Rock Creek

in the Georgetown section of Washington, D.C., westward to Point of Rocks, Md. See plates 2 to 7 for detailed geologic maps of this portion. The location of plate 6 is not shown because of the bend in the riverGeologic units shown in cross section are generalized from the geologic map shown on plate 1. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

36 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Georgetown to Glen Echo Park Area (MM 0 to 

MM 7) 

The east end of the C&O Canal is at sea level at Tidewater Lock (Lock 1), which is located at the confluence of 

Rock Creek and the Potomac River (fig. 41 and plate 2) in 

the Georgetown section of Washington, D.C. Rock Creek was 

used as the canal for about 0.38 mi, beyond which the canal 

was excavated in bedrock. Near Lock 1 is a National Historical 

Marker attached to a block of the Lower Cambrian Sykesville 

Formation that shows rock fragments in a fine-grained matrix 

(fig. 42). Theodore Roosevelt Island and bluffs in Washington, D.C., and Arlington County, Va., are underlain by similar 

bedrock. Rocks of the Sykesville Formation (_s) first crop out 

in the canal bed at the National Park Service’s (NPS) Georgetown Visitor Center. Westward to near Three Sisters Islands 

are igneous rocks of the Early to Middle Ordovician Georgetown Intrusive Suite, which are mostly biotite-hornblende 

tonalite (Ogh), with some gabbro (Ogg) and biotite tonalite 

(Ogb) (fig. 43). Outcrops show drill holes from the canal excavation; locally, the canal berm was excavated in bedrock. 

The remains of the Alexandria Aqueduct, which carried 

boats across the Potomac River to Alexandria, Va., may be 

Figure 42. Part of a National Historical Marker where the C&O 

Canal enters Rock Creek. Dark and light olistoliths (rock fragments) occur in metagraywacke of the Lower Cambrian Sykesville 

Formation to which the marker is bolted. 

seen under Key Bridge. The lock stones consist of quartz fragments in a sandy matrix that are typical of the Sykesville Formation. Near MM 1.7, the canal is partially excavated into a 

bedrock terrace and the towpath is elevated; here the Capital 

Crescent Trail (a hike-and-bike trail) descends onto a younger 

lower terrace. Beginning about here are large bedrock terraces 

above and north of the canal that extend to near Widewater. 

From MM 2 to MM 5, the canal traverses rocks of the 

Sykesville Formation, which contains rare blocks of gabbro and amphibolite. The canal bed along this stretch locally 

was excavated in bedrock, but many cobbles of quartz derived 

from the Cretaceous and Pleistocene upland and terrace deposits litter the canal bottom (fig. 44A). Some of these cobbles 

are crossbedded sandstone (fig. 44B) that probably was transported by the Potomac River from the Valley and Ridge province west of Harpers Ferry (MM 110). 

Little Falls Dam at MM 5.8 is the first of the eight original dams that provided water to the canal. At about MM 6.5, 

good outcrops of light-colored monzogranite of the Early to 

Middle Ordovician Dalecarlia Intrusive Suite (Odm) may be 

seen along the canal. 

Glen Echo Park Area to Swains Lock (MM 7 to 

MM 16.6) 

From MM 7 to almost MM 10, the canal crosses rocks 

of the Lower Cambrian Sykesville Formation (_s) that locally 

were intruded by rocks of the Early to Middle Ordovician 

Georgetown and Dalecarlia Intrusive Suites (plate 3). From MM 

7.5 west to Interstate Route I–495, these rocks had to be drilled 

and blasted to create the canal. Between MM 9.5 and MM 10 

is a major rock boundary within the Potomac terrane (see figure 41). Metasedimentary rocks of the older Neoproterozoic 

and (or) Lower Cambrian Mather Gorge Formation were transported westward over rocks of the younger Sykesville Formation along the Plummers Island thrust fault. From here west to 

Violettes Lock (except for an area near MM 11.5) are metagraywacke and schist of the Mather Gorge Formation (_Zmg) that 

originated as turbidite deposits in deep marine water. Phyllonite 

(_Zmp), which was produced by shearing along the faults, and 

migmatite (_Zmm), which was produced by partially melting 

rock, also occur in this interval. Some of the best outcrops of the 

Mather Gorge Formation in this region are along this section of 

the Potomac River on Bear Island; to see them, take the Billy 

Goat Trail that begins near the east end of Widewater. Relatively 

undeformed rocks of the Sykesville Formation (fig. 45) become 

very sheared near the Plummers Island thrust fault (fig. 46). On 

bedrock islands in the Potomac River, the rocks are so sheared 

that the two formations cannot be readily distinguished. From 

about MM 9 to MM 11, the canal was excavated into the margin 

of a broad bedrock terrace above the Potomac River (at Carderock) and its berm is lined with rock. 

Building stone from the Sykesville Formation was quarried 

at the Potomac Granite Mill in the early 1900s near MM 11.7; 

the abandoned pits are immediately north of the canal. This 
Piedmont Province—Potomac Terrane 37 

A B
 

C D 

Figure 43. Igneous rocks of the Early to Middle Ordovician Georgetown Intrusive Suite near Key Bridge in Washington, D.C. The canal 

was excavated through these rocks, which include leucocratic and melanocratic varieties. A, Leucocratic (light-colored) granitoid. B, 

Closeup of A showing quartz and plagioclase crystals. Quarter for scale. C, Melanocratic (dark-colored) gabbro. D, Closeup of C showing the crystalline texture of amphibole and plagioclase crystals. Quarter for scale. 
 

 

 

 

 

 

 

 

 

 

38 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

A B 

Figure 44. Sandstone cobbles along canal. A, View of alluvial sandstone cobbles that litter the canal; cobbles were derived from terrace deposits near MM 5.5. B, Example of a cobble with crossbedded laminations. Cobbles probably were derived originally from early 

Paleozoic sandstone found west of Harpers Ferry, W. Va.; were carried by the Potomac River to this site; were next deposited as part of 

the terrace deposits; and, finally, were deposited in the canal by erosion of the terrace. Quarter for scale. 

Figure 45. Undeformed sedimentary mélange of the Lower Cambrian Sykesville Formation at MM 9.8 contains olistoliths (clasts) 

of older rocks derived from the Neoproterozoic and (or) Lower 

Cambrian Mather Gorge Formation. Lens cap for scale. 

Figure 46. Sheared mélange of the Lower Cambrian Sykesville 

Formation near MM 11.5. Note elongated fragments of quartzite 

and quartz in a matrix of quartz and feldspar above a bed of white 

quartzite. Knife and keys for scale. 

rock, which is a sedimentary mélange with large rock clasts, 

was long thought by geologists to be a granite that contained 

abundant xenoliths. The rocks of the Sykesville Formation, last 

seen just west of Interstate Route I–495, are exposed here again. 

The structurally overlying rocks of the Mather Gorge Formation have been eroded to expose the rocks beneath the Plummers Island thrust fault in a tectonic window (Drake and Froelich, 1997). As with the Plummers Island thrust fault near MM 

9.5, the fault that frames the tectonic window here is a penetrative shear zone in the rocks on either side of the fault. 

To the west of the parking area opposite Old Anglers Inn 

is Widewater (MM 13), which is a section of the canal that 

uses an abandoned channel of the Potomac River (figs. 12 and 

47). The towpath locally had to be constructed above the channel (see figure 39), but elsewhere it was excavated in the bedrock terrace. Bedrock is well exposed in this region (Reed and 

others, 1980). Folded vein quartz and phyllonite of the Mather 

Gorge Formation (_Zmp) show that deformation predates the 

intrusion of the Ordovician(?) Bear Island Granodiorite (Ob) 

(fig. 48). Upright antiforms consisting of isoclinal folds in meta­
A 

0 1 MILE N 

Great Falls Park 

Great Falls Tavern 

Towpath 

C&O Canal 

Widewater 

Bear Island 

Glade Hill 

B 

Figure 47. The Great Falls of the Potomac River. A, Stereoscopic 

pair of aerial photographs with some features labeled. B, Oblique 

aerial photograph (looking north). Photograph by Abbie Rowe 

(1959), courtesy of the National Park Service. 

 

 

Piedmont Province—Potomac Terrane 39 

Vein quartz 

Phyllonite 

A B
 

Granodiorite Phyllonite 

0 6 INCHES 

Figure 48. Phyllonite of the Neoproterozoic and (or) Lower Cambrian Mather Gorge Formation near MM 12.5. A, Phyllonite and folded 

vein quartz (white body outlined by black line). Binoculars for scale. B, Intrusion of undeformed Ordovician(?) Bear Island Granodiorite (Ob) 

(white body outlined by black line) into phyllonite . 
 

 

 

 

 

 

 

 

 

40 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

graywacke are seen opposite the concrete structure at the overflow of Widewater. Dismembered folds of migmatitic metagraywacke of the Mather Gorge Formation (_Zmg) are exposed on 

both sides of Widewater (fig. 49). Bodies of dark amphibolite 

(_Za) and light-colored Bear Island Granodiorite and pegmatite 

contrast with the gray migmatite (_Zmm) (fig. 50). The amphibolite is dark due to hornblende crystals (fig. 51). 

Vein quartz intruded the migmatite near Lock 15 (fig. 52). 

Between the canal and MacArthur Boulevard to the north, gold 

was discovered in similar vein quartz in 1861 and was mined 

sporadically until 1951 (fig. 53). Trenches and pits from this 

gold mining activity remain today (Reed and Reed, 1970). 

Near MM 13.8 is “Mary’s Wall” (fig. 33A) where a rock 

berm was required to keep water in the canal and floodwater of the Potomac River out of the canal. An elevated bedrock terrace formed across the islands and the opposite shore; 

the alluvial boulders deposited at the crest of a feature locally 

known as Glade Hill (figs. 13 and 14) in Great Falls Park, Va., 

are evidence of this terrace. On Bear Island and along Mather 

Gorge are some of the largest potholes formed by the ancient 

Potomac River (see figures 8 and 9). Rare Devonian lamprophyre dikes (Dl) composed of fine-grained biotite, quartz, and 

plagioclase are exposed in the bluff of Mather Gorge (fig. 54). 

The dikes intruded schist of the Mather Gorge Formation 

(_Zms) along a prominent set of fractures that have remained 

open for 360 m.y. in spite of intervening tectonic activity. 

The complex deformation that occurred before the Ordovician Taconian orogeny may be seen in many outcrops on 

Bear Island. The outcrops exhibit isoclinal folds of metagraywacke and schist that were refolded as upright antiforms 

and then intruded by Ordovician pegmatite and granodiorite 

(Drake, 1989) (fig. 55). From the approach to Great Falls Tavern and westward to MM 16, the canal was excavated in alluvium deposited on a bedrock terrace. 

Swains Lock to Seneca (MM 16.6 to MM 22.8) 

From Swains Lock to Violettes Lock (plate 4), bluffs 

consisting of metagraywacke and schist of the Neoproterozoic and (or) Lower Cambrian Mather Gorge Formation 

(_Zmg) were drilled and blasted to construct the canal. The 

best areas in which to see this feat of labor are near MM 19 

and at Blockhouse Point (MM 21), where bedded metagraywacke shows near-vertical drill holes (see figure 25B). The 

rocks along this section of the canal are complexly folded at 

all scales. Isoclinal folds of metagraywacke and schist are 

seen on the bluffs (fig. 56). Near Lock 23, the Upper Triassic rocks of the Culpeper basin unconformably overlie schist 

and metagraywacke of the Mather Gorge Formation. This 

contact represents an interval of more than 320 m.y. of geologic time. 

A B
 

Figure 49. Migmatite of the Neoproterozoic and (or) Lower Cambrian Mather Gorge Formation. A, Rootless, ductile folds. B, Thin layers 

of metagraywacke. Pocket knives for scale. 
A
 

B 

Piedmont Province—Potomac Terrane 41

Figure 50. Migmatite of the Neoproterozoic and (or) Lower Cambrian Mather Gorge Formation. A, Outcrop containing blocks of darkcolored amphibolite intruded by light-colored Ordovician(?) Bear Island Granodiorite on an island in Mather Gorge along the Potomac 

River south of MM 13. Photograph by David B. Usher (U.S. Geological Survey). B, Sketch of A with rock units labeled as follows: _Zmm, 

migmatite of the Mather Gorge Formation; _Za, amphibolite blocks within Mather Gorge Formation; Ob, Bear Island Granodiorite. 
42 

Amphibolite Pegmatite Amphibolite 

Pegmatite and 

vein quartz 

A B
 

Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Figure 51. Dark amphibolite within the Neoproterozoic and (or) Lower Cambrian Mather Gorge Formation intruded by white pegmatite 

of the Ordovician(?) Bear Island Granodiorite. A, Outcrop north of MM 13. B, Same outcrop as A shows amphibolite, which consists of 

crystals of hornblende (dark) and plagioclase feldspar (light); the light-colored veins of pegmatite from the Bear Island Granodiorite and 

quartz intrude the amphibolite. Pocket knife for scale. 

0 10 INCHES 

Figure 52. Milky white, 

fractured vein quartz intruding 

metagraywacke of the Neoproterozoic and (or) Lower Cambrian Mather Gorge Formation 

near Widewater near MM 13.5. 

Similar quartz contained gold 

veins and was mined to the 

immediate north. 
Trench along 

second vein 

Trench along 

main vein 

West-dipping 

Shaft 

MacArthur 

Shafts

Migmatite of the 

Mather Gorge 

Formation Main vein 

Tunnels 

N 

200 feet 

100 

200 feet 

100

200 feet 

100 

Main vein 

Boulevard 

50-foot level

100-foot level

135-foot level 

200-foot level 

main vein 

A 

B 

Figure 53. Underground gold mine north of Great Falls Tavern. 

A, Block diagram based on illustration from Reed and Reed (1970). 

B, Gold in a vein of quartz that measured 3 cm across. Photograph 

courtesy of National Park Service. 

Piedmont Province—Potomac Terrane 43 
 

Ob 

Ob 

Cleavage 

F1 

Intersection 

lineation 

Metagraywacke 

Schist 

Metagraywacke 

F2 

F2 

Schist 

Metagraywacke 

A B 

44 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Lamprophyre dikes in metagraywacke 

Figure 54. Late Devonian lamprophyre dikes intruding metagraywacke of the Neoproterozoic and (or) Lower Cambrian 

Mather Gorge Formation along 

a prominent fracture system 

(parallel to arrows) on Bear 

Island south of Lock 16 at MM 

13.8 (Reed and others, 1980). 

Photograph by David F. Usher 

(U.S. Geological Survey). 

Figure 55. Folding in metagraywacke and schist of the Neoproterozoic and (or) Lower Cambrian Mather Gorge Formation (south of 

MM 12.5 in Virginia). A, Outcrop revealing the following events: recumbent folding (F1), upright folding (F2), and then intrusion by Ordovician(?) Bear Island Granodiorite (Ob) which also includes pegmatite (Reed and others, 1980; Drake, 1989). Ruler for scale, left of center. 

Photograph by David F. Usher (U.S. Geological Survey). B, Sketch of A with features labeled as follows: Ob, Bear Island Granodiorite; F1, 

first-generation recumbent fold; F2, second-generation upright fold. 
 

Piedmont Province—Culpeper Basin, Westminster Terrane, and Frederick Valley 45 

A 

Metagraywacke 

beds 

Metagraywacke 

and schist 

beds 

Metagraywacke 

beds 

Metagraywacke 

and schist 

beds 

Fold axes 

Anticlinal axis 

B
 C
 

Figure 56. Folded metagraywacke of the Neoproterozoic and (or) Lower Cambrian Mather Gorge Formation, which is common from 

Great Falls Tavern to Blockhouse Point. A, Bluffs at MM 19. B, Isoclinal folds of quartzite at Blockhouse Point (MM 21). C, Same location 

as B, showing isoclinal upright folds. Lens cap for scale. 

Piedmont Province—Culpeper Basin, 

Westminster Terrane, and Frederick 

Valley 

Seneca, Md., to Point of Rocks, Md. (MM 22.8 to 

MM 48.2) 

Access Points 

Seneca, Md.
 

McKee-Besher Wildlife Management Area, Md.
 

Edwards Ferry, Md.
 

Whites Ferry, Md.
 

Dickerson Regional Park, Md.
 

Monocacy River Aqueduct, Md.
 

Nolands Ferry, Md.
 

Point of Rocks, Md.
 

Introduction 

From MM 22.8 to MM 48.2, the area of the C&O Canal 

National Historical Park encompasses three sections of the 

Piedmont province—the Culpeper basin, the Westminster terrane, and the Frederick Valley. The towpath actually traverses 

only the Culpeper basin and Frederick Valley, but other parts 

of the park are located in the Westminster terrane. A discussion of all three sections, from east to west, follows. 

Culpeper basin.—At the end of the late Paleozoic, a continental collision (the Alleghanian orogeny) formed the Appalachian Mountains. Some 10 m.y. later during the Mesozoic, extensional tectonics created rift basins on the continent, 

which eventually caused the Atlantic Ocean to open. These 

rift basins extended along the eastern edge of the Appalachian 

orogen from Florida to Newfoundland, Canada. One of these 

basins, the Culpeper basin, extends from central Virginia near 

Culpeper, to Frederick, Md. The Culpeper basin once was continuous with the Gettysburg basin to the north of Frederick, 

but the connecting rocks have since been eroded. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

46 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

The Culpeper basin is a half graben filled with strata that 

dip westward into a border fault at the base of Catoctin Mountain. Faulting occurred during and after sediment deposition in 

the basin. Much later, the Potomac River followed a southeasterly course from Nolands Ferry (MM 44.5) to the mouth of the 

Monocacy River (MM 42.2) along such a fault. 

The Upper Triassic rocks in this part of the Culpeper basin 

(fig. 41 and plates 4 through 7) consist of a basal conglomerate overlain by and interbedded with sandstone and siltstone that 

were deposited by rivers and lakes. There are two basal conglomerate units. The western part of the basin contains a variegated limestone conglomerate of the Tuscarora Creek Member 

of the Manassas Sandstone (^cmt). The limestone clasts within 

the conglomerate locally were derived from the Upper Cambrian 

Frederick Formation and the Upper Cambrian and Lower Ordovician Grove Formation. The basal conglomerate along the north 

and east margin of the basin is the Reston Member of the Manassas Sandstone (^cmr). This conglomerate is composed of locally 

derived quartz and schist of the Potomac and Westminster terranes. Overlying the Reston Member basal conglomerate is red 

sandstone of the Poolesville Member of the Manassas Sandstone 

(^cmp) (informally known as the “Seneca Red Sandstone”). This 

unit was quarried at Seneca and was one of the most widely used 

building stones along the C&O Canal; it also was used to build 

the original Smithsonian Institution building, popularly known 

as “the castle” in Washington, D.C. The grain size of the sandstone becomes finer upward in the section to become the Balls 

Bluff Member of the Bull Run Formation (^cbb) (Weems and 

Olsen, 1997). Interbedded with the Balls Bluff Member from 

near Whites Ferry, Md., to east of Furnace Mountain in Loudoun County, Va., is a variegated limestone conglomerate, the 

Leesburg Member of the Bull Run Formation (^cbl), which is 

similar to the Tuscarora Creek Member of the Manassas Sandstone. Historical maps show quarries in the Leesburg Member 

near Whites Ferry (Frye and Jefferson, 1775) and east of Point 

of Rocks, Md. (Keith, 1894; Scheel, 1995). These conglomeratic 

rocks are interpreted to be ancient debris-flow deposits on a large 

alluvial fan that extended eastward from a highland near presentday Catoctin Mountain (Smoot, 1989). Therefore, the limestone 

conglomerate of the Tuscarora Creek Member and the Leesburg Member may be continuous in the subsurface. A significant 

amount of Cambrian limestone must have been exposed on and 

west of Catoctin Mountain in order to supply the abundant limestone clasts for these units (Lindholm and others, 1979). 

Near-vertical planar dikes and thick subhorizontal sills of 

Early Jurassic diabase (Jd) intrude these Late Triassic sedimentary rocks. Outcrops are scarce and the dike rocks commonly 

occur only as a float of brown and gray cobbles and boulders. 

Locally, the sedimentary rocks are altered by contact metamorphism that occurred when the molten diabase was intruded. 

Within the Culpeper basin are some of the most extensive alluvial-terrace deposits of the Potomac River. About 2 

mi east of MM 37 and about 1 mi west of MM 41 in Loudoun 

County, Va., these deposits lie at 288 and 240 ft, respectively, 

above the Potomac River. Auger holes near Martinsburg, W. 

Va., reveal deposits that are more than 40 ft thick; relief on the 

terrace in Loudoun County suggests a thickness of over 100 ft 

(Southworth, 1998). 

