CONTENTS

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Abstract

Introduction and Geologic Setting

Middle Proterozoic Rocks

Garnet Graphite Gneiss

Hornblende Gneiss

Garnet Monzogranite

Biotite Gneiss

Late Proterozoic Rocks

Swift Run Formation

Metadiabase Dikes

Metarhyolite Dikes

Catoctin Formation

Paleozoic Rocks

Chilhowee Group

Loudoun Formation

Phyllite

Conglomerate

Weverton Formation

Buzzard Knob Member

Maryland Heights Member

Owens Creek Member

Harpers Formation

Antietam Formation

Tomstown Formation

Bolivar Heights Member

Fort Duncan Member

Benevola Member

Jurassic Diabase Dikes

Metamorphism

Structural Geology

Gneissic Foliation

Mylonitic and Phyllonitic Foliation

Bedding

Cleavage

Middle Proterozoic Rocks

Late Proterozoic and Lower Cambrian Rocks

Late Proterozoic Dikes

Crenulation Cleavage

Shear Band Cleavage

Lineations

Joints

Folds

Faults

Tectonic History

Middle Proterozoic

Late Proterozoic

Paleozoic

Mesozoic

Cenozoic

Quaternary Bedrock Landslides

Surficial Geology

Acknowledgments

References Cited

PLATE (See thumbnail)

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1. Geologic map of the Harpers Ferry quadrangle, Virginia, Maryland, and West Virginia.

FIGURES

1. Index map showing the location of the Harpers Ferry quadrangle and adjacent quadrangles within the
Blue Ridge-South Mountain anticlinorium, and type localities of the members of the Weverton and
Tomstown Formations

2. Quartz-alkali feldspar-plagioclase feldspar diagram showing modal compositions of Middle Proterozoic
rocks, and normative feldspar plot of Middle Proterozoic rocks in the Harpers Ferry quadrangle

3. Index map showing the locations of isotopically dated rock units discussed in text

4. Photograph showing garnet monzogranite and gneissic foliation defining Grenvillian folds

5. Photograph showing sericitic quartzite of the Swift Run Formation showing beds interpreted to be overturned
and cut by cleavage, and closeup showing cobbles and pebbles of siltstone and iron-rich sandstone

6. Hf/3-Th-Ta ternary diagram for tectonic setting of Late Proterozoic metadiabase dikes, extrusive metabasalt,
and Jurassic diabase dikes

7. Variation diagrams for Late Proterozoic metadiabase dikes, metabasalt of the Late Proterozoic Catoctin
Formation, and Early Jurassic diabase dikes

8. Chondrite-normalized, rare earth element plots of Late Proterozoic metadiabase and metarhyolite dikes,
Early Jurassic diabase dikes, and metabasalt of the Late Proterozoic Catoctin Formation

9-11. Photographs showing--

9. Metabasalt of the Catoctin Formation showing flows interpreted to be overturned, with epidosite bodies
that are separated from schistose metabasalt by tuffaceous phyllite and mud-lump breccia in recessive
interval, and clast of metabasalt and beds of tuffaceous metarhyolite interbedded with tuffaceous
phyllite and metabasalt of the Catoctin Formation

10. Conglomerate and quartzite of the Loudoun Formation between phyllite of the Loudoun Formation and quartzite of the Weverton Formation, north of Purcell Knob

11. Type locality of the Weverton Formation along U.S. Route 340 east of Weverton, Md.

12. Diagrammatic sketches showing stratigraphy and structure of the Weverton Formation at the Maryland
Heights section at the south end of Elk Ridge and at Weverton Cliffs at the southern terminus of South
Mountain, Md.

13. Photograph showing mylonitic marble of the Keedysville marble bed of the Bolivar Heights Member of the Tomstown Formation along the CSX railroad tracks west of Bolivar, W. Va., in the Charles Town quadrangle

14. Tectonic map of the Harpers Ferry quadrangle

15. Lower hemisphere equal-area projections of structural elements

16-20. Photographs showing--

16. First-generation cleavage cut by crenulation cleavage in metabasalt of the Catoctin Formation near
Loudoun Heights Trail on Blue Ridge

17. Shear band cleavage cutting phyllonitic foliation with east-over-west asymmetry in rocks of the Harpers Formation along U.S. Route 340 in the hanging wall of the Short Hill fault

18. F1 isoclinal syncline, facing downward to the west, and axial planar cleavage that is cut by southeast-
dipping crenulation cleavage

19. Recumbent anticlines of quartzite of the Buzzard Knob Member of the Weverton Formation exposed
on Elk Ridge north of the Potomac River, and southwest-plunging overturned anticline in the Potomac
River

20. Overturned anticline in the Harpers Formation showing bedding and cleavage relations in the Harpers
Ferry National Historical Park along the Shenandoah River

21. Structural map and photograph of drill core and photomicrograph of the plane of the Short Hill fault

22. Structural map showing the relation of the Short Hill fault to faults in Maryland

23. Photograph showing blocks of quartzite of the Weverton Formation in matrix of vein quartz along the White
Rock thrust fault

24. Photograph showing intraformational thrust fault in the Maryland Heights Member of the Weverton
Formation and tight to isoclinal folds in the footwall

25. Diagrammatic sketches showing Middle Proterozoic to Cenozoic tectonic history of the Blue Ridge-South
Mountain anticlinorium near Harpers Ferry, W.Va.

26. Photograph showing colluvium of quartzite of the Weverton Formation that forms boulder fields on the
lower slope where boulder streams coalesce

TABLES

1. Chemical analyses of rocks from the Harpers Ferry quadrangle, Virginia, Maryland, and West Virginia

2. CIPW norms of Middle Proterozoic basement rocks in table 1

3. Geochronology and tectonic history of the Blue Ridge-South Mountain anticlinorium, northern Virginia and
southern Maryland

METRIC CONVERSION FACTORS

 

Multiply	By	To obtain
Length
millimeter (mm)	0.0394	inch
centimeter (cm)	0.3937	inch
meter (m)	3.281	foot
kilometer (km)	0.6214	mile
Volume
cubic meter (m3)	1.30795	cubic yard
Temperature
degree Celsius (C)	1.8 and add 32	degree Fahrenheit (F)