DEPARTMENT OF THE INTERIOR

Ray Lyman Wilbnr, Secretary

U. S. GEOLOGICAL SURVEY 

George Otis Smith, Director

Bulletin 799

GEOLOGY OF THE 

McCALLS FEBEY-QUABBYVILLE DISTEICT

PENNSYLVANIA

BY

ELEANORA BLISS KNOPF

AND

ANNA I. JONAS

UNITED STATES

GOVERNMENT FEINTING OFFICE 

WASHINGTON : 1929
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CONTENTS

Pag*

1 Outline of report._._____-_-___--__-__-_--------_--_-.-_---_------- ix

Introduction..___________________-__-____--_-_-_------------------ 1

* Location of the district__-___.____-_-_--------_-------------_-- 1

^ Field work and acknowledgments.-___-__---____----_-_-_----_-- 3

Previous work_____-_-__-_-_------_-_-~----_---_--_-_-_------- 3

. Bibliography.._-------_---.------------------_------ ..---. 5

Geography __________-_-_-__-__-_-----_-_-_-------------_--------- 6

; Topography.._-.-___-_______.___-_-_-__-_..__.-------____-_-- 6

Drainage __---_--.----_.-.---_-----_-_------------.---_------ 8

4 Climate.__.____.- ____-_--_--_--- -------- ----- 9

General geology-______------_---_--_---------__-_-_-------------- 10

Outline of the geology of the Appalachian Mountains and the Pied-

_ mont province-------.-------------------------------------- 10

" Appalachian Mountains__-__--_-----_---_---------------_- 10

.j General features --_---_----_---_----_---------------- 10

Geology of the Reading Hills._._____-____-_--.-_-.-.___ 11

j Geology of Catoctin-South Mountain___________________ 13

Piedmont province-_-_--_--___--_-_-----------__--_----_-- 13

General geology of the Piedmont province in Pennsylvania. 14

Geology of the Piedmont upland in Maryland._.__.____.. 15

Baltimore gneiss.._.___--__--_--__-_-_--_--__-_.__ 15

Glenarm series.__--..__--_-__-._--_--._-_-._-_---___ 15

* Geology of the McCalls Ferry-Quarry ville district. ________________ 18

, Pre-Cambrian rocks of sedimentary origin..__________________ 18

Pre-Glenarm rocks of the Mine Ridge upland__.______._. 18

Baltimore gneiss._.___-__-_--__-____.-______----__ 18

Glenarm series in Pennsylvania._---______---.____-__.__ 24

Cockeysville marble.-.-.__________________________ 25

Wissahickon formation..______.____-___-__--__._-__ 25

 * Oligoclase-mica schist f acies _-___-_.-_.____-.___ 25

Diaphthoritic schists.-_---___-__________._-___ 27

Albite-chlorite schist f acies _--___._-________-___ 28

Composition...-.-.....-._._.__...____...._-__ 30

Metamorphism..-----.-_----..__.-_-__._..._-_-_.__ 32

Age---.---------.-.-.-.-.-----..-...-........-.-.... 33

Peters Creek schist._______________________________ 35

m Cardiff conglomerate,__----_-__---.__.._____-_____ 37

Peach Bottom slate.______________________________ 39

Paleozoic sedimentary rocks.-__._-___-.__.____-__._________ 41

Paleozoic stratigraphy of Welsh Mountain and vicinity.... 41

,t Paleozoic stratigraphy of Mine Ridge upland and vicinity._ 42 

Lower Cambrian rocks of the McCalls Ferry-Quarry ville district..-----._---_-_-..---------_----_---_-______._ 42

L Chickies quartzite.________._-_____________________ 42

Hellam conglomerate member-____.______.._____ 43

Quartzite member-______-_-__._____-.___-_--__ 44

Harpers and Antietam schists.............___..__.__ 45

in
IV CONTENTS

General geology Continued.

Geology of the McCalls Ferry-Quarryville district Continued. 

Paleozoic sedimentary rocks Continued.

Lower Cambrian rocks of the McCalls Ferry-Quarryville

district Continued. Page

Vintage dolomite.-________________-____-_-____- 50

Kinzers formation._______________________________ 52

Ledger dolomite.____________________-_--_-____- 53

Ordovician rocks.__.__________-________-____---___-_ 54

Conestoga limestone____-__-_-__-_______-_-____--__ 54

Igneous rocks.------------------ ,__________-____-_--__ 58

Pre-Cambrian igneous rocks.________________---__------ 59

Biotitie amphibolite schists.----------------------- 59 4

Metagabbro---_-----_--_-_-__.______-___--__- 59

Age of biotitic amphibolite schist and metagabbro.---- 60

Serpentine.._______----____-__-_____---_----_---_ 61 ^

Amphibolite and chlorite schist altered volcanic flows. 62 *

Igneous rocks of undetermined pre-Triassic age ___________ 62

Quartz monzonite-____-________-_-_____-_____._- 62

Quartz monzonite pegmatite and aplite_____-_____-_- 63 ^

Quartz diorite-_-___-_-_____-__-____----___--_--_- 64 *

Granitic pegmatite and granite____------____--__-_- 64 \_

Age of granite and quartz monzonite_____^____.____- 65

Triassic igneous rocks______--__-_____-----_--_--_---- 67 l.._

Diabase _________________________________________ 67

Correlation of rocks of McCalls Ferry-Quarryville district with

those of other areas-_-_-__-____-__--_-___-_--_--_______- 68

Structure _________________________________________________________ 71

Folds........... ---_-------_--_----_-__-_-__ _-_-_--__- 71

Faults___--_-__-_-------._-_--_-__-_._________--_--_-___ 74

Age of the structure_--_---_-----_____-__.____-___--__-______-_ 77

Pre-Cambrian folding.___-___-_______-__________,______-_--_-_ 77

Paleozoic folding and faulting_________ __..______ ___... 77

Age of the Martic overthrust.__________________________________ 79

Close folding of Mine Ridge anticline.___________________________ 80

Load.-.--.--.-----_-----_-___---_-______-____-----_-_--_-.. 80

Triassic structural features_____________'_-_______-_____-________ 81 ^

Geologic history._______ _________ ___ _______ -__._____ 81

Stratigraphic record___________________________________________ 81

Pre-Cambrian history____________________________________ 81

Paleozoic history_____--________-___________-_______-______ 85

General outline-____-__-_-________-_______--__-_-_-___ 85

Age of the post-Conestoga deformation and metamorphism. _ 86

Inferences drawn from the post-Chazy stratigraphy of 0

adjoining regions----__-___-_-______-_--_-___.___ 86

Inferences drawn from the probable method of development of observed structure-____________----_-____ 88

Summary of Paleozoic geologic history.._________________ 89 r'

Post-Paleozoic stratigraphic record __________________________ 90

Physiographic record________________________________________ 91

Evidence of former drainage courses..____________----_-____-_ 91 >

Superimposition_____________________________________ 92 ,

Cross profiles of stream valleys_______________-__-_.____ 94

Wind gaps__.________________________________________ 94

Longitudinal profiles of streams...--__-______--_________ 95
CONTENTS V

Geologic history Continued.

Physiographic record Continued. Page

Relative preservation of successive drainage courses__----_---_ 95

Study of interstream areas.______ _______________________ 96

Study of drainage modifications.____________________________ 100

Mine Ridge cycle of erosion______________._____________ 100

Honeybrook subcycle of erosion___._____________________ 103

Harrjglburg subcycle of erosion.._.__-______-___-____.___ 103

Bryn Mawr subcycle of erosion.________________________ 104

Brandywine subcycle of erosion_________________________ 105

Late Pleistocene erosion______________________________ 109

Significance of knickpoints in stream gradient________________ 109

Longitudinal profiles of Susquehanna River and minor streams.. 110

Sequence and correlation of erosion records.__________________ 116

Amount and duration of erosion____ __ ______________ 117

Periodicity of uplift.__-_-_______-_-___----_____ _________ 117

Age of erosion cycles._____________________________________ 118

Origin of metamorphism in the crystalline schists of the Piedmont province . 118 

Variable metamorphic intensity in the McCalls Ferry-Quarryville

district. ________.._________________.._____----. 118

Previous ideas on me'tamorphism of the Piedmont province.. _______ 119

Criteria for determination of metamorphic intensity____-____-__-__ 120

Metamorphic textures_-_____---__----_---___-__._---__--_- 121

Metamorphic minerals.______--___---__--___--__--_-___-__- 122

Progressive metamorphism-________________________________ 122

Retrogressive metamorphism-_____---__--___-_--__--_-----_ 124

Regional variation in metamorphic intensity..___________.__...___ 124

Early pre-Cambrian paragneisses (Baltimore gneiss).-__.__-_ 124

Later pre-Cambrian paragneisses (Glenarm series)___________ 125

Paleozoic rocks of sedimentary origin _______________________ 125

Causes of regional variation in metamorphic intensity..____.._____- 128

Causes of regional metamorphism.__________________________ 129

Dynamic and contact metamorphism in the oldest pre-Cambrian rocks-__---___--_-_-_________-_-__-__--_-____---_ 130

Load metamorphism and regionaj. contact metamorphism in the

Glenarm series____---_---______-__-_--__-__-_-____---_ 131

Double metamorphism in the Glenarm series_.___--_-___--___ 132

Variation in metamorphic intensity in the Paleozoic rocks...--. 132

Double metamorphism in the Paleozoic rocks.________________ 133

Development of older metamorphism in the Glenarm series and

in Paleozoic rocks._____________________________________ 134

Relative age of double metamorphism in the Glenarm series and

in the Paleozoic rocks__________________________________ 138

Summary of conclusions___--_-_--_____--___-______-___-______- 139

Economic geology.________________________________________________ 141

Nickel ore.___________________________________________________ 141

Iron ore________________________________.-____--_____. 150

Chrome ore._________________________________________________ 151

Silver ore.-_--_-____-____---___--_-_--______-_______-_____-__ 152

Roofing slate_--___-_-_-__-_--__--------__-______----___---_ 152

Limestone.._ __--__-_-_--_----___---_-_--__-_________---___--_ 153

Sand.____-____-_.._.______________________________________ 153

Index... _____________________ ____ _______________________ 155
ILLUSTRATIONS

Page 

PLATE 1. Geologic map and geologic structure sections of the Quarry -

ville and McCalls Ferry quadrangles. -_.-___-_--_____ In pocket.

2. Photomicrographs of schist: A, Harpers schist; B, Antietam

schist.._______________________________________________ 50

3. A, Upper part of the Vintage dolomite overlain by Kinzers 

shale, exposed in cut on the Pennsylvania Railroad near Vintage; B, Vintage dolomite in quarry at Kinzers..---------- 50

4. A, Kinzers pseudoconglomerate on Pennsylvania Railroad west 

of Kinzers; B, Basal Conestoga slate and limestone conglomerate, unconformably overlying Vintage dolomite, Bellemont 

quarry ________________________________________________ 50

5. A, Thick-bedded crystalline limestone passing into finely 

laminated slaty limestone typical of Conestoga limestone in 

quarry 1 mile northwest of Bellemont; B, Closely folded 

micaceous marble of Conestoga limestone in quarry half a 

mile northwest of Quarryville-_-_--_-_____-_----__-_-_-_- 51

6. A, Anticline in Antietam schist, road cut 2 miles west of Bellemont; B, Closely folded Antietam schist, road cut on east side 

of Oak Hill _________________________________________ 98

7. A, Projected profiles of area between Blue Mountain and the 

Coastal Plain; B, Profile of divide between Susquehanna and 

Schuylkill River drainage slopes_________________________ 99

8. A, Specimen of Harpers schist; B, specimen of Chickies banded

slate; C, Specimen of mica schist from upper beds of Chickies 

quartzite_ .-_-_-__--__--_-__--__-___-_____-__--_----_ 138

FIGURE 1. Index map showing location of McCalls Ferry and Quarryville quadrangles, Pa.-. _______________________________ 2

2. Physiographic provinces in the Middle Atlantic States-_.____ 10

3. Columnar section showing crystalline schists and sedimentary 

rocks of the McCalls Ferry-Quarryville district and adjoining region in Maryland and some of the igneous intrusions. 16

4. Sketch map showing principal pre-Cambrian areas of the 

Piedmont province and Appalachian Mountains in Pennsylvania and New Jersey._____-___--__________-_------_-- 69

5. Generalized structure of the region between Philadelphia and

Washington.-_._________..___.______.__________-.-_-_ 72

6. Projected profile of Blue Mountain showing cross profiles of

Lehigh Gap and Little Gap.__--_____.___-___------__-_ 102

7. Generalized topography of eastern Pennsylvania and Maryland

at the end of the Honeybrook subcycle of erosion.________ 105

8. Generalized topography of eastern Pennsylvania and Maryland

at the end of the Harrisburg subcycle of erosion,_________ 106

VI
ILLUSTRATIONS VII

Pago

FIGURE 9. Generalized topography of eastern Pennsylvania and Maryland

at the end of the Bryn Mawr subcycle of erosion________ 107

10. Generalized topography of eastern Pennsylvania and Maryland

at the end of the Brandy wine subcycle of erosion. ________ 108

,> 11. Longitudinal profiles of Susquehanna River from its mouth to 

a point 3 miles southwest of Sunbury, Pa., and of eight 

' minorstreams_------__---_____-_----------_--__-_---_ 112

12. Cross profiles of Octoraro Valley and of the wind gap above

the source of Octoraro Creek________________---___-_-__ 115

13. Diagrammatic map showing distribution of metamorphic 

intensity in the crystalline schists of the Piedmont province 

'<> of portions of Pennsylvania and Maryland_________----__ 120

14. Cross section through a wedge-shaped thrust block. _________ 136

15. Geologic map and cross sections of the Gap nickel mine.-.__- 144

OUTLINE OF THE REPOKT

The McCalls Ferry-Quarryville district, as the term is used in this report, 

includes the McCalls Ferry and Quarryville quadrangles, in southeastern 

Pennsylvania. It comprises parts of Lancaster, Chester, and York Counties 

and lies within a belt of metamorphosed pre-Cambrian and Paleozoic rocks 

that extends southwestward from Trenton, N. J., through eastern Pennsylvania 

and Maryland into the Southern States of the eastern Atlantic slope.

Topographically the area consists of a well-dissected upland and a gently 

rolling lowland. The country is fertile and is well adapted to agriculture, 

which is the most important industry. The mining importance belongs to the 

last century, when the district furnished a considerable amount of iron ore 

and for several years yielded, from the Gap nickel mine, about one-sixth of 

the world's annual output of nickel.

The geologic interest of the district centers in the nature and origin of its 

highly metamorphosed rocks and in the bearing of its geologic history on the 

interpretation of the stratigraphy and structure of the Piedmont province as 

a whole. Study of this district has furnished a clue for the solution of 

puzzling stratigraphic problems in the region on the northeast, but the interpretation of the stratigraphy and structure of the pre-Cambrian rocks in this 

area itself is dependent on previous work by the writers in Maryland, where 

the base of the stratigraphic column was established. Therefore, in this report the district has been treated in its relation to the geology of the Piedmont 

province as a whole.

The northeast corner of the Quarryville quadrangle is occupied by the southern spur of a wide anticline underlying the Honeybrook upland, which extends 

northeastward as far as Schuylkill River. The core of this anticlinal spur, 

which is known as the Mine Ridge anticline, is made up of the Baltimore 

gneiss of early pre-Cambrian age. This formation comprises intensely metamorphosed graphitic schists and gneisses with a few calcareous lentils and 

numerous interlaminated layers of hornblende schist, which are probably of 

igneous origin. The whole series has been invaded and injected by igneous 

rocks ranging in composition from granite to gabbro. The invading igneous 

rocks are presumably pre-Cambrian, because they have not been found to cut 

the intensely metamorphosed Paleozoic schists and quartzites that flank the 

uplift. Moreover, the fact that they have been altered by a strong dynamic 

metamorphism that was apparently contemporaneous with the first metamorphism of the Paleozoic rocks suggests that they were intruded during a 

period of igneous activity that preceded Cambrian sedimentation.

Lower Cambrian arenaceous strata unconformably overlie the pre-Cambrian 

core of the anticline and are about 2,200 feet thick. They are overlain by 

5,000 feet of calcareous and dolomitic rocks ranging in age from Lower Cambrian to Ordovician. These rocks were gently folded and lifted above sea 

level early in Ordovician time, and across their eroded edges the argillaceous 

Conestoga limestone, of probable Chazy age, was laid down. After the deposition of the Conestoga limestone the district was again folded with moderate in-
X OUTLINE OF THE REPORT

tensity, and probably during a subsequent deformation a block of the earth's 

crust that originally lay farther south was driven northward and westward across 

the folded Paleozoic strata. In the McCalls Ferry and Quarryville quadrangles 

the overriding rocks comprise the Cockeysville marble, Wissahickon formation, 

Peters Creek schist, Cardiff conglomerate, and Peach Bottom slate, all formations of the Gleuarm series, which was studied by the writers in Maryland, 

where it was determined to be of later pre-Cambrian age and younger than 

the Baltimore gneiss, which it overlies unconformably. The total thickness 

of the Glenarm series is probably between 8,000 and 10,000 feet, although no 

accurate estimate can be made, for the middle formations have been repeated 

by close folding.

The basal member of the series is the Setters formation, which is not exposed 

in the McCalls Ferry and Quarryville quadrangles but which occurs about 5 

miles southeast of the district in an anticline between Avondale and Doe Bun, 

where its thickness ranges from a few feet to 1,000 feet. It is composed of 

mica gneiss and mica schist with an intercalated quartzite member. Overlying 

the Setters formation in this anticline is the Cockeysville marble, a dolomitic 

and calcitic marble several hundred feet thick. In the McCalls Ferry quadrangle the Cockeysville marble crops out only in a small valley, where it is 

overlain by the Wissahickon albite-chlorite schist.

The Wissahickou formation occurs in two metaniorphic" facies an oligoclasemica schist and an albite-chlorite schist. The oligoclase-mica schist, which 

surrounds several anticlines north and northwest of Baltimore, Md., forms a 

belt along the southeastern border of the Piedmont province from Baltimore to 

Trenton, N. J. Lithologically it resembles the Setters formation, and it shows 

the same intense metamorphism. In the McCalls Ferry-Quarryville district 

it is largely replaced by the less intensely metamorphosed facies of the Wissahickon, the albite-chlorite schist, which is a heterogeneous formation comprising 

both feldspathic and chloritic schists with a subordinate amount of quartzite 

or quartzose gneiss. The difference in mineral constituents between the two 

facies of the Wissahickon appears to have resulted from a difference in conditions of metamorphism rather than from difference in stratigraphic position 

or in chemical composition. The thickness of the Wissahickon formation 

where it has been cut across by Susquehanna River must amount to several 

thousand feet, even after allowance has been made for the close folding. The 

two uppermost formations of the Glenarm series, the Cardiff conglomerate 

and the overlying Peach Bottom slate, aggregate between 1,500 and 2,000 feet 

in thickness. They have been included with the pre-Cambrian rather than with 

the Paleozoic section because it has been impossible to prove an unconformity 

at the base of the conglomerate, whereas the base of the Cambrian shows a 

distinct angular unconformity.

The overthrust block of Glenarm schists covers a little more than two-thirds of 

the McCalls Ferry-Quarryville district. After the thrusting the overridden and 

overlying blocks alike were folded along axes that were approximately parallel 

to the main structural lines of the preexistent folds. In some places the older 

structural features emerge from under the thrust block so that the preoverthrust deformation can be recognized. The last pre-Triassic deformation of 

the region is recorded in a cleavage that cuts across the older schistosity and 

folding. The folding of the Conestoga limestone, the thrusting of the preCambrian block, and the folding of the thrust plane took place between the end 

of Middle Ordovician time and the beginning of the Triassic, but it is impossible to fix the dates of the individual deformations with more exactitude. 

During Triassic time the region was broken by normal faults, which accompanied or followed the intrusion of narrow dikes of diabase in this district.
OUTLINE OF THE REPORT XI

The post-Triassic history is chronicled in the physiography, which gives 

evidence of progressive but intermittent uplifts. These successive uplifts 

interrupted the normal course of erosion, so that ten uncompleted cycles of 

erosion are recorded in the drainage history of the McCalls Ferry-Quarryville 

district and the adjoining region. The remnants of the old erosion surfaces 

that mark these partial cycles have been correlated by a study of present and 

preexistent drainage channels.

The main problems of the metamorphic history of the Piedmont province 

are embodied in the crystalline, schists of the McCalls Ferry-Quarryville district. 

The clue to the solution of certain of these problems lies within the district 

itself, and the answer to some other puzzling questions has come from the surrounding region. The most striking feature of the metamorphism of the 

Piedmont province as a whole is its extremely variable intensity, which con 

not be adequately explained by any one cause alone. Some of this variation 

is accounted for by the determination that the metamorphism of the region 

occurred at more than one time and that the metamorphic effects produced at

different times are to a large extent, distinct. Thus the pre-Cambrian basement complex, wherever it is exposed, shows an intense alteration. Such 

intensity might possibly be expected on the ground that the oldest rocks have 

been through the metamorphic mill the greatest number of times and therefore have naturally been the most altered. However, the recent metamorphic 

studies of European geologists have brought out the fact that repeated 

metamorphism is not necessarily cumulative in its effects but is commonly 

retrogressive, so that a rock that was originally metamorphosed into a coarsely 

crystalloblastic gneiss may later become a less highly metamorphic rock a. 

diaphthoritic or retrometamorphic schist. =The oldest pre-Cambrian gneiss shows 

no diaphthoritic effects and none of the superposed foliation that is so striking 

a feature of both the later pre-Cambrian and the Paleozoic schists. Therefore, it is probable that the earliest known crystalline schists have remained 

more or less inert since their formation, doubtless owing to their prolonged 

deep-seated position, which has preserved them from retrogressive metamorphism.

The earliest pre-Cambrian rocks (the Baltimore gneiss) may therefore be 

disregarded in .the study of the variations in metamorphic intensity. It is 

evident that the metamorphic intensity of the other crystalline schists of the 

Piedmont province in Maryland decreases continuously from the vicinity of 

Baltimore toward the west and northwest, as was pointed out many years 

ago by Williams. But north and northeast of Baltimore, instead of a continuous decrease in intensity, there is a series of approximately parallel belts 

of variable intensity, starting with a belt of maximum intensity along the 

eastern border of the Piedmont province. Toward the northwest this maximum diminishes to a minimum metamorphism, still registered in pre-Cambrian 

rocks. Still farther northwest the intensity rises to a second maximum in 

the Paleozoic schists that flank the anticline of Mine Ridge, and north and 

northwest of this anticline the metamorphism in the Paleozoic phyllites and 

calcareous rocks once more diminishes in the direction of the Triassic basin. 

These metamorphic belts run in general parallel to the trend lines of the 

formations, but they are not genetically connected with the folding, as highly 

altered rocks occur in the cores of open folds while the rocks on the limbs are in 

the condition of phyllites or mildly altered chlorite schists. Moreover, the 

second maximum of intensity, which is well shown in the section along Susquehanna River and northeastward from the river, diminishes not only toward the northwest but also toward the southwest, where it gradually dis-
.XII OUTLINE OF THE REPORT

appears in the decreased metamorphism between Baltimore and the Frederick 

Valley. : ..- -

Nor does repetition of the same stratigraphic horizon by folding explain 

these recurrent maxima and minima of metamorphism, for some of the most 

highly altered rocks are the autochthonous Paleozoic schists, whereas the 

metamorphic intensity wanes perceptibly in the overlying pre-Cambrian fault 

block. Moreover, in the pre-Cambrian Glenarm series the Wissahickon formation, which was presumably metamorphosed under a uniform load, occurs in 

two metamorphic facies of widely differing intensity. The reason for this 

difference in metamorphic intensity is not obvious, but it is noteworthy that 

the more intensely metamorphosed facies is penetrated by numerous large 

igneous intrusions, which are conspicuously absent from the facies of low.er 

intensity. The inference is that the high metamorphism of the one facies 

may have been caused by the thermal influence of the igneous invasion.

In the area around the Mine Ridge upland, where pre-Cambrian schists 

have ridden northwestward over Paleozoic rocks, the metamorphic gradient 

is abnormal, the maximum metamorphism occurring in the autochthonous 

Paleozoic strata and diminished intensity in the overthrust pre-Cainbrian 

rocks. Such a metamorphic gradient, which is the reverse of the relation found 

in many places where pre-Cambrian rocks have been thrust over younger 

formations, is a strong argument in favor of the belief that the metamorphism 

was younger than the thrusting. This belief is strengthened by the fact that 

the somewhat meager evidence that is available regarding the sole of the 

thrust plane indicates that the thrusting took place under conditions that 

were favorable to free molecular movement rather than to freedom of molar 

movement.