The eastern margin of the Culpeper basin coincides with 

the location of Dam 2 (MM 22.1), which was built where the 

shallow, broad bed of the Potomac River crosses the resistant 

metasedimentary rocks of the Mather Gorge Formation at Seneca Falls (plate 4). The remains of a rubble dam, which was 

constructed by placing local boulders on the bedrock ledges 

in the riverbed, may be seen at Dam 2. On the Virginia shore 

at Dam 2, a skirting canal that was variably called the “Seneca 

Bypass,” “Seneca cut,” “Seneca break,” or “Washington cut,” 

was used by George Washington’s Patowmack Company before 

construction of the C&O Canal (Garrett and Garrett, 1987). 

Westminster terrane.—Phyllite, metalimestone, and 

metabasalt of unknown age occur north of the confluence of 

the Potomac and Monocacy Rivers (plate 7). These rocks are 

assigned, respectively, to the Neoproterozoic(?) and Lower 

Cambrian(?) Ijamsville Phyllite (_Zi), Silver Run Limestone, 

and Sams Creek Formation, and they were transported here 

along the Martic thrust fault. These rocks are interpreted to be 

deepwater deposits of the Iapetus Ocean that were thrust westward onto metasiltstone of the Lower and Middle Cambrian 

Araby Formation (_ar) of the Frederick Valley. The Araby Formation and overlying Upper Cambrian Frederick Formation 

were deposited on the continental shelf and subsequently were 

folded into the Frederick Valley synclinorium (Southworth, 

1996, 1998). The highly folded phyllite (fig. 57) and metalimestone (fig. 58) of the Ijamsville Phyllite are best exposed along 

the bluff of the Monocacy River immediately north of the canal 

near MM 42.5, but there are rare outcrops within the flood 

plain at Indian Flats. These rocks are truncated to the east along 

a Mesozoic normal fault that juxtaposes Upper Triassic sandstone (Poolesville Member of the Manassas Sandstone) against 

them. The terrace deposits at the north margin of Indian Flats 

flood plain contain blocks of quartzite as much as 10 ft long 

that were transported by the Potomac River during flood stages. 

Frederick Valley.—Early to Late Cambrian metasedimentary rocks are juxtaposed against older rocks of the east 

limb of the Blue Ridge-South Mountain anticlinorium along 

a Mesozoic normal fault that runs 22 mi from Furnace Mountain, Va., to Catoctin Furnace, Md. (plate 6). These rocks constitute the Frederick Valley synclinorium that is interpreted to 

have formed along with the Blue Ridge-South Mountain anticlinorium during late Paleozoic deformation of the Alleghanian orogeny. The oldest rocks are quartz-rich metasiltstone 

of the Lower and Middle Cambrian Araby Formation (_ar), 

which was deposited in a basin (Reinhardt, 1974). The type 

locality is to the north near the Monocacy River at Araby Station. Overlying these rocks is limestone of the Upper Cambrian Frederick Formation (_f, _fa, _fr), which marks the 

beginning of the development of the passive continental margin during Cambrian and Ordovician time. Mesozoic normal 

faults along the Potomac River cause the bedrock in Virginia 

to be quite different from that in Maryland. 

East of Catoctin Mountain in Virginia and Maryland are 

Cambrian rocks of the Antietam, Tomstown, and Frederick 
Piedmont Province—Culpeper Basin, Westminster Terrane, and Frederick Valley 47 

Formations that underlie the Furnace Mountain area. The ironand manganese-rich arkosic sandstone of the Lower Cambrian 

Antietam Formation (_ca) was mined in the 1800s for manganese. With the exception of the dark-gray, graphitic phyllite exposed in excavations, this sequence of rocks is the same 

as the sequence exposed on the westernmost limb of the Blue 

Ridge-South Mountain anticlinorium west of Harpers Ferry, 

W. Va. 

Highly cleaved, tan and gray metasiltstone of the Lower 

and Middle Cambrian Araby Formation (_ar) is exposed in 

the railroad cut opposite the broad flood plain from MM 42.6 

to MM 44 (fig. 59). Gray limestone of the Upper Cambrian 

Frederick Formation occurs in several varieties: thin-bedded 

limestone (_f) is exposed along New Design Road and along 

Tuscarora Creek near Nolands Ferry (fig. 60A), limestone conglomeratic breccia (_fr) is exposed near the Monocacy River 

(fig. 60B), and thin, folded argillaceous limestone interbedded with shale (_fa) is exposed in Virginia (fig. 60C). Similar limestone was used to build the water treatment facility 

along the towpath near MM 44.5. North of Nolands Ferry, the 

limestone is overlain by patches of limestone conglomerate of 

the Upper Triassic Tuscarora Creek Member of the Manassas 

Sandstone (^cmt) as well as by coarse Quaternary alluvium. 

Of note is the conical hill covered with rounded cobbles of 

quartzite and sandstone north of the intersection of Maryland 

Route 28 and New Design Road (fig. 61). The conical shape of 

the hill and the cobble deposit resulted when coarse alluvium 

filled a sinkhole in limestone. The resistant quartz cobbles in 

the alluvium protected the otherwise soluble limestone below. 

As erosion progressed, the limestone elsewhere dissolved, 

except for the protected depression. This process resulted in a 

topographic inversion; that is, the depression became a hill. 

Seneca to Edwards Ferry (MM 22.8 to MM 30.8) 

The nature of the Potomac River changes dramatically 

here (fig. 62). To the west of Seneca Creek, the river valley is 

broad with many elevated terraces, but to the east, the Potomac 

has cut a gorge with abundant outcrops forming the falls and 

islands. 

The Upper Triassic basal conglomerate of the Reston 

Member of the Manassas Sandstone (^cmr) is poorly exposed 

north of River Road and contains cobbles of vein quartz and 

schist (derived from the underlying Neoproterozoic and (or) 

Lower Cambrian Mather Gorge Formation) in a matrix of 

arkosic sandstone (fig. 63). Near MM 23, the Seneca Creek 

Aqueduct (fig. 35), lock keeper’s house, and Seneca Stone 

Cutting Mill (fig. 28) are constructed from the red sandstone 

of the Upper Triassic Poolesville Member of the Manassas 

Sandstone (^cmp) (informally known as “Seneca Red Sandstone”). This rock was quarried extensively on the Potomac 

River bluffs west of the aqueduct. The quarries opened in 

1774 and operated until 1898 (Davies, 1999). The large water 

impoundment between the towpath and bluffs was constructed 

as a holding basin for the barges that transported the stone. 

Along the bluffs from here to MM 24 are abandoned sandstone quarries, at least one of which also contained copper 

deposits. 

Near MM 24, siltstone is more dominant than sandstone, 

so the rocks are assigned to the Upper Triassic Balls Bluff 

Figure 57. Sheared phyllite (to the right of the hammer) and 

folded white vein quartz (to the left of the hammer) of the Neoproterozoic(?) and Lower Cambrian(?) Ijamsville Phyllite that lies 

above the Martic thrust fault along the Monocacy River, north of 

Indian Flats campsite (MM 42.5). 

Figure 58. Tightly folded, thin-bedded limestone within the Ijamsville Phyllite along the Monocacy River. Hand lens for scale. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

48 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Figure 59. Metasiltstone of the Lower and Middle Cambrian Araby 

Formation showing foliation with near-vertical cleavage along the 

CSXT railroad north of MM 43. View is about 1 foot across. 

Member of the Bull Run Formation (^cbb). Finer grained 

siltstone is rather poorly exposed along the canal but can be 

seen at the member’s type locality in the bluffs of Balls Bluff 

National Cemetery in Virginia, across the river from MM 34. 

From MM 24 to MM 30, the canal and towpath cross the 

broad flood plain within the McKee-Besher Wildlife Management Area. The lowland required berms to be built above the 

canal to keep out floodwater and overbank deposits of silt. 

Near the Chisel Branch campsite, west of MM 30, is the 

only siltstone exposed until almost MM 38. At Chisel Branch, 

the rock had to be drilled and blasted to construct the canal. 

Just to the west, the Goose Creek River Lock (Lock 25) connected the C&O Canal with the Goose Creek and Little River 

Navigation (Trout, 1991), a canal that was used to transport 

goods as far away as Middleburg, Va. 

Edwards Ferry to Monocacy Aqueduct (MM 30.8 

to MM 42.2) 

Near MM 32 are the remains of the Broad Run Trunk 

(Aqueduct), the only aqueduct along the canal constructed 

from wood. To the west beyond MM 32.5, the canal and towpath cross the flood plain. Scattered on the flood plain are large 

blocks of quartzite that were transported by the Potomac River 

during floods. At Whites Ferry (MM 35.5) are the remains of a 

large bridge, constructed from red sandstone, that carried ferry 

users across the canal to the river. From near MM 38 to west of 

the Marble Quarry campsite (plate 6), graded beds of limestone 

conglomerate (Upper Triassic Leesburg Member of the Bull 

Run Formation, ^cbl) (fig. 64) are interbedded with siltstone and 

sandstone. These conglomerates are ancient distal debris flows 

that brought carbonate clasts derived from north and west of 

Catoctin Mountain into a basin of muddy silt. An Early Jurassic 

diabase dike (Jd) intrudes the conglomeratic rock. Also present 

within the Leesburg Member are limestone clasts derived from 

the Frederick Formation. These clasts were metamorphosed 

into white marble that was quarried for agricultural lime. Native 

Americans lived on the islands and along the shore of this section of the Potomac River; some of their most well preserved 

fish weirs (fig. 10) are found here (Scheel, 1995; Guzy, 2000). 

From near MM 41 north to the Monocacy River good 

outcrops of siltstone interbedded with sandstone of the Poolesville Member of the Manassas Sandstone (plate 7) may be 

seen. These outcrops exhibit drill holes and the remnants 

of small quarries (fig. 64). The Monocacy River Aqueduct 

(fig. 36) was constructed from white Lower Cambrian(?) Sugarloaf Mountain Quartzite quarried from the southern part of 

Sugarloaf Mountain in Frederick County, Md., and transported 

here by horse and wagon and by light rail. 

Monocacy Aqueduct to Point of Rocks (MM 42.2 

to MM 48.2) 

From MM 42 to near MM 47.5, the canal and towpath 

cross a broad flood-plain deposit on Triassic rocks that are, 

in part, downfaulted against older rocks of the Piedmont to 

the north (plate 7). Limestone conglomerate of the Upper 

Triassic Tuscarora Creek Member of the Manassas Sandstone (^cmt) crops out on the flood plain of Indian Flats and 

looks identical to conglomerate of the Upper Triassic Leesburg Member of the Bull Run Formation (^cbl) that occurs 

near MM 47. The Leesburg Member, which was informally 

called “Potomac marble” or “Calico rock” was quarried here 

for the columns of Statuary Hall in the U.S. Capitol building 

in Washington, D.C. (fig. 65B). Indian Flats was undoubtedly 

the site of a major village of Native Americans. Upstream 

of MM 43, a complex fish weir may be seen in the Potomac 

River (fig. 9). 

From about MM 47.5 to MM 48.2, the area is underlain 

by limestone of the Lower Cambrian Tomstown Formation 

(_t) (fig. 66A), which is exposed only in the Virginia portion 

of the map. Extensive alluvial and colluvial deposits (fig. 66B) 

mantle the bedrock so that only water-well records and drill 

core (Hoy and Schumaker, 1956) reveal its presence in Maryland. Kanawha Spring (also known as “Big Spring”) discharges from the limestone near MM 47.5. Stratigraphically 

beneath the Tomstown Formation is a dark, carbonaceous, 

and graphitic phyllite unit that produces a distinctive ashy soil 

where exposed in excavations (fig. 67A). Similar black shales 

are found locally within the limestone of the Upper Cambrian Frederick Formation (Southworth and Brezinski, 2003). 
C 

A B

Figure 60. Examples of the varieties of limestone in the Upper 

Cambrian Frederick Formation. A, Thin-bedded limestone is 

exposed in Tuscarora Creek north of Nolands Ferry. Pocket knife 

for scale, top center. B, Platy limestone conglomerate breccia 

near the Monocacy River, about 2 mi north of the map area. Brunton compass for scale. C, Folded, thin limestone beds interbedded 

with shale south of the Potomac River in Virginia, opposite MM 

47.5. 

Piedmont Province—Culpeper Basin, Westminster Terrane, and Frederick Valley 49 

A B 

Figure 61. Conical hill north 

of Nolands Ferry and Maryland Route 28 representing 

topographic inversion. A, 

Broad view of the hill showing 

its conical form. B, Closeup 

view of the slopes of the conical hill showing the cobbles of 

sandstone and quartzite that 

were deposited in a sinkhole 

in limestone. The surrounding limestone has since been 

eroded while the resistant 

sandstone and quartzite 

gravel remained and armor 

the hill. 
50 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

N 

Dam 2 

Canal basin 

Culpeper 

basin 

Fault 

Seneca, Md. 

Potomac 

terrane 

Blockhouse 

Point 

Seneca 

Break 

C&O Canal 

0 1 MILE 

Figure 62. Stereoscopic pair of aerial photographs showing the change in the morphology of the Potomac River valley in the Piedmont 

province across the boundary (shown by the solid line) between the Culpeper basin (west) and Potomac terrane (east). Note that the 

Potomac terrane is dissected and the Culpeper basin has a broader flood plain. The tonal change in the river is Seneca Falls and the 

rubble remains of Dam 2. Seneca Break was a skirting canal constructed around Seneca Falls by George Washington’s Patowmack 

Company; canal towpath indicated by dashed white line. 

Figure 63. Conglomerate of the Upper Triassic Reston Member 

of the Manassas Sandstone that forms the base of the Culpeper 

basin. The conglomerate includes rounded cobbles of vein quartz 

and metamorphic rocks. Knife and keys for scale. 
A B

Figure 64. Limestone conglomerate interbedded with red siltstone of the Upper Triassic Leesburg Member of the Balls Bluff Siltstone. 

A, Outcrop of Leesburg Member in bluffs opposite Marble Quarry campsite. Hammer’s head lies on an Early Jurassic diabase dike that 

intruded the conglomerate and locally metamorphosed the limestone clasts to marble. B, Conglomerate of the Leesburg Member, which 

was excavated to create the floor of the canal. Quarter for scale.

A B

Figure 65. Limestone conglomerate of the Upper Triassic Leesburg Member of the Bull Run Formation. A, Conglomerate near MM 47. 

The canal was excavated through this unit, and it was quarried to the immediate east. B, Ornamental columns of Statuary Hall in the 

U.S. Capitol Building formed from the Leesburg Member. Photograph courtesy of the National Park Service.

Piedmont Province—Culpeper Basin, Westminster Terrane, and Frederick Valley 51
52 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

The graphitic phyllites were deposited over metasandstone of 

the Lower Cambrian Antietam Formation (_ca). These dirty 

metasandstones (fig. 67B) are exposed in excavations along 

Furnace Mountain in Virginia. They are not composed of clean 

quartz such as the metasandstone of the Antietam Formation 

found west of Harpers Ferry, W. Va., but are more similar to 

the rocks of the Araby Formation. The facies change within 

the Antietam Formation across the Blue Ridge province probably reflects a depositional change from a shallow-marine to a 

deep-water-slope environment. 

A B
 

Figure 66. Lower Cambrian Tomstown Formation. A, Folded beds of limestone and marble of the Tomstown Formation, which only 

crops out south of the Potomac River in Loudoun County, Va. B, Colluvium of sandstone and vein quartz covering Tomstown Formation in 

Maryland. The Tomstown is known in Maryland only from water-well records and drill cores. 

A B
 

Figure 67. Lower Cambrian rocks of the Blue Ridge province. These two rock units are not exposed along the canal and river, but are 

within the map area along U.S. Route 15 in Loudoun County, Va. A, Dark-gray to black, carbonaceous phyllite unit which forms ashy soil. 

B, Arkosic metasandstone of the Antietam Formation, which was quarried for manganese in the 1700s along Furnace Mountain just 

south of the river. In Pennsylvania, this unit contains trilobites and is the oldest fossil-bearing rock known in this region. 
 

 

 

 

 

 

 

Blue Ridge Province 53 

Blue Ridge Province 

Point of Rocks, Md., to Fort Duncan (MM 48.2 to 

MM 63) 

Access Points 

Point of Rocks, Md. 

Lander, Md. 

Brunswick, Md. 

Weverton, Md. 

Sandy Hook, Md. 

Harpers Ferry, W. Va. 

Pleasantville, Md. 

Introduction 

Some of the oldest rocks in the Appalachian Mountains are exposed in the Blue Ridge province. The main structure is a large overturned fold known as the Blue Ridge-South 

Mountain anticlinorium. This anticlinorium is defined by one 

eastern and two western flanks that underlie Catoctin Mountain, South Mountain-Short Hill Mountain, and Blue RidgeElk Ridge, respectively (fig. 68); the latter two pairs of mountains are continuous mountain ridges breached by the Potomac 

River. The rocks were metamorphosed, folded, faulted, and 

transported westward during the late Paleozoic Alleghanian 

orogeny, which was the result of continental collision of North 

America with Africa. 

The rocks underlying Catoctin Mountain on the east limb 

of the anticlinorium consist of resistant metabasalt of the Neoproterozoic Catoctin Formation (Zcm) and quartzite of the 

Lower Cambrian Weverton Formation (_cw). Likewise, the 

same rocks underlie South Mountain-Short Hill Mountain and 

Blue Ridge-Elk Ridge to the west, but the strata there are more 

folded and are overturned. Blue Ridge-Elk Ridge is almost 

a replica of South Mountain-Short Hill Mountain because a 

regional fault duplicates the strata and resultant ridges. The 

core of this anticlinorium contains 1.1 Ga granitic rocks that 

were metamorphosed and deformed during the Mesoproterozoic Grenville orogeny (Burton and Southworth 1996; Southworth and others, 2006). These unnamed granitic rocks are 

gneissic, well layered, and include the following varieties: biotite granite gneiss (Ybg), leucocratic metagranite (Yg), garnetiferous leucocratic metagranite (Ygt), and hornblende monzonite 

gneiss (Yhg). The 1.1 Ga age of these rocks was determined by 

using uranium-lead (U-Pb) dating techniques on zircon crystals that formed when the intrusive granite crystallized (Aleinikoff and others, 2000). 

The first strata deposited over the granitic basement rocks 

were fluvial deposits that were later lithified and metamorphosed to form quartzite, metasandstone, schist (Zss); phyllite 

(Zs), and rare marble (Zsm) of the Neoproterozoic Swift Run 

Formation. The type locality of these rocks is Swift Run Gap 

in the Shenandoah National Park to the southwest. The Catoctin Formation, which overlies the Swift Run Formation, is 

composed of metabasalt (metamorphosed basalt) (Zcm), lightcolored metarhyolite, and some metasedimentary phyllite (Zcs) 

and marble similar to that found within the underlying Swift 

Run Formation. The formation is named after Catoctin Mountain in Maryland. The age of the volcanic rocks ranges from 

571 Ma (from a rhyolite dike exposed along the Potomac 

River opposite MM 52.5) to 565 Ma (from metabasalt in the 

Shenandoah National Park area) (Badger and Sinha, 1988; 

Aleinikoff and others, 1995). 

Many metadiabase dikes (Zmd) intruded the granitic 

rocks within the core of the anticlinorium. These dikes represent fissures that fed basaltic lava through the granite to the 

surface where it was extruded as lava flows. The lava flows 

on top of the clastic sediments that would later become rocks 

of the Swift Run Formation. The fissures and volcanic activity were the result of continental rifting of the land mass 

referred to as Laurentia. The rifting in turn resulted in the formation of the ancient Iapetus Ocean between approximately 

600 and 545 Ma. 

Fossil soil, volcanic tuff, silt, and gravel were lithified to 

collectively make up the rocks of the overlying Lower Cambrian Loudoun Formation (_clc, _clp). These rocks represent a 

dramatic change in the depositional environment from a volcanic landscape to one dominated by rivers and deltas. 

Fluvial sand and gravel shed from the continent eastward to its shore were lithified to form light and dark quartzite of the Lower Cambrian Weverton Formation (_cw), 

whose type locality is Weverton Cliffs, north of Lock 31. 

Silty rocks within the quartzite increase stratigraphically 

upward to form the Lower Cambrian Harpers Formation (_

ch, _chq), whose type locality is along the Shenandoah River 

in Harpers Ferry National Historical Park in West Virginia, 

south of MM 61. These dark, fine-grained rocks that form 

the bold cliffs along the Potomac River at Harpers Ferry 

probably are deltaic deposits (Southworth and Brezinski, 

1996a). The siltstones of the Harpers are interbedded with 

thin beds of sandstone near the top of the formation. Where 

the sandstone rocks become dominant, the rocks are assigned 

to the Lower Cambrian Antietam Formation (_ca), which is 

named after Antietam Creek to the north where there are typical exposures. The tan metasandstone of the Antietam contains the trace fossil Skolithos linearis. In rocks of the Antietam Formation in Pennsylvania, the Olenellus trilobite is the 

oldest shelly fossil recognized in all of the rocks of the Blue 

Ridge province. Collectively, the Loudoun, Weverton, Harpers, and Antietam Formations are part of the geographically 

more extensive Chilhowee Group, which is named after Chilhowee Mountain in Tennessee. The Olenellus trilobite is 

the basis for the Early Cambrian age designation (Stose and 

Stose, 1944). 

The western boundary of the Blue Ridge province generally is considered to be the contact between the Antietam 

Formation and the overlying limestone of the Lower Cam­
54 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

A 

Blue Ridge province 

Weverton 

Brunswick 

Point of Rocks 

Harpers 

Ferry 

Great 

Valley 

section 

N 

0 5 MILES 

Piedmont 

province 

A 

A' 

Bull Run Mountain fault 

EXPLANATION 

(for parts A and B) 

Balls Bluff Formation of the Bull Run Formation and the 

Manassas Formation, undivided (Upper Triassic) 

Catoctin Formation (Neoproterozoic) 

Leucocratic matagranite (Mesoproterozoic) 

Contact—Dashed where inferred 

Fault—Dashed where inferred. Bar and ball on downthrown 

side. Sawteeth on upper plate. Arrows show relative motion 

Fold axis—Showing trace only 

Province or section boundary 

Yg Yg Yg 

Zc 

?cw 

Zc 

?cw 

?ch 

?ch ?clp 

?ca 

?t 

?ch 

?ca 

A' FEET 

SEA 

LEVEL 

5,000 

10,000 

5,000 

10,000 

plate 8 

A 

Blue Ridge province 

Great Valley 

section 

figure 83 figure 77 

plate 9 

figure 73 

Harpers Ferry

Weverton

Brunswick

Point of Rocks

Piedmont province 

B 

Figure 68. Generalized structure map and cross section of a portion of the C&O Canal National Historical Park and Potomac River corridor as it crosses the westernmost part of the Piedmont province, the Blue Ridge province, and the easternmost part of the Great Valley 

section of the Valley and Ridge province. A, Structure map showing fold axes and major faults. B, Cross section showing folds and faults 

from Point of Rocks, Md., westward to just past Harpers Ferry, W. Va. See plates 8 and 9 for detailed geologic maps of this portion. Geologic units shown in cross section are generalized from the geologic map shown on plate 1. 
Blue Ridge Province 55 

brian Tomstown Formation (_t) of the Great Valley section. 

The eastern boundary of the province is the foot of the slope 

of Catoctin Mountain, west of the border fault of the Culpeper 

basin. The complex geologic history of the region is illustrated 

in figure 4. 

Point of Rocks to Brunswick (MM 48.2 to MM 55) 

Point of Rocks is a steep bluff of Neoproterozoic Catoctin Formation metabasalt (Zcm) that was exposed when the 

Potomac River cut a water gap through Catoctin Mountain 

(plates 7 and 8, fig. 69A). The Potomac River and flood plain 

widen as the less resistant rocks of the Piedmont are crossed 

(fig. 70). The steep bluff, which gives Point of Rocks its name, 

left little room for both a canal and railroad, so a tunnel was 

excavated through the metabasalt for the railroad. Just east of 

the bridge, folded rocks of the Lower Cambrian Harpers Formation (_ch) were juxtaposed against the Catoctin Formation along a normal fault at about 200 Ma. Metabasalt crops 

out in the middle of the Potomac River and is the dominant 

rock exposed from Point of Rocks to Lander (MM 51). At the 

railway tunnel (MM 48.4), both metabasalt with gas vesicles 

(fig. 69B) and intrusive vein quartz were folded and broken by 

faults (fig. 69C). 

The linear creek that drains into the Potomac River 

immediately east of Lander flows along the contact between 

metabasalt of the Catoctin Formation and Mesoproterozoic 

granite gneiss (Yg). Locally, the metabasalt contains small 

pods of marble (fig. 71). From here west to Brunswick and 

beyond to Weverton, the bedrock is a granite gneiss (Yg) 

intruded by metadiabase dikes (Zmd) (fig. 72). These poorly 

exposed rocks can be seen just east of Catoctin Creek in the 

railroad cut at MM 51.5. 

Quaternary cobbles and boulders of sandstone and 

quartzite that litter the embankment here were deposited by 

the Potomac River in one of its now-abandoned meanders. 

The Catoctin Creek Aqueduct (at about MM 51.6) collapsed 

in 1973 when floodwaters scoured its supports. The remains 

of the aqueduct, which was constructed from Ordovician Ellicott City Granodiorite, are present beneath the footbridge. 

From Catoctin Creek to Brunswick is a large rail yard that 

takes advantage of the broad low terrace of the flood plain of 

the Potomac River. 

Brunswick to Fort Duncan (MM 55 to MM 62.5) 

From Brunswick to Knoxville (at about MM 57) and 

beyond to Weverton (at about MM 58), the canal and towpath 

are constructed on a broad low terrace of the Potomac River 

(plate 9). Rocks of the Lower Cambrian Weverton Formation 

(_cw) are located at Weverton Cliffs, which is at the southern 

end of South Mountain, east of MM 58, where the Potomac 

River cut through the western limb of the Blue Ridge-South 

Mountain anticlinorium. This ridge is underlain by folded and 

A
 

B
 

C
 

Figure 69. Neoproterozoic Catoctin Formation. A, Historical 

photograph (around 1920) of Point of Rocks, Md., showing cliffs of 

the Catoctin Formation that form the north side of the water gap 

of Catoctin Mountain. Note railroad tunnel entrance on right side 

of photograph. Photograph by Consolidated Coal Co.; courtesy of 

National Park Service. The Catoctin contains a variety of rocks 

as illustrated in B and C. B, Vesicular metabasalt. C, Schist and 

phyllite containing folded vein quartz. The vein quartz intruded the 

Catoctin sometime before the late Paleozoic Alleghanian orogeny. 
56 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

C&O Canal 

Point of Rocks 

0 ~1 MILE N 

Blue Ridge province 

Piedmont province 

Figure 70. Stereoscopic pair 

of aerial photographs and 

sketch centered on Point of 

Rocks, Md., which is on the 

north side of the water gap 

in Catoctin Mountain. A fault 

separates the Blue Ridge and 

Piedmont provinces; bar and 

ball are on downthrown block. 

Figure 71. Pods of marble in the Neoproterozoic Swift Run Formation found along the creek northeast of Lander, Md., and east of 

Short Hill Mountain to the west. The pods probably resulted from 

deposits of shallow-water limestone. 

Figure 72. Basement rocks of the Blue Ridge province. The core 

of the anticlinorium in this area consists of Mesoproterozoic light 

granitic gneiss intruded by numerous dark metadiabase dikes that 

probably fed the basalt flows of the Neoproterozoic Catoctin Formation. Camera lens for scale. Photograph by William C. Burton 

(U.S. Geological Survey). 

faulted quartzite and metasiltstone of the Weverton Formation and the rocks are mostly overturned to the west (fig. 73) 

(Southworth and Brezinski, 1996b). Coarse pebble conglomerate of the Lower Cambrian Loudoun Formation (_clc) (fig. 74) 

is found at the base of the Weverton Formation north of the 

Potomac River, but north of MM 57.5, only phyllite (_clp) 

is exposed. The clasts in the conglomerate consist mostly of 

quartz, but red jasper and metabasalt also are present and probably were derived from erosion of the underlying Neoproterozoic Catoctin Formation. 

The stone remains of Weverton Mills (MM 57.8), which 

was constructed in 1834 for manufacturing, may be seen along 

the Potomac River (lower left part of figure 73A). The bedrock 

ledge in the river and the mill ruins consist of quartzite. The 

sandy flood plain from here to Knoxville was the location of a 

“hobo jungle” during the Great Depression. 

Just west of Lock 31, the poorly exposed trace of the 

Short Hill-South Mountain fault duplicates the stratigraphic 

sequence of the west limb of the Blue Ridge-South Mountain 

anticlinorium by placing phyllitic metasiltstone of the Harpers Formation (_ch) over garnetiferous leucocratic granite 

(Yg) to the west. Drill core from a spot immediately north of 

the canal indicates that the fault is an early Paleozoic normal 

fault that was folded and reactivated as a thrust fault in the late 
Blue Ridge Province 57 

A
 

West East 

Colluvium 

Cleavage 

Bedding 

Thrust 

fault 

Cleaved 

metasiltstone 

of the Harpers 

Formation 

Quartzite and siltstone 

of the Weverton Formation 

Phyllite of 

the Catoctin 

Formation 

B 

Figure 73. Weverton Cliffs at the southern end of South Mountain in Maryland. A, White rocks are outcrops of quartzite beds of the 

Lower Cambrian Weverton Formation. In the left foreground of the photograph are the stone ruins of the Weverton mills. B, Sketch of 

Weverton Cliffs showing geologic units and other features. 

Paleozoic (Southworth and Brezinski, 1996a,b). This fault is 

unique to the region in that there is no other evidence of a normal fault and extensional event between the Neoproterozoic 

(760–542 Ma) and Jurassic (about 220–200 Ma) Periods. 

From MM 58 westward to Harpers Ferry, W. Va. (MM 

60.5), the rock sequence that is crossed from Lander to Weverton (granitic gneiss intruded by metadiabase dikes, overlain by 

metabasalt, quartzite, and metasiltstone) is repeated. Outcrops 

of gneiss and intrusive metadiabase dikes are covered with 

soot along the adjacent railroad tracks to the north, but the 

Mesoproterozoic (1 Ga) folded, light-colored, garnetiferous 

granite (Ygt) can be seen in fresh exposures nearby (fig. 75). 

The rust-colored massive body of rock exposed near MM 59 is 

a Jurassic diabase dike (Jd) that intruded the older rocks when 

North America was rifting apart to create the Atlantic Ocean; 

a pyroxene crystal in the diabase here was dated at 200 Ma 
58 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

using the technique of analyzing the ratio of argon isotopes 

(Kunk and others, 1992). 

The flat uplands above the bluffs in Maryland and Virginia resemble river terraces, but no such deposits have been 

found. Chips of Ordovician Ellicott City Granodiorite along 

the towpath near Lock 32 (about MM 60.5) suggest that some 

of the dimension stone was transported here from Ellicott City, 

Md., as rough blocks that were dressed at the site. 

The Harpers Ferry area is a spectacular water gap, 

described by Thomas Jefferson as “worth the voyage across 

the Atlantic” (Jefferson, 1982). A stereoscopic pair of aerial 

photographs (fig. 76) shows the three-dimensional perspective of the gorge created where the Potomac and Shenandoah 

Rivers converge to cross the Great Valley section into the Blue 

Ridge province (fig. 76). Maryland Heights and Blue RidgeElk Ridge are composed of erosion-resistant, hard quartzite and metagraywacke of the Weverton Formation. Exposures of folded and faulted rocks and coarse colluvium may 

be seen in the blufs above the river (figs. 77 to 80). Figure 81 

shows the westernmost, southward-plunging, overturned anticline exposed during low-water conditions in the Potomac 

River (see figure 76). The crossbedded quartzite provides good 

examples of the overturned, folded strata (fig. 82). Much of 

the canal across this section was excavated in this quartzite. 

According to local folklore, workers built fires on the quartzite bedrock and then quenched them with cold water hauled 

from the river; this process shattered the quartzite. A tunnel 

was required to accommodate both the canal and the B&O 

Railroad. The west portal to the tunnel (MM 60.5) is along the 

near-vertical contact between the Weverton Formation and the 

Figure 74. Polymictic conglomerate of the Lower Cambrian Loudoun Formation north of the Potomac River on the east flank of 

South Mountain. This unit was deposited locally stratigraphically 

above phyllite and metabasalt of the Neoproterozoic Catoctin 

Formation. The conglomerate may represent alluvial-fan and 

channel deposits that preceded the deposition of the quartz-rich 

sediments of the Lower Cambrian Weverton Formation. 

A B 

Figure 75. Mesoproterozoic rocks between Weverton, Md., and Harpers Ferry, W. Va. A, Historical photograph (date unknown) showing bluffs of Mesoproterozoic granitic gneiss along the B&O railroad and C&O Canal west of Weverton. These rocks are now covered 

with soot. Photograph courtesy of the National Park Service. B, Fresh exposure of garnetiferous leucocratic granite in the roadcut 

of southbound U.S. Route 340, north of Potomac River. Uranium-lead dating of zircon crystals (Aleinikoff and others, 2000) shows that 

this granite formed over 1 billion years ago and was then metamorphosed and folded during the Grenville orogeny. Shiny graphite and 

almandine garnet in this rock suggest that an older suite of metasedimentary rocks melted to form the granite. 
Blue Ridge Province 59 

0 ~1 MILE 

N 

Elk Ridge 

Harpers 

Ferry 

Dam 3 

C&O Canal 

Blue Ridge 

Potomac 

River

Shenandoah 

River 

Potomac 

River 

Figure 76. Stereoscopic pair of aerial photographs and sketch centered on Harpers Ferry, W. Va., showing the Potomac River gorge 

and water gap. 

stratigraphically overlying Lower Cambrian Harpers Formation (_ch). 

The point of land at the confluence of the Potomac and 

Shenandoah Rivers was subjected to extensive erosion during 

the numerous floods that affected the area. Historical photographs (Hahn, 1995) show many structures of the canal at this 

location that no longer exist because they were destroyed by 

flooding. 

From Harpers Ferry westward to Lock 35 (MM 62.2), the 

rocks consist of highly cleaved and folded metasiltstone of the 

Harpers Formation (fig. 83). The canal along this stretch was 

excavated locally in the metasiltstone and drill holes can still 

be seen in the outcrops that served as canal berms (fig. 84A). 

Slabby, consolidated colluvium was used to build walls around 

the outcrops. A good example of bedding and cleavage may be 

seen in an outcrop of Harpers Formation along the berm of the 

canal near MM 61 (fig. 84B). Locally, the cleavage is folded 

by later folding or faulting, such as seen above the canal near 

MM 61.1 (fig. 85). 

Lumpy, swirly patterns may be seen in blocks of limestone from the Lower Cambrian Tomstown Formation in 

Locks 34 to 36. These features are bioturbation structures that 

formed when marine organisms mixed the sediments while 

they were still soft and unconsolidated. The few blocks of 

white Keedysville marble (informal name) in Locks 34 to 36 

are from the Bolivar Heights Member at the base of the Tomstown Formation. The Keedsyville marble originally was a 

limestone that became detached from the underlying Lower 

Cambrian Antietam Formation and recrystallized as it moved 

along a regional fault (Brezinski and others, 1996). All of the 
West East 

Blue Ridge Bedding Maryland 

Cleaved Heights 

metasiltstone 

of the Phyllite of Harpers the Loudoun Formation Colluvium 

Formation 

Quartzite and metasiltstone 

of the Weverton Formation 

Area of figure 78 Railroad tunnels 0 1000 FEET 

 

Figure 77. Sketch of the bluffs of Elk Ridge north of the C&O Canal showing folded quartzite beds within the Lower Cambrian Weverton 

Formation that comprise the overturned west limb of the Blue Ridge-South Mountain anticlinorium. The two large folds on the east side 

in the Buzzard Knob Member are shown in figure 78. 

NW SE 

60 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Figure 78. View of eastern end of Elk Ridge showing overturned pairs of anticlines and synclines (traces shown by white lines) in 

quartzite of the Lower Cambrian Buzzard Knob Member of the Weverton Formation. 
Blue Ridge Province 61 

Figure 79. Historical photograph (around 1912) of the westernmost overturned anticline of quartzite in the Lower Cambrian Buzzard Knob Member of the Weverton Formation as shown in figure 

78. Photograph by George W. Stose (U.S. Geological Survey). 

Figure 80. Historical photograph (around 1912) showing the easternmost overturned anticline in quartzite of the Lower Cambrian 

Buzzard Knob Member of the Weverton Formation as shown in figure 78. Photograph by George W. Stose (U.S. Geological Survey). 

NW SE 

Figure 81. View of the 

westernmost overturned anticline shown in figure 80. The 

anticline plunges southward 

toward the Potomac River and 

can be seen in the river bed 

during low-water conditions 

from Loudoun Heights, Va., as 

well as from the air (fig. 76). 
 

figure 86B 

600 FEET 

80 FEET 

0 

figure 86A 

Lower Cambrian Harpers Formation 

Cleavage Harpers Ferry Road

Hoffmeister Road 

Cover 

Bedding

MM Lock

EAST 

62 34 figure 85 figure 84B 

62 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Figure 82. Crossbedding in quartzite of the Lower Cambrian Weverton Formation at Maryland Heights. Crossbeds are defined by layers 

of dark heavy minerals in light quartz sand. The truncation of the beds is used to determine whether the rocks are upright or overturned. 

A, Upright beds. B, Overturned beds. Mechanical pencil for scale in both photographs. 

Figure 83. Geologic sketch of the bluffs from about MM 61 to the Great Valley west of Lock 36 (MM 62.5). Rocks of the Lower Cambrian 

Harpers Formation show penetrative cleavage, folds that are defined by bedding, and folded cleavage by deformation near thrust faults. 

The rocks of the Lower Cambrian Antietam and Tomstown Formations are oriented either vertically or steeply dipping (overturned) to the 

southeast. 
Drill holes 

Drill holes 

Bedding Drill holes

Drill hole 

Canal 

berm 

Bedding 

Figure 84. Folded and cleaved metasiltstone of the Lower Cambrian Harpers Formation, which was drilled and blasted to excavate the 

canal near MM 61. A, Pervasive slaty cleavage dips at a low angle to the southeast, but bedding and drill-hole scars are nearly vertical. 

B, In another outcrop near A, bedding dips to the northwest. The rightmost one-third of the photograph shows slabs of Harpers Formation stacked to construct the berm of the canal. 

Blue Ridge Province 63 

overlying rocks of the Great Valley section were transported 

westward along this fault surface for some unknown distance. 

The fault was active early in the tectonic history of the region 

because it has been folded with the anticlinorium and is overturned and dips to the east. 

From Lock 35 (MM 62.3) to Dam 3, stone berms were 

constructed to elevate the canal and towpath above the flood 

plain. Good exposures of cleaved metasiltstone interbedded 

with metasandstone at the transitional contact of the Harpers and stratigraphically overlying Antietam Formation (_ca) 

(fig. 86) may be seen near the remains of Dam 3 and behind 

Lock 35. The dark lines with raised centers that are perpendicular to bedding in metasandstone of the Antietam Formation are the remains of the trace fossil Skolithos linearis. On 

top of the hill of the meander bend above Lock 36 is the strategic location of Fort Duncan, which was built by Union forces 

in 1862 to defend Harpers Ferry. The foundation of the fort’s 

ramparts is still preserved. 
64 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

A B
 

Bedding 

Burrows 

Figure 85. Cleavage (nearly 

horizontal) in metasiltstone of 

the Lower Cambrian Harpers 

Formation folded as the result 

of motion along a thrust fault. 

Spaced crenulation cleavage 

dips to the northwest, parallel 

to the late fold axes. Outcrop 

is west of Hoffmeister Road, 

immediately north of the canal 

near MM 61.5. Pocket knife in 

center for scale. 

Figure 86. Outcrops of Lower Cambrian Harpers and Antietam Formations. A, Outcrop showing the transition from metasiltstone of the 

Harpers Formation upward into metasandstone of the Antietam Formation between Dam 3 and Lock 35, just west of MM 62. Here, 2- to 

3-inch-thick beds of clean metasandstone are interbedded with darker metasiltstone. B, Outcrop of Antietam Formation showing dark 

lines (perpendicular to the laminated bedding) which indicate the presence of the trace fossil Skolithos linearis (tubular worm burrows 

in sand that were filled in with finer grained silt in shallow water). 
 

 

 

 

 

Valley and Ridge Province—Great Valley Section 65 

Valley and Ridge Province—Great 

Valley Section 

Fort Duncan to McCoys Ferry (MM 62.5 to 

MM 110) 

Access Points 

Pleasantville (Harpers Ferry and Hoffmeister Roads) 

Dargan Bend 

Lock 37 

Antietam 

Lock 38 

Snyders Landing 

Taylors Landing 

Dam 4 

Charles Mill Road 

Falling Waters Road 

Williamsport 

Dam 5 

Two Locks 

Four Locks 

McCoys Ferry 

Introduction 

The Great Valley section (locally called the Shenandoah 

Valley in Virginia, Hagerstown Valley in Maryland, or Cumberland Valley in Pennsylvania) of the Valley and Ridge section is the broad area of low relief between the mountainous regions of the Blue Ridge province to the east and the 

remainder of the Valley and Ridge province (west of the 

North Mountain thrust fault) to the west (fig. 87 and plate 

10). Because the Great Valley is underlain primarily by limestone and dolomite, caves and sinkholes are abundant (see 

fig. 15). 