At first sight it might appear possible to explain the abnormal and localized 

intensity of metamorphism in the autochthonous Paleozoic schists around the 

Mine Ridge upland -as static metamorphism, or as dynamic metamorphism 

under the normal stratigraphic load augmented by the weight of the thrust 

block. It is perfectly evident that the metamorphism was not produced under 

the causal influence of the normal stratigraphic load alone, as the Lower Cambrian sediments in the McCalls Ferry-Quarryville district were nearer to the

Paleozoic shore line and therefore under lighter load than the sediments of 

the same age in the deeper part of the geosyncline 10 or 20 miles farther northwest. Nor does a dying out of the folding explain this difference in metamorphism, because the slaty Conestoga limestone near Lancaster is folded 

just as strongly as its metamorphosed equivalent near Quarryville. The additional weight of 10,000 feet of rock thrust over the normal stratigraphic column 

would produce a rise in geothermal temperature of only 100° C., which would 

not be adequate to explain the development of garnetiferous biotite schists and 

micaceous marbles near Quarryville in place of phyllites and slaty limestones 

20 miles farther northwest. Both the autochthonous and the overriding rocks 

in Carroll County, Md., are more feebly metamorphosed than the corresponding 

rocks in Pennsylvania, although the folding is equally close in both regions. 

Thus if the weight of the overriding fault block were the dominant f actor x in 

the intense metamorphism in Pennsylvania this would necessitate the conclusion 

that the thickness of the fault block was appreciably less in°Maryland than in 

Pennsylvania, so that folding and the attendant metamorphism would have 

taken place at a lesser depth and under conditions of less intensity.

Such a localized rise in geothermal temperature as is indicated by the 

Paleozoic schists that flank the Mine Ridge anticline suggests still another 

causal factor in folding namely, the influence of underlying igneous magma.
OUTLINE OF THE REPORT XIII

Folding that took place under such thermal influence, combined with the locally 

augmented load of the overriding fault block, would doubtless account for 

the intense metamorphism in the autochthonous Paleozoic block and for the 

upward waning of intensity in the pre-Cambrian rocks.

This hypothesis of regional alteration under the influence of an underlying 

tatholith is attractive in that it satisfactorily explains the peculiar localization of 

metamorphic intensity, but unfortunately it is virtually insusceptible of proof, 

except in so far as the widespread tourmalinization and pegmatitization of the 

Paleozoic rocks indicate the passage of magmatic emanations from an underlying igneous mass. This evidence is so conclusive that the presence of the 

underlying batholith is certain, but whether the influence of the batholith 

is sufficient to account for the intense metamorphism in the belt of highly 

altered Paleozoic rocks is speculative in the light of present evidence. In 

fact, from our present knowledge of the metamorphic history of the region 

it is impossible to ascribe the localized intensity of the metamorphism in the 

vicinity of the Mine Ridge upland to one rather than to the other of the two

possible causes that have been outlined. It can only be said that both possibilities answer the requirements of working hypotheses, although the first 

hypothesis seems inadequate to account for the extremely local rise in metamorphism. It remains for these hypotheses to be later eliminated or substantiated by more detailed and more extended work on the recurrent metamorphism of the Piedmont province.

GEOLOGY OF THE McCALLS FERRY-QUARBYV1LLE 

DISTRICT, PENNSYLVANIA

By ELEANORA BLISS KNOPF and ANNA I. JONAS

INTRODUCTION

LOCATION OF THE DISTRICT

The McCalls Ferry and Quarryville quadrangles mapped by the 

United States Geological Survey are in southeastern Pennsylvania, 

in York, Lancaster, and Chester Counties, between parallels 39° 45' 

and 40° and meridians 76° and 76° 30'. (See fig. 1.) The geology 

of these quadrangles1 is so intimately related ta the geology of the 

adjoining portions of Pennsylvania and Maryland in which the 

writers have worked that this report describes not only the area included in the quadrangles but also a considerable part of the adjoining 

region. The McCalls Ferry quadrangle is crossed from north to 

south by Susquehanna River, whose right bank forms the boundary 

line between York County on the west and Lancaster County on the 

east. Fully two-thirds of the two quadrangles lies in Lancaster 

County, and the greater part of the remainder is in York County, 

only the extreme southeast corner belonging to Chester County. 

There are no large towns in the district. Quarryville, a borough 

of several hundred inhabitants, is 12 miles southeast of Lancaster 

and 20 miles southwest of Coatesville.

Four through railroads and a small branch line enter the McCalls 

Ferry and Quarryville quadrangles. The northeast corner is crossed 

for a distance of 6 miles by the Philadelphia division of the Pennsylvania Railroad, which is a part of the main line between Philadelphia and Pittsburgh. The central part of the district is crossed 

from east to west by the Atglen & Susquehanna branch of the 

Pennsylvania Railroad, which turns north at Shenks Ferry station 

and follows the east bank of the Susquehanna to Harrisburg. This 

line, which carries only freight, is locally known as the " low-grade 

freight line," because it maintains an approximately uniform grade 

of about 5 feet to the mile between Philadelphia and Harrisburg. 

The Columbia & Port Deposit branch of the Pennsylvania Railroad

1
McCALLS FEEEY-QTJAEEYVILLE DISTEICT, PA.

runs along the east bank of Susquehanna River. Its northern 

terminus is Columbia, and it connects at Perryville with the Philadelphia, Wilmington & Baltimore division of the Pennsylvania 

Railroad. The Maryland & Pennsylvania Railroad follows the 

winding course of Muddy Creek in the southwestern part of the 

district. It starts in Baltimore, Md., and its .nprthern terminus is 

York, Pa. The Strasburg Railroad is a branch dine of the Pennsylvania Railroad which runs from the main Philadelphia division at 

Leaman Place 4 miles southwest to Strasburg. A narrow-gage road

FIGURE 1. Index map showing location of McCalls Ferry and Quarryville quadrangles, Pennsylvania

known as the Lancaster, Oxford & Southern Railroad, which served 

to connect Susquehanna River at Peach Bottom with Quarryville 

to the north and Oxford to the east, was abandoned in 1921. A 

standard-gage freight line between Quarryville and Lancaster, a 

branch of the Pennsylvania Railroad, is still in operation. An extensive trolley system, under the management of the Conestoga 

Traction Co., radiates from Lancaster, and three branches of this 

system terminate within the district, at Millersville, Quarryville, and 

Strasburg. A fourth branch of this trolley system crosses the district between Kinzers and Christiana and terminates at Coatesville, 

9 miles east of Christiana. The branch of the electric railroad of the 

Conestoga Traction Co. that terminates at Millersville connects
INTRODUCTION 3

with Susquehanna River by the Millersville & Pequea Electric Kailroad.

Susquehanna River is crossed by motor ferries at Pequea and 

McCalls Ferry and by a steam ferry at Peach Bottom. The river 

is navigable for small boats between Shenks Ferry and Holtwood, 

where the installation of a large dam has impounded the river water 

into Tucquan Lake. Below the tail race of the Holtwood dam it 

is navigable during the high-water season. -

FIELD WORK AND ACKNOWLEDGMENTS

The field work on the area described in this report was started by 

the authors during the field season of 1915. It was interrupted by 

the World War and was resumed and almost completed during portions of the field seasons of 1919, 1920, and 1921. The final detailed 

mapping of the Paleozoic rocks in the northern part of the Quarryville and McCalls Ferry quadrangles was done by A. I. Jonas during 

part of the field seasons of 1921 and 1922. The accompanying geologic map (pi. 1) is the result of this field work.

The geological survey of the McCalls Ferry and Quarryville 

quadrangles was planned in 1915 by Mr. Arthur Keith, chief of the 

section of areal geology. In February, 1915, Dr. E. B. Mathews, to 

whom the area had been originally assigned before his resignation 

from the United States Geological Survey, turned over to the Geological Survey a reconnaissance map and some reconnaissance notes 

made by Mr. W. J. Miller while he was a student at Johns Hopkins 

University. The area was then assigned to Eleanora F. Bliss (E. B. 

Knopf) for mapping. The work was carried on in full cooperation 

with A. I. Jonas, of the Pennsylvania State Geological Survey. 

Mr. Sidney Paige, who was chief of the section of areal geology 

during the progress of the later part of the field work, inspected the 

work, and acknowledgments ar6 due to him for his advice on several 

occasions during the preparation of the manuscript.

Acknowledgments are especially due to Mr. G. W. Stose for his 

collaboration on the stratigraphy of Lancaster Valley and to Mr. 

Adolph Knopf for his assistance on the metamorphic problems 

of the district.

PREVIOUS WORK

The earliest geologic work in the McCalls Ferry-Quarryville district consisted of the general investigations by the geological surveys 

of Pennsylvania, whose results were published in the several county 

reports. In 1895 Kemp studied the geology of the Gap nickel-ore 

deposit but confined his attention to the rocks immediately associated

6136 29 2
4 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

with the ore. In 1905 W. J. Miller made a brief reconnaissance, but 

the results were never published.

Although little has been written on the McCalls Ferry-Quarryville 

district in particular, the geology of the belt of highly metamorphosed 

rocks in which the district lies has long been a subject for discussion. 

The tendency among the early writers, such as Rogers, Hall, and 

Lesley, was to place practically all the crystalline schists of the region 

in the pre-Cambrian. The later work of Maryland geologists placed 

the series that overlies the pre-Cambrian Baltimore gneiss in the 

Paleozoic. The tentative conclusions reached by Mathews 1 in 1905 

were that the Paleozoic section probably begins with the Setters formation ; that the overlying Cockeysville marble is probably " CambroOrdovician" and represents the same horizon as the limestone of 

Chester Valley in the Philadelphia area; and that the Wissahickon 

formation, overlying the marble, is probably Ordovician. But even 

before this Bascom 2 had recognized the possibility that the Wissahickon gneiss might prove to be pre-Cambrian, and in 1909,3 at the 

time that the Philadelphia folio was published, she decided that the 

age of the Wissahickon gneiss is pre-Cambrian first, because of certain apparent stratigraphic relations between the Wissahickon gneiss 

and the adjoining sedimentary rocks in Pennsylvania, and second, on 

the ground that it contains intrusive igneous material generally considered to be absent from the Paleozoic rocks of the region. A series 

of phyllites and schists, termed the " Octoraro schist," was separated 

by Bascom from the Wissahickon and placed in the Ordovician, because these rocks were considered to be less coarsety crystalline than 

the Wissahickon and to conformably overlie limestone of Beekmantown age in Chester Valley. The contact of the Wissahickon and

the " Octoraro schist " was regarded as a thrust fault.

Early study of the Doe Run and Avondale district, 12 miles east 

of Quarryville, brought the writers 4 also to the conclusion that the 

Wissahickon gneiss is a different [formation from the " Octoraro 

schist" and that it must be of pre-Cambrian age, thrust over the 

so-called " Cambro-Ordovician " limestone and over the " Octoraro 

schist," then considered to be Ordovician. The sinuous line of contact between the gneiss and the limestone and between the gneiss and 

the schist was considered to represent the eroded plane of the over1 Mathews, E. B., Correlation of Maryland and Pennsylvania Piedmont formations : 

Geol. Soc. America Bull., vol. 16, pp. 329-346, 1905.

2 Bascom, F., The geology of the crystalline rocks of Cecil County: Maryland Geol. Sur 

vey, Cecil County, pp. 107-108, 1902.

3 Bascom, F., U. S. Geol. Survey Geol. Atlas, Philadelphia folio (No. 162), p. 5, 1909.

* Bliss, E. F./ and Jonas, A. I., Relation of the Wissahickon mica gneiss to the Shenandoah limestone and the Octoraro schist of the Doe Bun and Avondale region, Chester 

County, Pa., private publication, February, 1914; U. S. Geol. Survey Prof. Paper 98, pp. 

9-34, 1916.
INTRODUCTION 5

thrust which had been folded in a period of deformation subsequent 

to the faulting.

A similar explanation of the relations of the Wissahiekon gneiss 

to the other formations northeast of Baltimore was given by 

Mathews 5 in 1917. The pre-Cambrian Wissahiekon gneiss is described as thrust over the Baltimore gneiss, Setters formation, and 

Cockeysville marble along such a tfault plane as was believed to explain the anomalous position of the Wissahiekon gneiss in Pennsylvania.

During the field seasons of 1919 and 1920 spent in Baltimore and 

Harford Counties, in Maryland, the writers were.able to determine 

the position of the Wissahiekon gneiss in the stratigraphic column 

and to show that the limestones of the Doe Kun and Avondale district 

of Pennsylvania and the Cockeysville marble of Maryland are preCambrian, not Shenandoah, and that they had been erroneously 

correlated with the limestone series of Chester Valley. The preCambrian section established by the investigation is described in 

some detail in this report.

BIBLIOGRAPHY

The number of recent publications on the McCalls Ferry-Quarryville district has not been large, but the geology of the district was 

discussed in considerable detail in the reports of the Second Geological Survey of Pennsylvania. The following bibliography includes 

only the more important of the earlier papers on the district.

1876. Frazer, Persifor, jr., Report of progress in the district of York and

Adams Counties: Pennsylvania Second Geol. Survey, Kept. C. 

1878. Frazer, Persifor, jr., Geological section along the Susquehanna in Lancaster County, Pa.: Am. Philos. Soc. Proc., vol. 17, pp. 719-720. 

1880. Frazer, Persifor, jr., The geology of Lancaster County: Pennsylvania

Second Geol. Survey, Kept. CCC. 

Frazer, Persifor, jr., On age and position of the Peach Bottom slates:

Am. Philos. Soc. Proc., vol. 18, pp. 366-369.

Lesley, J. P., On fossils in the Peach Bottom roofing slates of Pennsylvania: Am. Philos. Soc. Proc., vol. 18, pp. 365-366.

Lesley, J. P., A Hudson River fossil plant in the roofing slate that is 

associated with chlorite slate and metamorphic limestone in Maryland, 

adjoining York and Lancaster Counties, Pa.: Am. Jour. Sci., 3d ser., 

vol. 19, pp. 71-72.

1883. Lesley, J. P., The geology of Chester County: Pennsylvania Second Geol. 

Survey, Rept. Ct.

1884. Frazer, Persifor, jr., The Peach Bottom slates of southeastern York and

southern Lancaster Counties, Pa.: Am. Inst. Min. Bng. Trans., vol. 12.

pp. 355-359. 

1895. Kemp. J. F., The nickel mine at Lancaster Gap, Pa,, and the pyrrhotite

deposit at Anthony's Nose on the Hudson: Am. Inst. Min.- Bng. Trans.,

vol. 24, pp. 620-633, 888.

5 Mathews, B. B.. TI. S. Geol. Survey Geol. Atlas, Tolchester folio (No. 204), p. 3, 1917.
6 McCALLS FEREY-QUAEBYVILLE DISTBIOT, PA.

1900. Rand, T. D., Notes on the geology of southeastern Pennsylvania: Acad.

Nat. Sci., Philadelphia, Proc. for 1900, pp. 160-338. 

1902. Bascom, F., The crystalline rocks of Cecil County: Maryland Geol. Survey,

Cecil County, pp. 83-143. 

1905. Bascom, F., Piedmont district of Pennsylvania: Geol. Soc. America Bull.,

vol. 16, pp. 289-328. 

Mathews, B. B., Correlation of Maryland and Pennsylvania Piedmont

formation: Geol. Soc. America Bull., vol. 16, pp. 329-346. 

Miller, W. J., Report on reconnaissance work in southern Lancaster

County, Pa. (unpublished manuscript).

1909. Bascom, F., U. S. Geol. Survey Geol. Atlas, Philadelphia folio (No. 162). 

1914. Bliss, E. F., and Jonas, A. I., Relation of the Wissahickon mica gneiss to

the Shenandoah limestone and Octoraro schist of the Doe Run and Avondale region, Chester County, Pa., private publication, 1914; U. S.

Geol. Survey Prof. Paper 98, pp. 9-34, 1916. 

1917. Mathews, E. B., U. S. Geol. Survey Geol. Atlas, Tolchester folio (No.

204), p. 3. 

Mathews, B. B., Submerged " deeps" in the Susquehanna River: GeoL

Soc. America Bull., vol. 28, pp. 335-346. 

Jonas, A. I., Pre-Cambrian and Triassic diabase in eastern Pennsylvania:

Am. Mus. Nat. Hist. Bull., vol. 37, pp. 173-181. 

1920. Bascon, F., U. S. Geol. Survey Geol. Atlas, Elkton-Wilmington folio

(No. 211).

1922. Stose, G. W., and Jonas, A. I., The lower Paleozoic section of southeastern Pennsylvania: Washington Acad. Sci. Jour., vol. 12, pp. 358-366 

Wherry, E. T., and Shannon, E. W., Crocidolite from eastern Pennsylvania: Washington Acad. Sci. Jour., vol. 12, pp. 242-244.

1923. Knopf, E. B., and Jonas, A. I., Stratigraphy of the crystalline schists of 

Pennsylvania and Maryland: Am. Jour. Sci., 5th ser., vol. 5, pp. 40-62. 

Stose, G. W., and Jonas, A. I., Ordovician overlap in the Piedmont province of Pennsylvania and Maryland: Geol. Soc. America Bull., vol. 34, 

pp. 507-524.

1924. Jonas, A. I., Pre-Cambrian rocks of the western Piedmont of Maryland:

Geol. Soc. America Bull., vol. 35, pp. 355-364.

<?- Hawkins, A. C., Alternative interpretation of some crystalline schists in

southeastern Pennsylvania: Am. Jour. Sci., 5th ser., vol. 7, pp. 355-364.

Phemister, T. C., A note on the Lancaster Gap mine, Pennsylvania: Jour.

Geology, vol. 32, pp. 498-510. 

Knopf, E. B., Correlation of residual erosion surfaces in the eastern

Appalachian Highlands: Geol. Soc. America Bull., vol. 35, pp. 633-666. 

Bascom, F., The Resuscitation of the term Bryn Mawr gravel: U. S. Geol.

Survey Prof. Paper 132, pp. 117-119.

1927. Stose, G. W., and Jonas, A. I., Ordovician shale and associated lava 

in southeastern Pennsylvania: Geol. Soc. America Bull., vol. 38, pp. 

505-^536.

GEOGRAHPY

TOPOGRAPHY

The McCalls Ferry-Quarryville district is part of the physiographic division of the eastern United States that is known as the 

Piedmont province. The topography of the district falls into two 

distinct types a well-dissected upland, whose interstream areas are
GEOGRAPHY 7

wide, flat hill summits, and a gently rolling lowland, broken here 

and there by long, low hills trending northeast. The lowland occupies the northern and northwestern portion of the district and widens 

nortlnvard into Lancaster Valley. To the south it cuts across the 

upland surface in a depression about 1 mile wide, near Quarryville. 

From Quarryville toward the northeast a long, narrow valley, known 

as Chester Valley, separates the upland into a northern portion, which 

includes the Mine Ridge upland and a southern portion known 

as the Sbuth Valley Hills.

Chester Valley is a well-defined topographic feature, controlled in 

shape and extent by the easy solubility of the underlying rock, which 

is limestone and dolomite. It has a linear extent of 66 miles and 

finally disappears north of Philadelphia, where the limestone outcrop 

ends on the crest of a pitching fold. The floor of the valley is flat, 

about a quarter of a mile in width at the narrowest part, and widening northeastward to a couple of miles. It slopes from an altitude 

of 580 feet at Quarryville to 100 feet near Philadelphia and is occupied by a succession of small runs, some flowing westward and some 

eastward. There is no continuous stream by whose agency the valley 

might have been formed. The bounding ridges that delimit the valley rise in steep slopes 150 to 250 feet above the valley floor.

The northern portion of the upland rises 350 feet above the level 

of Lancaster Valley and 140 to 200 feet above the floor of Chester 

Valley. To a distant observer the upland appears to be flat, but a 

close examination of topographic detail reveals the fact that the 

surface is more diversified than would at first appear. The western 

and northwestern rims of the upland rise appreciably above the 

general upland level in the narrow Mine Ridge proper. This ridge, 

which presents to the eye of an observer in Lancaster Valley an 

approximately even crest line similar to the crest of Blue Mountain, 

attains a maximum altitude of 920 feet and descends on both sides 

of this maximum to surfaces that maintain for several miles altitudes ranging from 820 to 880 feet. Similar altitudes ranging 

from 820 to 900 feet reappear in the upland surfaces on the west 

side of the Quarryville Valley and to the southwest across Susquehanna River in York County, forming the highest and flat 

summits of the upland country. These hills at 820 and 900 feet 

form the divide between northward and southward flowing streams, 

which is breached by Susquehanna River in a deep gorge from 

northwest to southeast. The steep rugged slopes of this gorge are 

covered with a dense growth of timber, and in the valley bottom, 

which ranges in width from a quarter of a mile to more than a mile, 

the river rushes in a deep and narrow channel past many rocky 

islets, which occupy most of the valley floor. Even now, to a person 

standing in the gorge at the mouth of one of the tributaries that
8 McCALLS FEEEY-QUARBYVILLE DISTRICT, PA.

empty into the main stream through ravines dense with laurel and 

rhododendron, the wildness of the scene recalls the days when the 

Susquehanna furnished to the Indian canoe a means of travel 

through a well-nigh impassable wilderness. But on the upland 

summits the atmosphere of the wilderness is lost, and the lonely 

wooded gorge of the river forms a sharp contrast to the fertile 

farming land that occupies the wide upland of comparatively slight 

relief. '

DRAINAGE

The district is drained by Susquehanna River and numerous tributaries. The length of the Susquehanna from its source in Lake 

Otsego, N. Y., to the head of Chesapeake Bay is 422 miles, and it 

drains an area of 27,400 square miles. 6 The portion that is included, 

in the district here described lies 380 miles from the source and 17 

miles from the mouth and has an average fall of 5.3 feet to the mile. 

The largest tributaries to Susquehanna River are Fishing and Muddy 

Creeks on the York County side and Conestoga and Pequea Creekson the Lancaster County side. The flow of water in Susquehanna 

River ,is extremely variable. Between 1911 and 1916 the average 

flow below the Holtwood Dam was 20,000 second-feet, the maximum 

440,000 second-feet, and the minimum 2,000 second-feet.

The amount of water carried by the river is small in comparison 

to the size of its valley, the river being in this sense an underfit 

stream. The explanation is doubtless to be found ,in the fact that 

at one time during the retreat of the ice of the glacial epoch the 

drainage from the Finger Lakes of New York went chiefly southward 

through Susquehanna Kiver. The volume of water carried by the

river at that time therefore was probably much larger than it is at 

present.

A curious feature of the channel is the occurrence of the " deeps " 

or long depressions below the level of the channel that have been 

described by Mathews. 7 Six of these deeps occur between Turkey 

Hill and Fites Eddy, where the channel is near the left bank. They 

range from 2 miles to slightly less than a mile in length and attain 

a maximum depth of 30 feet below sea level. They are confined 

to the eastern edge of the rock channel and are associated with large 

potholes. No entirely satisfactory explanation has been proposed 

to account for the origin of these remarkable holes. Mathews believed that they were the work of post-Talbot erosion during a time

8 Hollister, G. B., U. S. Geol. Survey Water-Supply Paper 108, p. 9, 1904. 

7 Mathews, E. B., Submerged " deeps " in the Susquehanna River: Geol. Soc. AmericaBull., vol. 28, pp. 335-346, 1917.
CLIMATE 9

when the river channel was either narrower or the volume considerably greater than at present. Such increase in volume might have 

been caused by increased drainage through the Susquehanna, but 

it is difficult to explain by increased volume the localization of the 

deeps. Daly 8 has suggested that a more probable reason for the 

formation of these depressions is an increased gradient of the stream 

deposits on the southerly slope of a peripheral bulge of the land 

south of the continental ice cap. But study of the tributary drainage 

in the zone of the assumed peripheral upwarping of the crust shows 

no such trenching as must have accompanied a steepening in gradient sufficient to account for the carving of the Susquehanna deeps. 

Moreover, the crest of the bulge must have been at no great distance 

from the foot of the ice sheet, which in this region was as much as 

110 miles from the Turkey Hill-Fites Eddy stretch of Susquehanna 

River.

CLIMATE

The mean temperatures for the eastern Piedmont of Pennsylvania 

are 31° F. for winter, 50° for spring, 72° for summer, and 55°, for 

fall. The maximum summer temperature rises above 95°, and the 

minimum winter temperature falls below zero, especially in the 

month of February. There is a difference of about 3° between the 

climate of the western part of the Piedmont area and that of Philadelphia. In spring the last killing frost occurs from April 6 to 

May 27, and in fall the first killing frost from September 17 to 

November 14, according to the season and to the altitude of the section. Sudden melting of the ice and snow in the mountains often 

causes the formation of ice gorges in the Delaware and Susquehanna 

River valleys. The ice gorges are due to an insufficiency of water 

to carry out the floating ice blocks, which are penned up in narrows 

of the river and refrozen into a solid mass by sudden drops in temperature. The resultant ice jams have attained great size in Susquehanna River below Columbia and have caused much damage to river 

property, both by ice work and by flooding of the shores of the 

mainland and tributary streams.

The mean annual rainfall is 40 inches, as shown by records at 

Lancaster covering 20 years. Records at Coatesville covering a 

period of 32 years show a mean annual rainfall of 48 inches, with a 

variation from 32 to 69 inches. Records at Holtwood covering the 

period from 1915 to 1920 show a mean annual rainfall of 37 inches, 

with a maximum of 46 inches.

8 Daly, R. A., Oscillations of level in the belts peripheral to the Pleistocene ice caps: 

Geol. Soc. America Bull., vol. 31, p. 314, 1920.
10 McCALLS FERRY-QUARRYVrLLE DISTRICT, PA.

GENERAL GEOLOGY

OUTLINE OF THE GEOLOGY OF THE APPALACHIAN MOUNTAINS 

AND THE PIEDMONT PROVINCE

The easternmost physiographic division of the Atlantic slope is 

the Coastal Plain, a gently sloping plain underlain by unconsolidated 

deposits. (See fig. 2.) Between the Coastal Plain on the east and 

the long parallel ridges and intermontane valleys of the Appalachian

FIGURE 2. Physiographic provinces in the Middle Atlantic States. The Jersey Highlands, including the Reading and Durham Hills, are the northern representative of 

the Blue Ridge province and are generally considered to extend northward into the 

New England upland. South Mountain in Pennsylvania forms the north end of the 

Blue Ridge province

Ridge and Valley province on the west lies the Piedmont province. 

The western boundary of the Piedmont province is formed by the 

Appalachian Mountains, an interrupted mountainous belt of moderate 

relief with maximum altitudes of about 6,000 feet.

APPALACHIAN MOUNTAINS 

GENERAL FEATURES

The northern portion of the Appalachian Range comprises the 

Green Mountains, in Vermont and western Massachusetts, and the 

Hoosac Eange, in southwestern Massachusetts and Connecticut. Its 

southwestward extension through New York crosses Hudson River
GENERAL GEOLOGY 11

between Fishkill and Peekskill and continues through New Jersey 

as the Highlands. This range crosses Delaware River near Easton 

and forms in, Pennsylvania the Reading and Durham Hills, whose 

highest summits stand at 1,000 to 1,100 feet. The Beading and 

Durham Hills terminate abruptly in Penn and Neversink Mountains, 

which overlook Schuylkill River at Reading. In Lebanon and 

Berks Counties about 6 miles southwest of Reading an isolated group 

of hills, known as South Mountain, marks the south end of the 

northern Appalachians. From these hills southwestward for a distance of 50 miles there is a topographic break and disappearance 

of the ridge that marks the boundary between the CumberlandLebanon Valley of the Appalachian Ridge and Valley province on 

the west and the Piedmont province on the east.

South of Carlisle the Appalachian Mountains reappear in the 

larger South Mountain, completely disconnected from the South 

Mountain in Lebanon and Berks Counties, near Reading. The 

South Mountain of southern Pennsylvania is a wide area made up 

of many narrow ridges with a maximum altitude of 2,100 feet which 

extend southward into South Mountain and Catoctin Mountain, 

Maryland. The Appalachian Mountains in Virginia lie between 

Catoctin Mountain and the western front of the Blue Ridge. The 

Appalachian Mountains widen in the Southern States, forming a 

belt about 50 miles in width, which comprises the Unaka and Great 

Smoky Mountains and many others.

The topography of the Appalachian Mountains is determined by 

the nature of the underlying rocks. The range is composed of 

schists and gneisses of both sedimentary and igneous origin. In the 

Highlands of New Jersey and in the Reading and Durham Hills 

the hills that are composed of feldspathic gneisses are of rounded or 

irregular outlines, with more or less gently sloping sides, and the 

hills that consist of quartzite form ridges elongated along the strike, 

whose steep rugged slopes are covered with fragments of broken 

quartzite.

GEOLOGY OF THE BEADING HILLS

The writers have studied the crystalline schists of the Appalachian mountains in some detail in the Reading Hills, the southward 

continuation into Pennsylvania of the New York and New Jersey 

Highlands. The oldest rocks of sedimentary origin in these hills 

are graphite-bearing biotite gneisses containing intercalated layers 

of marble and quartzite. The graphitic gneisses are associated with 

hornblende schists that may belong to the sedimentary series, although it has not yet been possible to delimit them from other hornblende schists of undoubted igneous origin. The sedimentary series
12 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

is lithologically similar to the Franklin limestone and interbedded 

graphitic schists of the New Jersey Highlands, and like those rocks 

it has been invaded by a series of igneous rocks ranging in composition from granite to gabbro. Numerous small isolated patches of 

sedimentary gneiss completely surrounded by igneous rock probably 

represent rifted blocks or roof pendants. In a region of moderate 

relief it is impossible to establish the presence of roof pendants because the depth of outcrop is not sufficient to expose the downward 

prolongation of the pendant into the intruding igneous material. 

The metamorphosed sediments are granoblastic in texture as a result 

of a deep-seated metamorphism that has developed various heavy 

minerals such as garnet, sillimanite, hornblende, and plagioclase. 

Undulose extinction is almost universally present and indicates that 

a mechanical deformation has been superimposed upon the deepseated metamorphism. In contrast to the crystalloblastic texture of 

the sedimentary gneisses, the igneous gneisses show distinct igneous 

texture. The dynamic deformation that affected the sediments was 

later than the igneous intrusion, for it has left its mark upon the 

igneous rocks in undulatory extinction, granulation, and twisting of 

the mineral constituents. In certain rather well-defined zones the 

deformation has been so intense that the rocks involved show all 

stages of cataclastic action from a strong brecciation to a complete 

mylonitization through which the original character of the rock 

can be only faintly discerned. The shattering of the rock complex 

was accompanied by a certain amount of mineralization, as shown 

by the presence of crocidolitized zones,9 and by the passage of orebearing solutions.

The presence of certain fine-grained and notably undeformed 

bostonitic dikes in the magnetite district of Rittenhouse Gap suggests 

the presence of an alkalic magma younger than the magma that 

furnished the rocks of the strongly deformed igneous complex. 

The Reading and Durham Hills are penetrated by dikes of Triassic 

diabase 10 and broken by numerous faults of Triassic age, and the 

possible genetic relationship of the alkalic dikes to the nephelite 

syenite magma of Beemerville, N. J., suggests that the considerable 

deformation during late Triassic time in the Reading and Durham 

Hills was accompanied by the intrusion of both diabasic and alkalic 

magma. Such an idea is in accord with the suggestion made by 

Wherry" that the hydrothermal solutions by which the gneisses 

were crocidolitized originated in the Triassic diabase magma. The

8 Wherry, E. T., and Shannon, E. V., Crocidolite from eastern Pennsylvania : Washington 

Acad. Sci. Jour., vol. 12, pp. 242-244, 1922.

10 Jonas, A. I., Pre-Cambrian and Triassic diabase in eastern Pennsylvania: Am. Mus. 

Nat. Hist. Bull.,-vol. 37, pp. 173-181, 1917.

11 Wherry, E. T., and Shannon, E. V., op. clt., p. 244.
GENERAL GEOLOGY 13

 crystalline schists of the Reading and Durham Hills and the associated igneous complex are unconformably overlain by a slightly 

altered series of Paleozoic sediments. Evidence of the unconformity 

is afforded by the discordance in structure between the Paleozoic 

.and the pre-Cambrian rocks and by the presence in the basal Cambrian beds of reworked material derived from the underlying igneous 

rocks.

GEOLOGY OF CATOCTIN-SOTTTH MOUNTAIN

The pre-Cambrian crystalline rocks of the New York and New 

Jersey Highlands and of \ the Reading and Durham Hills, as far 

south as the South Mountain in Lebanon County, Pa., form a continuous belt with similar lithologic character. There is a distinct

topographic break west and southwest of this South Mountain in 

Lebanon-Berks County, where the rugged hills of crystalline schist 

Are replaced by the limestone lowland and rolling shale hills of the 

.Lebanon-Cumberland Valley. South of Carlisle, where the Appalachian Mountains reappear in the South Mountain-Catoctin Mountain ridges of southern Pennsylvania and Maryland, the pre-Cambrian core of the range is quite different in lithologic character from 

the crystalline core of the Reading and Durham Hills. In Catoctin 

Mountain and South Mountain a series of devitrified rhyolites and 

epidotized basaltic flows 12 intruded by granite are overlain unconformably by Cambrian sedimentary rocks that show pebbles of the 

underlying lava in the basal conglomerate. 13

Up to the present time it has been impossible to establish any 

geologic relationship between the lavas and the plutonic intrusions of the Reading Hills. Recent work in the pre-Cambrian stratigraphy of the adjoining Piedmont Province, however, has furnished some suggestions concerning possible correlation, which will 

be presented in the following pages.

PIEDMONT PROVINCE

The rugged hill country of the Appalachian Mountain belt slopes 

rapidly downward for about 1,500 feet to the level of the Piedmont 

province, which extends eastward if or 30 miles to the western margin 

of the Coastal Plain. The boundary between the Piedmont province 

.and Coastal Plain runs through New Jersey from Perth Amboy 

south westward to Trenton; south of this point it follows Delaware 

River through Philadelphia south westward to Wilmington, thence

« Keith, Arthur, Geology of the Catoctin belt: U. S. Geol. Survey Fourteenth Ann. Rept., 

pt. 2, pp. 285>-395, 1894. Bascom, F., The ancient volcanic rocks of South Mountain, Pa.: 

U. S. Geol. Survey Bull. 136, 1896.

13 Bascom, F., op. cit., p. 28.
14 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

through Elkton, Havre de Grace, and Baltimore to Washington. 

This boundary is really a border zone, which is known as the " fall 

line " because here the stream courses are turbulent and full of rapids. 

The extreme width of the Piedmont province is 120 miles; its mean 

width is 60 miles. Its general slope is eastward and southeastward. 

In New Jersey it slopes from 400 feet above sea level at the western 

margin to an altitude at the margin of the Coastal Plain which ranges 

from 100 rfeet on Delaware River to sea level on Newark Bay. Between Susquehanna and Delaware Rivers the higher summits of the 

Piedmont province attain an altitude of 1,100 feet in Welsh Mountain 

and slope down to about 300 feet near the fall line. The Piedmont 

province rises to its culminating point in South Carolina and Georgia, 

where it has an altitude of 1,800 to 1,900 feet along the base of the 

Blue Ridge.

GENEEAL GEOLOGY OF THE PIEDMONT PROVINCE IN PENNSYLVANIA

The Piedmont province is divided into the Triassic Lowland and 

the Piedmont Upland. In Pennsylvania the Triassic deposits were 

laid down in a basin that now extends westward and southwestward 

along the northwestern edge of the province. The Triassic Lowland 

ranges in width from 40 miles at Delaware River to 4 miles near 

Cornwall. The Triassic shales and sandstones, dipping gently westward, form a rolling surface diversified by rugged hills of conglomerate and by rounded knoblike hills and long, curving ridges of 

diabase and indurated sediments that rise to 1,200 and 1,300 feet on 

both sides of Susquehanna River.

Southeast of the Triassic area maturely»dissected hills with a

maximum altitude of 900 to 1,080 feet form the gentle topography of 

the Piedmont Upland. The country is well dissected by streams that 

flow in broad valleys having composite slopes. The interstream 

areas are flat-topped hills, which are remnants of partial peneplains 

not entirely removed by erosion since their formation. The shape of 

the hills is largely dependent on the character o»f the underlying rocks. 

The Piedmont Upland is underlain by crystalline schists and less 

altered slates, phyllites, and limestones. Crystalline schists and 

gneisses, including a few areas of marble, form the easternmost belt 

of the upland from Trenton, N. J., southward to Washington. For 

a large part of the westernmost belt the term " upland " is really a 

misnomer, inasmuch as this part is occupied by a rolling plain of low 

relief. This plain, which is underlain chiefly by limestones and 

dolomites, is known as the Lancaster, York, and Hanover Valleys in 

Pennsylvania and the Frederick Valley in Maryland. It has a wide 

range in width from a maximum of 20 miles in the vicinity of Lancaster to a minimum of 1 mile on Potomac River near Frederick.
GENERAL GEOLOGY 15

Southeast of Lancaster, in the vicinity of Quarryville, the valley cuts 

across the northeastward trend of the schist hills and continues 

through the middle of the Piedmont Upland in a narrow depression 

known as Chester Valley, which 8 miles north of Philadelphia 

merges into the wide plain of the Triassic Lowland. The area of .the 

Lancaster and York Valleys is interspersed by several anticlinal 

ridges underlain for the most part by phyllites and quartzites.

Post-Triassic sediments overlap the margin of the Piedmont province and occur in residual patches upon its surface. These sediments 

are unconsolidated deposits of sand, marl, and gravel that have been 

gently tilted southeastward. They range in age from Lower 

Cretaceous to Quaternary.

GEOLOGY OF THE PIEDMONT UPLAND IN MARYLAND

During a study of the crystalline rocks in Baltimore County, Md., 

and of their northeastward extension in Pennsylvania, begun in 1919, 

the writers have established the stratigraphic succession shown in 

Figure 3 for the pre-Cambrian crystalline schists of the eastern border 

of the Piedmont province in Maryland.

Baltimore gneiss. The oldest rock of the region is the Baltimore 

gneiss, a light-colored, thoroughly recrystallized rock of sedimentary 

origin, which is in some places a thickly bedded gneiss of granitic 

aspect and in others a thinly banded ribbon gneiss. The deep-seated 

nature of the metamorphism that has produced the foliation is shown 

by the granoblastic texture and by the nature of the constituent 

minerals. The gneiss is composed dominantly of quartz, oligoclase, 

microcline, and biotite. Garnet and tourmaline are characteristic 

accessories. The sedimentary origin of the gneiss is shown in the 

field by the distinct and persistent even bedding, which can be traced 

in some quarries completely across the outcrop, for 100 feet or more. 

Study of thin sections indicates that recrystallization is complete 

and no trace of original sedimentary texture has been preserved. 

Many of the zircons occur in well-worn, rounded grains that are 

indicative of water-borne material. This criterion, however, must 

be used with considerable caution, because it is possible that zircon 

in igneous rocks may be rounded by magmatic corrosion. The Baltimore gneiss has been intruded by a granite that has produced a 

lit-par-lit injection in the upper part of the formation.

Glenarm series. Unconformably overlying the Baltimore gneiss 

is a series of later pre-Cambrian sediments termed the Glenarm 

series, from its typical development near Glenarm, 13 miles northeast 

of Baltimore. The United States Geological Survey classifies the 

Baltimore gneiss as Archean and the Glenarm series as Algonkian. 

though in the writers' opinion there is lack of adequate evidence that
Bog re p "i 5"

= >-,.< CTQ

3 M*

J? S o 3 "q

ffi.o-1

§ s E B> Cj S

2. 2

a&
GENEBAL GEOLOGY 17

they are the precise equivalents of rocks referred to these systems 

in other regions.

The Glenarm series comprises the Setters formation, the Cockeysville marble, the Wissahickon formation, the Peters Creek schist, 

the Cardiff conglomerate, and the Peach Bottom slate. The total 

thickness of the series probably amounts to 8,000 or 10,000 feet, 

although no accurate estimate can be made, for the middle formations 

have been repeated by close folding. In some places the Wissahickon formation lies directly upon the eroded surface of the 

Baltimore gneiss, owing to the fact that the earlier formations of 

the series are overlapping shore deposits. So far as now known the 

deposition of Glenarm sediments was continuous and not interrupted 

by erosion or by erogenic deformation.

Only the upper formations of the Glenarm series are exposed in 

the McCalls Ferry-Quarryville district, but a brief description of 

the lower Glenarm section as established in Baltimore County is 

introduced here in order to make clear the stratigraphic relations of 

the whole series.

The basal formation is the Setters, which overlies the Baltimore 

gneiss unconformably but with no basal conglomerate. Williams " 

considered that the basal schist of the Setters formation was really a 

transition bed between the Baltimore gneiss and the overlying formation, but recently it has proved possible to establish the unconformity by several facts. A structural discordance between the two 

formations can be recognized, the divergence in average strike 

amounting to 20°. There is more evidence of deformation in the 

Baltimore gneiss than ,in the Setters formation, even though the 

members of the gneiss are more resistant to deformation than the 

argillaceous layers of the Setters. Moreover, the Setters is irregular 

in thickness and in some places is entirely lacking, so that the 

Cockeysvijle marble overlaps the Setters deposits and lies cjirectly 

upon the Baltimore gneiss. In the absence of positive evidence of 

erosion, it seems likely that the irregular thickness of the Setters 

formation and of the Cockeysville marble is due to transgression 

rather than to eros.ion. The Setters formation is composed of mica 

gneiss and mica schist with an intercalated quartzite member. The 

maximum thickness measured is 1,000 feet. The whole formation 

has been thoroughly recrystallized in the lowermost zone of metamorphism, with the production of deep-seated minerals such as 

garnet and biotite. Tourmaline is an abundant and characteristic 

accessory constituent, which is undoubtedly of pneumatolytic origin 

and associated with a granite known as the Gunpowder granite, 

which has invaded the Baltimore gneiss and the overlying Glenarm

u Williams, G. H., Geology of Baltimore and vicinity (extract from the Guidebook of 

Baltimore prepared for the American Institute of Mining Engineers), February, 1892.
IS McCALLS FEKRY-QUARRYVILLE DISTRICT, PA.

series. The Setters formation is characterized by an unusually high 

content of potash which has crystallized as microcline and muscovite, 

indicating that the original sediment was a mud or sand high in 

potash feldspar and white mica.

The Cockeysville marble consists of alternating dolomitic and 

calcitic layers. It overlaps the Setters formation and Baltimore 

gneiss and grades through a calcareous mica schist into the overlying Wissahickon formation. Its probable thickness is 400 feet. 

The Wissahickon formation occurs in two metamorphic facies an 

oligoclase-mica schist and an albite-chlorite schist. The oligoclasemica schist facies is well developed in the vicinity of Baltimore. It 

is lithologically similar to the Setters, but the quartzite beds are less 

conspicuous than those in the Setters because they are thinner and 

more evenly distributed throughout the formation, whereas in the 

Setters the quartzite is chiefly confined to the middle member. -The 

Wissahickon differs from the Setters also in its lower content of 

piotash, which has caused oligoclase rather than microcline to be the 

dominant feldspar. The Wissahickon oligoclase-mica schist is a 

series of thoroughly recrystaliized sediments that have been metamorphosed under deep-seated conditions with the production of 

garnetiferous muscovite-biotite schists, staurolite and kyanite schists, 

and biotite-plagioclase gneiss. In some places tourmaline is abundantly developed.

In northern Baltimore County, Md., and in the southern part of 

York and Lancaster Counties, Pa., metamorphism under different 

conditions has resulted in the formation of an albite schist that is 

considered to be the stratigraphic equivalent of the Wissahickon 

oligoclase-mica schist, the difference in mineral constituents being

caused by metamorphism of moderate intensity 'in contrast to the 

intense metamorphism of the oligoclase-mica schist.

The stratigraphic relations of the Wissahickon albite schist and 

the upper formation of the Glenarm series to the underlying and 

overlying rocks have been established mainly by work in the McCalls 

Ferry-Quarryville district. Their lithologic character and stratigraphic relations are described in detail in the following section.

, GEOLOGY OF THE McCALLS FERRY-QTJARBYVILLE DISTRICT

PRE-CAMBRIAN ROCKS OF SEDIMENTARY ORIGIN 

PRE-GLENARM ROCKS OF THE MINE RIDGE UPLAND

BALTIMORE GNEISS

North of Chester Valley an upland known as the Honeybrook 

upland extends from Valley Forge, on Schuylkill River, southwestward for a distance of 40 miles. The surface of this upland trun-
PRE-CAMBBIAN BOOKS 19

cates a wide anticlinal arch that ends in several westward-pitching 

anticlinal folds. These anticlines underlie three elongated hills 

Welsh Mountain, Barren Hill, and the Mine Ridge upland. The 

Mine Ridge upland has been studied in detail by the writers, but 

the results are unsatisfactory, owing to the unusual scarcity of outcrops, exceptional even in a region that has long been the bane of 

geologists on account of its deep, fertile soil and intensive cultivation. The only places where more than isolated outcrops are available for study are sections exposed in rock cuts along the railroads 

that cross the upland in three places. These three sections are (1) 

that between Christiana and Gap on the main line of the Pennsylvania System between Philadelphia and Harrisburg; (2) that 

between Coatesville and Brandywine Manor on the Wilmington 

division of the Philadelphia & Reading Railroad; (3) that north of 

Downingtown on the New Holland branch of the Pennsylvania 

Railroad.

A gneiss of sedimentary origin occurs in the core of the Mine 

Ridge upland below the Cambrian gneisses, quartzites, and schists 

that flank the uplift. Metamorphism has completely altered both 

Paleozoic and pre-Paleozoic sediments into highly crystalline schists 

and gneisses, in which the schistosity is so well marked that it has 

more or less obliterated original structural differences. But the 

unconformity between the Cambrian rocks and the underlying gneiss 

is well marked on the Philadelphia & Reading Railroad 1 mile north 

of Coatesville, where the Cambrian schists and quartzites dip southward over the northward-dipping pre-Cambrian schists and gneisses. 

Additional evidence of unconformity consists in the constant repetition of layers of hornblende schist or amphibolite in the underlying 

gneiss, combined with the absence of all such interlayered amphibolites in the Cambrian sediments.

The upper beds of the pre-Cambrian sedimentary series are biotitealbite schists. The writers have so far never succeeded in finding 

in this district an exposure showing the actual contact of pre-Cambrian 

and Cambrian rocks. Four miles northeast of Christiana and about 

700 feet south of the Steprock quarry, in Cambrian quartzite, the 

rock that lies within a few feet beneath Cambrian schists is a gray 

finely banded biotite schist in which numerous narrow layers of 

light-colored granitic rock suggest an injection of a highly biotitic 

schist by a granitic magma.

A succession of quartzose and biotitic gneisses, in many places 

graphitic, underlies the biotite-albite schist. These rocks are interlaminated with fine-grained biotitic hornblende schists and amphibolites that appear to be dynamically altered basic sills or flows, 

6136 29 3
20 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

although the formation of uralite is so complete .that the original 

character of the rock can not be determined. The amphibolites and 

interlayered sediments are intruded by a light:colored rock of 

granitic texture that contains much saussuritized feldspar. The 

light-colored rock in many places near the upper part of the preCambrian section is injected into the sedimentary series and the 

amphibolites, forming a banded lit-par-lit injection zone.

The biotite-albite schists are distinctly foliated rocks, composed of 

quartz, albite, biotite, and muscovite, with strong parallel dimensionalism. These constituents surround numerous lenticular areas 

of albite or albite and quartz. It is not easy to decide whether the 

rock was produced by the recrystallization of an original arkosic 

sandstone or by the injection of a monzonitic magma into a shaly 

sediment followed by strong metamorphism of the mixed rock. The 

albite, which is nearly pure (about Ab95Ang ) is crowded with fairly 

coarse inclusions of muscovite, biotite, and quartz. The whole rock 

shows a strong dynamic deformation superimposed upon an original 

crystalloblastic texture.

The quartzose and biotitic gneisses that underlie the biotite-albite 

schists are strongly foliated rocks in which layers of sutured quartz 

alternate with bands of biotite and muscovite. The crystalloblastic 

texture and the development of garnet, epidote, and biotite indicate 

a recrystallization under deep-seated conditions. Undulatory extinction and granulation of quartz show a cataclastic deformation 

subsequent to the recrystallization. - Arkosic beds of the pre-Cambrian gneissic series contain a considerable amount of plagioclase filled with muscovite and zoisite. 

The species of the plagioclase could not be accurately determined in 

the three sections studied, but the index and birefringence indicate 

the composition of a sodic oligoclase or albite.

As yet it is impossible to make a conclusive correlation between 

the Baltimore gneiss of the type locality in Baltimore, Md., and 

the pre-Cambrian gneiss of the Mine Ridge uplift, but the existing 

evidence seems to justify the belief that the core of Mine Ridge 

anticline is of pre-Glenarm age. About 12 miles southeast of Coatesville, between Pocopson and Chadds Ford, Brandywine Creek cuts 

through the Avondale-Kennett anticline, which exposes a quartzose 

biotite gneiss in the core and injection gneisses on the flank. The 

anticline is in the same line of strike as the anticlinal uplift of 

Baltimore gneiss north of Baltimore, Md., and the rocks of the 

anticline in Pennsylvania recall by their lithology the Baltimore 

gneiss of Maryland with its accompanying injection zone. Moreover, the sedimentary series on the flanks of the Avondale-Kennett
PEE-CAMBRIAN ROCKS 21

uplift resembles in lithology and thickness the members of the Glenarm series in Maryland. The lowest formation overlying the biotite 

gneiss is a series of mica schists, mica gneisses, and quartzites of 

variable thickness, ranging from a few feet to at least 800 feet. 

It is overlain by crystalline marbles, also of variable thickness, and 

these in turn are overlain by garnetiferous mica schists and more or 

less quartzose mica gneisses.