Structurally, the Great Valley is a large syncline called the 

Massanutten synclinorium, named after Massanutten Mountain in the core or center of the fold where the youngest rocks 

are found. The youngest rocks along the C&O Canal in this 

province belong to the Middle and Upper Ordovician Martinsburg Formation (Om), which consists primarily of shale with 

lesser amounts of sandstone. The eastern flank of the Massanutten synclinorium generally dips southeastward rather than 

westward toward the center of the syncline; the overturned 

fold is a limb of the Blue Ridge-South Mountain anticlinorium. Westward across the Great Valley, the rocks become less 

intensely folded and faulted. 

The western edge of the Great Valley is marked by the 

North Mountain fault, which places Cambrian and Ordovician carbonate rocks over younger shale and sandstone of the 

remainder of the Valley and Ridge province. The rocks of 

the Great Valley section, Blue Ridge province, and Piedmont 

province have been transported tens of miles westward along 

this fault. 

Fort Duncan to Antietam Creek (MM 62.5 to 

MM 69) 

The bedrock along the meander bends of the Potomac 

River is the Lower Cambrian Tomstown Formation (_t) 

(plate 10). Between MM 63 and MM 63.5, the outcrop of 

dark-gray dolomite is the type section of the Fort Duncan 

Member (_tf) of the Tomstown. Some of the limestone near 

MM 63.4 is assigned to the Bolivar Heights Member (_tbh) 

of the Tomstown Formation. The type section of the Bolivar Heights Member is along the CSXT railroad tracks about 

3 mi to the south in West Virginia. The red “terra rosa” soil 

exposed here characterizes the residuum that developed from 

the weathered carbonate rock. 

Between MM 63.5 and 65.2, shale and thin sandstone of 

the Lower Cambrian Harpers Formation (_ch) are exposed. 

By strict geologic definition, the Harpers actually is part of the 

Blue Ridge province; therefore, along this stretch, the boundary between the Blue Ridge province and Great Valley section 

is crossed twice. The lime kiln at MM 65.2 burned high-calcium limestone from the Bolivar Heights Member, which was 

quarried from 1876 to 1910 just to the west (fig. 88). A sinkhole filled with colluvium consisting of sandstone blocks of 

the Lower Cambrian Antietam Formation in a red-brown mud 

and clay matrix is located near the northeastern part of the 

quarry. Blocks of the sandstone and fragments of the limestone 

contain dark, nodular manganese that was discovered here during excavation of the canal and then was quarried and used 

to make steel (Hahn, 1985). The locations of additional small 

quarries to the north were all confined to the Bolivar Heights 

Member because the overlying Fort Duncan Member was not 

suitable for the needs of the lime kiln operators. 

Near MM 66, riprap fill for the towpath 6 contains blocks 

of yellow jasper (a variety of quartz) that is usually found near 

the contact between the Antietam and Tomstown Formations. 

The jasper was prized by Native Americans for making tools. 

The Bolivar Heights Member was quarried in order to 

construct Lock 37 (Mountain Lock) to the north. The weathered surface of the cut stone has a notable zigzag pattern (see 

figure 32). These structures resulted when the small dolomitized burrows within individual beds of limestone were folded 

and aligned along penetrative cleavage. Outcrops near MM 

67.5 show a similar relationship of bedding, cleavage, and the 

distorted burrows (fig. 89). 

The collapse breccia and calcium-carbonate-rich tufa 

deposits of the Fort Duncan Member are exposed on the bluff 

near MM 68 (fig. 90). Just west of MM 68, within the upper 

part of the Bolivar Heights Member, is a tight fold where bedding is obscured by cleavage. This cleavage is well developed 

within the limestone of the Bolivar Heights Member (see figures 32 and 89), but is absent within the dolomite of the overlying Fort Duncan Member. A little farther to the west are 
Keedysville fault 

Valley and Great Valley section of the 

Ridge province Valley and Ridge province 

McCoys Ferry, Md. 

Williamsport, Md. A 

North Mountain
 

thrust fault
 

Potomac River 

and C&O Canal 

Shepherdstown, 

W. Va. 

N Great 

Valley 

section 

c 

Blue 

Ridge 

Province 

Keedysville 

fault 

0 5 MILES 

Harpers 

Ferry, W. Va. 

A 

A' 

North Mountain

Thrust Fault

Blue Ridge 

FEET A 

10,000 

5,000 

SEA 

LEVEL 

5,000 

10,000 

Great Valley section of the Province 

Valley and Ridge province A'
 

figure 100B figure 100A figure 95 figure 88 MM 

B plate 13 plate 12 plate 11 plate 10 

61 

66 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Figure 87. Generalized structure map and cross section of a portion of the C&O Canal National Historical Park and Potomac River 

corridor as it crosses the western part of the Blue Ridge province and the Great Valley section of the Valley and Ridge province. A, 

Structure map showing fold axes and major faults. B, Cross section showing folds, faults, and other features from west of Harpers Ferry, 

W. Va., westward to the North Mountain fault. See plates 10 to 13 for detailed geologic maps of this portion. Geologic units shown in 

cross section are generalized from the geologic map shown on plate 1. 
Valley and Ridge Province—Great Valley Section 67 

Figure 88. Geologic sketch from about MM 65.2 westward, across the overturned thrust fault contact (Keedysville detachment) of 

metasandstone of the Antietam Formation (_a) with limestone of the Bolivar Heights Member of the Tomstown Formation (_tbh), and 

another thrust fault cutting an upright anticline of the Bolivar Heights, Fort Duncan (_tf) and Benevola (_tb) Members of the Tomstown 

Formation. Both thrust faults are marked by mylonite, marble, and cleavage. The lime kiln near MM 65.2 was for cooking locally mined 

limestone in order to make cement and fertilizer. Manganese discovered during excavation of the canal was also mined near the overturned contact, near the sinkhole that is filled with colluvium. 

Cleavage 

Bedding 

Joint 

A B
 

Cleavage 

Bedding 

Knife 

Burrowed 

rock 

Figure 89. Limestone of the Lower Cambrian Bolivar Heights Member of the Tomstown Formation near MM 68. A, Outcrop shows 

pervasive cleavage dipping at a low angle and beds dipping steeply to the northwest. B, Closeup of A showing that fossil burrows 

(originally oriented perpendicular to beds) are disrupted and realigned by pervasive, nearly horizontal cleavage. Three-inch-long knife 

for scale. 

(1) outcrops of massive, light-gray dolomite of the Benevola 

Member (_tb) of the Tomstown; and (2) the type section of the 

medium-bedded dolomite and limestone of the Dargan Member (_td) (Brezinski, 1992), which is the uppermost member 

of the Tomstown. 

From MM 68.2 northwestward, the flood plain widens 

and is covered with alluvium, thus hiding the Lower Cambrian Waynesboro Formation (fig. 91) that underlies this section. The Waynesboro Formation is made up of three members (in ascending order): (1) the Red Run Member (_war), 

which consists of interbedded tan sandstone and dolomite; (2) 

the Cavetown Member (_wak), which is composed mainly 

of limestone and dolomite; and (3) the Chewsville Member (_wac), which consists of interbedded red shale, sandstone and tan dolomite (Brezinski, 1992). Near MM 69, the 

canal crosses the contact of the Waynesboro Formation with 

the overlying Middle and Upper Cambrian Elbrook Limestone 

(_e), which also is not exposed. 
 

 

68 

C 

A B
 

Figure 90. Examples of karst 

features in the Fort Duncan 

Member of the Tomstown 

Formation near MM 68. A, 

Collapse breccia of limestone 

cemented by calcium carbonate. B, Tufa deposits. C, One of 

many small caves in the bluffs 

of the Great Valley. 

Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Antietam Creek to Big Slackwater (MM 69 to 

MM 85.5) 

The Antietam Creek Aqueduct was built in 1834 of limestone and dolomite (Cavetown Member of the Lower Cambrian Waynesboro Formation) that was quarried about 0.75 mi 

to the southeast (Davies, 1989). The rock is very finely laminated and was deposited on an intertidal mudflat at about 530 

Ma. Each lamination represents a single tidal cycle or storm 

event. Blocks along the top of the aqueduct have fossil mud 

cracks that also formed in a tidal-flat environment. 

From the aqueduct to MM 73, opposite Shepherdstown, 

W. Va., the Potomac River flood plain is wide and alluvium 

obscures the interbedded thin limestone and shaly tan dolomite 

of the Elbrook Limestone (_e) (Brezinski, 1996a) (plate 11). 

Abundant algal stromatolites are common in the Elbrook; on 

the West Virginia side of the Potomac River, opposite MM 

71.5, good exposures of stromatolites may be seen along the 

road that runs near the river. There are also abundant sinkholes 

and caves developed in the Elbrook Limestone in this region. 

To the north and south of the Potomac River, rounded 

Quaternary gravel, cobbles, and boulders of sandstone and 

conglomerate cover the hills. These are river-channel deposits 

left by the Potomac River as it cut down to the modern channel. Kilns and abandoned limestone quarries were located on 

both sides of the river near MM 71.5. The Botelers and Reynolds Cement Mill (fig. 92) operated from 1828 to 1834 in 

West Virginia, and the Potomac Cement Company operated in 

Maryland from 1888 to 1903 (Davies, 1999). 

From MM 72 to MM 85, nearly all of the bedrock is 

the Upper Cambrian and Lower Ordovician Conococheague 

Limestone (O_c). These rocks are characterized by ribbonlike, thin layers of gray limestone (0.25 to l.0 inches thick) 

interbedded with layers of tan dolomite or dolomitic limestone. Bedrock is exposed along steep bluffs that show abundant drill holes and small caves. Shenandoah River Lock, near 
B

Figure 91. Outcrops of Lower and Middle Cambrian Waynesboro 

Formation in the vicinity of the C&O Canal. A, Shale, sandstone, 

and dolomite of the Chewsville Member northeast of MM 69. 

Clipboard for scale. B, Limestone and dolomite of the Cavetown 

Member in an outcrop on the Virginia side of the Potomac River 

opposite MM 69.5. Jacobs staff (1 meter or about 3 ft long) for 

A scale. Along the canal, the Waynesboro is covered with alluvium.

Figure 92. Historical photograph (about 1895) showing 

Botelers and Reynolds Cement 

Mill’s quarries in near-vertical 

limestone (in West Virginia 

opposite MM 71.6) and 

Botelers and Reynolds Dam 

(on right) for mill race for the 

cement mill (left background). 

Before the Round Top Cement 

Company plant was built, this 

plant supplied the cement for 

the canal masonry. Photograph courtesy of National 

Park Service.

MM 72.6, provided access to Shepherdstown in the same manner as the Goose Creek and Alexandria Locks by linking the 

C&O Canal to commerce centers to the east.

On top of the bluffs seen east of MM 73 are large blocks 

and cobbles of round sandstone and quartzite that were deposited by the ancestral Potomac River during the Pleistocene. The 

meander bends of the current river have extensive exposures on 

the north (upstream) side where erosion is most severe. In these 

areas, the berms to the towpath are often lined with stone for 

protection. The south sides of the meanders have a gentler slope 

and the terrace deposits are better preserved.

Near MM 75.7, a large overhang in the Conococheague 

Limestone (known as Killiansburg Cave) served as a Civil War 

shelter (fig. 93). Flowstone, or travertine, may be seen in the 

cave. Several more caves may be found between here and MM 

76.2 (Davies, 1999). Tufa deposits may be seen in the tributaries of the Potomac River near Snyders Landing (MM 76.6). 

On the shore in West Virginia, opposite MM 81, the land that 

is enclosed by a large meander loop is known as Terrapin 

Neck. This is the location of the National Conservation Training Center of the U.S. Fish and Wildlife Service. West of MM 

83, the Potomac River cuts across the strike of the ConocoValley and Ridge Province—Great Valley Section 69
 

 

 

 

 

 

 

70 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

cheague Limestone. Between MM 83 and Dam 4, the Conococheague is tightly folded. 

At MM 83.3, the cave (locally known as Bergen’s Cave) 

at Dam 4 discharges spring water at canal level (fig. 16). In the 

bluffs at Dam 4, the near-flat-lying uppermost part of the Conococheague Limestone shows drill holes and a stuck drill rod 

(see fig. 25A). 

Big Slackwater to Williamsport (MM 85.5 to 

MM 99.5) 

Because bluffs prevented the construction of a canal, the 

canal boats entered the Potomac River (fig. 94) at MM 85.5. 

This stretch is known as the Big Slackwater because the water 

moved slowly enough to allow the canal boats to use the river. 

Limestone and dolomite of the Upper Cambrian and Lower 

Ordovician Conococheague Limestone (O_c) and Lower Ordovician Stonehenge Limestone (Obs) along this section are well 

exposed (plate 12 and fig. 95). Between Guard Lock 4 (MM 

85.5) and McMahons Mill (MM 88), flooding severely eroded 

the canal towpath on the river bank. Numerous small caves may 

be found between MM 86.8 to MM 92 (Davies, 1999). 

West of McMahons Mill (built in 1778) at the end of 

Charles Mill Road, is a 400-ft interval of medium-bedded 

limestone of the Stonehenge Limestone followed by a 120­

ft-wide interval of the Stoufferstown Member of the Stonehenge (Obss). The rocks are characterized by paper-thin, parallel, calcite and clay laminations (fig. 96), which resulted from 

mylonitic foliation due to faulting. 

Westward to Lock 41 (near MM 88.8), the bedrock consists of the Lower and Middle Ordovician Rockdale Run Formation (Obrr) (fig. 95), which is characterizied by gray limestone interbedded with tan to brown dolomite. These lithologies 

are repeated in vertical succession from ribbony to sandy limestone to tan, laminated dolomite. Although the relative thickness 

Figure 93. Historical sketch 

(about 1867) of soldiers and 

citizens in and around Killiansburg cave near MM 75.7. The 

sketch suggests that the cave 

is situated in the hinge of an 

anticline of the Upper Cambrian and Lower Ordovician 

Conococheague Limestone, 

but it is actually developed in 

the formation’s east-dipping 

beds. From Hahn (1985); used 

with permission. 

Figure 94. Historical photograph (date unknown) showing location north of Dam 4 

where canal boats entered 

the Potomac River because 

limestone cliffs prevented 

construction of a canal (as 

with Big Slackwater to the 

west). Photograph courtesy of 

National Park Service. 
 

 

 

 

 

 

 

 

Valley and Ridge Province—Great Valley Section 71 

of the individual limestone or dolomite beds varies through the 

succession, the sequence of limestone to dolomite is repeated 

through the formation. The lithologic cycles reflect a change in 

depositional environments from shallow subtidal (limestone) to 

supratidal (dolomite). The cycles seen within these rocks indicates that, during the time of deposition, sea level rose and fell 

with considerable frequency and thus produced produce the 

repeated sequence of lithologies. The original depositional features (such as chert, burrows, crossbedding, graded beds, and 

ripple marks (fig. 97)) may be seen within the limestones; mud 

cracks are found within the dolomites. 

Upstream of the Opequon Junction campsite and MM 

91 are exposures of folded rocks of the Stonehenge Limestone and Rockdale Run Formation. These rocks contain 

abundant small caves (such as what is locally known as Dellinger’s Cave at MM 92.14) and dissolution features (Davies, 

1999). This is the midway point of the 184.5-mi-long canal. 

To the west, the rock is Middle Ordovician Chambersburg 

Limestone (Oc). A thrust fault places the rocks of the Rockdale Run Formation above the younger rocks of the Chambersburg Limestone near MM 92. A stream valley overlies 

the fault between the two units. Springs and tufa deposits are 

common in this area. 

WEST 

The Chambersburg is the youngest carbonate rock of the 

Great Valley section and perhaps the most fossiliferous. The 

rocks contain abundant brachiopods, bryozoans, trilobites, 

and, near its base, abundant remains of the echinoderm Echinosphaerites (fig. 20, item 16). 

The Chambersburg grades into shale of the overlying 

Middle and Upper Ordovician Martinsburg Formation (Om) 

west of MM 92.5. The Martinsburg consists of gray to brown 

shale in its lower part and interbedded sandstone and shale in 

its upper part. This shaly interval marks the end of the depositional epoch that spans Early Cambrian to late Middle Ordovician time when only carbonate rocks were deposited. The 

environment on the shelf of the continent was similar to that 

east of Florida today. This stable environment resulted in 

the deposition of a carbonate rock sequence that is nearly 2 

mi thick over a period of approximately 100 m.y. Shale of 

the Martinsburg marked the beginning of clastic-dominated 

(sandstone-siltstone-shale) deposition that continued through 

most of the middle and late Paleozoic over a period of about 

200 m.y. The initiation of the shale deposition also represents 

the beginning of a tectonic event known as the Taconian orogeny, which occurred near the end of the Ordovician Period. 

During this period, a mountainous highland formed in the eastEAST 

Limestone FEET Thrust McMahons 

0 500 FEET Mylonitic Stonehenge limestone Limestone 

quarry Rockdale Run Chert Syncline fault Mill 

Formation Quarry 

Caves MM 88 

Canal figure 97 Stoufferstown figure 96 Member of Lock 41 

Bedding 

Figure 95. Geologic sketch from McMahons Mill near MM 88 westward to Lock 41 depicting Ordovician rocks of the older Stoufferstown Member of the Stonehenge Limestone thrust above limestone of the younger Rockdale Run Formation. The rocks are folded into a 

syncline beneath the fault. There are several limestone quarries and caves in the bluffs, some of which are shown in this sketch. 

Figure 96. Mylonitic limestone within the Lower Ordovician 

Stoufferstown Member of the Stonehenge Limestone, immediately west of MM 88 and McMahons Mill. The mylonitic foliation 

was produced as the rocks were thrust westward above rocks of 

the Lower and Middle Ordovician Rockdale Run Formation. The 

mylonitic foliation is defined by clay seams and recrystallized 

limestone (parallel to the rock hammer handle). The mylonitic 

foliation is kinked by later folds. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

72 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Bedding 

Stylolite 

Burrows 

Fractures 

A B
 

Ripple marks on 

bedding plane 

Figure 97. Depositional features in the Lower and Middle Ordovician Rockdale Run Formation. A, Dark chert and limestone along the 

bluffs near MM 88.5. Note the stylolites (irregular, bedding-parallel seams of concentrated insoluble material formed by compaction; 

shown by arrow) that are parallel to beds. B, Ripple marks (3-inch spacing) on bedding plane in shaly limestone exposed on the bluff 

near MM 88.5. The ripple marks indicate that the rocks were deposited in a shallow tidal-water environment. 

ern Piedmont of Maryland that was the source of the sand and 

clay in the Martinsburg Formation. 

The contact between the Chambersburg Limestone and 

Martinsburg Formation may be seen in a small shale quarry 

east of MM 96. Scarce fossil remains of the trilobite Cryptolithus and fragments of graptolites occur here. These animals 

favored habitation within the deepwater environments where 

shale was deposited. The Martinsburg Formation is the youngest and only noncarbonate rock in the northern Shenandoah 

Valley and it forms the core of the Massanutten synclinorium. 

Because of the meandering nature of the Potomac River, the 

canal crosses the axial region (or center) of the Massanutten 

synclinorium three times (near MM 93.5, MM 96, and at about 

MM 100). A dramatic change in both physiography and land 

use (from agriculture to pasture and woodland) occurs in the 

area underlain by Martinsburg Formation because of the contrast in materials that result from weathering. 

Good exposures of shale in the lower part of the Martinsburg Formation may be seen from MM 95 to MM 96.5. At the 

footbridge, dolomite of the Rockdale Run Formation (Obrr) is 

thrust-faulted above shale of the younger Martinsburg Formation, and rocks of the Middle Ordovician St. Paul Group and 

Chambersburg Limestone are missing. 

At MM 97.1 are remnants of the piers for the old railroad trestle which were made of Triassic red sandstone similar 

to the Upper Triassic Manassas Sandstone quarried at Seneca, 

Md. These rocks most likely were quarried from the Gettysburg basin near Emmittsburg, Pa., and were transported here 

by railroad. 

The Stonehenge Limestone (Obs) is exposed from MM 

97 to MM 99 near Williamsport (plate 13). Immediately east of 

the powerplant (MM 99), shale of the Martinsburg Formation is 

exposed beneath the same fault encountered near Lock 43 and 

the footbridge near MM 96.5. This regional fault can be traced 

northward into Pennsylvania and southward into West Virginia. 

The steel drawbridge immediately north of the power plant 

allowed coal trains to cross the canal to supply the plant. 

A feature locally known as the Cushwa basin at the 

National Park Service’s C&O Canal Visitor Center at Williamsport was a transfer station for canal cargo, which was principally 

coal from west of Cumberland (fig. 98). Like the basins at Seneca and Cumberland, this basin was a large holding area where 

canal boats and barges loaded or unloaded cargo. High (1997) 

provides a photograph of a canal boat that crashed through the 

Conococheague Creek Aqueduct at Williamsport in 1920. 