Hawkins 15 is of the opinion that the sedimentary series which 

flanks the Avondale-Kennett anticline around Avondale is of the 

same age as the sedimentary series of Chester Valley. He bases 

his opinion on the bel,ief that " the sedimentary succession and thickness of the various members in the Avondale series 16 are closely 

similar to those of the sedimentary series of accepted CambroOrdovician age in the adjacent Chester Valley," which he describes 

as a " basal quartzite followed by blue crystalline limestone, overlain 

by phyllite, all apparently conformable," and also on the belief that 

the apparent dissimilarity between the two series is the result of a 

regional contact metamorphism exerted by " a large buried intrusive 

mass below, whose roof cover has not yet been removed by erosion 

and which has but barely reached in its metamorphic action the 

series of the Chester Valley." In the first statement Hawkins makes 

the error of comparing the section of the sedimentary series at 

Avondale, not with the complete Paleozoic section around the Mine 

Ridge upland and northward, but with the incomplete section west 

of Coatesville, where a post-Beekmantown unconformity 17 has removed a considerable thickness of Cambrian and Lower Ordovician 

beds. As a result he ignores the fact that the full sequence of the 

Paleozoic sedimentary formations in the Chester Valley-Mine Ridge 

upland section is entirely different from that of the sedimentary 

series at Avondale, even if we accept his own estimate as to the 

thickness and structure of the latter series. For purposes of comparison two stratigraphic sections are given below; the first shows 

the section for the Glenarm series near Avondale, and the second 

shows the section for the sedimentary series in the vicinity of the 

Mine Ridge upland, as determined by the work of Stose, Jonas, and 

Knopf.

18 Hawkins, A. C., Alternative interpretations of some crystalline schists In southeastern 

Pennsylvania: Am. Jour. Sci., 5th ser., vol. 7, pp. 355-364, 1924.

10 "Avondale series " is used to denote " the Glenarm sedimentary series in the vicinity 

of Avondale, Chester County, Pa."

17 Stose, G. W., and Jonas, A. I., Ordovician overlap in the Piedmont province of Pennsylvania and Maryland: Geol. Soc. America Bull., vol. 34, pp. 507-524, 1923.
22 McCALLS FERRY-QUABRYVILLE DISTRICT, PA. 

Sections near Avondale and Mine Ridge anticline

Pre-Cambrian Glenarm series of Avondale

Formation

Wissahickon schist (correlated 

by Hawkins « with Ordovician phylh'te) .

Marble __________ . .....

Thickness 

(feet) "

(?) 

600± 

1,200-0

Paleozoic succession of Mine Ridge upland and northward1

Thickness 

(feet)

1,000+

500+ 

2,000± 

900± 

500±

1,000± 

150 

600-0

1,500 

500±

Formation

Cocalico shale. 

Unconformity.

Conestoga limestone. 

Unconformity. 

Beekmantown limestone.

Conococheague limestone.

Elbrook dolomite. 

Unconformity.

Ledger dolomite. 

Kinzers formation. 

Vintage dolomite. 

fAntietam schist. 

(.Harpers schist. 

Chickies quartzite, including 

Hellam conglomerate member (150 feet) at base.

Age

Ordovician.

Upper Cambrian.

Middle Cambrian.

Lower Cambrian.

0 Knopf, E. B., and Jonas, A. I., Stratigraphy of the crystalline schists of Pennsylvania and Maryland: 

Am. Jour. Sci., 5th ser., vol. 5, pp. 40-62,1923.

6 Hawkins, A. C., Alternative interpretations of some crystalline schists in southeastern Pennsylvania: 

Am. Jour. Sci., 5th ser., vol. 7, p. 357,1924.

"Idem, p. 363.

The series around Avondale is a continuous stratigraphic sequence 

with no evidence of unconformity between the individual formations, although the younger deposits overlap the older in some places 

owing to the irregularity in deposition of the quartzite and the limestone. Thus in some places the marble overlies the Baltimore gneiss 

directly, and in others the Wissahickon formation overlies the Baltimore gneiss. The highly garnetiferous schist beds of the Wissahickon that overlie the Baltimore gneiss on the north flank of the 

Avondale-Kennett anticline are apparently the same beds that overlie the marble at all places where the contact between the marble 

and the Wissahickon formation is exposed. This suggests that the 

relation between the Wissahickon and the Baltimore gneiss where 

the marble and quartzite ate absent is an overlap rather than a 

fault. The same irregularity in deposition of Cockeysville marble 

and Setters formation is seen in Maryland on the flanks of the 

Phoenix anticline in Baltimore County, where the Wissahickon 

overlies in some places the Cockeysville marble, in some the Setters 

formation, and in some the Baltimore gneiss.

Around the Mine Ridge upland, on the other hand, there is a 

marked unconformity,18 as a result of which the Conestoga limestone, probably of Chazy age, overlies Lower Cambrian dolomites 

and quartzites. North and west of Coatesville the Conestoga limestone overlies the Lower Cambrian Harpers schist, but east of

18 Stose, G. W., and Jonas, A. I., Ordovician overlap in the Piedmont province of Pennsylvania and Maryland: Geol. Soc. America Bull., vol. 34, pp. 507-524, 1923.
PEE-OAMBBIAN BOOKS 23

Coatesville the Conestoga limestone overlies the Upper Cambrian 

Conococheague limestone.19

If the marble at Avondale is the equivalent of the fossiliferous 

Vintage dolomite, which normally overlies the Lower Cambrian 

arenaceous and argillaceous rocks, the Wissahickon formation might 

be considered the stratigraphic equivalent of the Cocalico shale, 

which in the area north of Lancaster Valley overlies the Beekmantown limestone.20 But the Vintage dolomite everywhere rests upon 

the Lower Cambrian arenaceous and argillaceous rocks and nowhere 

lies directly upon the Baltimore gneiss. Moreover, the aggregate 

thickness of the Wissahickon formation and the Peters Creek schist 

as exposed along Susquehanna Kiver must be 8,000 to 10,000 feet, 

whereas the total thickness of all the Ordovician argillaceous sediments exposed south of Blue Mountain is probably of an order of 

magnitude of only 3,000 feet.

Nor could the marble at Avondale be explained as Conestoga limestone overlying Harpers schist, because although the absence of 

about 5,000 feet of calcareous sediments under the Conestoga and. 

above the Harpers might be explained, as it is north and west of 

Coatesville, by erosion before the Conestoga was deposited, it is not 

so easy to explain the absence both of the Conestoga itself and of 

the Harpers schist in those places where the W.issahickon directly 

overjies the Baltimore. Nor is it easy to explain why the Cocalico 

shale, which nowhere overlies the Conestoga limestone north of 

Chester Valley, should overlie it around Avondale. Therefore " the 

sedimentary succession and thickness of the various members ,in the 

Avondale series " are not, as asserted by Hawkins, " closely similar 

to those of the sedimentary series of accepted Cambro-Ordovician 

age in the adjacent Chester Valley."

In the second statement concerning the metamorphic effects of 

a large buried intrus,ive mass, which has altered the series at Avondale but which has barely affected the series in Chester ValJ.ey, 

Hawkins ignores the fact that the Paleozoic rocks in the vicinity 

of the Mine Ridge upland are themselves highly metamorphosed and 

in that respect are strictly comparable to the rocks of the Glenarm 

series. The degree of metamorphism attained by the micaceous marbles of the Quarry ville and Chester Valleys is analogous to that of the 

Cockeysville marble near Avondale. Therefore the dissimilarity between the two series can not be explained by a weakening of metamorphic action in the rocks of Chester Valley. The Paleozoic schjsts, 

quartzites, and marbles are cut by granitic intrusions, indicated by 

extensive pegmatitization, thus showing that there was igneous

19 Stose,, G. W., and Jonas, A. I., op. cit., p. 518.

20 Idem,, pi 521.
24 McCALLS FERBY-QUARBYVILLE DISTRICT, PA.

activity in the Paleozo.ic era, but the interlayered amphibolites that 

are characteristic of the pre-Cambrian core of the Mine Ridge upland 

occur throughout both the Glenarm series and the underlying Baltimore gneiss near Avondale and are absent in the Paleo/oic sediments 

that flank the Mine Ridge upland. It seems evident, .therefore, that 

the post-Glenarm basic intrusions must have been intruded prior to 

the deposition of the sediments flanking the Mine Ridge upland, 

because the amphibolitic layers occur with striking regularity and 

persistence interlaminated in the pre-Cambrian core of the Mine 

Ridge upland within a few feet of the base of the Cambrian. There 

is strong evidence for the pre-Cambrian age of the recrystall.ized 

sedimentary series near Avondale, which is stratigraphically continuous with the Glenarm series that underlies the basal Cambrian 

sediments in the western Piedmont belt of Maryland,21 and ,it is 

reasonable to correlate the pre-Glenarm core of the Avondale-Kennett 

anticline with the Baltimore gneiss of Maryland.

The stratigraphic equivalence of the core of the Mine Ridge uplift 

and the Baltimore gneiss in the Avondale-Kennett anticline is difficult to prove because of the absence of the Glenarm series underlying 

the Paleozoic section on the flanks of the Mine Ridge upland. However, the pre-Cambrian sedimentary formations in the Mine Ridge 

anticline are strikingly similar in lithology to the Baltimore gneiss 

in the Avondale-Kennett anticline. About a mile north of Chadds 

Ford there is interwelded within the Baltimore gneiss a lentil of 

graphitic marble that is identical in structure and lithology with the 

lenticular areas of Franklin limestone which are in many places interbedded with the graphitic mica schists in the northeastward extension of the Mine Ridge Upland known as the Honeybrook upland and 

in the graphitic schists of the Reading and Durham Hills and New 

Jersey Highlands. Therefore, it seems reasonable to assume that 

the metamorphosed sediments in the Mine Ridge upland are the 

equivalent of the Baltimore gneiss in the Avondale-Kennett anticline and are thus of pre-Glenarm age.

GLENARM SERIES IN PENNSYLVANIA

The oligoclase-mica schist facies of the Wissahickon formation, 

which overlies the Setters formation in Maryland, dips northwest on 

the northern flank of the Phoenix anticline north of Baltimore and 

passes under the Peters Creek schist. The Peters Creek schist underlies in apparent conformity Cardiff conglomerate and Peach Bottom 

slate in a tightly compressed syncline known as the Peach Bottom 

syncline. The Cardiff conglomerate and Peach Bottom slate crop

21 Jonas, A. I., Pre-Cambrian rocks of the western Piedmont of Maryland: Geol. Soc. 

America Bull., vol. 35, pp. 355-364, 1924.
PRE-CAMBRIAN ROCKS 25

out in an area in the center of the s'yncline 18 miles long and half a 

mile to a mile in width, which extends from northern Maryland into 

southern Pennsylvania. The cleavage of the slate in the trough of 

the syncline is nearly vertical and is so strongly developed that the 

bedding is obscure. But it is possible to show that the bedding is 

parallel to the cleavage on the southeast limb of the fold, where the 

slate, for a distance of 100 feet, is clearly seen to be interbedded with 

the conglomerate. Farther northwest along Susquehanna River the 

Peters Creek schist forms the northwest limb of this compressed 

syncline and conformably overlies a wide anticline of albite-chlorite 

schist which appears from its position to be the stratigraphic equivalent of the "Wissahickon oligoclase-mica schist that adjoins the Peters 

Creek schist on the southeast in Maryland.

COOKEYSVILLE MARBLE

About 1 mile south of Chanceford, York County, there is a small 

valley underlain by marble, which crops out only in a few isolated 

boulders. It is a coarsely crystalline micaceous marble with a considerable admixture of epidote that imparts a greenish color to the 

rock. No structure is discernible in the marble outcrops, but the 

rock shows a lithologic resemblance to the Cockeysville marble, 

which underlies the Wissahickon albite-chlorite schist farther southwest, in Carroll and Frederick Counties, Md. The Chanceford occurrence lies along the northern fold of the Mine Eidge-Tucquan 

anticline and is probably an anticline of Cockeysville marble exposed 

by the erosion of overlying Wissahickon albite-chlorite schist.

WISSAHIOKON1 FORMATION

Oligoclase-mica schist fades. The southern facies of the Wissahickon formation, which is characterized by strong intensity of metamorphism, occurs only in a small area in the extreme southeast 

corner of the McCalls Ferry-Quarryville district. It is well developed to the east and northeast of the district, along the eastern 

boundary of the Piedmont province northeastward to Trenton, N. J., 

and was named by Bascom from its fine exposure along the banks of 

Wissahickon Creek in Philadelphia. In the small area of oligoclasemica schist in the McCalls Ferry-Quarryville district outcrops of 

fresh rock are almost lacking. A few isolated exposures show the 

typical fine-grained " pepper and salt" biotite gneiss and the white 

quartzite of the strongly metamorphosed facies. The mica schist 

beds, dominantly composed of biotite and muscovite, are almost entirely lacking in fresh outcrop, but their presence is indicated by the 

characteristic soil derived from the mica schist, a deep, unctuous 

clay, sparkling brilliantly with numerous minute flakes of mica.
26 McCALLS FERRY-QUABRYVILLE DISTRICT, PA.

The gneissic layers are intensely metamorphosed, consisting chiefly 

of quartz, biotite, and plagioclase, with some orthoclase. The plagioclase is oligoclase (Ab70An30 ). Coarse-grained layers in which 

quartz predominates alternate with fine-grained layers made up of 

biotite, plagioclase, and small interlocking grains of interstitial 

quartz. Strain shadows in the quartz indicate a certain amount of 

deformation subsequent to the recrystallization, but the rock in this 

area does not show the strong cataclastic action that has affected 

the Wissahickon formation south of Chadds Ford, 20 miles farther 

east. Characteristic accessories are garnet, apatite, zircon, and magnetite. Many of the garnets are poikilitic porphyroblasts containing 

quartz inclusions; where the garnets have been broken the fragments 

are healed by secondary quartz.

The quartzitic layers crop out about half a mile south of Tweedale, 

on the road that follows the left bank of Tweed Creek, where the 

rock is composed chiefly of recrystallized quartz, a little fine-grained 

biotite, and a small amount of feldspar.

The mica schist beds of the Wissahickon are finely plicated and 

coarse to medium grained. Biotite, muscovite, and quartz are dominant, associated with a variable content of oligoclase. The mica 

occurs in large blades that spangle the cleavage surfaces and wrap 

around lenticular areas of quartz or large pink garnets, some of which 

attain half an inch in diameter. Under the° microscope the blades 

of mica are seen to be much twisted and frayed. The garnet is 

usually very abundant, standing out in knobs upon the cleavage surface of the weathered rock. In Maryland a notable feature of the 

mica schist is the abundant development of staurolite and kyanite. 

The rapid alternation of beds in the deposits that make up the 

Wissahickon formation makes it impossible to establish any bed as 

a key horizon. Although in numerous localities the garnetiferous 

schist overlies the Cockeysville marble, the lithologic variation is 

so great that the absence of garnetiferous schist over the marble can 

not be considered an evidence of faulting. In the gneissic and schistose members of the Wissahickon there is a well-marked foliation that 

is parallel to the bedding, accompanied by a secondary strain-slip 

cleavage that is parallel to the drag folds.

The oligoclase-mica schist shows, by its complete recrystallization 

and by its content of such minerals as garnet, staurolite, kyanite, and 

sillimanite, that the present condition has been developed by metamorphic agencies characteristic of a deep-seated zone. That the 

original material was a sediment may be inferred from the heterogeneous character of the formation and the presence of abundant 

rounded grains of zircon, suggestive of waterworn grains; from the 

occurrence of mineral high in alumina, such as staurolite, kyanite, 

and sillimanite; and from the lack of correspondence between the
1 PRE-CAMBRIAN ROCKS 27

s, chemical composition of the rock and any known igneous type. The 

J. development of tourmaline along a certain well-defined zone indicates

 that the rock has been subjected to pneumatolytic action subsequent 

to its deep-seated metamorphism, and this conclusion is supported by 

the large amount of secondary quartz that penetrates the Wissahickon

.* formation in lenses and stringers. The undulatory extinction in the 

quartz and feldspar is the effect of a stress to which the rock has

V been subjected subsequent to the formation of its metamorphic

% minerals. '

l' Diaphthoritic schists. In the vicinity of Octoraro Creek below the

\ confluence of the East and West Branches there are several out-

 crops of a peculiar schist that indicates diaphthoritic metamorphism.--

On Black Run, V/2 miles east of its confluence with Octoraro Creek,

J there is a fine example of such a diaphthorite. The rock is a garnet-

? iferous schist, in many respects identical Avith the typical gar- 

»: netiferous mica schist of the Wissahickon. However, in this rock

 v the biotite of the typical deep-seated schist has been partly replaced

> by chlorite. The chlorite fibers are intimately intertwined and show

\ in places remnants of the original brown color and polarization of

o- biotite. Associated with the chlorite is a large amount of iron oxide

 liberated from the original biotite during the formation of chlorite. 

.". Another instance of the breaking down of a mineral formed under 

I deep-seated conditions is seen in the garnet porphyroblasts, which are 

;i surrounded by rims of chlorite, showing that the chemical constituents of the garnet have adapted themselves to new conditions of 

<f metamorphism by the formation of chlorite, which is stable under 

the new environments. Such diaphthorites have hitherto been de- 

; scribed only in Europe, where they occur in the lower part of the 

~ overriding block in areas of overthrusting. They are called by 

5 Suess 23 "deep-seated diaphthorites," because the retrogression in 

$ metamorphism induced by friction along the fault plane has taken 

: place under deep-seated conditions. In this connection it is sig- 

', nificant that three specimens of a cataclastic rock collected near

 Glenroy suggest a mylonitized quartz conglomerate by the presence 

of large lenticular areas of granulated quartz in a matrix of 

chlorite, muscovite, quartz, a little biotite, and abundant iron oxide. 

The southwestward extension of the diaphthoritic zone is inter-

: rupted by igneous masses that are of wide extent in the vicinity

23 Diaphthorite is a term that was introduced by Becke to describe a schist that has 

been recrystallized under deep-seated conditions and later carried into an upper tectonic 

zone, where it has undergone a reversal of metamorphism, indicated by certain new minerals that are characteristic of the upper zone of metamorphism and that have been 

formed at the expense of the heavier minerals of the deep-seated zone. Becke, F., fiber 

Diaphthorite: Min. pet: Mitt., Baiidi 28, pp. 369^375, 1909.

28 Suess, F. E., Moravische Fenater: K. Akad. Wiss. Wien, Math.-naturwiss. Klaese, 

Anzeiger, Jahrgang 47, pp. 428-432, 1910.
28 McCALLS FERRY-QUARBYVILLE DISTRICT, PA.

of the Maryland-Pennsylvania State line. The diaphthorite can 

not be traced in these igneous rocks, but still farther south, in Cecil 

County, Md., along Basin Run 1 mile south of Liberty Grove, 

Grimsley 24 and Bascom 25 have found what is apparently the reappearance of the Pennsylvania diaphthorites. The rock here is a 

garnetiferous-staurolite mica schist in which the garnets have partly 

altered to chlorite and the staurolite has almost entirely altered to 

muscovite, One mile southeast of Liberty Grove E. B. Knopf has 

found a brecciated rock in which numerous fragments of biotite 

schist and biotitic quartzite 1 to 2 inches in length stand out upon 

the weathered surface in long prismoids that are arranged roughly 

parallel to the plications of the schistose matrix. The rock is thickly 

studded with garnets and is penetrated by quartz veinlets.

East of the Quarryville district, in the West Chester quadrangle, 

Knopf has found a narrow belt of mylonitized schists, which are 

dense, schistose dark-colored rocks interleaved with paper-thin layers 

and elongated lenses of feldspathic material. The rock occurs near 

the contact between Baltimore gneiss and Wissahickon formation 

on the southern flank of the Avondale-Kennett uplift. The Wissahickon formation for at least a mile southward from the belt of 

mylonitization is a highly cataclastic, sillimanite-bearing oligoclasebiotite schist filled with lenses and stringers of pink granite material. 

The features described suggest strong mechanical deformation by 

which a biotite schist injected with granitic material was first mashed 

and finally rolled out so that the original constituents have been 

pulverized. Although the inference is strong that the presence of 

these diaphthoritic schists indicates a zone of overthrusting, it has 

so far been impossible to establish the overthrust by any other 

evidence.

Albite-chlorite schist fades. The albite-chlorite schist facies of the 

Wissahickon formation is widespread on the northwest side of the 

Peach Bottom syncline, where it underlies almost half of the McCalls 

Ferry quadrangle. It is particularly well exposed along Susquehanna River, where the steep river gorge is cut into rugged, heavily 

wooded cliffs of albite schist for almost 17 miles between Turkey 

Hill and Fites Eddy. As in the southern oligoclase-mica schist 

facies of the Wissahickon, individual beds vary widely in lithologic character. The formation as a whole comprises a porphyroblastic feldspathic type and a micaceous schistose type, with a subordinate amount of quartzose gneiss in both types.

The first type, which is in some places massive and almost igneous 

in appearance, is characterized by poikilitic metacrysts of albite,

24 Grimsley, G. P., The granites of Cecil County, In northeastern Maryland: Cincinnati 

Soc. Nat. Hist. Jour., vol. 17, pp. 66-67, 1894.

25 Bascom, F., The crystalline rocks of Cecil County: Maryland Geol. Survey, Cecil 

County, p. 115, 1902.
PRE-OAMBRIAN BOOKS 29

embedded in a schistose matrix that is made up of chlorite and muscovite in varying proportion. The lower beds exposed in the core of 

the anticline between Safe Harbor and McCalls Ferry contain much 

biotite in deep-brown, strongly pleochroic individuals, some of which 

are intergrown with chlorite. The chlorite is a grass-green, strongly 

pleochroic clinochlore, and clinochlore, biotite, and muscovite alike 

contain numerous fine pleochroic haloes surrounding inclusions of 

zircon. The albite metacrysts are dull white to pinkish, some are as 

large as 10 millimeters in diameter, and they are most conspicuous on 

rough surfaces normal to the plane of schistosity. They occur in 

subhedral or anhedral individuals with ragged boundaries and commonly show Carlsbad twins, rarely polysynthetic twinning. They 

inclose poikilitically numerous rather large inclusions of garnet, 

epidote, magnetite, biotite, muscovite, calcite, and rarely tourmaline. 

The inclusions generally show a certain alinement, which is parallel 

to the schistosity of the groundmass in some specimens and inclined 

to it in others. (See pi. 2, A.) Out of eight thin sections studied 

only one showed a random arrangement of the inclusions. The inclined relation of schistosity of the groundmass to the alinement of 

the inclusions in the metacrysts has been considered by some writers 

to indicate that the metacrysts have been formed after the crystallization of the groundmass has ceased and that they have rotated during 

the later stages of their formation. In support of this view Bailey 26 

has figured albites from the albite schists of the Scottish Highlands, 

in which the ends of the linear inclusions are curved in opposite directions, thus presumably indicating the direction of rotation. Such a 

curvature in the alinement of the inclusions has been interpreted by 

Clough 27 to show that the albite has been formed later than the main 

schistosity of the groundmass, which is fossilized, as it were, in the 

winding stream of inclusions that represents the original puckered 

foliation planes of the rock. The inclusions in the albite metacrysts 

of the Wissahickon schist show no evidence of crumpling or distortion in the linearity. It is hard to escape the conclusion that .the 

alinement is caused by a rotation of the metacrysts by which the 

albites, in a final stage of growth, adjusted themselves in a certain 

way to their physical environment. However, it is difficult to understand why this adjustment to environment did not keep pace with the 

development of the foliation, manifesting itself in an elongation of 

the metacryst parallel to the foliation plane rather than by a bodily 

movement of the individual. The fact that some of the albite metacrysts are so elongated in response to the stress under which they

28 Bailey, E. B., The metamorphism of the southwest Highlands: Geol. Mag., vol. 60, p. 

329, 1923.

^Gunn, W., Clough, C. T., and Hill, J. B., The geology of Cowal (Scotland Geol. Survey 

Mem.), p. 40, 1897.
30 McCALLS FEEEY-QUAREYVILLE DISTRICT, PA.

were formed suggests that the rotation of the metacrysts may be 

connected with a second period of metamorphism subsequent to that 

which produced foliation parallel to the bedding. The existence of 

such a later deformation is clearly indicated by the presence of a fine 

foliation that crosses the first foliation at a steep angle. This secondary foliation is best developed in the soft phyllitic beds, where it 

corresponds to the direction of the minute drag folds. It is excellently exposed on the north flank of Turkey Hill in the cuts of the 

Pennsylvania Kailroad along Susquehanna River. The rock is filled 

with twisted and broken veinlets of quartz, which are in some places 

cut across by quartz veins of a later age than the deformed veins.

The schistose type of the albite-chlorite schist is characterized by 

an abundance of chlorite, which is deep-green clinochlore, and a variable quantity of sericite. Albite is almost universally present, but is 

subordinate in amount to the micaceous minerals, so that the rock is 

highly foliated.