Williamsport to McCoys Ferry (MM 99.5 to 

MM 110) 

Over the next two miles, the wide flood plain of the 

Potomac River overlies the Middle and Upper Ordovician 

Martinsburg Formation (plate 13). Incompetent, dark-gray, 

silty shale of the unnamed lower member of the Martinsburg 

Formation (Om) that was folded and faulted more easily than 

the adjacent competent carbonate rocks may be seen along 

the CSXT railroad tracks to the north. Approximately 2,000 

ft of shale grades upward into more sandy beds that mark the 

base of the unnamed upper member. The upper member of 

the Martinsburg Formation consists of gray, silty shale interbedded with gray to brown, fine- to medium-grained sandstone. The sandstone beds typically are less than 1 ft thick 

(fig. 99A) in the lower part of the member and gradually 

increase in thickness to more than 20 ft near the top. The 

thinner beds exhibit basal scours and lineations (fig. 99B) 

that are typical of distal deposits resulting from high energy 
 

Valley and Ridge Province—Great Valley Section 73 

caused by deepwater turbidity currents (McBride, 1962). The 

thicker sandstones higher in the member were deposited in 

shallower water, closer to the sand source, as the depositional 

basin was filling. 

The last shale outcrop that marks the eastern limb of the 

Massanutten synclinorium may be seen near MM 102. From 

this point westward to McCoys Ferry (MM 110), the same 

sequence of bedrock formations occurs on the eastern flank of 

the fold. Rocks underlying the western limb of the synclinorium were not as severely deformed as those of the overturned 

eastern limb; therefore, sedimentary structures and fossils are 

better preserved in the western limb. 

Figure 98. Historical photograph (about 1920) of Cushwa 

basin at Williamsport (MM 100) 

showing coal being loaded 

into horse-drawn wagons. The 

coal was transported down 

the canal from Cumberland. 

Photograph by Consolidated 

Coal Co.; courtesy of National 

Park Service). 

Sandstone 

bed 

Shale 

bed 

A B
 

Figure 99. Upper part of Middle Ordovician Martinsburg Formation along the railroad north of MM 101. A, Thin-bedded sandstone and 

shale represent turbidites, which were deposited in a deepening basin at the onset of the Taconian orogeny during the Late Ordovician. 

They were the first clastic rocks deposited after almost 100 million years of carbonate deposition on the continental shelf. B, Flute casts 

at the base of a sandstone bed. These features formed where a trough, which formed as turbulent water scoured the surface, later filled 

with sand. The water flowed parallel to the direction of the mechanical pencil. 
WEST EAST
 

Chambersburg


New Market Limestone Martinsburg
 Stonehenge Limestone Rockdale Run Formation Pinesburg Station Limestone Formation Dolomite 

Quarry Quarry 20 FEET

0 

MM 103 Canal Culvert MM 102 0 120 FEET Fault 

A

WEST
 

Stoufferstown
 Big Spring Station Member of the Member of the Conococheague Limestone Stonehenge Conococheague

Culvert 

Limestone Stonehenge Limestone Conococheague Limestone Limestone Cover Quarry 

20 FEET0

Canal figure 113 Culvert Lock 0 120 FEET Lock MM 107 Lock 45 MM 108 46 Fault Dam 5 47 

Big Spring Station

Member of the

Conococheague EAST Conococheague Limestone Limestone Elbrook Ruins of Filled cavern Syncline Limestone Charles Mill 

20 FEET0 Cave Four Locks Canal MM 108 Culvert 0 120 FEET 

B

74 

Figure 100. Geologic sketches showing structures in Cambrian and Ordovician Conococheague Limestone is thrust on Rockdale Run Formation. From there the rocks

rocks of the Great Valley section of the Valley and Ridge province. 

A, Section from form a broad anticline cored by Elbrook Formation; the axis of the anticline is at MM

Pinesburg Station (MM 102) west to Millers Bend (MM 103) illustrating the folded 108. West of the ruins of Charles Mill, the Conococheague Formation is folded to form Ordovician bedrock of the west limb of the Massanutten syncline, from the shale of a broad syncline. The other bedrock units are the Big Springs Station Member of thethe Martinsburg Formation (east) downsection to the Stonehenge Limestone (west). Conococheague Limestone, Stonehenge Limestone, and Stoufferstown Member ofIntervening limestone rocks are Rockdale Run Formation, Pinesburg Station Dolomite, the Stonehenge Limestone. There are several caves and limestone quarries along thisSt. Paul Group (undivided), New Market Limestone, and Chambersburg Limestone. 

B, section. Thin dashed lines indicate bedding.

Two-part section from Dam 5, westward to near Four Locks. Just west of Dam 5, the 

Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
 

 

 

 

 

 

 

 

 

 

 

 

Valley and Ridge Province—Great Valley Section 75 

A B
 

Mud-filled crack 

Mud crack 

Figure 101. Mudcracks on Upper Cambrian and Lower Ordovician Conococheague Limestone at MM 107.8. A, Plan view of mudcracks 

developed on the top of the limestone. The polygonal shape (4 inches across) is very similar to present-day mudcracks that form in the 

silty deposits after the Potomac River floods. B, Cross section of laminated Conococheague Limestone with mudcracks. 

Westward from MM 102 to MM 103.2, the section contains a nearly continuous bedrock exposure starting near the 

base of the Martinsburg Formation and descending stratigraphically (westward) to the top of the Lower Ordovician 

Stonehenge Limestone (Obs) (fig. 100). The Middle Ordovician Chambersburg Limestone (Oc), below the Martinsburg, here consists of medium- to dark-gray, wavy- to nodular-bedded, fossiliferous limestone that is shaly at its base 

near the eastern wall of the active quarry immediately to the 

north. To the west, the more gently dipping strata are part 

of the Middle Ordovician St. Paul Group (Osnr) rocks (Neuman, 1951), which are actively being quarried. The St. Paul 

Group, which consists of the Row Park and New Market 

Limestones mapped as an undivided unit, consists mainly 

of light-gray, thick-bedded limestone with thin interbeds of 

tan dolomite. An interval of gray, fractured dolomite of the 

Middle Ordovician Pinesburg Station Dolomite (Op) underlies the St. Paul Group and is exposed at the parking area at 

MM 103.2. Along the meander immediately to the south are 

scattered outcrops of folded rocks within the Conococheague 

and Stonehenge Limestones. 

At Dam 5 and Lock 45 north of MM 107, the exposed 

rocks are the Upper Cambrian and Lower Ordovician Conococheague Limestone (O_c). Quaternary boulders and gravel litter a low river terrace. Approximately 1,000 ft west of Lock 45, 

the rocks are highly cleaved due to a southeast-dipping thrust 

fault that places the Conococheague Limestone over the Lower 

and Middle Ordovician Rockdale Run Formation (Obrr). Immediately to the west (MM 106.8) are the underlying rocks of the 

Lower Ordovician Stonehenge Limestone (Obs); a complete 

section (from top to bottom) of the Stonehenge is exposed from 

here to the area between Locks 46 and 47 (MM 107.2). From 

Lock 47 westward, the Conococheague Limestone is exposed 

relatively continuously until MM 108, where the upper strata 

of the underlying rocks of the Middle and Upper Cambrian 

Elbrook Limestone (_e) occur in an anticline. Laminated Conococheague Limestone with mud cracks is exposed near MM 

107.8 (fig. 101). The Conococheague is exposed all the way to 

Four Locks (MM 108.5) in a broad syncline (fig. 100). 

At Four Locks, canal boats were lifted to pass through 

the neck of a meander, thereby obviating about 4.5 mi of canal 

construction. The elevated valley occupied by the canal and 

towpath is an abandoned Pleistocene channel of a former tributary to the Potomac River. Shaly and algal limestones with 

stromatolites (fig. 102) exposed near the North Mountain 

campsite characterize the Elbrook Limestone along the western limb of the Massanutten synclinorium (Brezinski, 1996a). 

At MM 110 (plate 14) is an outcrop of very light gray to 

white quartzite of the Upper Ordovician and Lower Silurian 

Tuscarora Quartzite (SOt) (fig. 103). The Tuscarora Quartzite 

and the Elbrook Limestone are separated by the North Mountain thrust fault, which is perhaps the most regionally significant thrust fault in the central Appalachian region. This 

thrust fault transported all of the rocks between here and 

Figure 102. Stromatolites (fossilized algal remains) in the Middle 

and Upper Cambrian Elbrook Limestone east of McCoys Ferry, 

near MM 109.5. Knife for scale. 
 

 

 

76 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Georgetown in Washington, D.C., westward over Silurian 

and Devonian rocks. For the next 74.5 mi to the western terminus of the canal at Cumberland, the Potomac River transects the rest of the Valley and Ridge province. 

A
 

B
 

Figure 103. Upper Ordovician and Lower Silurian Tuscarora 

Quartzite. The Tuscarora is highly fractured along the North 

Mountain thrust fault. Rocks to the east were transported westward, structurally above the fractured and brecciated quartzite. 

To the south, this resistant quartzite underlies North Mountain. A, 

Tuscarora Quartzite at McCoys Ferry (MM 110). B, Fractured and 

brecciated Tuscarora Quartzite. Quarter for scale. 

Valley and Ridge Province—West of 

North Mountain Thrust Fault 

McCoys Ferry to Cumberland, Md. (MM 110 to 

MM 184.5) 

Access Points 

McCoys Ferry 

Fort Frederick State Park 

Big Pool 

Little Pool 

Hancock 

Cohill 

Woodmont 

Pearre 

Little Orleans 

Green Ridge/Kasecamp Road 

Twigg Hollow Road 

Paw Paw Tunnel 

Keifers 

Town Creek 

Oldtown 

Spring Gap 

North Branch 

Wiley Ford 

Cumberland 

Introduction 

The Valley and Ridge physiographic province west of the 

Great Valley section consists of Ordovician to Permian sandstone, siltstone, shale, and limestone that were folded into 

anticlinoria and synclinoria west of the North Mountain thrust 

fault during the late Paleozoic Alleghanian orogeny (fig. 104). 

Silurian, Devonian, and Mississippian rocks are exposed along 

the C&O Canal; Ordovician rocks are exposed in the core of 

the Wills Mountain anticline west of Cumberland; and Pennsylvanian and Permian rocks are exposed in the Appalachian Plateaus farther to the west. The limbs of the anticlinoria are underlain by resistant Upper Ordovician and Lower 

Silurian Tuscarora Quartzite (SOt) and Lower Devonian Oriskany Sandstone (Do) that make up ridges. These rocks exhibit 

bedforms such as ripple marks, crossbeds, and worm burrows 

(Skolithos linearis) that indicate a beach environment that is 

similar to that found along the Atlantic Coast today. Most of 

the younger rocks are Devonian and consist of siltstone and 

shale that form mostly broad valleys with low ridges underlain by sandstone. The youngest rocks are the Upper Devonian and Lower Mississippian Rockwell Formation (MDr) 

and the Lower Mississippian Purslane Formation (Mp), both 

of which are preserved in the Sideling Hill syncline and are 
Sideling Hill 

Mountain 

T

Cacapon 

Mountain 

A 

Meadows

Ground 

S 

uscarora

Patterson 

A 

Broad Top A 

Town Hill 

S

Whip Cove 

A 

S 

Georges Creek 

S 

S 

S 

Wills Mountain 

A 

Evitts 

Creek 

Creek 

North Moutain fault

Meadows Branch 

BearPond

Mountain

A 

A
 

Hancock, Md. 

Cumberland, Md. 

McCoys 

Ferry 

Oldtown, Md. 

B 

Paw Paw 

Tunnel 

FEET 

10,000 

Wills Mountain 

anticline Sideling Hill 

syncline 

Devil’s 

Eyebrow 

Great Valley section 

North Mountain 

thrust fault 

5,000 M Do SOt 

SEA LEVEL 

C 

5,000 

10,000 

plate 20 plate 19 

D Dh 

plates 17 and 18 

O 

plate 16 plate 15 

S 

plate 14 

?

Valley and Ridge Province—West of North Mountain Thrust Fault 77 

Figure 104. Side-looking airborne radar (SLAR) image, generalized structure map, and cross section of a portion of the C&O Canal 

National Historical Park and Potomac River corridor as it crosses the Valley and Ridge province, including the westernmost part of the 

Great Valley section. A, SLAR image illustrating the namesake physiography of the Valley and Ridge province. Erosion-resistant rocks 

on the limbs of anticlines and synclines underlie the hills, ridges, and mountains. Easily eroded shale and limestone are in the valleys. 

B, Structure map showing fold axes and major faults. C, Cross section showing folds and faults from the North Mountain fault westward 

to the Wills Mountain anticline and the edge of the Appalachian Plateaus province. See plates 14 to 20 for detailed geologic maps of this 

portion. Geologic units shown in cross section are generalized from the geologic map shown on plate 1. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

78 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

best exposed in the Interstate Route 68 roadcut at the State of 

Maryland’s Sideling Hill Exhibit Center. 

The oldest rocks along the C&O Canal in the Valley and 

Ridge province are shale and sandstone of the Upper Ordovician Juniata Formation (Oj) which is exposed in the core of the 

Wills Mountain anticline in The Narrows, a water gap northwest of Cumberland. Silurian and Lower Devonian rocks are 

exposed on North Mountain in the footwall of the North Mountain thrust fault and in the following anticlinoria (from east to 

west): Cacapon Mountain anticlinorium (between Tonoloway 

Ridge and Warm Springs Ridge), Irons Mountain anticline of 

the Patterson Creek anticlinoria, and Wills Mountain anticline 

(fig. 104). The mostly Middle and Upper Devonian siltstone and 

shale units are exposed (east to west) in the Meadow Branch 

synclinorium, Sideling Hill syncline, Whip Cove anticlinorium, 

Town Hill syncline, and Evitts Creek syncline. Erosion of these 

rocks and their structures resulted in the classic valleys and 

ridges that give this physiographic province its name. 

The oldest rocks exposed along the canal west of North 

Mountain are Silurian and range in age from about 443 to 418 

Ma (Tucker and others, 1998). Shale and sandstone of the Middle Silurian Rose Hill Formation (Sr) are exposed along the canal 

at Leopards Mill. These rocks are overlain by (in ascending 

order) white Middle Silurian Keefer Sandstone (Sk), shaly limestone of the Middle and Upper Silurian McKenzie Formation 

(Sm), and red shale and sandstone of the Upper Silurian Bloomsburg Formation (Sb). The clastic rocks are overlain by shale of 

the Upper Silurian Wills Creek Formation (Sw), Upper Silurian Tonoloway Limestone (Stl), and Upper Silurian and Lower 

Devonian Keyser Limestone (DSk). The boundary between the 

Silurian and Devonian Periods is within the Keyser. These rocks 

record a shallow marine sea that transgressed over and submerged much of the Appalachian basin during the Late Silurian. 

Devonian rocks range in age from about 418 to 362 Ma 

(Tucker and others, 1998) and consist of the Lower Devonian 

Oriskany Sandstone (Do), Lower and Middle Devonian Needmore Shale and Middle Devonian Marcellus Shale (mapped 

undivided as Dmn), Middle Devonian Mahantango Formation (Dm), Upper Devonian Brallier Shale (Db), Upper Devonian Foreknobs Formation (Df), and Upper Devonian Hampshire Formation (Dh). The boundary between the Devonian 

and Mississippian periods is located within the Rockwell Formation (MDr), which is the youngest unit exposed along the 

canal in the Valley and Ridge province. Sideling Hill and Purslane Mountain are underlain by younger rocks (350 Ma) of the 

Lower Mississippian Purslane Formation (Mp). 

The Oriskany Sandstone is interpreted as a beach sand 

deposited during the regression of the sea that deposited the 

underlying Keyser Limestone. Overlying the Oriskany Sandstone are shale, siltstone, and sandstone of the Needmore 

Shale, Marcellus Shale, and Mahantango Formation that were 

deposited as turbidites when another Early Devonian sea transgressed over the region. During the Late Devonian, regional 

regression of the sea led to the development of a broad 

swampy lowland with deltas and rivers. Some of these rivers 

carried debris from a rising highland to the east that may have 

resulted from the Acadian orogeny (a tectonic event). Fragments of metamorphic rocks and granitic gneiss that constitute 

a diamicton (a bouldery deposit) at the Devonian-Mississippian boundary may be derived from rocks of the Blue Ridge 

and Piedmont that were exposed as the result of this orogeny; 

the diamicton may be seen at the Sideling Hill roadcut along 

Interstate Route 68 to the north of the canal. 

All of the Devonian shale, siltstone, and sandstone 

that was deposited above the Oriskany Sandstone is a rather 

monotonous section of rock. The turbidite deposits are not laterally extensive and there are abundant facies changes both 

along and across strike that make defining unit contacts difficult. The transitional nature of these units and the inconsistent 

definition and mapping of them is manifested in the varying 

interpretations published in the geologic literature. 

The Ordovician to Mississippian rocks were folded and 

faulted during the late Paleozoic Alleghanian orogeny when 

the North American tectonic plate collided with the African 

tectonic plate. The resultant forces folded the rocks and transported them westward above thrust faults that are in the subsurface (figs. 4 and 104). 

McCoys Ferry to Licking Creek Aqueduct 

(MM 110 to MM 116) 

Along this section of the C&O Canal, the Potomac River 

has eroded a broad flood plain with sloping terraces along the 

north shore that overlie mostly Devonian shales and siltstones 

(plate 14). Abundant sandstone colluvium mantles the ridge 

slopes. The combination of colluvium and alluvium on the 

lower slopes forms an extensive Quaternary deposit that conceals most of the bedrock. The canal along this section was 

excavated in alluvium. 

Much of the towpath was raised to prevent repeated damage from floods. The large sandstone boulders that remain 

along the lower terrace near MM 111 are evidence of ancient 

floods in the region. Similar boulders deposited by the Potomac 

River litter the uplands as much as 140 ft above the current 

river level. They were used as building stones for the construction of Fort Frederick in 1756, north of MM 112 (fig. 105). 

Big Pool (MM 112.5 to MM 114), located in a formerly 

swampy area, is a feature unique to the C&O Canal. At this 

spot, engineers used the swamps of the broad flood plain to 

make the canal (fig. 106). Levees were constructed and water 

was impounded within the enclosed swampy lowland. Little 

Pool to the west is a similar feature. The swampy area near the 

campground at Fort Frederick is probably what the areas near 

Big Pool and Little Pool looked like prior to canal construction. 

Licking Creek Aqueduct to Round Top Cement 

Mill (MM 116 to MM 127.5) 

Fossilized mud cracks, similar to those shown in figure 101, occur on top of several limestone blocks used in con­
Valley and Ridge Province—West of North Mountain Thrust Fault 79 

Big Pool 

Towpath 

Fort Frederick 

A B
 

Figure 105. Fort Frederick and Big Pool at MM 112. A, Aerial view of Fort Frederick (north of MM 112), which was built in 1756. View 

looks to the southwest. Big Pool, the water body to the south, was a lowland that engineers used for the canal. Photograph by Abbie 

Rowe, 1955; courtesy of the National Park Service. B, Colluvial and alluvial sandstone boulders that were gathered from the fields for 

construction of the fort’s walls. 

Figure 106. Historical photograph (date unknown, probably 

late 19th century) of workers and mules scraping the floor of Big 

Pool after it was drained to remove silt. Photograph courtesy of 

National Park Service. 

structing the Licking Creek Aqueduct (near MM 116) (fig. 

107A). Between the aqueduct and the overpass for Interstate 

Route 70, limestone and chert of the Upper Silurian and Lower 

Devonian Keyser Limestone (DSk) (fig. 107B) are thrust over 

shale of the Middle Devonian Mahantango Formation (Dm). 

The thrust fault is not exposed. From here to the Tonoloway 

Creek Aqueduct (plate 15) the canal was excavated in alluvium and the only rock that is exposed is the Upper Devonian 

Brallier Shale (Db) at MM 119 (fig. 108) and Foreknobs Formation (Df) at MM 120. Little Pool, a swampy lowland used 

for part of the canal channel, is located at MM 119 (see previous discussion). 

Near MM 122, the towpath crosses the shale and siltstone 

of the Upper Devonian Foreknobs and Hampshire Formations 

(Dh) (fig. 109) in the Meadows Ground syncline. At the southeast abutment of the Tonoloway Creek Aqueduct (MM 123) 

is an outcrop of sandstone of the upper part of the Foreknobs 

Formation. This location is a good example of the geomorphology of the flood plain. The modern flood plain was deposited on shallow bedrock of the Foreknobs Formation, which 

was leveled by the meandering Potomac River; the bedrock 

was later exposed by downcutting of Tonoloway Creek, a tributary of the Potomac. To the immediate north is the National 

Park Service’s C&O Canal Hancock Visitor Center. Hancock 

is situated on the east limb of the Cacapon Mountain anticlinorium (plate 15), a regional fold that extends for over 45 mi 

from Virginia to Pennsylvania. The east limb of the Cacapon 

Mountain anticlinorium is Warm Springs Ridge, which is 

underlain by the Lower Devonian Oriskany Sandstone (Do). 

This pure quartz sandstone is quarried commercially in West 

Virginia for making glass. The Oriskany Sandstone also is a 

major subsurface reservoir for natural gas throughout the central Appalachians. Natural warm-water springs, such as Berkeley Springs to the south in West Virginia, are situated where 

radiogenically heated ground water rises within the sandstone 

and discharges at the surface. Dark, fissile shale of the Middle 

Devonian Needmore Shale (Dmn) and the Lower and Middle 

Devonian Marcellus Shale stratigraphically overlie the Oriskany. The uppermost part of the Marcellus contains limestone 

nodules (fig. 110). 

The Potomac River eroded through and deposited alluvium over the Oriskany Sandstone and older rocks from Little 

Tonoloway Creek (MM 124.3) west to MM 127. White cliffs 

of Oriskany Sandstone, locally known as Lovers Leap, crop 

out southeast of MM 126 in West Virginia. North of the canal 

near MM 127 (west of White Rock campsite), the creek flows 

across broadly folded beds of white Middle Silurian Keefer 

Sandstone (Sk). 

The remains of the Round Top Cement Company’s mill 

(fig. 34), as well as good outcrops of folded and faulted rock, 

are located at about MM 127.5. As summarized by Glaser 

(1987), the studies of these folds, cleavages, and faults here 

and along the abandoned railroad cut above the canal by Cloos 

(1951, 1958, 1964) and Geiser (1970, 1974) have advanced the 

understanding of the mechanisms of structural geology. The 
 

 

 

 

 

 

 

 

 

80 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

A
 

B
 

Figure 107. Upper Silurian and Lower Devonian Helderberg 

Limestone and Lower Devonian Shriver Chert. A, Helderberg 

Limestone and limestone within the Shriver Chert, exposed north 

of MM 116, are located above a blind thrust fault that places them 

over shale of the Middle Devonian Mahantango Formation (not 

shown). Note the two caverns (located in the Helderberg) on the 

left side and center of the photograph. B, Licking Creek Aqueduct 

at MM 116 was constructed from limestone blocks. Beneath the 

aqueduct and alluvium is the Mahantango Formation, which at 

this location also is located below the thrust fault. 

anticline (locally called Devil’s Eyebrow) (figs. 111, 112, and 

113) and adjacent folds (fig. 113) are in red sandstone, siltstone, and shale of the Upper Silurian Bloomsburg Formation 

(Sb). Calcareous shale and limestone have weathered away to 

form a cavelike recessed area in the hinge of the Devil’s EyeSandstone bed Shale bed 

Figure 108. Thin-bedded sandstone and shale of the Upper 

Devonian Brallier Shale, east of the access trail to Little Pool (at 

about MM 120.2), resemble turbidites of the Middle and Upper 

Ordovician Martinsburg Formation near Williamsport (see figure 

99). The stone berm of the canal is visible in the lower left of the 

photograph. 

brow anticline (fig. 112). High-calcium limestone of the overlying Upper Silurian Wills Creek Formation (Sw) was mined 

(fig. 114) and processed for cement beginning in 1837. 

Round Top Cement Mill to Sideling Hill Aqueduct 

(MM 127.5 to about MM 137) 

From MM 127.5 to MM 133, the Potomac River meanders 

through complexly folded rocks in the core and along the northwest limb of the Cacapon Mountain anticlinorium (plate 16). 

The nearly flat-lying rocks excavated along the canal northeast 

of Leopards Mill are from the axial region (or crest) of the main 

anticline. The oldest rocks, Late Ordovician and Early Silurian 

Tuscarora Quartzite (SOt), are exposed opposite the river near 

Sir Johns Run in West Virginia. Outcrops of shale and sandstone of the Middle Silurian Rose Hill Formation (Sr) (fig. 115) 

and Keefer Sandstone (Sk) are located along the road and creek 

northwest of MM 130; elsewhere, Quaternary colluvium and 

alluvium cover the gentle slope and obscure bedrock. Near 

MM 131.8, shale of the Rose Hill Formation contains drill holes 

that resulted from excavation of the canal. 

The Potomac River cuts across the strike of bedding 

from near MM 133 to the Sideling Hill syncline near MM 

137 (fig. 116). Folded Silurian and Devonian rocks on the 

northwest limb of the Cacapon Mountain anticlinorium may 

be seen from MM 133 west to Dam 6 and Tonoloway Ridge 

(MM 134) (fig. 117). An anticline of tan Keefer Sandstone 

(Sk) is located near MM 133 (fig. 118A). Also present in this 

area are thrust faults that truncate sandstone beds (fig. 118B). 
C 

A B
 

Sandstone

bed 

Figure 109. Upper Devonian Foreknobs Formation. A, 

Fissile, red shale interbedded with thin sandstone in the 

Upper Devonian Foreknobs Formation. B, Thin-bedded, 

red sandstone of the Foreknobs Formation. Both A and B 

are characteristic of the Foreknobs, which is best exposed 

south of the Potomac River on the limbs of the Meadows 

Ground syncline. The sandstone beds become more massive 

and thicker upsection into the Upper Devonian Hampshire 

Formation in the core of the syncline. C, Thick red shale of 

the Foreknobs Formation shown transitioning to thin-bedded 

sandstone and shale of the Upper Devonian Brallier Shale, 

which was last seen near MM 120.2 (see fig. 108). 

Valley and Ridge Province—West of North Mountain Thrust Fault 81 

Drill holes in the sandstone are oriented perpendicular to the 

beds. Tubelike lines of trace fossil Skolithos linearis also are 

present (fig. 118C). The intricately folded rocks along this 

section of the canal (fig. 119) are best exposed to the south 

along the Cacapon River at Fluted Rocks, W. Va. (fig. 120). 

Here, the folds are secondary to the larger fold that underlies Cacapon Mountain to the east where the white Tuscarora 

Quartzite is exposed. The folds in these fluted rocks show how 

contrasting rock types, such as sandstone and shale, respond 

differently to deformation. Differences in the geometry of the 

folds (Lessing, 1988) reflect this competency contrast. In the 

Potomac River near MM 133.5, Davies (1999) noted a fish 

weir constructed by Native Americans. 

Resistant, cherty, fossiliferous limestone of the Upper 

Silurian and Lower Devonian Keyser Limestone (DSk) 

(fig. 121) and overlying Lower Devonian Oriskany Sandstone 

(Do) underlie Tonoloway Ridge west of MM 134. The Oriskany was quarried locally for silica-rich sand and dimension 

stone. Like Dam 3 at Harpers Ferry, feeder Dam 6 was built 

above resistant sandstone ledges and shallow waterfalls. Sandstone outcrops with fossil brachiopod molds (fig. 122) form 

the base to the dam abutment. The original dam was a timber crib filled with sandstone rubble and then covered with 

wooden planks, but only the rubble remains (fig. 38). Opposite 

Dam 6 are well-developed fluvial terraces of both the Potomac 

and Cacapon Rivers. 

Stratigraphically above the Oriskany Sandstone are shale, 

siltstone, and sandstone turbidite deposits of the Lower and 

Middle Devonian Needmore Shale and Middle Devonian Marcellus Shale (Dmn) (fig. 123). Westward from MM 134, the 

siltstone and sandstone are folded (fig. 124) (Cloos, 1951). 

The axial region of the Sideling Hill syncline is crossed at the 

Sideling Hill Creek Aqueduct. This syncline is best exposed 

to the north along the Interstate Route 68 roadcut at the State 

of Maryland’s Sideling Hill Exhibit Center (fig. 125). The east 

end of the aqueduct is made up of sandstone taken from the 
 

A 

Bedding 

Cleavage 

B 

Figure 110. Middle Devonian Marcellus Shale southeast of MM 

125 in West Virginia. A, Fissile black shale dipping east on the limb 

of the Cacapon Mountain anticlinorium and displaying west-dipping 

cleavage. B, Local concretions of limestone in the Marcellus that 

were deposited above the Lower Devonian Oriskany Sandstone. 

82 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Figure 111. Historical photograph (1897) of the Devil’s 

Eyebrow, an anticline in rocks 

of the Upper Silurian Bloomsburg Formation at MM 127. 

The shallow cave in the core 

of the anticline behind the 

seated man was formed from 

the weathering of limy shale 

and limestone. Photograph 

by Charles D. Walcott (U.S. 

Geological Survey). 
Silurian Wills Creek Formation Silurian Bloomsburg Formation 

Ruins of Round Top Cleavage Devil’s Eyebrow 

Western Maryland 

Railroad 

North side of railroad Bedding

Covered 

covered 

figure 127A Level of canal figure 127B 

South side of railroad 

Cave 

60 FEET 

0 

Cement Company mill and towpath figure 125 

0 100 FEET 

Valley and Ridge Province—West of North Mountain Thrust Fault 83 

Figure 112. Sketch of the Devil’s Eyebrow and ruins of Round Top Cement Company mill west of Hancock, Md. (modified from Davies, 

1989). 

A B
 

Figure 113. Folds in Upper Silurian Bloomsburg and Wills Creek Formations. In both photographs, note that the cleavage is nearly perpendicular to the folded beds. A, Lime kilns of the Round Top Cement Company mill situated above an anticline of Bloomsburg Formation 

west of MM 127 and the Devil’s Eyebrow, which is also composed of Bloomsburg. B, Historical photograph (1897) showing intervening 

syncline in Wills Creek Formation. Photograph by Charles D. Walcott (U.S. Geological Survey). 

Bedding 

Anticline axis 

Figure 114. Historical photograph (1897) looking 

down the axis of an anticline. Limestone in the core 

of the fold was mined on the side of Round Top. 

Timber was used to support the rock during mining. 

Photograph by Charles D. Walcott (U.S. Geological 

Survey). 
84 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Figure 115. Shale of the Middle Silurian Rose Hill Formation, 

which was drilled and excavated near Leopards Mill (MM 130). 

These are some of the oldest rocks exposed along the C&O Canal 

in the Valley and Ridge province. Note that bedding dips to the left 

and joints dip to the right. 

Figure 116. Historical photograph (date unknown) from the overlook on Cacapon Mountain in West Virginia, looking west toward the town 

of Great Cacapon, W. Va. (on left side of photograph). Note the fresh rock exposed along the canal and railroad excavations in the lower 

right (illustrated in figure 117). The nearly flat open pastures amid forests on the hills on either side of the Potomac River are terraces of the 

ancestral river deposited during early Quaternary or possibly Tertiary time. Similar terraces south of the Potomac River (to the left, beyond 

Great Cacapon), are younger in age and lower in elevation. Dam 6 and Lock 51 are located at the bend of the river at the end of Tonoloway 

Ridge, in the center of the photograph. Photograph courtesy of the National Park Service. 
West 

A A' 

Wills Creek 

Formation 

Helderberg 

Limestone 

Oriskany Sandstone 

Dam 6 Lock 51 

Rock covered with 

vegetation or colluvium 

figure 119 

East 

A' A" 

Bloomsburg 

Formation 

McKenzie 

Formation 

Western 

Maryland 

Railroad 

MM 133 figure 118 Canal Keefer 

0 100 FEET towpath Sandstone 

Figure 117. Geologic sketch of the bedrock along the section of the C&O Canal from east of MM 133 west to Dam 6 and Tonoloway 

Ridge (see figure 116), showing the folded and faulted Silurian and Devonian rocks on the west limb of the Cacapon Mountain anticlinorium. Locations of figures 118 and 119 are shown. 

Canal 

Thrust fault 

Anticline axis 

Bedding 

Quarter 

for scale 

Skolithos linearis 

(worm burrows) 

A B
 

C 

Bedding 

Cleavage 

Fault zone Figure 118. Middle Silurian Keefer Sandstone. A, An anticline of 

Keefer Sandstone exposed along the canal east of MM 133 (locations shown on figure 117). Compare the style of folding seen here 

with the chevron folds shown in figure 120. The thrust fault on the 

eastern limb of the fold is shown in B. Outcrop is about 15 ft high. 

B, Thrust fault, which formed during flexural-slip folding of the 

beds. Sandstone beds are fractured and truncated beneath the 

fault, which is in the more ductile and cleaved shale. C, Outcrop 

showing Skolithos linearis (trace fossil worm burrows) perpendicular to sandstone beds. Burrows are similar to those seen in 

the Lower Cambrian Antietam Formation near Harpers Ferry, W. 

Va. (fig. 86B). Quarter for scale. 

Valley and Ridge Province—West of North Mountain Thrust Fault 85 
 

Cleavage

WEST Wills Creek Formation Anticline

Thrust fault 

Bedding Cleavage Syncline EAST 

Figure 119. Geologic sketch (after Cloos, 1951) of folded and faulted rocks of the Upper Silurian Wills Creek Formation east of MM 134. 

Outcrop is about 15 ft high. 

West East 

McKenzie 

Formation 

Keefer 

Sandstone 

Location of 

part B

 0 100 200 300 400 500 FEET 

Fault 

Rose Hill 

Formation 

300 

200 

100 

FEET 

0 

Location of part C 

A
 

B 

C 

86 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Figure 120. Folded and faulted strata at Fluted Rocks, along the 

Cacapon River in West Virginia. A, Geologic sketch showing Silurian 

rocks of the Rose Hill Formation, Keefer Sandstone, and McKenzie 

Formation folded into anticlines and synclines (modified from Lessing, 1988). B and C are historical photographs (around 1910) showing intricately folded and faulted rocks. Photographs by George W. 

Stose (U.S. Geological Survey). 
 

 

Valley and Ridge Province—West of North Mountain Thrust Fault 87 

Chert 

A B
 

Figure 121. Upper Silurian and Lower Devonian Helderberg Limestone. A, Chert within the Helderberg Limestone. B, Anthozoans (coral 

fossils) are abundant in the Helderberg Limestone. 

A B 

Figure 122. Brachiopods 

in Lower Devonian Oriskany 

Sandstone underlying Tonoloway Ridge at Dam 6. A, The 

Oriskany Sandstone contains 

abundant molds of Spiriferida 

shells, which were deposited 

on a sandy beach. Quarter 

for scale. B, Cross sections of 

brachiopod molds. 

Shale 

Sandstone

Sandstone 

Sandstone

Figure 123. Outcrop of Lower and Middle Devonian Needmore 

Shale, which was the first shale unit deposited as part of the thick 

Devonian clastic sequence. The Needmore contains thin sandstone beds and is exposed above the Lower Devonian Oriskany 

Sandstone immediately west of Tonoloway Ridge near MM 134.2. 

Figure 124. Historical photograph (undated) showing folded and 

faulted shale and sandstone of the Upper Devonian Brallier Shale 

exposed above the C&O Canal along the railroad at MM 135 near 

the Woodmont station. Chevron folds are in sandstone beds about 

3 in thick. Photograph by George W. Stose (U.S. Geological Survey). 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

88 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

adjacent outcrop of the basal part of the Upper Devonian and 

Lower Mississippian Rockwell Formation (MDr) (fig. 126), 

which is the youngest unit seen along the canal west of Point 

of Rocks, Md. The best exposures of the Rockwell are along 

the abandoned railroad just north of the canal. 

Rockwell 

Formation 

Purslane 

Formation 

Coal 

Figure 125. Mississippian rocks of the Sideling Hill syncline 

exposed in the Interstate Route 68 roadcut through Sideling Hill 

at the State of Maryland’s Sideling Hill Exhibit Center. The black 

seams are the oldest beds of coal deposited in the region. 

Figure 126. Sandstone in the Upper Devonian and Lower Mississippian Rockwell Formation exposed along the Sideling Hill Creek 

Aqueduct at MM 136.5. The sandstone consists of cemented 

vein-quartz gravel which may have been derived from rocks of 

the Piedmont province exposed and eroded by tectonic activity 

associated with the Acadian orogeny. 

Sideling Hill Aqueduct to Paw Paw Tunnel 

(MM 137 to MM 156) 

The Broadtop synclinorium (the largest fold structure of 

this region) occurs between the Cacapon Mountain anticlinorium (MM 133) and the Broadtop anticlinorium (MM 167 at 

Oldtown) (fig. 104). The Broadtop synclinorium consists of 

numerous secondary folds such as the Sideling Hill syncline, 

Whip Cove anticlinoria, and Town Hill syncline. The Potomac 

River meanders across the rocks of the Broadtop synclinorium to form the incised channels east of Paw Paw, W. Va. (fig. 

104). A composite geologic sketch shows parts of the structure (fig. 127 and plate 17). From MM 137 to MM 138 are 

east-dipping rocks of the Upper Devonian Hampshire Formation (Dh) on the west limb of the Sideling Hill syncline. 

West of Indigo Neck campsite are the underlying rocks of the 

Upper Devonian Foreknobs Formation (Df). From about MM 

140 west to the old railroad bridge are folded rocks of the 

Upper Devonian Brallier Shale (Db) that form an anticlinorium. Figure 127 also illustrates a tight anticline exposed west 

of MM 143. Like the Devil’s Eyebrow anticline at the Round 

Top Cement Company mill (MM 127.5), the hinge of this anticline forms a “cave,” which actually is an overhang. In this 

region, the shale bedrock and thin soil form a unique ecosystem known locally as the shale barrens. Plants such as cactus 

prefer the stratum and relatively dry climate. To the west, large 

folds of the Foreknobs and Brallier are exposed along the cliffs 

at MMs 145, 152 and 154. 

Beyond Lock 62 (near MM 154.2) is the Paw Paw Tunnel, which is perhaps the most famous manmade structure associated with the C&O Canal. The approach to the tunnel follows 

the route of a former tributary through three lift locks, two of 

which have fraction designations (Locks 631⁄3, 642⁄3, and 66). 

The tunnel was a solution to bypass the construction of a canal 

along bedrock cliffs on four meanders of the Potomac River. In 

order to retain the lock numbering system already in use to the 

west, fraction numbers were required for the additional locks. 

South of Lock 65 near MM 155, the canal was excavated at an 

oblique angle to folded shale and sandstone beds of the Brallier 

Shale. A cross section of the anticline is exposed at the north 

portal to the tunnel (fig. 26). The towpath cuts across the axis 

of the fold, but it mostly follows the northwest limb of the anticline and the beds dip toward it (fig. 128A). Steel rock bolts 

were installed by NPS to prevent bedrock landslides along bedding planes onto the towpath. On the opposite wall of the canal, 

the rocks dip into the slope. These rocks occasionally break 

along joints and fall into the canal. These slope failures suggest 

that the 12-year construction of the 3,118-ft-long tunnel was 

both a major accomplishment and hazardous. Exposed along 

the towpath are ancient mud cracks that are accentuated by 

cleavage that refracts through them (fig. 128B). The shale beds 

in the Brallier have fossil burrows and tracks (fig. 128C) made 

by Devonian organisms that crawled through the mud before 

it was lithified. Slickensides of vein quartz, which is parallel 

to the shale beds, formed as the beds flexed and slipped during 

folding (fig. 128D). 
Valley and Ridge Province—West of North Mountain Thrust Fault 

Foreknobs Foreknobs Hampshire Brallier Shale Formation Formation Formation 

MM 143.3 Canal MM 143 MM 140 Lock 57 MM 138 MM 137 

Location of part B Indigo Tunnel 

A 

Location of part C Brallier Shale 

MM 143.3 

MM 143 

B 

Shale 

Sandstone 

Figure 127. Structures in the 

Upper Devonian Brallier Shale. 

A, Composite geologic sketch 

Small from MM 137 west to MM 138, 

thrust from Lock 57 west to the west 

fault portal of Indigo Tunnel, and 

from MM 143 west to the old 

railroad bridge (MM 143.5), 

illustrating the folded strata on 

the west limb of the Sideling 

Hill syncline. B, Detailed sketch 

of the anticlinorium of Brallier 

Shale centered on MM 143. 

See part C for enlargement of 

center fold west of MM 143. 

C, Photograph and sketch of a Canal tight, upright anticline in thin 

sandstone and shale of the 

C Brallier Shale west of MM 143. 

89 
 

Bedding

plane 

Canal 

Cleavage Bedding 

B 

Rock bolts 

Towpath 

C 

A
 

Figure 128. Upper Devonian Brallier Shale near MM 155. A, West limb 

of an anticline, looking down from above the north portal of the Paw Paw 

Tunnel. The towpath and canal transect the anticline (see figure 26) so that 

bedrock of the west limb dips into the towpath and canal. The bedrock on 

the dip slope required bolts to prevent gravity slides along the bedding plane. 