Quartz is fairly abundant in both types. It fills the interstices 

between the albite, chlorite, and mica, and in some of the schists it 

occurs in layers, alternating with micaceous bands. Characteristic 

accessories are magnetite, zircon, carbonates (calcite or dolomite), 

and tourmaline. Other common constituents are pyrite, epidote, garnet, apatite, titanite, and hematite. The carbonate is in many specimens abundant and appears to have crystallized at the same time as 

the other constituents. Magnetite is of universal occurrence and in 

many places forms large porphyroblasts that show under the microscope a distinctly ragged outline.

The quartzose members of this formation occur in subordinate 

amount. Quartzite beds that probably belong in the Wissahickon

albite-chlorite schist are well exposed in a railroad cut along Brandywine Creek in Coatesville, where they occur as layers from 4 to 8 

feet in thickness, containing numerous closely cemented grains of 

bluish quartz. Quartzite layers in the albite-chlorite schist occur also 

at Martic Forge and at several places in the section exposed along 

Susquehanna Kiver.

Both facies of the Wissahickon formation weather readily into a 

micaceous clay soil, which has the consistency of putty when wet and 

holds water for a long time. This clay forms deep ruts, thereby making roads that are almost impassable in rainy weather. The unctuous 

clay soil derived from the oligoclase-mica schist sparkles brilliantly, 

owing to the presence of numerous mica flakes. The soil derived 

from the albite-chlorite schist shows less mica and is full of silvery 

schist fragments.

Composition. The chemical composition of the two facies is illustrated by the following analyses:
PRE-CAMBRIAN ROCKS 31 

Chemical composition of schists and gneisses from the Wissahickon formation

SiOa. ......

AlaOs.. .

FeaOs-.. 

FeO. ......

MgO.......

CaO. ......

NaaO......

KaO. ......

1

70.26

12.87

2.61

2.70

1.49

2.19

2.15

2

66.13

15.11

2.52

3.19

2.42

1.87

2.86

3

79.60

9.48

1.77

1.49

.76

.72

1 QQ

1.54

4

77.00

10.17

2.41

2.35

1.26

2.02

1.14

1.94

5

54.99

18.74

2.90

5.49

2.81

.85

5.36

HjO......

HaO+- 

TiOa ....

C02 ........

PaOj.......

MnO-.-..

1

} 1.10

1.85

.11

.05

99.93

2

f 1.55

X -24

.82

.22

.20

99.84

3

,

| 1.66

.71

.19

.67

100. 42

4

f 0.01

\ 1.18

.96

.08

.10

.09

100.71

5

} 5.10

1.66

.62

.04

100. 42

1. Wissahickon formation (oligoclase-mica schist, gneissic variety, with fine schist beds and quartz 

veinlet), 1 mile northwest of Monkton, Md. Analysis made for Maryland Geological Survey by Penniman & Browne. Represents an arkosic sediment high in lime. Rock is composed of quartz, oligoclase, 

and biotite, with small amount of orthoclase and muscovite.

2. Wissahickon gneiss, composite sample from four different localities. Bascom, F., U. S. Qeol. Survey 

Qeol. Atlas, Philadelphia folio (No. 162), p. 4,1909. Analyst, W. F. Hillebrand. Represents an arkosic 

sediment.

3. Wissahickon gneiss (quartzose variety), Elm (Narberth) station, Pennsylvania Railroad, Montgomery 

County, Pa. Idem, p. 135. Analyst, F. A. Genth, jr. Represents a sandstone high in soda. Rock is

described as containing quartz, feldspar, and a little mica.

4. Wissahickon formation (fine-grained biotite gneiss). Rock is composed of quartz, oligoclase, and 

biotite.

5. Wissahickon formation (albite-chlorite schist, schistose variety), 1 mile south of Shenks Ferry station, 

Columbia & Port Deposit branch, Pennsylvania Railroad, Lancaster County, Pa. Analyst, R. K. Bailey, 

U. S. Geological Survey, 1920. Represents a potassic argillite. Rock is composed of muscovite, clinochlore, quartz, and albite.

The mineral composition of these rocks has been calculated from 

the chemical analyses in such a way as to correspond as nearly as 

possible with the mineral constituents of the rocks determined under 

the microscope.

Mineral composition of schists and gneisses from the Wissahickon formation

Quartz... ..................

Orthoclase _________

Chlorite.. ..................

Titanite. ........... _ .. ...

Hematite. ___ . ...........

Apatite... ..................

1

40

7.5

13

2.5

2.5

97.00

2

32

1.5

25

4

.5

3

97.00

3

62

10

4

3

1

98.00

4

54

12

15

2

3

101

5

21

Abioo 14

45

13

4

3

1

101

The variable content of Na2O and K20 in these analyses is such 

as would be expected from the heterogeneous nature of the original 

sediment. The 5.36 per cent K2O of the albite-chlorite schist is high 

for a normal shale. In the computation of mineral composition this 

potash is recalculated as 45 per cent muscovite, which agrees very 

well with the content of muscovite estimated in the microscopic section. The computation of 45 per cent muscovite that is demanded 

by the high potash content and by the absence of biotite and orthoclase requires more alumina than is indicated by the analysis, and 

it seems probable that the alumina content of the analysis is a little 

low. Normal shale is calculated to contain an average of 3.60 per
32 McCALLS FERBY-QUAERYVILLE DISTRICT, PA.

cent K2 0, as determined by a composite analysis of 51 samples of 

Paleozoic shale weighted in reference to the mass of the formation 

represented.28 The high potash content of the albite-chlorite schist 

is not, however, incompatible with a sedimentary origin, because 

some sediments and slates of known sedimentary origin contain an 

unusually large amount of potash. The slate in the Cartersville 

formation of Georgia, for example, shows as much as 8.26 per cent 

K2O in. an average of 59 analyses.29 Therefore although the potash 

content of the muscovitic variety of the Wissahickon albite-chlorite 

schist is high, it does not necessarily denote an igneous origin for 

the schist but rather indicates that the original sediment was a feldspathic mud rich in potash feldspar and white mica and similar 

in origin to the highly potassic sediments of the Setters formation 

in Maryland and of the Harpers schist in Pennsylvania.

Metamorphism. The nature of the mineral constituents in the two 

facies of the Wissahickon formation shows that the metamorphism 

of the oligoclase-mica schist took place under conditions of strong 

intensity and that the albite-chlorite schist was recrystallized in an 

upper zone of metamorphism. The lithologic characteristics of the 

two facies are essentially similar, in spite of the heterogeneity that 

is to be expected in widespread sedimentary deposits. In both facies 

mica schist alternates with more feldspathic gneisses, and quartzites 

or quartzose gneisses occur in subordinate amounts. The difference 

between the two facies appears, therefore, to be conditioned by a 

difference in intensity of metamorphism rather than by difference 

in stratigraphic position or in chemical composition. The constituents that have formed oligoclase under conditions of intense metamorphism have crystallized under more moderate metamorphic intensity as albite and calcite.

The gradation from rocks that were formed under moderate metar 

morphic intensity to rocks that have been formed by an intense 

metamorphism is well shown about 12 miles east of the McCalls 

Ferry-Quarryville district, where the Wilmington division of the 

Philadelphia & Beading Railroad follows the West Branch of 

Brandywine Creek southward from Coatesville to Embreeville. 

Within the borough limits of Coatesville, on the west bank of the 

West Branch of Brandywine Creek and along the north face of 

the South Valley Hills, there is a zone of much shattered rock that 

shows an alternation of quartzose and slaty beds. This rock overlies the Conestoga limestone in Chester Valley. From the lithology 

it is hard to determine whether the rock belongs in the upper part 

of the Wissahickon albite-chlorite schist or in the Peters Creek

28 Clarke, F. W., Analyses of rocks and minerals, 1880 to 1914: U. S. Geol. Survey Bull. 

591, p. 23, 1915.

29 Knopf, E. B., and Jonas, A. I., Geology of the crystalline rocks of Baltimore County: 

Maryland Geol. Survey, Baltimore County (in press).
PKE-CAMBRIAN BOOKS 33

formation. Metamorphism, which has been of low intensity, has not 

progressed far enough to destroy the original clastic grains of blue 

quartz in the quartzose beds, and the argillaceous layers have scarcely 

passed the stage of slates. About half a mile south of Coatesville 

this strongly shattered zone is succeeded by a belt about 1,200 feet 

wide of steeply dipping slates and slaty schists, impregnated with 

quartz veinlets. A mile farther south the rock is a chloritic quartz 

schist, and about 5 miles southeast of Coatesville it has become a 

quartzose muscovite schist or an albite-chlorite schist. Metamorphism has advanced but is still of moderately low intensity. Still 

farther south the rock becomes coarser grained, and.biotite begins to 

appear in association with the muscovite. From this point southward the rock gradually assumes the lithologic characteristics of the 

Wissahickon oligoclase-biotite schist, and the metamorphic intensity 

continues to increase until, half a mile north of Embreeville and 7 

miles south of Coatesville, the rock shows every evidence of metamorphism under conditions of strong intensity. It is a fairly coarsegrained, highly garnetiferous muscovite-biotite schist and contains 

short crumpled augen of quartz that show no connection with channels through which siliceous solutions might penetrate the rock. 

Such detached lenses of quartz have been explained by Holmquist as 

a result of segregation from the surrounding rock during the ultrametamorphism of schistose and folded gneisses. 30 In the Wissahickon mica schists, however, there is no indication of ultrametamorphism, which implies the vanishing of the schistose structure. As the whole Wissahickon formation is traversed by innumerable veinlets of tourmaliniferous quartz and feldspathic quartz, it is 

evident that the rock has been permeated by pneumatolytic vapors 

at some time during its metamorphism, and the quartz augen described above have probably been introduced in connection with 

pneumatolysis. It is clear from the section between Coatesville and 

Embreeville that there is a progressive increase of metamorphism 

from the rock overlying the Conestoga limestone in Coatesville, 

which is probably upper Wissahickon albite-chlorite schist, through 

an apparently synclinal area where the rock has the lithologic 

characteristics of the Peters Creek formation to an underlying mica 

schist, which becomes, north of Embreeville, the intensely metamorphosed facies of the Wissahickon.

Age. In the earlier work in this region the two mineralogic facies 

of the Wissahickon were considered parts of the same formation, but 

at that time the albite-chlorite schist that underlies the South Valley 

Hills was thought to be Ordovician because it appeared to overlie

30 Holmquist, P. J., Swedish Arcbean structures and their meaning: Geol. Inst Upsala 

Bull. 15, pp. 141, 142, 1916.
34 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

conformably Beekmantown limestone in Chester Valley, the limestones of Chester Valley being at that time all mapped under the 

name Shenandoah.81 Later the southern facies of the Wissahickon, 

the oligoclase-mica schist, was called Wissahickon mica gneiss and 

shown to be pre-Cambrian.32 The rock underlying the South Valley 

Hills was still assigned to the Ordovician and called " Octoraro schist," 

and the name " Octoraro " was used for all the rock in the area lying 

between the formation then mapped as Wissahickon gneiss and the 

limestone of Chester Valley. The " Octoraro schist" thus comprised 

both what is now mapped as the albite-chlorite schist facies of the 

Wissahickon formation and part of the overlying Peters Creek formation. Part of what is now mapped as Peters Creek was then 

called Wissahickon gneiss.

The interpretation of the " Octoraro schist" as Ordovician led to 

grave difficulties when it was discovered that the Wissahickon gneiss 

is pre-Cambrian, because the rock mapped as " Octoraro schist" 

grades imperceptibly into the Wissahickon gneiss. It was necessary 

to assume an overthrust fault along the contact between the Wissahickon gneiss and the " Octoraro schist" in order to explain the 

anomalous position of Wissahickon gneiss on top of the so-called 

Ordovician strata south of Chester Valley. The presence of the fault 

was apparently demanded by the stratigraphy, but no structural 

evidence of such a fault could be found.

Later work by the writers both in Maryland and in Pennsylvania, 

along the Susquehanna River section, shows that the pre-Cambrian 

Wissahickon gneiss north of the Phoenix anticline in Maryland, 

where the formation is an oligoclase-biotite schist, dips under a synclinal fold of Peters Creek schist and reappears from under the 

northern limb of the syncline in a different mineralogic facies, the 

albite-chlorite schist.33 It was thus shown that the " Octoraro schist" 

is not a stratigraphic unit and that the formations which made up 

the old " Octoraro schist" are part of a conformable sequence of preCambrian rocks known as the Glenarm series. Confirmation of the 

pre-Cambrian age of this series is the discovery by Jonas that basal 

Cambrian rocks rest unconformably upon the volcanic rocks and the 

Cockeysville marble of the Glenarm series in Frederick County, 

Md.,34 and upon Wissahickon albite-chlorite schist in Carroll County, 

Md., and southern York County, Pa.

31 Bascom, F., Piedmont district of Pennsylvania: Geol. Soc. America Bull., vol. 16, 

p. 305, 1905.

» Bascom, F., U. S. Geol. Survey Geol. Atlas, Philadelphia folio (No. 162), p. 3, 1909.

83 Knopf, E. B., and Jonas, A. I., Stratigraphy of the crystalline schists of Pennsylvania 

and Maryland: Am. Jour. Sci., 5th eer., vol. 5, pp. 40-62, 1923.

84 Jonas, A. I., Pre-Cambrian rocks of the western Piedmont of Maryland: Geol. Soc. 

America Bull., vol. 35, p. 363, 1924.
PEE-CAMBRIAN ROCKS 35

It thus became evident that the contact between the pre-Cambrian 

Wissahickon albite-chlorite schist (formerly mapped as "Octoraro 

schist") and the so-called "Cambro-Ordovician" Shenandoah limej stone of Chester Valley could not be conformable. The establishment of the lower Paleozoic stratigraphic column in the Lancaster,

\ Quarryville, and Chester Valleys 35 showed that the Shenandoah 

limestone compr,ises several different formations ranging in age from 

Lower Cambrian dolomite to Ordovician limestone of probable

I Chazy age. Moreover, it was discovered by Stose and Jonas that 

the Ordovician Conestoga limestone unconformably overlies the 

Beekmantown.86 Therefore the albite-chlorite schist underlying the 

South Valley Hills must be faulted over the Paleozoic rocks in Chester Valley. The existence of the overthrust is difficult to demonstrate 

on account of the fact that the fault plane itself has been seen in

' only a few localities, and there the structural relations are obscure. 

No mylonite has been observed in the few localities where the overthrust plane is visible. The absence of brecciated and mylonitized 

rock along such a plane of movement is not as surprising as it m;ight 

at first seem, because a strong metamorphism has succeeded or accompanied the overthrusting. Mylonites if ever present may have been 

obscured by this later metamorphism. It is more probable, however, 

as suggested by Bailey 37 in his paper on the Scottish Highlands, 

where there is a similar absence of mylonites along a plane of overthrust, that the overthrusting took place in a deep zone where the 

temperature was sufficiently high to allow recrystallization to take 

place at the same time as the overthrusting, hence mylonites would 

not be formed. But the absence of structural evidence of faulting 

along the overthrust plane is offset by the fact that the writers have 

discovered, in the hills west of Quarryville and north of Shenks 

Ferry, Wissahickon albite-chlorite schist and Peters Creek schist 

overlying previously folded rocks, the Vintage dolomite, of Lower 

Cambrian age, and the Conestoga limestone, of Ordovician age. 

This discordant contact of Glenarm rocks against folded strata of 

different stratigraphic horizons is structural evidence of the overthrust relation of the pre-Cambrian rocks to the underlying Paleozoic 

formations.

PETERS CREEK SCHIST

The Peters Creek schist underlies a belt about 6 miles wide that 

covers most of the southeast corner of the McCalls Ferry-Quarry88 Stose, G. W., and Jonas, A. I., The lower Paleozoic section of southeastern Pennsylvania : Washington Acad. Sci. Jour., vol. 12, pp. 358-366, 1922.

88 Stose, G. W., and Jonas, A. I., Ordovician overlap in the Piedmont province of Pennsylvania and Maryland: Geol. Soc. America Bull., vol. 34, pp. 507-524, 1923.

87 Bailey, B. B., The metamorphism of the southwest Highlands: Geol. Mag., vol. 60, 

p. 317, 1923.

6186 29 4
36 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

ville district. It overlies the southeast flank of the wide double - 

crested anticline of Wissahickon albite-chlorite schist that crosses 

Susquehanna Biver at Tucquan, is eroded from the crest of the anticline, and reappears on the northwest flank in a narrow belt on the 

north side of Turkey Hill and Long Level. The southeastern belt of 

Peters Creek schist contains the narrow synclinal area that is underlain by Cardiff conglomerate and Peach Bottom slate.

The Peters Creek schist was named by the writers on account of 

its typical exposures along Peters Creek, which flows into Susquefaanna River at Peach Bottom, Lancaster County. The formation 

lias been traced, with unchanged characteristics, along the strike 

from Pennsylvania southwestward into Maryland across Baltimore 

and Carroll Counties. It comprises part of what was originally 

mapped as Wissahickon gneiss and part of the " Octoraro schist."

The formation consists of a series of chloritic and sericitic quartz 

schists interbedded with chlorite-sericite schist and grading toward 

the top into mildly metamorphosed quartzose and conglomeratic 

sediments. The sericitic and chlor.itic schists are most abundant in 

the lower beds that overlie the Wissahickon albite-chlorite schist. 

These lower beds of the Peters Creek schist are well exposed along 

Susquehanna River between Fites Eddy and Fishing Creek Station, 

where the rock is a light greenish-gray straight-bedded fine-grained 

quartz-chlorite schist. The banding is produced by an alternation of 

quartzose.layers with thin layers of chlorite and sericite, which give 

a silvery luster on planes parallel to the bedding. The lowermost 

beds in which chlorite-sericite schist predominates are difficult to 

separate from the schistose beds of the Wissahickon albite-chlorite 

schist, which immediately underlies the Peters Creek. In fact, there 

is so perfect a lithologic gradation between the upper beds of the 

Wissahickon and the lower beds of the Peters Creek that the boundary 

between the two formations can not be accurately traced in the wellcultivated upland country above the valley of the Susquehanna. It 

has been represented on the map by a dotted line. Under the microscope the sericitic and chloritic quartz schists of the Peters Creek formation are seen to be almost completely recrystallized. The quartz, 

which predominates, is fine grained and clear, with local inclusions 

of rutile in minute rosettes. A fine foliation is imparted by the sericite and chlorite (a deep-green clinochlore). Albite, which is usually 

untwinned, is fairly abundant, amounting to about 20 per cent, and is 

very difficult to distinguish from quartz. Measurements on sections 

perpendicular to an optic axis indicate a species about Abn5Anv It 

differs from the albite of the Wissahickon albite schist in that it 

is finer grained, not noticeably porphyroblastic, and not spongiform. 

Magnetite and titanite are abundant accessories, and apatite, pyrite,
PKE-CAMBRIAN ROCKS 37

tourmaline, and a carbonate (presumably dolomite) are of common 

occurrence.

In the upper members of the Peters Creek formation recrystallization is not complete, and there are numerous clastic grains of clear 

blue quartz scattered throughout the rock. In some places toward 

the top of the formation these quartz pebbles become so abundant 

that the rock might almost be called a sandstone. Under the microscope these quartz pebbles show up plainly, as deformed lenticular 

areas that are in some places clear, with slight strain shadowing, and 

in other places completely granulated. The argillaceous matrix surrounding the pebbles is completely recrystallized to chlorite or seri- 

 cite. Some of the clastic fragments are feldspar, both a sodic albite 

and a microcline. The scarcity of microcline in the lower beds of the 

Peters Creek is probably due to the completeness of the recrystallization, which has taken place under a low metamorphic intensity, 

thereby using the potash of the microcline to form sericite.

The chemical and mineral composition of the lower schistose beds 

of the Peters Creek formation is given below.

Analysis of Peters Greek schist, Benton, Lancaster County, Pa.

Chemical composition 

[Qlenn V. Brown, analyst]

Si02 __'_____________ 73. 00

ALOa_________________ 10. 55

Fea08 _______________ 2. 42

FeO ________________ 2.75

MgO________________ 1. 55

CaO________________ 1.30

Na,O _,_____________ 3. 32

K.O _________________ 2.10

H.O+ _______________ 2. 01

Ti02 _________________ . 90

CO,_________________ . 25

P2O8_ ____ ________ . 07

MnO ________________ . 05

Mineral composition

Quartz_______________ 42. 84 

Albite (Abo5An8 )_________ 29.44 

Orthoclase ____________ 8.34 

Chlorite______________ 6.72 

Muscovite __ ___ _ 5.57 

Magnetite_____________ 3.02 

Titanite______________ 2.16 

Dolomite and apatite______ .89

100.27

CARDIFF CONGLOMERATE

The Cardiff conglomerate directly overlies the Peters Creek schist. 

It occurs in a belt off variable thickness about 18 miles long, extending southwestward from Fairmount, in Lancaster County, Pa., to 

Pylesville, in Harford County, Md., arid surrounds an elliptical area 

of black roofing slates about 6 miles long and half a mile wide. The 

name Cardiff has been given to this conglomerate by the Maryland 

Geological Survey from the town of Cardiff, on the Maryland State 

line, which is the direct continuation of the town of Delta, in Penn-
38 McCALLS FERRY-QTJARRYVILLE DISTRICT, PA.

sylvania, well known as a slate locality. In the lower beds of the 

conglomerate there is an alternation of schistose and conglomeratic 

layers, marking the transition from the Peters Creek pebbly sandstones to the Cardiff sediments. The Cardiff conglomerate, like the 

Peters Creek sandstones, is characterized by the predominance otf 

quartz in the pebbles. Most of the pebbles are completely granulated 

and in many places are deformed and considerably elongated. Some 

of the pebbles, however, show a core of clear glassy quartz, commonly 

of a bluish tinge, and some of the smaller pebbles are clear all the 

way through. In one place a granulated quartz contained within 

it an albite crystal, which suggests that the detrital material was 

derived from an albitic igneous rock. The matrix of the conglomerate is schistose, composed of fine quartz and fibers of sericite and 

chlorite. Chloritoid, which occurs rather abundantly in several specimens, is in coarser fibers than the muscovite and is packed closely 

around the quartz pebbles. Magnetite is very abundant in some 

specimens, and titanite commonly occurs. In some places the magnetite is associated with an opaque whitish mineral, probably 

leucoxene.

The Cardiff conglomerate was for some time tentatively considered 

to be Ordovician and to rest upon so-called Ordovician phyllite and 

to underlie the Peach Bottom slate, then considered of doubfeful Ordovician age.38 The grounds for belief in its Ordovician age were made 

questionable when the phyllite was shown to be pre-Cambrian. The 

Cardiff conglomerate resembles the Lower Cambrian Hellam conglomerate member of the Chickies quartzite on the Mine Ridge upland, and on lithologic grounds it might be interpreted as basal Cambrian. However, the Cardiff conglomerate is overlain by more than

900 feet of black slates, and the Hellam conglomerate on the Mine 

Ridge upland and vicinity is overlain by more than 350 feet of white 

quartzite.

The Lower Cambrian series exposed in the area south of Lancaster 

Valley resembles the Cardiff conglomerate and Peach Bottom slate 

sequence more closely than it does the succession of Cambrian strata 

on the flanks of the Mine Ridge upland. Jonas has studied the area 

south of the Lancaster-York Valley in Pennsylvania and in its southwestward extension in Maryland and has found that the basal Cambrian conglomerate is overlain by black slate containing thin quartzite 

layers. This slate and quartzite series represents the quartzite member of the Chickies quartzite of the area that lies north of Chester 

Valley, where the siliceous beds are thicker than in the area south 

of Chester Valley.

88 Mathews, E. B., Correlation of Maryland and Pennsylvania Piedmont formations: 

Geol. Soc. America Bull., vol. 16, p. 338, 1905.
PEE-CAMBRIAN ROOKS 39

The only places known in Pennsylvania or Maryland where recognized basal Cambrian is preserved on the overthrust mass are in 

the Sugarloaf syncline of Frederick County, Md., and in the Wentz 

syncline of Carroll County, Md., and York County, Pa.89 There 

Lower Cambrian rocks rest unconf ormably upon Cockeysvilje marble 

and volcanic flows and Wissahickon albite-chlorite schist, of lower 

Glenarm age. The fact that the upper part of the Glenarm series 

had been removed in Maryland before the deposition of the basal 

Cambrian indicates deeper pre-Cambrian erosion ,in the western part 

of the Piedmont than in the southeastern area.

In Pennsylvania there is no difference in trend between the Cardiff 

conglomerate and Peach Bottom slate and the underlying Peters 

Creek formation, and there is an apparent gradation from fine pebbly 

sandstones in the upper beds of the Peters Creek formation through 

coarser beds into the Cardiff conglomerate. Therefore, if a depositional break occurred between the two formations the field evidence 

indicates that it was not an angular unconformity. In the absence 

of any definite proof that it is Cambrian, the Cardiff conglomerate 

has been .included in the pre-Cambrian Glenarm series.

PEACH BOTTOM SLATE

The Cardiff conglomerate grades upward through beds of alternating conglomerate and slate into roofing slates, which form an area 

about 6 miles long and half a mile wide. These slates, which are 

well known to the trade as the Peach Bottom " black roofing " slate, 

have a probable thickness of 1,000 feet and show a uniform steep 

south dip, which coincides with the cleavage. On its southeast 

boundary the slate is distinctly interbedded with the conglomerate, 

and here it is possible to show that bedding and cleavage are parallel. 

The section measured from north to south on the road north of 

Peach Bottom station is as follows:

Section north of Peach Bottom station

Feet

Black slate______________________________ 45 

Slate with interbedded conglomerate, 3 inches to 1 foot thick 15 

Slate______..__________________________ 20 

Conglomerate_____________________________ 60

The Peach Bottom slate is dark bluish gray, nonfading, and of a 

strong luster on cleavage surfaces. It has a high crushing strength, 

11,260 pounds to the square inch, and is exceedingly tough. The 

microscope shows a distinctly crystalline texture approaching that 

of the phyllitic stage of metamorphism. The essential constituents

88 Jonas, A. I., Pre-Cambrian rocks of the western Piedmont of Maryland: Geol. Soc 

America Bull., vol. 35, p. 361, 1924.
40 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

are muscovite, quartz, andalusite, and graphite with variable 

amounts of magnetite and pyrite. The following table shows the 

chemical analyses of four specimens of Peach Bottom slate. The 

sedimentary or.igin of the slate is shown by the strong dominance 

of alumina which exceeds the molecular proportion of the combined 

lime and alkalies by over 100 per cent.

Analyses of Peach Bottom slate

SiOa.- ... ......

AlaOs ... ... ....

Fea03 . 

FeO....... ._____.

MgO.............

CaO........ ......

NaaO.... .........

KaO.. ............

HaO... ...........

C..... ............

1

55.88

21.85

9.03

1.50

.16

.46 3 64

3! 39

1.79

2

58.37

21.99

t 10. 66

1.20

.30

} 1.93

4.03

.93

3

60.32

23.10

7.05

.87

.49

3.83

4.08

4

44.15

30.84

14.87

.27

.48

{

M

} 4.49

TiOa..............

C02 . .............

SOa ..... ... ....

S.. ...............

MnO... ..........

FeSj... ...........

CoO.... ..........

1

1.'27

.02

.59

.05

99.63

2

.39

.11

99.91

3

99.74

4

99.97

Specific gravity 2.89.

1. Peach Bottom "roofing" slate. J. Humphrey & Co.'s quarry, half a mile east of Delta, York County, 

Pa. Analysis by A. S. McCreath, Pennsylvania Second Geol. Survey, Kept. CCC, pp. 269-270, 1880.

2. Peach Bottom slate (perfectly fresh). J. Humphrey & Co.'s quarry. Analysis by Booth, Garrett, 

and Blair, Philadelphia, Pa., 1895. Mathews, E. B., Maryland Geol. Survey Kept., vol. 2, p. 226,1898.

3. Slate, Lancaster County, Pa. Merrill, G. P., A treatise on rocks, rock weathering, and soils, p. 119, 

1906.

4. Slate, Harford County, Md. Merrill, G. P., idem, p. 214.

In 1876 I. N. Rendell, president of Lincoln University, near 

Oxford, Pa., obtained from the dump pile of the Peach Bottom slate 

quarries on Susquehanna River six or seven slabs of roofing slate 

bearing markings that suggested fossils. These slabs were sent to 

Leo Lesquereux, who reported in 1879 that the markings were 

Buthotrephis flescuosa.40 Later the supposed fossils were sent to 

James Hall, who reported in 1883 that " the conspicuous fossils have 

the character of marine vegetation and the aspect of algae." 41 He 

acknowledged that these markings may well be dendritic markings of 

inorganic origin. He then mentioned other forms that are " distinctly organic in their character and mode of occurrence and 

belong to some algoid form." He also described faint irregular 

marking and said, " In certain light I think I can see by the naked 

eye faint markings as if of branches, radiating in an irregular 

manner from the center and bifurcating in their course. This 

reminds me strongly of some graptolitic forms."

On the basis of the supposed ButfiotrepMs and supposed graptolites the Peach Bottom slates were assigned to the " Hudson Eiver 

shale " horizon of the Upper Ordovician. However, the most careful 

and persistent search through the Peach Bottom slates by numerous

*° Frazer, Persifor, jr., Age and position of the Peach Bottom slates: Am. Philos. Soc. 

Proc., vol. 18, pp. 366-369, 1879.

"Frazer, Persifor, Jr., The Peach Bottom slates of southeastern York and southern Lancaster Counties: Am. Inst. Min. Eng. Trans., vol. 12, pp. 353-359, 1884.
PALEOZOIC SEDIMENTARY BOOKS 41

geologists has failed to reveal any further fossil markings that would 

confirm the reported find of graptolites. The deep refuse piles that 

surround the quarries, both in Pennsylvania and in Maryland, contain 

many specimens with distinct dendritic markings that might easily 

suggest fossil forms, but are all of inorganic origin. It may be that 

the dubious graptolites mentioned by Hall were in reality nonorganic 

markings and that if any of the so-called fossils discovered in 1879 

were really of organic origin they were algae, not necessarily Ordovician or even Paleozoic. If the Cardiff conglomerate underlying 

the slates is a basal conglomerate, then the slates are in all probability Paleozoic. The Cardiff conglomerate occurs in a region where 

conditions subsequent to deposition have been favorable for molecular rearrangement which would have obliterated evidence of unconformity, Nevertheless it has been possible to prove an unconformity 

at the base of the Glenarm series where basal conglomerates are 

lacking, and it is somewhat surprising that an unconformity at the 

base of the Cardiff conglomerate can not be demonstrated by field 

evidence. Therefore it seems best to include both the Cardiff and 

Peach Bottom in the pre-Cambrian, because it is impossible to prove 

that there is an unconformity below the Cardiff.

PALEOZOIC SEDIMENTARY ROCKS

The Honeybrook upland lies north of the Chester Valley and 

extends southwestward from Valley Forge, on Schuylkill Kiver, to 

Quarryville on the south and from Phoenixville to a point north of 

Honeybrook on the north. It is cut off on the northeast and north 

sides by the sediments of the Triassic basin, which trends northwestward from Valley Forge to Phoenixville and Elverson. Its 

greatest width from north to south is about 12 miles. The upland 

terminates on the west in three anticlines, named Welsh Mountain, 

Barren Hill, and Mine Ridge, from the ridges of Lower Cambrian 

quartzite which partly inclose the pre-Cambrian crystalline rocks 

that form the core of the upland. These anticlines plunge southwestward and carry the Lower Cambrian arenaceous series under the 

Cambrian and Ordovician limestones of the Quarryville, Lancaster, 

and New Holland Valleys.

The Lower Cambrian series, which unconformably overlies the 

pre-Cambrian core of the Welsh Mountain and Barren Hill anticlines, is composed of the Chickies quartzite, including its basal 

Hellam conglomerate member, the Harpers phyllite, and the Antietam quartzite. This series is 2,200 feet thick in Welsh Mountain and 

is little metamorphosed. The calcareous Antietam quartzite grades 

upward into and is overlain by the Vintage dolomite, which together
42 McCALLS FERRY-QTJARRYVILLE DISTRICT, PA.

with the Kinzers formation and Ledger dolomite compose the equivalent of the Lower Cambrian Tomstown dolomite of Cumberland 

Valley. The Elbrook, Conococheague, and Beekmantown limestones overlie the Lower Cambrian limestone in the synclinal valley 

areas both north and south of the Welsh Mountain-Chestnut Hill 

axis, which crosses the Lancaster and New Holland Valleys. The 

Cocalico shale lies only north of the above-mentioned axis of uplift.

PALEOZOIC STRATIGRAPHY OF MINE RIDGE UPLAND AND VICINITY

The Mine Ridge upland is a double anticline lying partly in the 

northeastern part of the Quarryville quadrangle and is the southernmost of the three anticlinal folds that underlie the Honeybrook upland. The anticline is 30 miles long and 3 miles wide and extends 

'from a point east of Downingtown southwestward to Quarryville. 

At both ends it plunges under the limestones of the Chester-Quarry - 

ville Valley, and it is separated from the anticlinal area of Barren 

Hill, to the north, by normal faults. Like the Welsh Mountain and 

Barren Hill anticlines, the Mine Ridge fold is formed of a core of 

pre-Cambrian rocks bordered by Lower Cambrian arenaceous rocks 

overlain by Paleozoic limestones. The Paleozoic rocks of the Mine 

Ridge upland and vicinity have been greatly metamorphosed and 

closely folded and do not closely resemble the less altered Paleozoic 

rocks north and west of them. Their relations and age have been 

worked out, however, in connection with a study of sequence of the 

slightly altered fossiliferous Lower Cambrian arenaceous rocks of 

Welsh Mountain, Chestnut Hill, and the Hellam Hills and the fossiliferous limestone series of the adjoining New Holland, Lancaster, 

and York Valleys.42

LOWER CAMBRIAN ROCKS OF THE McCALLS FERRY-QVARRYVTLLE DISTRICT

CHICKIES QUARTZITE

The basal Cambrian formation of the McCalls Ferry-Quarryville 

district is the Chickies quartzite. It consists of the Hellam conglomerate member at the base, overlain by a considerable thickness of 

white vitreous quartzite, which is succeeded by a greater thickness 

of sericitic quartzite interbedded with quartz-mica schist and quartz 

schist. The Chickies quartzite was named in 1878 by Lesley. and 

Frazer from outcrops of quartzite at Chickies Rock, in the gorge 

of Susquehanna River north of Columbia, Pa. At the type locality 

the quartzite is exposed in an anticline which does not bring to the 

surface the underlying Hellam conglomerate member. The quartzite

42 Stose, G. W., and Jonas, A. I., The lower Paleozoic section of southeastern Pennsylvania : Washington Acad. Sci. Jour., vol. 12, pp. 358-366, 1922.
PALEOZOIC SEDIMENTARY ROCKS 43

i here is a heavy-bedded white vitreous quartzite at the base and a 

, pebbly quartzite with black slate partings near the top. 

/ Hellam conglomerate member. The Hellam conglomerate member 

of the Chickies quartzite forms the base of the Cambrian and is 

exposed on the flanks of Mine Ridge. It was named 48 from the 

Hellam Hills, York County, Pa., where it is well developed and 

gives rise to high ridges.

i In the Quarryville area the Hellam conglomerate is exposed as 

' a narrow band around the pre-Cambrian rocks of the Mine Ridge 

anticline. The conglomerate together with the overlying quartzite 

forms a ridge which surrounds the central area underlain by preCambrian rocks, above which the bordering quartzites rise 100 feet 

on the north and southwest sides. The conglomerate is well exposed 

in several places on both the north and south sides of the ridge. 

I The freshest outcrops are in the valley of Nickel Mines Run north of 

Greentree and along Houston Run southwest of Gap. At the latter 

place the rock stands up as ledges 25 feet high exposing 40 feet of 

thick beds of pebbly quartzite with sericitic schist partings.

The Hellam conglomerate on the Mine Ridge upland is a coarse 

to fine grained siliceous conglomerate interbedded with mica schist. 

The conglomerate is a pebbly quartzite and quartz conglomerate composed of thoroughly recemented and recrystallized quartz grains with 

green muscovite developed on bedding planes. It contains small 

amounts of microcline and albite, with accessory zircon, magnetite, 

hematite, and titanite. The pebbles are milky and blue quartz, and 

have been so much granulated by pressure that weathering reduces 

them to an aggregate of sand grains, thus obscuring the conglomeratic character of the rock in weathered exposures. These pebbles 

have been flattened and elongated, and their edges interlocked with 

the quartz of the groundmass.

The quartz conglomerate occurs in beds a foot or two thick intercalated with thinner beds of crumpled mica schist. The schist beds 

represent the metamorphosed equivalent of the fine sericitic quartzite 

\ layers that occur between conglomerate beds of the Hellam conglom- 

(' erate of Welsh Mountain. The effect of metamorphism on the quartz 

conglomerate beds has been elongation and granulation of the pebbles 

and the development of biotite and muscovite, producing a schistose 

texture in the rock.

The maximum thickness of the Hellam conglomerate of the Mine 

Ridge upland is 90 feet, measured north of Greentree. In the type 

locality of the Hellam Hills a thickness of more than 600 feet has 

been estimated. The Hellam conglomerate is thinner around the 

Mine Ridge upland than it is a few miles to the north, on Welsh Moun48 Stose, G. W., and Jonas, A. I., op. cit, p. 96.
44 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

tain. This difference is due partly to difference in original thickness 

and partly to a considerable reduction in thickness of the formation 

around the Mine Ridge upland as a result of the intense compression 

which has flattened the pebbles in the harder beds and rendered the 

softer beds schistose. In the writers' opinion the Hellam member 

corresponds in general with the Loudoun formation of South Mountain and Catoctin Mountain and the basal Cambrian conglomerate 

of the Reading Hills.

The Hellam conglomerate is the basal member of the Chickies 

quartzite. It contains no fossils but unconformably overlies preCambrian rocks throughout its extent. It contains pebbles of blue 

quartz, probably derived from quartz veins in the underlying preCambrian gabbro exposed northeast of Gap and from the pre-Cambrian granite of the region, which also contains blue quartz.

Quartzite member. In the McCalls Ferry-Quarryville district the 

quartzite member of the Chickies quartzite is exposed in the Mine 

Ridge upland, 15 miles east of the type locality. It encircles the 

upland and together with the underlying basal conglomerate forms 

a prominent ridge on the flanks of the uplift. Being less resistant 

to erosion, the underlying pre-Cambrian rocks have been reduced to 

a level below that of the quartzite on the northern and western flank 

of the anticline.

The quartzite has been quarried south of Gap in a quarry cut across 

the strike, where TO feet of hard vitreous quartzite with a dip of N. 

40° W. is exposed. On the southwest nose of the ridge, a mile north 

of Quarryville, the quartzite has disintegrated into fine white sand 

streaked with sericitic clay beds. This sand is dug in small pits and 

is used for concrete and plaster.

The quartzite member of the Chickies quartzite is made up of 

white vitreous quartzite, some of which is massive and some thin 

bedded, with sericite developed on bedding planes. The upper twothirds of the quartzite member consists of thin-bedded crumbly white 

sericite quartzite interbedded with quartz-mica schist and quartz 

schist. The quartzite is seen in thin section to be made up chiefly of 

interlocking areas of quartz, which show strain shadows. The other 

constituents are straw-colored blades of muscovite, a little microcline, 

irregular albites with fine inclusions, well-rounded zircons, and magnetite. Black tourmaline, which is a characteristic constituent of the 

rocks on the flanks of the Mine Ridge anticline, is especially abundant 

in the quartzite. The original sand, which was rather pure, has been 

thoroughly recrystallized, but the stretching of the tourmaline would 

indicate that the rock has been subjected to dynamic disturbance 

subsequent to recrystallization.

The mica schist is a fine-grained schistose aggregate of biotite, 

quartz, and subordinate feldspar.
PALEOZOIC SEDIMENTARY ROCKS 45

Of the following analyses No. 1 represents the quartzite at Chickic? 

Station, Pa., 'and No. 2 the Montalto quartzite of South Mountain. 

Both analyses are those of a pure quartzite. The rock from South 

Mountain contained some sericite and epidote.

Atialyses of quartzite

&\Qi..... ........................

AljOs -- -- - -

TeO.. ........................

MgO. ...........................

'CaO......... ..... . ...

NtaO.. ........... ...............

1

97.71

1.39

1.25

iq

.18

2

» 92. 00

4.21

1.80

.04

.1C

K8 0... ..........................

TiOa...... ......... ..............

P|05-.... . ........... ........

1

100. 66

2

1.16

.96

.14

.21

100.68

0 7.87 per cent is combined silica.

1. Pirsson, L. V., Rocks and rock minerals, 1st ed.. p. 367,1910.

2. Quartzite specimen No. 1157, lJ/£ miles south of burned sawmill on the Conocooheague. Analysis by 

IF. A. Genth. Pennsylvania Second Geol. Survey Kept. CCC, p. 363,1880.

The thickness of the quartzite member of the Chickies overlying 

the Hellam member on the flanks of the Mine Kidge anticline has 

' been estimated as 350 feet. The full section is not exposed at any 

 one place in that area, hence the thickness can not be determined 

.accurately. It is somewhat thinner on the Mine Ridge anticline than 

in Welsh Mountain or the Hellam and Chickies Hills.

The basal member of the quartzite, the Hellam conglomerate, unconformably overlies pre-Cambrian rocks. The quartzite is part of an 

rarenaceous series that contains Lower Cambrian trilobite and brachiopod remains in its upper part, although the only fossil remains that 

have been collected from the quartzite member are scolithus tubes. 

Therefore both stratigraphic relations and fossil content establish 

the age of the quartzite member of the Chickies as Lower Cambrian. 

It is similar to the Montalto quartzite member of the Harpers schist 

of South Mountain, Pa., but as it lies at the base of the Harpers and 

not in the midst it is probably not the exact equivalent of the 

Montalto.

HABPEBS AND ANTIBTAM SCHISTS

The Harpers schist 44 received its name from Harpers Ferry, 

~W. Va., where it is well exposed in the gorge of Potomac and Shen- 

.andoah Rivers. It lies on the flanks of the Blue Ridge, and an eastern belt of the formation extends northward along South Mountain 

into Pennsylvania.

The Harpers formation is widely exposed in the Piedmont of 

Maryland and Pennsylvania south of the Hanover-York Valley and 

in the Pigeon, Hellam, and Chickies Hills and Welsh Mountain.

"Keith, Arthur, Geology of the Catoctln belt: U. S. Geol. Survey Fourteenth Ann. 

Sept., pt. 2, p. 333, 1894.
46 McCALLS FEKRY-QUARRYVILLE DISTRICT, PA.

The Harpers of this area is a greenish-gray phyllite composed of 

albite, muscovite, and quartz with some biotite. It has not, however, advanced beyond a phyllitic stage of metamorphism and should 

be called a phyllite. In its type locality near Harpers Ferry the 

rock is a shale. In South Mountain it is a phyllite like much of the 

Harpers of the Piedmont north and northwest of the Mine Ridge 

upland. From Welsh Mountain southward to the Mine Ridge upland 

there is a gradual increase in metamorphism of the Paleozoic sediments, and the Harpers phyllite has been altered into an albite-mica 

schist and the overlying Antietam quartzite into a quartzose albite 

schist.

The Antietam sandstone (quartzite), which overlies the Harpers 

formation in Virginia, West Virginia, and Maryland, was named by 

Keith 45 from Antietam Creek, on the tributaries of which it is well 

shown. The Harpers phyllite of Pennsylvania is overlain by a 

quartzite which is the equivalent of the Antietam sandstone of the 

Blue Ridge and South Mountain. The upper beds of the formation 

are a light-gray, somewhat calcareous quartzite containing angular 

grains of glassy quartz. These upper quartzite beds weather to a 

laminated, porous, highly ferruginous rock whose bedding surfaces 

show numerous rusty molds of Obolella and trilobite fragments. 

These beds are similar to fossiliferous beds characteristic of the 

Antietam sandstone of South Mountain and are excellent horizon 

markers. The fossiliferous quartzite is interbedded with gray quartz 

phyllite in which weathering produces a rusty banding. The Antietam of Welsh Mountain is1 about 120 feet thick and is an easily identified formation between the Harpers phyllite and Vintage dolomite. 

Metamorphism of this formation in the McCalls Ferry-Quarryville 

district has produced a quartzose albite schist which is difficult to 

separate from the underlying Harpers albite schist. As it has been 

possible to recognize the ferruginous quartzite beds only in a few 

places near the Mine Ridge upland and in the hills to the west, the 

Harpers and Antietam have been in part mapped together in this 

report.

Areal distribution. The Harpers and Antietam schists of the 

McCalls Ferry-Quarryville district overlie the Chickies quartzite on 

the flanks of the Mine Ridge anticline, where they are exposed in a 

border around the Chickies. These rocks are brought to the surface 

also in a series of anticlinal hills extending from the Mine Ridge 

anticline west for 15 miles, nearly to Susquehanna River at Safe

«Keith, Arthur, as reported by Williams, G. H., and Clark, W. B., Maryland, its 

resources, industries, and institutions, p. 68, 1893. The formation was described, but not 

named, by Keith in Am. Geologist, vol. 10, p. 365, 1892.
PALEOZOIC SEDIMENTARY BOCKS 47

Harbor. These hills, to many of which local names have been 

given, such as Bunker Hill and Oak Hill, trend N. 75° E., in a 

direction roughly parallel to the strike of the formation. They rise 

about 150 feet above the valley level but show few fresh outcrops 

except in stream gorges cut, for the most part, north and south across 

the hills. Quarries have been opened in a few places and the stone 

is used locally for building. The largest quarry, a mile north of 

Safe Harbor, yields fresh and well-jointed blocks, but is not operated 

at present. ,

Lithologic character. Two facies of micaceous schist can be distinguished on the Mine Ridge upland. The lower overlies the Chickies quartzite and is certainly part of the Harpers formation. It will 

be called the blue garnetiferous chlorite schist. It is overlain by an

albite-mica schist that includes the upper part of the Harpers and 

the overlying Antietam formation.

The blue chlorite schist has been mapped separately on the flanks 

of the Mine Eidge anticline. It forms a narrow band on the south 

face and widens considerably on the northwestern nose of the ridge, 

and extends northeastward toward Gap, where it is cut off by a fault. 

Erosion has not gone deep enough to expose this basal part of the 

Harpers in the outlying hills west of the Mine Eidge upland.

The blue chlorite schist is a fine-grained closely crumpled schist 

containing chlorite and some biotite with subordinate quartz and 

feldspar. The chlorite and mica are in large blades that give a 

spangled appearance to the rock and wrap around large red garnets. 

In weathered rock the garnets stand out upon the foliation planes, 

producing a knobby surface. The bluish color is due to the chlorite 

content of the schist. Thin sections of the rock show that the feldspar is sericitized. Chlorite occurs in large green blades without 

orientation. Quartz, some secondary albite, garnet, and magnetite 

make up the rest of the rock.

The overlying albite schist is a gray coarse-grained sparkling muscovite-biotite schist spotted with dull-white porphyroblasts of albite. 

The albite crystals are most conspicuous on rough surfaces normal to 

the plane of schistosity. Calcite, tourmaline, zircon, and apatite are 

constituents of common occurrence. Albite occurs in spongiform individuals filled with inclusions of the other constituents. These iuelusions usually show a parallel arrangement, which may be either 

inclined or parallel to the foliation. (See pi. 2, A.} Quartz is fairly 

abundant. Calcite appears to have crystallized at the same time as 

the other constituents.

The albite mica schist includes the rest of the Harpers above the 

blue chlorite schist and the Antietam. The upper part of the Antietam
48 McCALLS FEKRY-QUARRYVILLE DISTRICT, PA.

is an albite schist but is more calcitic and dolomi'tic and contains lessalbite than the underlying beds. It is a fine-grained light-gray schist 

speckled with brown biotite containing many small needles of black 

tourmaline. Microscopically the rock shows abundant calcite, dolomite, and biotite; some of the biotite occurs as porphyroblastic bladeswith inclusions of the groundmass. Quartz, albite, titanite, and 

pyrite are present in small amounts. Tourmaline crystals which contain inclusions of the rock minerals show that the mineral was formed 

as a result of pneumatolytic action subsequent to recrystallization of 

the rock. (See pi. 2, B.)

At first sight such sparkling crystalline rocks as the albite-mica 

schists just described show absolutely no resemblance to the smooth 

phyllite or impure quartzite that occurs in the Hellam-Chickies anticline and in Welsh Mountain, but it has been possible to trace a 

graduation in metamorphism from phyllite to schist. The gradational belt is only 5 miles wide but has been shortened considerably 

by normal faulting. In the gradational belt the Harpers and Antietam are brought to the surface by several anticlines in which the 

phyllite progressively shows an increase of metamorphism from the 

flanks of Welsh Mountain southward. Metamorphism has for 

the most part obscured the fossil remains usually found in the ferruginous quartzose beds of the Antietam, but in many places, even in 

the area of most intense anamorphism, the ferruginous beds can be 

detected. These beds weather to a biotitic rusty-banded calcareous 

schist that contains albit© and is streaked with limonite and that 

probably represents ferruginous fossiliferous layers of the less metamorphosed Antietam quartzite.

The Harpers and Antietam schists weather readily because of their

content of calcite and form a deep, loose soil. The steep slopes of 

the hills are scarred by deep gullies that render the land difficult of 

cultivation. Because of the character of the soil these hills are locally 

called " the sand hills."

Chemical composition. The following table gives analyses of the 

coarse albite Harpers schist and of Antietam calcareous albite schist,, 

both from the Quarryville quadrangle; also a partial analysis of the 

Harpers phyllite from a point a short distance to the north.
PALEOZOIC SEDIMENTABY BOCKS 

Analyses of Harpers and Antietam, schists

Chemical composition

SiOa. .........'.........