B, Shale beds along the towpath north of the Paw Paw Tunnel with mudcracks accentuated by cleavage. Pen for scale. C, Burrows and trails made 

by organisms before the mud was lithified to rock. Pen for scale. D, Quartz 

slickensides along fault surfaces that are parallel to bedding. Pen for scale. D 

Slickensides 

90 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Paw Paw Tunnel to Oldtown (MM 156 to MM 167) 

The Potomac River branches into the South Branch 

West of the exit of the Paw Paw Tunnel, the area is under- Potomac River and the North Branch Potomac River west 

lain by the same Devonian shale, siltstone, and sandstone of MM 164.5. Like the Cacapon (MM 133.5) and Little 

sequence described between MM 135 and MM 156 (plate 18). Cacapon (MM 159) Rivers, there are well-developed incised 

Abandoned incised meanders and gravel deposits from the meanders near the confluence of these major tributaries and 

ancestral Potomac River and Little Cacapon River are found the Potomac River. Near MM 161.5, the canal and towpath 

in the valley of Purslane Run (MM 157.3) and at Reckley Flat cross the axial region of the Town Hill syncline. The red, fis­

(around MM 159) (figs. 11 and 129) (see the previous dis- sile shale and thin sandstone (fig. 130A), as well as crossbedcussion in the section on Abandoned Entrenched Meanders). ded sandstone (fig. 130B) are rocks of the Upper Devonian 

Folds on the west-dipping east limb of the Town Hill syn- Hampshire Formation (Dh). The Potomac River cut through 

cline are seen at MM 159.5 before the axial region (center) is an anticline of the Lower Devonian Oriskany Sandstone (Do) 

crossed at about MM 161. From here to Lock 68 (Crabtrees near MM 165 and parts of its northwest-dipping flank were 

Lock at MM 164.7), the Devonian rocks dip east on the west drilled and blasted to create the canal (fig. 131). Note the 

limb of the Town Hill syncline. pine trees and mountain laurel that prefer to grow on the thin 
Valley and Ridge Province—West of North Mountain Thrust Fault 91 

A B
 

Figure 129. Reckley Flat, north of MM 160. A, Reckley Flat is a terrace, as shown by the broad, open landscape. B, Fluvial silt, sand, 

and gravel derived from sandstone above the shale bedrock that underlies Reckley Flat. 

A B
 

Figure 130. Devonian Hampshire Formation, exposed east of Town Creek on the limb of the Town Creek syncline, north of MM 161.5 

along Maryland State Route 51. A, Red, fissile shale and thin sandstone beds. B, Thicker beds of crossbedded sandstone. Rocks shown 

in A grade upward into the thicker beds shown in B. 

Figure 131. Lower Devonian Oriskany Sandstone on the west 

limb of the Broadtop anticline. The sandstone was excavated 

locally to construct the C&O Canal and towpath east of Oldtown 

near MM 148. Pine trees and mountain laurel prefer the thin sandy 

soil that develops on the sandstone. 

sandy soil. From about MM 165.5, the canal diverges from 

the Potomac River west to Oldtown, an early settlement and 

river crossing for the Great Warrior Path, which was a principal Native American east-west migration route over the 

mountains. 

Oldtown to Spring Gap (MM 167 to MM 173.5) 

West of Oldtown, the Potomac River cuts across the strike 

of Devonian rocks that form an unnamed syncline and rocks of 

the Patterson Creek anticlinorium (plate 19). The canal at Oldtown was excavated in shale of the Middle Devonian Mahantango Formation (Dm) in an abandoned valley of Mill Run 

because there are bluffs of shale along the Potomac River that 

prevented construction of a canal and towpath. North of MM 

168, both the canal and towpath were excavated through fissile Lower and Middle Devonian Needmore Shale and Middle Devonian Marcellus Shale (Dmn) in order to rejoin the bank 

of the Potomac River (fig. 40B; also see Engineering section). 

From here west to Spring Gap (MM 173.5), the area is characterized by shale barrens and a broad flood plain developed on an 

anticline and syncline. There are good outcrops of folded Upper 

Devonian Brallier Shale (Db) on the limb of the syncline near 

MM 172 (fig. 132). Near Spring Gap, the canal crosses anticlines (part of the Patterson Creek anticlinorium) underlain by 

resistant Lower Devonian Oriskany Sandstone (Do) 

Spring Gap to Cumberland (MM 173.5 to 

MM 184.5) 

Alluvial-terrace deposits left by the Potomac River, when 

it flowed almost 100 ft higher in elevation than at present, may 
 

Syncline 

Anticline 

Rail bed 

figure 132B 

Canal 

A
 

B 

92 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Figure 132. Upper Devonian Brallier Shale near MM 172. A, 

Geologic sketch showing folded Brallier Shale (after Davies, 

1999). Distance between C&O Canal and Western Maryland 

Railroad is about 20 ft. B, Recessed area of anticline illustrated 

on left side of A. 

Figure 133. Historical photograph (around 1920) showing 

the C&O Canal and wharf area 

at Cumberland where boats 

were loaded with coal for 

transport eastward. Photograph by Consolidated Coal 

Company, courtesy of National 

Park Service. 
 

 

 

References Cited 93 

be seen north of MM 174 (plate 20). The Lower Devonian 

Oriskany Sandstone (Do) is exposed again at MM 175. From 

the west flank of the westernmost anticline, the Potomac River 

meanders across the Upper Devonian Brallier Shale (Db) of the 

broad Evitts Creek-Bedford syncline. 

For the next 3 mi, the canal crosses a large meander of 

the Potomac River now occupied by Mexico Farms, whose 

surface contains large, rounded sandstone boulders deposited by the river during the Pleistocene. To the south in West 

Virginia, above the Potomac River, is an elevated abandoned 

meander called Death Valley. To the north, the city of Cumberland and its municipal airport are built on similar elevated terraces. North of MM 180, hundreds of feet above the current 

river level, are excellent roadcuts that reveal Quaternary alluvial deposits on bedrock (see figure 10). From MM 181 to the 

National Park Service Cumberland Visitor Center, the canal 

follows a meander bend and the only bedrock units exposed 

are the Brallier Shale (Db) and the Middle and Lower Devonian Needmore and Marcellus Shales (Dmn) near MM 184. 

Historical photographs are all that remain to show what 

this region looked like when the canal was in operation 

(fig. 133) because much of the canal’s operating equipment 

has been removed and the canal has been filled in. Although 

this point represents the end of the east-to-west traverse, to 

many of its past travelers, Cumberland was an outpost on the 

western frontier where coal-filled canal boats began their journey eastward to supply the National capital region with heating fuel. 

Acknowledgments 

The authors gratefully acknowledge the work of many 

people that helped with this project: Douglas Faries, William 

Spinrad, and Marie Frias (National Park Service (NPS)-Chesapeake and Ohio Canal National Historical Park); Barbara 

Perdew (NPS-Great Falls Park); David Fox and William Hebb 

(NPS-Harpers Ferry National Historical Park); John Glaser 

(Maryland Geological Survey, retired); Robert E. Weems (U.S. 

Geological Survey (USGS)); Avery A. Drake, Jr., (USGS, 

Scientist Emeritus); E-an Zen (USGS, Scientist Emeritus); 

and Stanley Dickinson (USGS, Volunteer For Science). The 

authors thank Erin Caulfield, Peter Chirico, Carrie Fingeret, 

Angela Jacobini, Vicki Keegan, Kerry Lagueux, Remo Nardini, James Reddy, Phillip Reiss, and Christina Spiker for 

computer graphics and geographic information systems (GIS) 

work. Technical reviews by Arthur P. Schultz (USGS) and 

David B. Stewart (USGS, Scientist Emeritus) and editorial 

review by Elizabeth D. Koozmin (USGS) greatly improved the 

map and report. The late William E. Davies (USGS) is especially acknowledged for providing inspiration for this project. 

References Cited 

Aleinikoff, J.N., Burton, W.C., Lyttle, P.T., Nelson, A.E., and 

Southworth, C.S., 2000, U-Pb geochronology of zircon and 

monazite from Mesoproterozoic granitic gneisses of the 

northern Blue Ridge, Virginia and Maryland, USA: Precambrian Research, v. 99, no. 1–2, p. 113–146. 

Aleinikoff, J.N., Horton, J.W., Jr., Drake, A.A., Jr., and Fanning, C.M., 2002, SHRIMP and conventional U-Pb ages 

of Ordovician granites and tonalites in the central Appalachian piedmont; implications for Paleozoic tectonic events: 

American Journal of Science, v. 302, p. 50–75. 

Aleinikoff, J.N., Zartman, R.E., Walter, Marianne, Rankin, 

D.W., Lyttle, P.T., and Burton, W.C., 1995, U-Pb ages of 

metarhyolites of the Catoctin and Mount Rogers Formations, central and southern Appalachians; evidence for two 

pulses of Iapetan rifting: American Journal of Science, 

v. 295, no. 4, p. 428–454. 

Badger, R.L., and Sinha, A.K., 1988, Age and Sr isotopic signature of the Catoctin volcanic province; implications for subcrustal mantle evolution: Geology, v. 16, no. 8, p. 692–695. 

Bassler, R.S., 1919, Cambrian and Ordovician: Baltimore, Md., 

Maryland Geological Survey Systematic Report, 424 p. 

Becker, J.L., Kunk, M.J., Wintsch, R.P., and Drake, A.A., 

Jr., 1993, Evidence for pre-Taconic metamorphism in the 

Potomac terrane, Maryland and Virginia; hornblende and 

muscovite 40Ar/39Ar results [abs.]: Geological Society of 

America Abstracts with Programs, v. 25, no. 2, p. 5. 

Boy Scouts of America, 1983, 184 miles of adventure; hiker’s 

guide to the C&O Canal: Washington, D.C., Boy Scouts of 

America, 48 p. 

Braun, D.D., 1976, The relation of rock resistance to incised 

meander form in the Appalachian Valley and Ridge Province: Baltimore, Md., Johns Hopkins University, unpublished Ph.D. thesis, 246 p. 

Brezinski, D.K., 1988, The Mississippian System in Maryland: Maryland Geological Survey Report of Investigations 

52, 75 p. 

Brezinski, D.K., 1992, Lithostratigraphy of the western Blue 

Ridge cover rocks in Maryland: Maryland Geological Survey Report of Investigations 55, 69 p. 

Brezinski, D.K., 1996a, Stratigraphy of the Elbrook Formation in Maryland and adjacent states, in Studies in Maryland 

geology: Maryland Geological Survey Special Publication 

3, p. 165–185. 

Brezinski, D.K., 1996b, Carbonate ramps and reefs, Guidebook 

for the Sixth North American Paleontological Convention: 

Maryland Geological Survey Fieldtrip Guidebook 6, 25 p. 
 

 

 

 

 

94 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Brezinski, D.K., Anderson, T.H., and Campbell, P., 1996, 

Evidence for a regional detachment at the base of the Great 

Valley sequence in the central Appalachians: Maryland 

Geological Survey Special Publication 3, p. 223–230. 

Brezinski, D.K., Repetski, J.E., and Taylor, J.F., 1999, Stratigraphic and paleontologic record of the Sauk III regression in 

the central Appalachians, in Santucci, V.L., and McClelland, 

Lindsay, eds., National Park Service, Paleontological Research, 

v. 4: National Park Service Geologic Resources Division Technical Report NPS/NRGRD/GRDTR-99/03, p. 32–41. 

Brown, A.C., 1963, America’s greatest eighteenth century engineering achievement: Virginia Cavalcade, v. 12, p. 40–47. 

Burton, W.C., Froelich, A.J., Pomeroy, J.S., and Lee, K.Y., 

1995, Geology of the Waterford quadrangle, Virginia and 

Maryland, and the Virginia part of the Point of Rocks quadrangle: U.S. Geological Survey Bulletin 2095, 30 p., scale 

1:24,000. 

Burton, W.C., and Southworth, Scott, 1996, Basement rocks of 

the Blue Ridge Province in Maryland, in Studies in Maryland geology: Maryland Geological Survey Special Publication 3, p. 187–203. 

Clague, William, 1977, A collection of maps of the Chesapeake 

and Ohio Canal: Bethesda, Md., [privately published], 84 p. 

Cloos, Ernst, 1951, The physical features of Washington 

County: Baltimore, Maryland Department of Geology, 

Mines, and Water Resources, 333 p. 

Cloos, Ernst, 1958, Structural geology of South Mountain 

and Appalachians in Maryland, Guidebooks 4 and 5: Johns 

Hopkins University Studies in Geology 17, p. 1–76. 

Cloos, Ernst, 1964, Wedging, bedding plane slips, and gravity tectonics in the Appalachians, in Tectonics of the southern Appalachians: Virginia Polytechnic Institute and State University, 

Department of Geological Sciences Memoir 1, p. 63–70. 

Davies, W.E., 1971, Historical engineering geology of the 

Chesapeake and Ohio Canal [guidebook]: Washington, 

D.C., Geological Society of Washington, 10 p. 

Davies, W.E., 1989, Highlights of the geology and engineering 

of the Chesapeake and Ohio Canal, in Hanshaw, P.M., ed., 

28th International Geological Congress Guidebook T206: 

Washington, D.C., American Geophysical Union, 25 p. 

Davies, W.E., 1999, The geology and engineering structures 

of the Chesapeake and Ohio Canal: Glen Echo, Md., C&O 

Canal Association, 617 p. 

Dean, S.L., Kulander, B.R., and Lessing, Peter, 1987, Geology of the Hedgesville, Keedysville, Martinsburg, Shepherdstown, and Williamsport quadrangles, Berkeley and 

Jefferson Counties, West Virginia: West Virginia Geological 

and Economic Survey Map WV–31, scale 1:24,000. 

Dean, S.L., Lessing, Peter, and Kulander, B.R., 1995, Geology 

of the Hancock quadrangle, Morgan County, West Virginia: 

West Virginia Geological and Economic Survey Publication 

OF–9503, scale 1:24,000, 10 p. 

Dean, S.L., Lessing, Peter, Kulander, B.R., and Langenderfer, 

E.A., 1995, Geology of the Cherry Run quadrangle, Morgan 

County, West Virginia: West Virginia Geological and Economic Survey Publication OF–9505, scale 1:24,000, 10 p. 

Denkler, K.E., and Harris, A.G., 1988, Conodont-based determination of the Silurian-Devonian boundary in the Valley 

and Ridge province, northern and central Appalachians: 

U.S. Geological Survey Bulletin 1837, p. B1–B17. 

De Witt, Wallace, Jr., and Colton, G.W., 1964, Bedrock geology of the Evitts Creek and Pattersons Creek quadrangles, 

Maryland, Pennsylvania, and West Virginia: U.S. Geological Survey Bulletin 1173, 90 p., scale 1:24,000. 

Drake, A.A., Jr., 1989, Metamorphic rocks of the Potomac 

Terrane in the Potomac Valley of Virginia and Maryland, in 

Hanshaw, P.M., ed., 28th International Geological Congress 

Field Trip Guidebook T202: Washington, D.C., American 

Geophysical Union, 22 p. 

Drake, A.A., Jr., and Froelich, A.J., 1997, Geologic map of 

the Falls Church quadrangle, Fairfax and Arlington Counties, and the city of Falls Church, Virginia, and Montgomery 

County, Maryland: U.S. Geological Survey Geologic Quadrangle Map GQ–1734, scale 1:24,000. 

Drake, A.A., Jr., and Lee, K.Y., 1989, Geologic map of the 

Vienna quadrangle, Fairfax County, Virginia: U.S. Geological Survey Geologic Quadrangle Map GQ–1670, scale 

1:24,000. 

Drake, A.A., Jr., Lee, K.Y., and Southworth, Scott, 1999, 

Geologic map of the Seneca quadrangle, Montgomery 

County, Maryland, and Loudoun and Fairfax Counties, 

Virginia: U.S. Geological Survey Geologic Quadrangle Map 

GQ–1802, scale 1:24,000. 

Drake, A.A., Jr., and Morgan, B.A., 1981, The Piney Branch 

Complex—A metamorphosed fragment of the central Appalachian ophiolite in northern Virginia: American Journal of 

Science, v. 281, no. 4, p. 484–508. 

Drake, A.A., Jr., Sinha, A.K., Laird, Jo, and Guy, R.E., 1989, 

The Taconic orogen, in Hatcher, R.D., Jr., Thomas, W.V., and 

Viele, G.W., eds., The Appalachian-Ouachita orogen in the 

United States: Denver, Colo., Geological Society of America, 

The Geology of North America, v. F–2, p. 362–366. 

Fitzpatrick, M.F., 1987, Active and abandoned incised 

meanders of the Potomac River, south of Little Orleans, 

Maryland, Geological Society of America Centennial Field 

Guide—Northeastern Section: Boulder, Colo., Geological 

Society of America, p. 1–4. 
References Cited 95 

Fleming, A.H., Drake, A.A., Jr., and McCartan, Lucy, 1994, 

Geologic map of the Washington West quadrangle, District 

of Columbia, Montgomery and Prince Georges Counties, 

Maryland, and Arlington and Fairfax Counties, Virginia: U.S. Geological Survey Geologic Quadrangle Map 

GQ–1748, scale 1:24,000. 

Franz, Richard, and Slifer, Dennis, 1976, Caves of Maryland: 

Maryland Geological Survey Educational Series 3, 120 p. 

Froelich, A.J., 1975, Thickness of overburden map of Montgomery County, Maryland. U.S. Geological Survey Miscellaneous Investigations Series Map I–920–B, scale 1:62,500. 

Froelich, A.J., Hoffman, M.F., Taunton, S., and Phelan, D.J., 

1974, Cleavage in some sedimentary rocks of the central 

Valley and Ridge Province, Maryland: Geological Society 

of America Bulletin, v. 85, no. 9, p. 1399–1412. 

Froelich, A.J., Hoffman, M.F., Taunton, S., and Phelan, D.J., 

1992, Preliminary results of coring surficial deposits in the 

Winchester 30 X 60 minute quadrangle, West Virginia and 

Virginia: U.S. Geological Survey Open-File Report 92–395, 

46 p. 

Frye, Joshua, and Jefferson, Peter, 1775, A map of the most 

inhabited part of Virginia: Richmond, Va., Virginia State 

Library. 

Garrett, W.E., and Garrett, Kenneth, 1987, George Washington’s Patowmack Canal: National Geographic, v. 171, no. 6, 

p. 716–753. 

Geiser, P.A., 1970, Deformation of the Bloomsburg Formation 

in the Cacapon Mountains anticline, Hancock, Maryland: 

Baltimore, Md., Johns Hopkins University, unpublished 

Ph.D. thesis, 233 p. 

Glaser, J.D., 1987, The Silurian section at Roundtop Hill near 

Hancock, Maryland, Geological Society of America Centennial Field Guide—Northeastern Section: Boulder, Colo., 

Geological Society of America, p. 5–8. 

Glaser, J.D., 1994a, Geologic map of the Bellegrove quadrangle, Allegany and Washington Counties, Maryland: Baltimore, Md., Maryland Geological Survey, scale 1:24,000. 

Glaser, J.D., 1994b, Geologic map of the Cresaptown quadrangle, Allegany County, Maryland: Baltimore, Md., Maryland 

Geological Survey, scale 1:24,000. 

Glaser, J.D., 1994c, Geologic map of the Oldtown quadrangle, 

Maryland: Baltimore, Md., Maryland Geological Survey, 

scale 1:24,000. 

Glaser, J.D., 1994d, Geologic map of the Paw Paw quadrangle, 

Allegany County, Maryland: Baltimore, Md., Maryland 

Geological Survey, scale 1:24,000. 

Glaser, J.D., and Brezinski, D.K., 1994, Geologic map of the 

Cumberland quadrangle, Allegany County, Maryland: Baltimore, Md., Maryland Geological Survey, scale 1:24,000. 

Gore, P.J.W., 1988, Late Triassic and Early Jurassic lacustrine sedimentation in the Culpeper basin, Virginia, in 

Manspeizer, Warren, ed., Triassic-Jurassic rifting; continental breakup and the origin of the Atlantic Ocean and 

passive margins: Developments in Geotectonics 22, pt. A, 

p. 369–400. 

Guzy, Dan, 1999, Fish weirs in the upper Potomac River: 

Maryland Archeology, v. 35, no. 1, p. 1–24. 

Hahn, T.F., 1985, The Chesapeake and Ohio Canal old picture 

album: Shepherdstown, W. Va., American Canal and Transportation Center, 103 p. 

Hahn, T.F., 1995, Towpath guide to the Chesapeake & Ohio 

Canal: Shepherdstown, W. Va., American Canal and Transportation Center, 226 p. 

Hahn, T.F., and Kemp, E.L., 1994, Cement mills along the 

Potomac River: Institute for the History of Technology 

and Industrial Archaeology Monograph Series, v. 2, no. 1, 

90 p. 

Hall, A.J., 1991, L’Enfant’s Washington: National Geographic, 

v. 180, no. 2, p. 122–134. 

Harris, A.G., Stamm, N.R., Weary, D.J., Repetski, J.E., 

Stamm, R.G., and Parker, R.A., 1994, Conodont color 

alteration index (CAI) map and conodont-based age 

determinations for the Winchester 30’ × 60’ quadrangle, 

Virginia, West Virginia, and Maryland: U.S. Geological 

Survey Miscellaneous Field Studies Map MF–2239, 40 p., 

1 sheet. 