AhOs.. ................

FeiOa. .................

FeO.. .................

MgO...................

CaO. ..................

NaaO. .................

K2 0. _. ................

H2 0-. ................

H2 0+.. ...............

TiOs... ................

C08 .... ................

PjOt. ..................

MnO. .................

1

64.40 

12.62 

1.80 

2.56 

1.74 

2.20 

2.34 

5.12

J 1.08

1.13 

1.34 

.50 

.06

100. 89

2

57.36 

11.84 

1.26 

3.15 

4.52 

5.92 

1.06 

5.10 

f .01 

\ .95 

.88 

7.28 

.02 

.12

99.47

3

67.38

2.18 

5.79

Mineral composition (2)

Quartz ........................

Biotite ........................

Albite--.. ......r _.. ........

34 

18 

12 

10 

10 

7 

8 

2

101

1. Harpers albite schist, State road, three-fourths of a mile west of Camargo, Lancaster County, Pa. 

Analyst, R. K. Bailey, U. S. Geol. Survey, 1920.

2. Antietam calcareous albite schist, 1 mile south of Lime Valley, Lancaster County, Pa. Analyst 

Olenn V. Brown, U, S. Geql. Survey, 1920.

3. Harpers phyllite, l mile south of Mountville, Lancaster County, Pa. Analyst, J. G. Fairchild, 

U. S. Geol. Survey, 1922.

Character of the metamorphism in the Harpers and Antietam 

albite schist. In the garnetiferous chlorite schist metamorphism has 

apparently taken place in the upper part of the deep-seated zone, as 

is shown by the formation of such heavy minerals as garnet and 

plagioclase, which are characteristic of zones of high pressure and 

temperature, associated with chlorite, which forms in the upper zones 

of metamorphism. In the albite-mica schist also metamorphism has 

taken place in a transition zone, albite being characteristic of the 

upper zone and biotite of the lower zone. The lime content of the 

albite-mica schist, instead of uniting with soda to form plagioclase, 

has crystallized as calcite, and the soda as albite. All the minerals 

in the path of each growing albite crystal that were not needed in 

forming calcite were inclosed by the metacryst, which has irregular 

spongiform boundaries.

Thickness. The thickness of the Harpers phyllite has been estimated to be 1,000 feet in the Hellam and Chickies Hills and about 

1,500 feet in Welsh Mountain. The overlying Antietam quartzite is 

about 120 feet thick in Welsh Mountain. The albite-mica schist representative of the Harpers and Antietam formations on the flanks of 

the Mine Kidge upland probably attains a thickness .comparable to 

that of the formation on Welsh Mountain, a few miles to the north, 

but the thickness of the schist can not be measured. Cleavage has 

developed to such an extent that the bedding planes are rarely discernible. It is not only difficult to trace the original bedding, but 

repeated minor folding makes it impossible to estimate the thickness 

of the formation with any exactitude.
50 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

Age. The resemblance of the Harpers albite schist to the albite 

schist facies of the Wissahickon is very striking, so that in earlier 

work the writers considered the rocks to be identical. This resemblance is especially marked between the Harpers albite schist and the 

biotitic albite facies of the Wissahickon that occurs along Susquehanna River near Safe Harbor and Shenks Ferry. However, the 

albite schist on the flanks of the Mine Ridge anticline and farther 

west has been proved on stratigraphic grounds to be Lower Cambrian 

Harpers schist and Antietam quartzose schist. The chlorite schist 

and albite schist overlie Chickies quartzite on the flanks of the Mine 

Ridge anticline. The albite and quartzose calcareous schist in the 

valley have been established as anticlinal and have been shown to be 

conformable under Vintage dolomite, into which they also grade 

through a calcareous schist. The gradation in fresh rock cuts has 

been seen at several localities. An excellent exposure three-fourths 

of a mile south of Conestoga shows in a thickness of 6 feet passage 

from a biotite dolomite schist through rusty-banded dolomite 

speckled with biotite into a cream to dove-colored dolomite free of 

mica and belonging to the Vintage formation. Metamorphism has 

almost entirely obliterated fossils, but fossils have been found in the 

Antietam half a mile north of Gap. Therefore, on structural and 

stratigraphic grounds, as well as by fossil content in adjacent areas, 

the albite-mica schist has been found to be identical with the Harpers 

phyllite and the calcareous biotite schist with the Antietam quartzite 

and hence Lower Cambrian.

VINTAGE DOLOMITE

The arenaceous series of Welsh Mountain and of the McCalls 

Ferry-Quarryville district is overlain by limestones, the oldest of 

which is the Vintage dolomite,46 named from a small village 15 miles 

east of Lancaster and just north of the Quarryville quadrangle, 

where most of the section is excellently exposed in a cut of the 

Pennsylvania Railroad. (See pi. 3, A.)

Distribution. The Vintage dolomite is exposed north and west of 

Mine Ridge upland on the slopes of the "sand hills," made up of 

Antietam albite schist, which it surrounds. It occupies valleys also 

between the Cambrian schists and the blue slaty Conestoga limestone. North of Gap it underlies the valley west to Bellemont and 

Leaman Place and forms a narrow valley that extends 8 miles west 

of Bellemont. The best exposures lie in the cuts of the Pennsylvania 

Railroad from Leaman Place to Kinzers. There is a large quarry 

in Vintage dolomite (see pi. 3, B] now operating at Kinzers, north

* Stose, G. W., and Jonas, A. I., Lower Paleozoic section of southeastern Pennsylvania: 

Washington Acad. Sci. Jour., vol. 12, No. 15, p. 362, 1922.
U. S. GEOLOGICAL SURVEY BULLETIN 799 PLATE 2

yl. HARPERS SCHIST

The albite is crowded with inclusions in a roughly parallel orientation that is inclined to the

foliation of the rock

B. ANTIETAM SCHIST 

Porphyroblastic hiotite, a, in a dolomitic matrix. The tourmaline, t, is epigenetic

PHOTOMICROGRAPHS OF SCHIST
IT. S. GEOLOGICAL SURVEY BULLETIN 798 PLATE 3

A. UPPER PART OF THE VINTAGE DOLOMITE OVERLAIA l!1 MAZERS SHALE 

EXPOSEDlIN CUT ON THE PENNSYLVANIA RAILROAD NEAR VINTAGE

Ji. VINTAGE DOLOMITE IN QUARRY AT KINZERS 

Shows well-bedded character of lower part of Vintage dolomite
U. S. GEOLOGICAL SUBVEY BULLETIN 799 PLATE 4

to

.1. MINZKKS 1'SKUDOCONGLOMERATE ON PENMSYLVANIA RAILROAD WEST

OF KINZERS

Produced by differential weathering of white marble and durk impure dolomite

B. HASAL CONKSTOGA SLATE AND LIMESTONE CONGLOMERATE, UNCONFORMABLY OVERLYING VINTAGE DOLOMITE, BELLEMONT QUARRY
U. S. GEOLOGICAL SURVEY BULLETIN 799 PLATE 5

A. THICK-BEDDED CRYSTALLINE LIMESTONE PASSING UNTO FINELY 

LAMINATED SLATY LIMESTONE TYPICAL OF CONESTOGA LIMESTONE IN 

QUARRY 1 MILE NORTHWEST OF BELLEMONT

B. CLOSELY FOLDED MICACEOUS \IARRLE OF CONESTOGA LIMESTONE IN 

QUARRY HALF A MILE NORTHWEST OF QUARRYVILLE
PALEOZOIC SEDIMENTARY BOCKS 0.1.

of the Lincoln Highway, where the rock quarried is crushed for road 

metal. In the valley of Pequea Creek north and east of Lime Valley 

there are many good outcrops of dolomite in stream cuts a.nid old 

quarries. It is well exposed farther southwest along Pequea Creek 

near Burnt Mills. No quarries are now operated in the Vintage 

dolomite in the McCalls Ferry-Quarryville district, except the one 

at Kinzers. . ' ' ' '"''.''.'' '

Lithology. In general the 'Vintage consists of massive 'glistening 

dark-gray dolomite, weathering whitish, with scattered coarse c'rys-' 

talline blebs, and dark-blue dolomite with argillaceous partings, 

weathering knotty. At'the base, is a cream-colored schistose 'thinbeolded dolomite containing muscovite flakes, with overlying knotty 

blue and white dolomite weathering to a rough surface. Higher up 

there are some dark-blue beds with argillaceous partings, in places

micaceous and wavy. In the upper part the rock is a massive 

sparkling gray to blue mottled dolomite in thick beds. It weathers 

white with siliceous and calcareous blebs. The uppermost beds are 

very massive, as is well shown in the Pennsylvania Railroad cut, 

where the Vintage underlies shales of the succeeding formation. 

The rock weathers to a deep-red clay soil and occupies the lower 

slopes of the sand hills. The red color of the soil derived from the 

dolomite contrasts sharply with that of the gray sandy soil derived 

from the Harpers and Antietam schists.

TMckn&ss. At the type locality, just north of the Quarryville 

quadrangle, the thickness of the Vintage dolomite has been estimated 

as 654 feet, although the base of the formation is not exposed at this 

place. The Vintage dolomite thins southward toward the Mine 

Ridge upland. At Lime Valley it is 150 feet thick, 2 miles farther 

south it is 50 feet thick, and south of Oak Hill it is absent. This 

irregularity of thickness and southward thinning is due to erosion 

subsequent to deposition and is discussed on page 57. The section at 

Vintage is given below.

Section of Vintage dolomite in cut on main line of Pennsylvania Railroad west

of Vintage, Pa.

Feet

Argillaceous dolomite; weathers rusty and earthy______ 7 

Massive, sparkling, gray-blue mottled dolomite; weathers

chalky, with siliceous and calcareous blebs in relief-___ 30 

Blue dolomite, wavy bedding, with mica partings_______ 25 

Thin shale, micaceous_______________________ 5 

Blue argillaceous dolomite; weathers to thin slivers, with

some massive blue dolomite bands._____________ 15 

Granular dolomite, light gray-blue mottled, in 5-foot beds__ 35 

Dense darker blue massive dolomite, with some blebs, argillaceous partings; weathers chalky_,____________ 100 

. 6136 29 5
52 McGALLS FEERY-QUAREYVILLE DISTRICT, PA.

Feet

Light and dark mottled dolomite. 35 

Gray, wavy, thin-bedded dolomite; weathers chalky ; 148 

Covered, probably dolomite 234 

Dark, glistening dolomite; weathers white (base not exposed) - 20

±654

Age (md correlation. The Vintage dolomite has not yielded any 

fossils in the McCalls Ferry-Quarryville district. Stose and Jonas 

have collected remains of SaLterella, trilobites, and brachiopods and 

plates of cystids from the formation at several localities in the York 

Valley. The section that has yielded the most fossils lies south of 

Emigsville and has been described by Walcott.47

The fossils and stratigraphic relations of the Vintage dolomite 

show its Lower Cambrian age. It overlies the Antietam schist, 

which grades up into the dolomite, and it is overlain by a shale that 

contains Lower Cambrian fossils. Lithologically it resembles the 

Tomstown dolomite of Cumberland County, Pa., and it is considered 

to represent the basal part of the Tomstown.

KINZEBS FORMATION

Name. The Kinzers formation takes its name 48 from an excellent 

exposure in the Pennsylvania Railroad cut at Kinzers, just east of 

Vintage. Here it is exposed in an almost complete section overlying 

Vintage dolomite.

Distribution. The Kinzers formation in the McCalls Ferry-Quar-> 

ryville district occurs in two synclinal folds in the Vintage dolomite 

2 miles north of Gap. It is more widely exposed to the north, in the 

valley west of Barren Hill and Welsh Mountain, and north and northwest of Lancaster. Being a thin, shaly formation between two dolomites, it forms narrow hills by differential erosion. No quarries 

have been operated in it in the vicinity of the Mine Ridge upland 

and Welsh Mountain, but north of Lancaster, where the formation is 

largely a shale, it has been quarried extensively.

LitJiologic character. At the base of the formation there are a 

few thin beds of impure dolomite that weather to an earthy tripoli 

containing at many .places remains of Salterella^ brachiopods, and 

trilobites. These beds are followed by a variable thickness of blue 

hackly shale as much as 50 feet thick in places. The earthy tripoli 

and shale beds are shown in Plate 3, A, overlying Vintage dolomite. 

Above the shale is a variable series, of dark banded argillaceous 

dolomites that weather to tough buff ribbed argillaceous rocks,

« Walcott, C. D., Cambrian rocks of Pennsylvania: U. S. Geol. Survey Bull. 134, pp. 

15-16, 1896.

48 Stose, Q. W., and Jonas, A. I., The Lower Paleozoic section of southeastern Pennsylvania : Washington Acad. Sci. Jour., vol. 12, p. 362, 1922.
PALEOZOIC SEDIMENTARY ROCKS . 53

sparingly fossiliferous. Some beds are an intimate mixture of 

nodular white granular marble and dark impure dolomite that 

weathers to a knotty pseudoconglomerate of white marble. (See 

pi. 4, A.)

Thickness. The thickness at the type locality is about 132 feet. 

South of Welsh Mountain the Kinzers formation is much thinner, in 

places not more than 25 feet, and the shale is not pfom'ineflt.

The section from the type locality is as follows:

Partial section of Einzers formation, in railroad cut at Kinzers, Pa.

Feet 

Dark-olue limestone with wavy impure partings__ _______ 10

Thick-bedded light-gray dolomite 12 

Dark-blue limestone with wavy impure partings_______ 6 

White spotted marble with wavy buff dolomite partings S 

Blue limestone banded with slightly wavy siliceous layers__ 10 

Highly siliceous banded dark limestone, weathering to skeleton of buff siliceous network__ _____ _ 8

Impure thick-bedded dolomite, weathering to dense buff

tripoli ____._________ ________________ 3 

White spotted marble with even buff dolomite banding.'__'__ 8 ; 

Wavy banded blue limestone, numerous argillaceous partings- 10 

Crumbly fissile gray shale___ _________________ 50±

Impure dolomite, weathering to buff, tripoli and containing 

few trilobate fragments and Satterella___________________ 7

Massive light-blue dolomite (Vintage). 

132±

Age. The Kinzers formation contains Lower Cambrian fossils 

OlenelluS) Salterella, and brachiopods. The trilobites, chiefly Olenellus thompsoni (Hall), occur abundantly in the shale and were 

described by Walcott and extensively collected in the LancasterYork Valley by Professor Koddy, of Millersville, Pa., and other 

local collectors. In the McCalls Ferry-Quarryville district fragments of trilobites and Salterella occur in the basal Kinzers beds of 

the Kinzers cut at a horizon which is abundantly fossiliferous elsewhere. The Kinzers is an argillaceous fossil-bearing formation, 

which is represented by dolomite or limestone in the Lower Cambrian (Tomstown) of the Cumberland and Lebanon Valleys.

LEDGER DOLOMITE

Name and distribution. The upper part of the Tomstown dolomite is represented in this region by the Ledger dolomite. It received its name from Ledger,49 3 miles northeast of Kinzers, and 

although it is not exposed in the McCalls Ferry-Quarryville district, 

it underlies most of the wide Valley just to the north, from Barren 

Hill to Lancaster.

48 Stose, G. W., and Jonas, A. I., op. cit., p. 363.
54 McOALLS FEEEY-QUARRYVILLE DISTRICT, PA.

Litkology. The Ledger is a, granular gray to white dolomite, gerieraliy thick bedded, with few'bedding planes.' Outcrops are few 

because of the readiness with which the dolomite weathers to a 

granular red clay soil. It is quarried considerably 2 miles east qf 

Gap, at Limeville, and northward. It has been used as building 

storie to a slight degree, but it weathers too''readily to be satisfactory 

for that purpose. - ' >;. .» .

Thickness and age'. The thickness of the Ledger has been estimated .^t 1,000 feet. Fossils have not been found in it, in the Lancaster Valley, and its Lower Cambrian age was there determined on 

stratigraphic grounds. In 1922 Stose and. Jonas found a Lower 

Cambrian brachiopod, Nasivsia festirmta . Billings, in this formation 

west of Hanover. The presence of this brachiopod .confirms the 

determination of Lower Cambrian age.

ORDOVICIAN ROCKS

CONESTOGA -LIMESTONE

In the northern part of. the McCalls FerryrQuarryville district 

there is a great thickness of dark-blue slaty crystalline limestone and / 

limestone conglomerate, which is called Conestoga limestone by the \} 

writers because of excellent outcrops along Conestoga Creek, south of ,' 

Lancaster. \ 

.Distribution. The Conestoga limestone lies south of the Welsh \ 

Mountain-Hellam Hills uplift and Lancaster and occupies most of 

the limestone valley area of the northern part of the McCalls FerryQuarryville district. South of Lancaster, which is its northwesternmost locality in the valley, it extends to the Martic Hills and to

Quarry ville. East of Quarryville it covers the entire narrow floor 

of Chester Valley nearly to Coatesville and borders the south side 

of the valley to Schuylkill River, 50 miles east of this area. It , extends westward from a point near Gap, its most easterly known / 

occurrence north of Mine Ridge upland, to Susquehanna River. It ^ 

occurs on the west side of the river at Shenks Ferry in outcrops at j 

the water's edge, and north of Long Level it underlies a valley area } 

that extends to the northern edge of the McCalls Ferry quadrangle ( 

and is part of the wider valley included in the York quadrangle west 

of Long Level. The. Conestoga limestone is bordered on the south 

by the albite-chlorite schist facies of the Wissahickon formation all 

the way from the eastern edge of the Quarryville quadrangle westward to Susquehanna River at Shenks Ferry. From Shenks Ferry 

north to Safe Harbor it adjoins on the south in turn the Vintage 

dolomite, Wissahickon schist, and Harpers schist. > 

Its northern limit lies north of this district. It underlies the J 

northern part of the valley north of the Mine Ridge upland from a ,'
PALEOZOIC SEDIMENTARY POCKIS - ;;

point south of .Kinzers through Strasburg to the Avest side .o|; Sus- 

.quehanna River. South of London Grove and Willow Street it occupies narrow valleys between areas of Cambrian schist and Vin.ta.ge 

dolomite. From the northwest side of the Mine Ridge upland s,outhAvard and eastward the Conestoga limestone is in contact Avith' the 

Antietam and Harpers schists of the Mine Ridge upland and,contains, in the valley north of Quarryville, hills of the same schists. 

The apparently anomalous relations it sustains to other formations, 

both pre-Cambrian and Cambrian, are explained beloAv in connection 

Avith the age of the Conestoga. . .

Lithology. The Conestoga limestone is made up of thin-bedded 

dark slaty limestone, coarse conglomerate or breccia of limestone and 

marble pebbles and fragments, thin-bedded blue crystalline limeStone, and thin dark graphitic slate. The limestone conglomerates 

occur at or near the base of the formation but are irregular in 

occurrence and thickness along the strike. They may occur in single 

beds or repeated many times through the lower part of the formation. The conglomerate is composed of both coarse and fine gray to 

Avhite marble pebbles and angular fragments of dolomitic and siliceous limestone in a blue slaty matrix. Some of the fragments measure as much as 5 feet across, but for the most part the conglomerate 

is made up of fragments a few inches to a foot in length. Although 

most of the marble fragments are light in a dark-blue matrix, in 

some of the conglomerate this condition is reversed dark pebbles 

occurring in a lighter-colored matrix.

The conglomerate may form the base of the formation but usually 

has beneath it a thin layer of black slate. In some places this underlying bed is a thick, hackly, blue-black slate Avhich replaces the conglomerate and other basal beds. The slate is usually banded and 

micaceous, Avith abundant pyrite, Avhich weathers to a rusty stain on' 

the blue slate.

The greater part of the Conestoga limestone is blue crystalline 

limestone Avith slaty partings. (See pi. 5, A.) At some places there 

are dolomitic beds and beds of light-gray limestone containing quartz 

in round glassy grains. This rock Aveathers to a buff porous siliceous 

skeleton by removal of the lime.

The conglomerate is well exposed at many places in the northern 

part of the Quarryville quadrangle in the Bellemont quarry, south 

of Vintage, at Lime Valley and north of it on Pequea Creek, where 

its outcrops are repeated many times by folding, also near Pequea 

Valley and Burnt Mills.

Southward from the northern edge of the district to the Mine 

Ridge anticline there is a gradual increase in the metamorphism of 

Paleozoic sediments and consequently of the Conestoga limestone.
56 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

South of Bunker Hill, Refton, and Marticville the Conestoga limestone is a coarsely crystalline marble with considerable brown mica 

developed on the partings and throughout the rock. It is closely 

folded and crumpled and converted to a crystalline schist, like the 

underlying Harpers and Antietam schists. West and south of the 

Mine Ridge upland the metamorphism and folding have been most 

intense, so that no individual beds of the Conestoga can be traced any 

distance and no sequence can be determined. Recrystallization has 

obliterated all trace of limestone conglomerate beds and produced a 

series of gray and blue marbles in which the slate beds have been 

altered to micaceous banded marble. (See pi. 5, B.) The mica in 

this marble is phlogopite. There are many good exposures of 

micaceous marble, highly folded or crumpled, in cuts of the Atglen 

and Susquehanna branch of the Pennsylvania Railroad north and 

east of Quarryville and for 2 miles east of Shenks Ferry. Here beds 

of marble and siliceous marble alternate with beds of micaceous 

calcareous schist. Cleavage diagonal to the bedding, which is marked 

in the micaceous layers, is absent in purer marble beds made up of 

calcite and without the platy mineral, mica.

Thickness. The thickness of the Conestoga limestone is not known 

but is probably several hundred feet. The beds are so variable from 

place to place that no consecutive section has been recognized for 

any distance. There is no place where a full section can be obtained, 

and the upper beds have been removed by erosion. -\

Utilisation. Many quarries have been opened in the Conestoga 

limestone, both in the limestone conglomerate and in the crystalline 

limestone. The largest quarries in the limestone are those west of 

Strasburg, just east of Strasburg, near Letort, east of Kockhill, and

near New Providence. There are quarries in the conglomerate at 

Bellemont, south of London Grove, south of Wheatland Mills, and 

both north and south of Lime Valley along Pequea Creek. Quarryville, as its name indicates, was the center of a considerable quarrying 

industry, and the marble taken out near by has been used for building 

stone in Quarryville, Lancaster, and vicinity, and burned for lime. 

/Surface form. The Conestoga limestone weathers readily because 

of its easy solubility and forms a valley country in contrast to that 

produced by the harder Cambrian and pre-Cambrian schists. Its 

slaty beds furnish a certain amount of resistance to solution, and the 

country underlain by the Conestoga is rolling valley land with low 

hills that rise higher than the valleys underlain by the less resistant 

Vintage dolomite. On weathering the Conestoga limestone of the 

northern part of the district produces a slaty soil to which the 

presence of numerous slate fragments gives a blue to black color. 

The crystalline limestone and marble in the belt south of Bunker
PALEOZOIC SEDIMENTABY BOOKS 57

Hill weather to a red micaceous soil more nearly resembling that 

derived from the Vintage dolomite.

Age and correlation. As yet the Conestoga limestone has yielded 

no fossils in the McCalls Ferry-Quarryville district. Small crinoid 

stem segments and a brachiopod, determined by Bassler as Strophomena stosei, were collected by Stose and Jonas from slaty limestones 

interbedded with conglomerate near the base of the formation at the 

York Lime & Stone Co.'s quarry, 6 miles northeast of York, Pa. 

Bassler 60 described this form from the Frederick limestone as of 

probable Chazy age. Hence the Conestoga limestone is of probable 

Chazy age and in part, at least, of the same age as the Frederick 

limestone of Maryland.

Walcott in his study of the limestones of the York-Lancaster Valley called these limestone conglomerates and associated beds probably 

Lower Cambrian. Before Ordovician fossils were found near York 

detailed study of the York-Lancaster Valley by Stose and Jonas 

brought out the facts that these conglomerates are part of a younger 

formation that unconformably overlies the older limestones and that 

the Conestoga limestone laps progressively southeastward from the 

Ledger dolomite, north of Lancaster, down to the Harpers schist, on 

the south flank of the Mine Kidge anticline.

Southeast of Lancaster the Conestoga overlaps lower and lower 

beds of the Ledger until near Leaman Place, just north of Vintage, 

it is nearly down on the Kinzers formation. At the Bellemont 

quarries, south of Vintage (see pi. 4, #), limestone conglomerate and 

associated slaty limestone of the Conestoga rest on Vintage dolomite, 

and the unconformity can t$ seen between the basal slate of the Conestoga and the beveled edges of the underlying beds of Vintage dolomite. The Conestoga overlies the Vintage dolomite throughout the 

northern part of the McCalls Ferry-Quarryville district. Farther

south, on the north flank of the Mine Kidge anticline and south of 

Refton and Marticville, the Conestoga almost completely overlaps 

the Vintage dolomite. On the south flank of the Mine Ridge anticline and in areas north of the Martic Hills west to Prospect Hill 

School the Conestoga rests on Antietam and Harpers schists. The 

Conestoga limestone in the Norristown quadrangle is said to overlie 

Beekmantown limestone. The Frederick limestone luiconformably 

overlies Beekmantown limestone, hence the age of the unconformity 

is post-Beekmantown.