Hatcher, R.D., Jr., Thomas, W.A., and Viele, G.W., eds., 1989, 

The Appalachian-Ouachita orogen in the United States: 

Boulder, Colo., Geological Society of America, The Geology of North America, v. F–2, 767 p. 

High, Mike, 1997, The C&O Canal companion: Baltimore, 

Md., The Johns Hopkins University Press, 269 p. 

Hoy, R.B., and Schumacher, R.L., 1956, Fault in Paleozoic 

rocks near Frederick, Maryland: Geological Society of 

America Bulletin, v. 67, no. 11, p. 1521–1528. 

Jefferson, Thomas, 1982, Notes on the State of Virginia: New 

York, Norton Publishers, 315 p. 

Keith, Arthur, 1894, Description of the Harpers Ferry quadrangle [Va.-Md.-W. Va.]: U.S. Geological Survey, Geologic 

Atlas, Folio 10, 5 p., 4 map sheets, scale 1:125,000. 

Kranz, Peter, 1989, Dinosaurs in Maryland: Maryland Geological Survey Educational Series 6, 34 p. 

Kulander, B.R., Dean, S.L., and Lessing, P., 1997, Geology of 

the Oldtown quadrangle, Hampshire County, West Virginia: 

West Virginia Geological and Economic Survey Open-File 

Report OF–9703, 6 p., scale 1:24,000. 
 

 

96 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Kulander, B.R., Lessing, P., and Dean, S.L., 1997, Geology 

of the Paw Paw and Artemas quadrangles, Hampshire and 

Morgan Counties, West Virginia: West Virginia Geological and Economic Survey Open-File Report OF–9702, 6 p., 

scale 1:24,000. 

Kunk, M.J., Froelich, A.J., and Gottfried, David, 1992, Timing 

of emplacement of diabase dikes and sheets in the Culpeper 

basin and vicinity, Virginia and Maryland; 40Ar/39Ar age 

spectrum results from hornblende and K-feldspar in granophyres [abs.]: Geological Society of America Abstracts with 

Programs, v. 24, no. 2, p. 125. 

Landing, Ed, 1994, Precambrian-Cambrian boundary global 

stratotype ratified and a new perspective of Cambrian time: 

Geology, v. 22, no. 2, p. 179–182. 

Lee, K.Y., 1979, Triassic-Jurassic geology of the northern part 

of the Culpeper basin, Virginia and Maryland: U.S. Geological Survey Open-File Report 79–1557, 29 p., 16 sheets, 

scale 1:24,000. 

Lessing, Peter, 1988, Fluted rocks; a magnificent exposure to 

the forces of mother nature: Mountain State Geology, v. 1987, 

p. 20–28. 

Lessing, P., Dean, S.L., Kulander, B.R., and Langenderfer, 

E.A., 1995, Geology of the Big Pool quadrangle, Berkeley 

and Morgan Counties, West Virginia: West Virginia Geological and Economic Survey Open-File Report OF–9502, 

14 p., scale 1:24,000. 

Lessing, P., Dean, S.L., and Kulander, B.R., 1997, Geology 

of the Great Cacapon and Bellegrove quadrangles, Morgan County, West Virginia: West Virginia Geological and 

Economic Survey Open-File Report OF–9701, 6 p., scale 

1:24,000. 

Lindholm, R.C., Hazlett, J.M., and Fagin, S.W., 1979, Petrology of Triassic-Jurassic conglomerates in the Culpeper 

basin, Virginia: Journal of Sedimentary Petrology, v. 49, 

no. 4, p. 1245–1262. 

McBride, E.F., 1962, Flysch and associated beds of the Martinsburg Formation (Ordovician), central Appalachians: 

Journal of Sedimentary Petrology, v. 32, no. 1, p. 39–91. 

National Park Service, 1991, Chesapeake and Ohio Canal 

Official National Park Handbook 142: Washington, D.C., 

National Park Service, 111 p. 

Neuman, R.B., 1951, St. Paul Group; a revision of the “Stones 

River” group of Maryland and adjacent states: Geological 

Society of America Bulletin, v. 62, no. 3, p. 267–324. 

Nickelsen, R.P., 1956, Geology of the Blue Ridge near Harpers Ferry, West Virginia: Geological Society of America 

Bulletin, v. 67, no. 3, p. 239–269. 

Olsen, P.E., McCune, A.R., and Thomson, K.S., 1982, Correlation of the early Mesozoic Newark Supergroup by 

vertebrates, principally fishes: American Journal of Science, 

v. 282, no. 1, p. 1–44. 

Reed, J.C., Jr., 1981, Disequilibrium profile of the Potomac 

River near Washington, D.C.; a result of lowered base level 

or Quaternary tectonics along the Fall Line?: Geology, v. 9, 

no. 10, p. 445–450. 

Reed, J.C., Jr., and Reed, J.C., 1970, Gold veins near Great 

Falls, Maryland: U.S. Geological Survey Bulletin 1286, 22 p. 

Reed, J.C., Jr., Sigafoos, R.S., and Fisher, G.W., 1980, The 

river and the rocks—The geologic story of Great Falls and 

the Potomac River gorge: U.S. Geological Survey Bulletin 

1471, 75 p. 

Reinhardt, Juergen, 1974, Cambrian off-shelf sedimentation, 

central Appalachians: Society of Economic Paleontologists 

and Mineralogists Special Publication 25, p. 83–112. 

Sando, W.J., 1957, Beekmantown Group (Lower Ordovician) 

of Maryland: Geological Society of America Memoir 68, 

161 p. 

Scheel, E.M., 1995, The Potomac River from Brunswick to 

Van Deventer’s Island and environs of Sugarloaf, Poolesville, Leesburg, Waterford, and The German Settlement: 

Waterford, Va., [privately published], scale about 1:42,000. 

Schwartz, C.K., Prouty, W.F., Ulrich, E.O., and Bassler, R.S., 

1923, Silurian: Baltimore, Md., Maryland Geological Survey, Systematic Report, 794 p. 

Schwartz, C.K., Schuchert, C., Prosser, C.S., Poole, T., and 

Rowe, R.B., 1913a, Lower Devonian; plates: Baltimore, 

Md., Maryland Geological Survey, Systematic Report, 83 p. 

Schwartz, C.K., Schuchert, C., Prosser, C.S., Poole, T., and 

Rowe, R.B., 1913b, Middle and Upper Devonian; plates: 

Baltimore, Md., Maryland Geological Survey, Systematic 

Report, 72 p. 

Smoot, J.P., 1989, Fluvial and lacustrine facies of the early 

Mesozoic Culpeper basin, Virginia and Maryland, in Hanshaw, P.M., ed., 28th International Geological Congress 

Field Trip Guidebook T213: Washington, D.C., American 

Geophysical Union, 15 p. 

Southworth, Scott, 1996, The Martic fault in Maryland and 

its tectonic setting in the central Appalachians, in Studies 

in Maryland geology: Maryland Geological Survey Special 

Publication 3, p. 205–221. 

Southworth, Scott, 1998, Geologic map of the Poolesville 

quadrangle, Frederick and Montgomery Counties, Maryland, and Loudoun County, Virginia: U.S. Geological Survey Geologic Quadrangle Map GQ–1761, scale 1:24,000. 
References Cited 97 

Southworth, Scott, and Brezinski, D.K., 1996a, How the Blue 

Ridge anticlinorium in Virginia becomes the South Mountain anticlinorium in Maryland, in Studies in Maryland 

geology: Maryland Geological Survey Special Publication 

3, p. 253–275. 

Southworth, Scott, and Brezinski, D.K., 1996b, Geology of 

the Harpers Ferry quadrangle, Virginia, Maryland, and West 

Virginia: U.S. Geological Survey Bulletin 2123, 33 p., scale 

1:24,000. 

Southworth, Scott, and Brezinski, D.K., 2003, Geologic map 

of the Buckeystown quadrangle, Frederick and Montgomery 

Counties, Maryland, and Loudoun County, Virginia: U.S. 

Geological Survey Geologic Quadrangle Map GQ–1800, 

scale 1:24,000. 

Southworth, Scott, Burton, W.C., Schindler, J.S., Froelich, 

A.J., Aleinikoff, J.N., and Drake, A.A., Jr., 2006, Geologic 

map of Loudoun County, Virginia: U.S. Geological Survey 

Geologic Investigations Map I–2533, scale 1:50,000. 

Southworth, Scott, Fingeret, Carrie, and Weik, Thomas, 2000, 

Geologic map of the Potomac River gorge; Great Falls Park, 

Virginia, and part of the C&O Canal National Historical 

Park, Maryland: U.S. Geological Survey Open-File Report 

00–264, 1 CD-ROM, scale 1:10,000. 

Stose, G.W., and Stose, A.J., 1944, The Chilhowee group and 

Ocoee series of the Southern Appalachians: American Journal of Science, v. 242, p. 367–416. 

Taylor, J.F., Repetski, J.E., and Roebuck, C.A., 1996, Stratigraphic significance of trilobite and conodont faunas from 

the Cambrian-Ordovician shelfbreak facies in the Frederick 

Valley, Maryland, in Brezinski, D.K., and Reger, J.P., eds., 

Studies in Maryland geology: Maryland Geological Survey 

Special Publication 3, p. 141–164. 

Tucker, R.D., Bradley, D.C., Ver Straeten, C.A., Harris, 

A.G., Ebert, J.R., and McCutcheon, S.R., 1998, New U-Pb 

zircon ages and the duration and division of Devonian 

time: Earth and Planetary Science Letters, v. 158, no. 3–4, 

p. 175–186. 

Trout, W.E., III, 1991, The Goose Creek Scenic River atlas: Lexington, Va., Virginia Canals and Navigations Society, 18 p. 

Weary, D.J., and Harris, A.G., 1994, Early Frasnian (Late 

Devonian) conodonts from the Harrell Shale, western 

foreland fold-and-thrust belt, West Virginia, Maryland, and 

Pennsylvania Appalachians, U.S.A.: Courier Forschungsinstitut Senckenberg 168, p. 195–225. 

Weems, R.E., 1979, A large parasuchian (phytosaur) from the 

Upper Triassic portion of the Culpeper basin of Virginia 

(USA): Proceedings of the Biological Society of Washington, v. 92, no. 4, p. 682–688. 

Weems, R.E., 1987, A Late Triassic footprint fauna from the 

Culpeper basin, northern Virginia (USA): American Philosophical Society Transactions, v. 77, no. 1, p. 1–79. 

Weems, R.E., 1992, A re-evaluation of the taxonomy of Newark Supergroup saurischian dinosaur tracks, using extensive 

statistical data from a recently exposed tracksite near Culpeper, Virginia, in Sweet, Palmer, ed., Proceedings, Twentysixth forum on the geology of industrial minerals: Virginia 

Division of Mineral Resources Publication 119, p. 113–127. 

Weems, R.E., 1993, Stratigraphic distribution and bibliography of fossil fish, amphibians, and reptiles from Virginia: 

U.S. Geological Survey Open-File Report 93–222, 49 p. 

Weems, R.E., and Kimmel, P.G., 1993, Upper Triassic reptile 

footprints and a coelacanth fish scale from the Culpeper 

basin, Virginia: Proceedings of the Biological Society of 

Washington, v. 106, no. 2, p. 390–401. 

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

terrains: New York, Oxford University Press, 464 p. 

Zen, E-an, 1997a, The seven-story 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: U.S. 

Geological Survey Open-File Report 97–60, 142 p. 

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

Potomac River between Great Falls, Maryland, and Hampshire, West Virginia: U.S. Geological Survey Open-File 

Report 97–480, 92 p. 

Appendix 1
 

Appendix 1 

Sources of Data—Geologic Maps of 7.5-Minute Quadrangles 

[Abbreviations are as follows: MGS, Maryland Geological Survey; USGS, U.S. Geological Survey; unpub., unpublished. See References Cited for complete 

reference citation.] 

Artemas 

Kulander and others (1997) 

J.D. Glaser (MGS) and S. Southworth (USGS) (unpub. data, 1996– 

1997) 

Bellegrove 

Glaser (1994a)
 

Lessing and others (1997)
 

D.K. Brezinski (MGS) and S. Southworth (USGS) (unpub. data, 

1996–1997) 

Big Pool 

Lessing and others (1995) 

J.D. Glaser (MGS, unpub. data, 1996–1997) 

D.K. Brezinski (MGS) and S. Southworth (USGS) (unpub. data, 

1996–1997) 

Buckeystown 

Southworth and Brezinski (2003) 

Charles Town 

Nickelsen (1956) 

D.K. Brezinski (MGS), R.C. Orndorff (USGS), and S. Southworth 

(USGS) (unpub. data, 1996–1997) 

Cherry Run 

Dean and others (1995) 

J.D. Glaser (MGS, unpub. data, 1996–1997) 

S. Southworth (USGS, unpub. data, 1996–1997) 

Cresaptown 

Glaser (1994b) 

S. Southworth (USGS, unpub. data, 1996–1997) 

Cumberland 

Glaser and Brezinski (1994) 

S. Southworth (USGS, unpub. data, 1996–1997) 

Evitts Creek 

DeWitt and Colton (1964) 

S. Southworth (USGS, unpub. data, 1996–1997) 

Falls Church 

Drake and Froelich (1997) 

S. Southworth (USGS, unpub. data, 1996–1997) 

Great Cacapon 

Lessing and others (1997) 

S. Southworth (USGS, unpub. data, 1996–1997) 

Hancock 

Dean and others (1995) 

J.D. Glaser (MGS, unpub. data, 1996–1997) 

D.K. Brezinski (MGS), S. Dickinson (USGS volunteer), and S. 

Southworth (USGS) (unpub. data, 1996–1997) 

Harpers Ferry 

S. Southworth (USGS, unpub. data, 1996–1997) 

Hedgesville 

Dean and others (1987) 

J.D. Glaser (MGS, unpub. data, 1996–1997) 

D.K. Brezinski (MGS), R.C. Orndorff (USGS), and S. Southworth 

(USGS) (unpub. data, 1996–1997) 

Keedysville 

Dean and others (1987) 

Brezinski (1992) 

D.K. Brezinski (MGS) and S. Southworth (USGS) (unpub. data, 

1996–1997) 

Leesburg 

Lee (1979) 

S. Southworth (USGS, unpub. data, 1996–1997) 

Oldtown 

Glaser (1994c)
 

Kulander and others (1997)
 

Southworth and Brezinski (1996b)
 

Patterson Creek 

DeWitt and Colton (1964) 

S. Southworth (USGS, unpub. data, 1996–1997) 

Paw Paw 

Glaser (1994d)
 

Kulander and others (1997)
 

D.K. Brezinski (MGS), S. Dickinson (USGS volunteer), and S. 

Southworth (USGS) (unpub. data, 1996–1997) 
102 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Index map showing U.S. Geological Survey 7.5-minute topographic quadrangle coverage of the Chesapeake and Ohio Canal National Historical Park and the Potomac River corridor. 

Cumberland

Cresaptown Oldtown Patterson Creek Paw Paw Great Cacapon Big Pool Williamsport Hedgesville 

Evitts Creek Artemas Bellegrove Hancock Cherry Run Clear Spring 

Seneca

Vienna Washington West Falls Church 

Leesburg Sterling 

Charles Town Harpers Ferry Point of Rocks 

Rockville 

Waterford Poolesville 

Shepherdstown Keedysville 
Appendix 1 103 

Point of Rocks 

Burton and others (1995) 

Burton and Southworth (1996) 

D.K. Brezinski (MGS) and S. Southworth (USGS) (unpub. data, 

1996–1997) 

Poolesville 

Southworth (1998) 

Rockville 

A.A. Drake, Jr. (USGS Scientist Emeritus, unpub. data, 1996–1997) 

S. Southworth (USGS, unpub. data, 1996–1997) 

Seneca 

Drake and others (1999) 

S. Southworth (USGS, unpub. data, 1996–1997) 

Shepherdstown 

Dean and others (1987) 

D.K. Brezinski (MGS), R.C. Orndorff (USGS), and S. Southworth 

(USGS) (unpub. data, 1996–1997) 

Sterling 

Lee (1979) 

S. Southworth (USGS, unpub. data, 1996–1997) 

Vienna 

Drake and Lee (1989) 

S. Southworth (USGS, unpub. data, 1996–1997) 

Washington West 

Fleming and others (1994) 

S. Southworth (USGS, unpub. data, 1996–1997) 

Waterford 

Burton and others (1995) 

S. Southworth (USGS, unpub. data, 1996–1997) 

Williamsport 

Dean and others (1987) 

D.K. Brezinski (MGS), R.C. Orndorff (USGS), and S. Southworth 

(USGS) (unpub. data, 1996–1997) 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 105 

Plates 2–20. 

Plates 2 through 20 are detailed geologic maps that roughly correspond to sections described in the Geologic Guide. An index to 

the plates is found on plate 1 (in pocket). Sources of data are listed by 7.5-minute quadrangle in Appendix 1. Each plate consists 

of a geologic map on the right-hand page, and a description of map units and explanation of symbols on the left-hand page. 

The shaded-relief topography was derived from U.S. Geological Survey Digital Elevation Model (DEM) files. Selected hydrography 

was derived from U.S. Geological Survey Digital Line Graph (DLG) files. Selected roads were based on the U.S. Geological 

Survey’s 1:100,000-scale planimetric maps of the Cumberland, Hagerstown, Winchester, Frederick, Washington West, and 

Washington East quadrangles. These maps are not intended for navigation. 

Plates 2–20.	 Geologic map of the Chesapeake and Ohio Canal National Historical Park and 

Potomac River Corridor from: 

2. Georgetown (MM 0) to near Glen Echo Park (about MM 7) 

3. Near Glen Echo Park (about MM 7) to Swains Lock (MM 17) 

4. Swains Lock (MM 17) to Seneca (MM 22.8) 

5. Seneca (MM 22.8) to Edwards Ferry (MM 30) 

6. Edwards Ferry (MM 30) to Monocacy River Aqueduct (MM 42.2) 

7. Monocacy River Aqueduct (MM 42.2) to Point of Rocks (MM 48.2) 

8. Point of Rocks (MM 48.2) to Brunswick (MM 55) 

9. Brunswick (MM 55) to Fort Duncan (MM 62.5) 

10. Fort Duncan (MM 62.5) to Antietam Creek (MM 69) 

11. Antietam Creek (MM 69) to Big Slackwater (MM 85.5) 

12. Big Slackwater (MM 85.5) to Williamsport (MM 99.5) 

13. Williamsport (MM 99.5) to McCoys Ferry (MM 110) 

14. McCoys Ferry (MM 110) to Licking Creek Aqueduct (MM 116) 

15. Licking Creek Aqueduct (MM 116) to Round Top Cement Company mill (MM 127.5) 

16. Round Top Cement Company mill (MM 127.5) to Little Orleans (MM 141) 

17. Little Orleans (MM 141) to Paw Paw Tunnel (MM 156) 

18. Paw Paw Tunnel (MM 156) to Oldtown (MM 167) 

19. Oldtown (MM 167) to Spring Gap (MM 173.5) 

20. Spring Gap (MM 173.5) to Cumberland (MM 184.5) 
Plate 2.
 

5 

106 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor
7 

6 

5 

1-4 

Plate 2 107
 
Plate 3. 

108 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Plate 3 109
 
Plate 4. 

110 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Plate 4 111
 
Plate 5. 

112 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Plate 5 113
 
Plate 6. 

114 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Plate 6 115
 
Plate 7. 

116 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Plate 7 117
 
Plate 8. 

118 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Plate 8 119
 
Plate 9. 

120 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Plate 9 121
 
Plate 10. 

122 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Limekiln Road 

H r a ep s r

Fer y r Rd. 

a H epr s r r eF y r

G or ev

d ao R

. d R

Rd. 

Rd. 

Back 

Mt. Lock Hill 

Chestnut 

Plate 10 123
 
Plate 11. 

124 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Plate 11 125
 
Plate 12. 

126 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Plate 12 127
 
Plate 13. 

128 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Plate 13 129
 
Plate 14. 

130 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Plate 14 131
 
Plate 15. 

132 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Plate 15 133
 
Plate 16. 

134 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Plate 16 135
 
Plate 17. 

136 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Plate 17 137
 
Plate 18. 

138 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Plate 18 139
 
Plate 19. 

140 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Plate 19 141
 
Plate 20. 

142 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 
Plate 20 143
 
144 Geology of the Chesapeake and Ohio Canal National Historical Park and Potomac River Corridor 

Manuscript approved for publication February 27, 2004.
 

Prepared by the Eastern Publications Group, Reston, VA.
 

Editing by Elizabeth D. Koozmin.
 

Photocomposition and design by Anna N. Glover.
 

Graphic art and cartography by Linda M. Masonic and Anna N. Glover.
 
Southworth and others—Geology of the C&O Canal and Potomac River Corridor, D.C., Md., W. Va., and Va.—Professional Paper 1691