It has been suggested 81 that the Cocalico shale is the northern representative of the Conestoga limestone. When graptolites were first

80 Bassler, E. S., Maryland Geol. Survey, Cambrian and Ordovician, p. 117, 1919.

81 Stose, G. W., and Jonas,, A. I., OrdoTician overlap In the Piedmont province of Pennsylvania and Maryland: Geol. Soc. America Bull., vol. 34, p. 520, 1923.
58 McCALLS FEREY-QUABRYVILLE DISTRICT, PA.

found in the shale, in 1921-1923, they were considered by Ulrich to 

be of Normanskill type and the shale probably of Chazyan age and 

older than the Martinsburg shale, of Trenton and Utica age. Further study by Ulrich of the graptolites found in the Cocalico shale has 

thrown doubt on the earlier identification of these poorly preserved 

fossils. He now believes that they are probably lower Trenton and 

that the Cocalico shale is probably equivalent to the Martinsburg 

shale of the main belt and outlying areas in region north of the 

Triassic rocks. 52 The Cocalico shale occurs only north of an axis 

of uplift represented by the Honeybrook-Welsh Mountain upland 

and the Chickies and Hellam. Hills, and the Conestoga limestone 

occurs only south of this1 axis. North of Lancaster the Conestoga 

limestone and the Cocalico shale are only 5 miles apart across the 

strike, and the uplift which separates them is structurally low at this 

point. The suggestion of the probable equivalence of the shale and 

argillaceous limestone was a natural one in view of the fact that both

were considered to be of Chazy age.

The Cocalico shale is now considered to be younger than Chazy 

on fossil evidence. It unconformably overlies limestones of lower 

Trenton and upper Beekmantown age, and therefore was deposited 

after a period of uplift and erosion which was later than the limestone of lower Trenton age. If the fossils' of £he Conestoga limestone 

are correctly interpreted and that limestone is Chazyan, its deposition followed an older period of uplift and erosion, the post-Beekmantown. It is possible, therefore, that the uplift represented by 

the Honeybrook upland and Hellam Hills acted as a partial or 

complete barrier between the two basins whose erosional and depositional history after Beekmantown time was'so different.

IGNEOUS ROCKS

The pre-Cambrian sedimentary core of the Mine Kidge upland 

is intimately penetrated by igneous rocks that range in composition 

from amphibolite to diorite and granite. Owing to the paucity and 

weathered condition of the outcrops it is difficult to establish the 

structural relations of these rocks, and, as in studying the stratigraphy of the Baltimore gneiss, the geologist, in interpreting the 

sequence of igneous intrusion, is forced to rely almost entirely upon 

a few railroad cuts. Mapping of the underlying rock in the highly 

cultivated farm country that intervenes between stream valleys becomes almost entirely a matter of the successful delimitation of areas 

characterized by float of varying composition. The probable 

sequence of igneous intrusion may be outlined as follows:

B2 Stose, G. W., and Jonas, A. I., Ordovician shale and associated lava in southeastern 

Pennsylvania: Geol. Soc. America. Bull.; vol. 38. pp. 505^536, 1927.
IGNEOUS ROCKS 59

1. The intrusion of basic sills or the extrusion of flows in the early 

pre-Cambrian sedimentary series. These sills or flows, which were 

later metamorphosed to amphibolite schist, probably belong to an 

earlier period of igneous activity than the post-Glenarm gabbro, 

which covers so wide an area in southern Pennsylvania, Delaware, 

and Maryland.

2. The intrusion of gabbro, probably the equivalent of the postGlenarm gabbroic invasions.

3. The intrusion of diorite, probably associated with granodiorite 

or quartz monzonite, which forms a widespread invasion of the sedimentary and igneous complex of the Mine Kidge upland, accompanied by pegmatitization, tourmalinization, and an intimate injection along bedding or cleavage planes in the interlayered sedimentary 

and hornblende schists. The pre-Cambrian or post-Cambrian age 

of this granodiorite, which has been highly deformed subsequent to 

consolidation, is a matter of doubt.

4. The intrusion of granite cut by dikelets of pegmatite, quartz, 

and ore-bearing solutions, all strongly tourmalinized.

5. Diabase intrusion of Triassic age.

PRE-CAMBRIAN IGNEOUS ROCKS

In the Mine Ridge anticline the biotitic amphibolite schists are 

interlaminated with the pre-Cambrian sediments and are not separated on the map from the Baltimore gneiss. They are completely 

recrystallized and strongly foliated, the planes of foliation being 

parallel to the bedding of the inclosing mica schists. The dominant 

constituents are hornblende, biotite, quartz, and plagioclase. Epidote 

(in some places surrounding allanite), clinozoisite, and garnet occur 

in considerable quantities, and leucoxene is abundant. Some biotite 

plates show distinct pleochroic haloes. Interlayered with the sedimentary beds are a few bands of a light-colored rock that is characterized by actinolite instead of hornblende, in association with biotite.

Complete recrystallization of the original constituents makes it 

difficult to determine the origin of these amphibolite schists, but the 

mineral composition and the prevalence of leucoxene are strongly 

suggestive of derivation from a basic igneous rock rich in ilmenite.

METAGABBBO

No unmetamorphosed gabbro is exposed in Mine Ridge, because 

the igneous rocks together with the including sedimentary series have 

been profoundly altered. Biotite-hornblende gabbro or metagabbro 

occurs in a massive rock whose outcrops are few. This rock is best 

seen at the Gap nickel mine, where the metagabbro is a coarsely crys-
60 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

talline, dark-gray, and brilliantly lustrous rock, composed chiefly of 

green hornblende, with subordinate plagioclase and a considerable 

amount of brown biotite. Under the microscope the original igneous 

texture is apparent, although there remains no trace of the mineral 

from which the hornblende was derived, and it is possible that a large 

part of the hornblende is pyrogenetic. Kemp 53 has found remains 

of orthorhombic pyroxene in his study of the ore rock, which he calls 

amphibolite. Biotite, of a deep reddish-brown color, appears to be 

universally present, though showing a considerable range in amount. 

Both hornblende and biotite contain numerous large and distinct 

pleochroic haloes. The radioactive minerals that form the cores of 

these haloes are zircon and a deeply colored prismatic mineral, possibly allanite. Titaniferous magnetite or ilmenite is of common 

occurrence in places surrounded by rims of titanite. The abundance 

of large apatite crystals is noteworthy. Chalcopyrite is a common 

accessory.

AGE OF BIOTITIC AMPHIBOLITE SCHIST AND METAGABBRO

A strongly foliated variety of fine-grained micaceous amphibolite 

schist found upon the dump at the Gap nickel mine is composed of 

hornblende, biotite, plagioclase (a saussuritic albite), some quartz, 

and both iron oxides and iron sulphides. This amphibolite schist 

may represent a sheared border of the gabbro that has yielded to 

dynamic action more completely than the central portion. On the 

other hand, it may be a layer in the inclosing sedimentary series and 

thus older than the cross-cutting gabbro rock. The similarity between the biotitic amphibolite interlaminated with the ancient preCambrian sediments and the amphibolite at the Gap nickel mine

would suggest that the two rocks are of the same age, and there is no 

evidence from field relations to show whether the amphibolite specimen from the abandoned nickel mine is really a part of the intrusive 

stock or a part of the intruded series. But along the Philadelphia & 

Heading Railroad north of Coatesville, where there are good outcrops of amphibolite schists, interlaminated with the ancient preCambrian sediments, the amphibolites appear to be an integral part 

of the intensely metamorphosed complex and older than the intrusions of massive metagabbro on the Siousca Road about a quarter of 

a mile farther north. The massive metagabbro is in every respect 

identical with the gabbro at the nickel mine and is undoubtedly of 

the same age.

In the Reading and Durham Hills there is a similar suggestion of 

two ages of gabbroic intrusion, the older represented by thin layers

53 Kemp, J. P., The nickel mine at Lancaster Gap, Pa.: Am. Inst. Min. Eng. Trans., 

vol. 24, p. 626, 1895.
; .IGNEOUS ROCKS 01

of highly foliated biotitic amphibolites or hornblende schists, intimately interleaved with the sedimentary schists, and the younger represented by the slightly deformed hornblende norites and hornblende 

gabbros. But even in a comparatively detailed study of the area! 

geology of these hills that was made by the writers in 1912 to 1914 

it was not possible to delimit the two types or to find a place where 

the massive rock cuts the amphibolite schists. Therefore until extremely detailed field work is done it is impossible to do more than 

state that a basic rock occurs in the pre-Cambrian sediments in cross-' 

cutting bodies and in thin layers, which have intimately interleaved 

or interpenetrated the sedimentary layers. In these thin layers, 

which may have originated as sills or as basaltic flows, fine-grained 

amphibolite schists have resulted. The intense shearing of these 

schists is not in itself indicative of older or more intense metamorphism, for the massive metagabbro occupies the center of larger 

igneous masses where the bulk of the intrusion has somewhat protected the rock from shearing action. Fine-grained amphibolite 

schists occur also in association with gabbro intrusions, where they 

probably represent peripheral zones in which the rock has yielded 

readily to dynamic metamorphism. The sills or flows may be of the 

same age as the schists associated with the gabbro or they may 

belong to different periods of igneous activity. The gabbro in 

Mine Ridge is in every respect lithologically identical with the widespread intrusive bodies of post-Glenarm gabbro in the southeastern 

Piedmont of Pennsylvania an,d Maryland and is probably part of the 

same igneous intrusion.

SERPENTINE

A few pieces of float comprising serpentine and talcose schists were 

found in the Mine Ridge anticline at isolated localities. As no contacts between serpentine and the inclosing rock could be found it was 

impossible to determine the structural relations of this serpentine, 

although its association with pre-Cambrian schists at a locality 1 

mile north of Christiana, on the Lancaster-Christiana trolley line, 

suggests that it is probably intrusive in the pre-Cambrian. In York 

County, about 2 miles northwest of Peach Bottom, fragments of serpentine and talc schist denote the existence of a pyroxenite or peridotite surrounded by schist. The areal distribution of the serpentines 

of Maryland indicates that they are intrusive in the Wissahickon and 

Peters Creek formations and in the gabbro. It is probable, therefore, 

that the serpentines in the McCalls Ferry-Quarryville district are 

younger than the metagabbro in which the nickel deposit occurs.
62 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

AMPHIBOLITH AND OHLORITB SCHIST ALTERED VOLCANIC FLOWS

A mile and a quarter southwest of Woodbine, in York County, on 

Bald Eagle Creek, there is an isolated outcrop of a dense and massive 

dull-green rock speckled with lighter-colored areas. Under the 

'microscope this rock is seen to be a garnetiferous amphibolite, in 

which the large garnets are surrounded by rims of chlorite. The 

rock is doubtless derived from the deep-seated metamorphism of a 

diabase or basalt. Subsequent dynamic metamorphism at higher 

tectonic horizons has crushed the garnet and produced a reversal of 

metamorphism indicated by the alteration of the garnet to chlorite. 

There are no visible relations between the amphibolite and the surrounding rock.

A similar occurrence is found in Chester County half a mile west 

of Tweedale, on the north bank of Tweed Creek near its junction 

with the East Branch of Octoraro Creek, where a dense green schist 

with light-colored lenses and streaks is in contact with the quartzose 

beds of the Peters Creek schist. The contact, which is obscure; suggests a crosscutting dike in the Peters Creek schist, but the greenstone schist under the microscope suggests an altered basic lava by 

the presence of chlorite, epidote, quartz, and plagioclase in a fine 

groundmass together with lenticular areas of calcite and epidote and 

quartz that may be amygdules. The rock contains a large amount of 

ilmenite altering to leucoxene and ilmenite intergrown with pyrite.

At a third locality, in Lancaster County near the mouth of Fishing Creek, green schist crops out in ledges of sharply inclined rock 

infolded in Peters Creek schist. The mineral composition of the 

green schist, calcite, epidote, and chlorite, with some albite, magnetite, and actinolite, is suggestive of tuffaceous layers interbedded 

with the Peters Creek sediments. It may represent a calcareous bed 

in the original sediment, but the absence of similar beds elsewhere 

in the Peters Creek schist makes the sedimentary origin of the green 

schist somewhat improbable.

The three outcrops described above show no field relations to the 

surrounding rock but suggest in their composition and manner of 

occurrence that they may belong to the altered lava flows that are 

more extensively developed in the Glenarm series farther southwest.54

IGNEOUS ROCKS OF UNDETERMINED PRE-TRIASSIC AGE 

QUARTZ MONZONITE

Throughout the Mine Kidge anticline there are fragments of a 

light-colored rock of granitic texture, composed of quartz, feldspar, 

and a variable quantity of biotite that imparts an irregular gneissic

64 Jonas, A. I., Pre-Cambrian rocks of western Piedmont of Maryland: Geol. Soc. 

America Bull., vol. 35, pp. 355-384, 1924.
IGNEOUS ROCKS 63

banding to the rock where present in considerable amount. The 

rock usually appears in isolated occurrences as stray boulders, in the 

meadows or along roadsides, but in a few places, such as in the Pennsylvania Railroad cut north of Christiana, it forms layers of variable 

thickness in the amphibolites. In the cuts of the Philadelphia & 

Reading Railroad about 1 mile north of Coatesville it cuts across 

the'schistosity of. the.pre-Cambrian schist, and penetrates^tfrat^ roqk 

in stringers, forming an undoubted igneous contact with the schist 

of sedimentary origin.

In many hand specimens it is possible to recognize the repeated 

twinning striation of a plagioclase feldspar, and under the microscope the constituents are seen to be albite, or albite and microcline, 

quartz, and more or less biotite. Some specimens are practically 

lacking in dark-colored constituents and might be termed alaskite.

In other places there is as much as 25 per cent biotite and considerable muscovite. The rock everywhere shows evidence of strong 

mechanical deformation in the granulation of the quartz and in the 

numerous inclusions of zoisite, epidote, and quartz with which the 

albite individuals are crowded. The association of calcic minerals 

with albite points to the breaking down under dynamic action of a 

plagioclase that was originally more calcic than albite and the 

secondary formation of the albite. The variable percentage of 

microcline suggests that the original rock ranged in composition 

from a quartz monzonite to a quartz diorite.

A peculiar light-colored gneiss found upon the dump of the Gap 

nickel mine showed a pronounced development of " schachbrett" or 

chessboard albite, together with a high percentage of normal albite 

and some microcline. The occurrence in one or two other specimens 

of albite that shows indication of a " schachbrett" texture suggests 

that the microcline in some of the quartz monzonite has been replaced by albite.

QUARTZ MONZONITE PEGMATITE AND APLITE

In several localities, notably in the Pennsylvania Railroad cut north 

of Christiana, the quartz monzonite is cut by satellitic pegmatites 

that have been strongly deformed. The albite crystals in these 

pegmatites are filled with inclusions that indicate saussuritization 

of a calcic plagioclase. Like the magma with which they are genetically associated, the pegmatites contain varying amounts of microcline together with albite. The relations of these monzonitic rocks 

to the sedimentary and igneous complex and to the gabbro is shown 

in the Pennsylvania Railroad cuts north of Christiana and along 

the Philadelphia & Reading Railroad north of Coatesville, where 

they intrude both the gabbro and the sedimentary and igneous complex. The massive gabbro shows lenses, stringers, and dikelets of
64 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

pegmatitic and aplitic material. In the foliated rocks the highly 

fluid magma has spread out in stringers that follow the foliation 

planes and form typical banded injection gneisses in the biotite 

schists and in the biotitic amphibolites. Large outcrops of these 

banded gneisses show the characteristics of lit-par-lit intrusion in 

the lenticular areas of granitic material that fray out on the ends of 

 the lenses into the including schist, in the pinching 1 and swelling of 

the granitic bands, and in the ptygmatic folding?? of the invading 

rock. In some places stringers, parallel to the foliation, can be 

traced into crosscutting dikelets. The evidence indicates, therefore, 

that after the invasion of the old complex by massive gabbro there 

was a batholithic intrusion of dioritic magma, whose highly fluid 

upper portion invaded both the massive rock and the foliates where 

well-established planes of schistosity offered easy pathways for 

intrusion.

QTTABTZ DIOBITE

A couple of isolated occurrences on the Mine Eidge upland show 

boulders of medium-grained light-gray rock of granitic texture, with 

a poorly defined gneissic banding. When examined under the microscope the rock shows considerable alteration. The original feldspar 

appears to have been chiefly oligoclase-andesine with so small an 

amount of potash feldspar that the rock may be termed a quartz 

diorite. Biotite is abundant, in some flakes accompanied by syngenetic chlorite. The biotite shows pleochroic haloes. A variable 

amount of quartz is present together with epidote, zoisite, and 

titanite.

The relation of the quartz diorite to the other igneous rocks and to 

the sediments is nowhere clearly shown, but at one place the diorite 

is intruded by a strongly deformed albitic pegmatite. It is highly 

probable that the quartz diorite is consanguineous with the quartz 

monzonite described above. It is impossible to separate these two 

rocks upon the areal map,

GRAN1TIO PEGMATITE AND GRANITE

There are no outcrops of true granite in the McCalls Ferry-Quarryville .district, but in the northeastward extension of the Mine Ridge 

uplift about iy2 miles north of Downingtown on the New Holland 

branch of the Pennsylvania Railroad, there is exposed a pink microcline granite with a notable content of deep-blue quartz. This granite cuts a green hornblende-quartz diorite. Some of the quartz in 

the diorite shows a striking blue color, and the diorite is filled with

«««Ptygmatic folding" (from irriiyna, fold) Is a term Introduced by J. J. Sederholm, 

who defined it as folding " due to undulatory movements in a semimolten medium" 

(Comm. sr6ol. Finlande Bull. 58, p. 85, 1923).
IGNEOUS BOOKS 65

blue quartz veins. North of Coatesville, on the branch of the Philadelphia & Reading Railroad that parallels the West Branch of 

Brandy wine Creek, massive gabbro and hornblende schists, injected 

.with monzonitic aplites, are cut by veins of blue quartz that are 

folded together with the including rocks. It is possible that the blue 

;quartz in the diorites and the blue quartz in the granites are integral 

parts of two different igneous intrusions, or it is possible that the 

blue quartz, which is better developed in the granite than in the 

diorite, is genetically related to the granite and is of secondary 

origin in the diorite. , Although granite outcrops are Jacking in the west end of the Mine 

.Ridge anticline, the abundant tourmalinization and pegmatitization 

throughout the flanking Paleozoic sediments is an evidence of underlying granite. In two localities, one about half a mile north of 

Atglen and the other half a mile north of Greentree, a pink microcline pegmatite heavily laden with magnetite and tourmaline cuts 

through the basal pebbly schistose beds of the Hellam conglomerate 

member of the Chickies quartzite. This pegmatite is accompanied 

by a large amount of pure quartz, and the quartz is loaded with 

tourmaline crystals. About 140 feet higher in the section tourmaline is abundantly developed along the bedding planes of white 

Chickies quartzite, interbedded with sericite schist carrying hematite 

crystals. One mile northeast of Simmonstown an albite-microcline 

pegmatite cuts across the bedding of basal Cambrian pebbly sericite 

schist.

AGE OF GRANITE AND QUARTZ MONZONITE

There are in the Mine Ridge anticline occurrences of igneous rocks 

that range in composition from quartz diorite to granite. The relations of the granite and quartz monzonite have not been determined, 

as the writers have nowhere seen them in contact, unless the hornblende-quartz diorite north of Downingtown is a part of the same 

intrusion as the quartz monzonite, in which case the granite is younger 

than the quartz monzonite. So far as known, with one possible 

exception, the outcrops of granite and quartz monzonite are confined 

to the pre-Cambrian sedimentary series, although pegmatites have 

been found in several places cutting Cambrian schists. The exception 

mentioned is in a cut where the Pennsylvania Railroad crosses the 

flanking sediments of the Mine Ridge anticline between Christiana 

and Atglen. Here a fine-grained rock of granitic texture occurs in 

the greenish sericite schist, with pebbly beds, that forms the base of 

the Paleozoic section. However, the outcrop is so much disintegrated 

that it is not possible to prove an intrusive contact between the 

Cambrian sediments and the granitic rock.
66 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

Nevertheless the universal distribution of epigenetic tourmaline 

throughout the Chickies quartzite, far too much to be merely recrystallized clastic tourmaline, and the presence in the Lower Cambrian schists of the magnetite-beating pegmatites described above, 

together with the common occurrence of hematitic quartz, give conclusive evidence that the Cambrian sediments have been permeated 

by hot solutions and fumarolic vapors. It is, of course, possible, but 

it seems improbable that the pegmatites and ore-bearing solutions 

that penetrate the Paleozoic rocks are the only evidence at the surface of this cycle of igneous activity. If we search for evidence of 

the parent magma among the igneous rocks that have been discovered 

in the pre-Cambrian sediments there are three possibilities. The 

post-Cambrian magma may have been the source of the quartz monzonite or of the granite or of both. If it was the source of both, the 

granite would be consanguineous with the quartz diorite and quartz 

monzonite, although probably intruded during a welling up from the 

magmatic reservoir that succeeded the consolidation of the dioritic 

portion of the magma, as suggested by the lack of mechanical deformation in the granite contrasted with the quartz monzonite.

Support of the hypothesis that granite and quartz monzonite are 

derived from the same post-Cambrian magma is furnished by the composition of the pegmatites that cut the Lower Cambrian beds and the 

association of pegmatites of similar composition with both granite 

and quartz monzonite facts that point to the derivation of granite, 

quartz monzonite, and pegmatite from a common magma. The fact 

that igneous rock of granitic texture has not been discovered in Paleozoic sediments except at one place, where the igneous contact could 

not be proved, may possibly be due to the lack of good exposures of 

the Lower Cambrian. It may also be accounted for by the hypothesis 

that the magma from which the quartz diorite, quartz monzonite, and 

granite have been differentiated has either invaded and largely swallowed up the pre-Cambrian sedimentary core of the Mine Kidge 

anticline or has been intruded as a concordant batholith whose 

upper limit is the. plane of unconformity that marks the boundary 

between pre-Cambrian and Cambrian sediments. The highly argillaceous sediments of the upper beds of the pre-Cambrian would 

yield readily to injection by the more fluid portion of the invading 

batholith, but the more resistant quartzose beds of the Lower Cambrian would yield only to the permeation of the vapors and superheated solutions that emanated from the underlying batholith. That 

some highly active vapors derived from, an underlying batholith penetrated upward for several thousand feet is shown by the presence 

of tourmaline nests in several places in the dolomites and limestones 

that overlie the Lower Cambrian arenaceous sediments. Such tour-
IGNEOUS ROCKS 67

maline nests have been found as far up as the Conestoga limestone, 

of probable Chazy age, thereby indicating that the igneous intrusions 

were post-Ordovician.

The hypothesis that the granite is post-Cambrian is somewhat 

shaken by the association of the granite with blue quartz, that seems 

to be the logical source for the blue quartz pebbles in the Cambrian 

conglomerate overlying the granite. Moreover, in the Reading and 

Durham Hills a granite that may be identical with the granite 

north of Downingtown furnishes material for the immediately overlying arkosic beds of the Lower Cambrian. However, the argument from the blue quartz is not conclusive, because the presence of 

blue quartz pebbles in both upper Glenarm and Paleozoic sediments 

shows that the blue quartz was a constituent of pre-Glenarm igneous 

rocks as well as of the post-Glenarm granites and thus was not confined to one period of igneous intrusion. If the quartz monzonite 

belongs to an earlier intrusion than the granite, the blue quartz associated with the quartz monzonite may be an original constituent of the 

diorite magma, and this may have furnished the quartz for the 

Paleozoic sediments.

In the absence of conclusive field evidence it is only possible to 

say that granite and quartz monzonite may well be differentiates of 

the same magma and that this parent magma may have been intruded 

in pre^Paleozoic time because of the absence of granite rocks in Paleozoic sediments and because of the presence in basal Cambrian sediments of blue quartz that may have been derived from the granite 

quartz. On the other hand granite was intruded in Paleozoic time, 

as shown by the effects of highly active vapors and solutions in 

Paleozoic sediments. Whether the quartz monzonite and granite 

in Mine Eidge are the source of these Paleozoic pegmatites or whether 

they are an older pre-Cambrian intrusion is still an open question.

DIABASE

Several diabase dikes cross the country with parallel northeasterly 

trends. These dikes are narrow and usually do not crop out. They 

weather into a trail of boulders, by means of which they may be 

traced across the country. In the few outcrops in which the diabase 

of the dikes can be measured it nowhere exceeds 100 feet in thickness.

In contrast to the other igneous rocks of the district the diabase 

is notably fresh, the only evidence of alteration being a slightly 

undulose extinction in some places and a little clouding of the 

feldspar and the formation of a small amount of chlorite and delessite. The essential constituents are labradorite, augite, and olivine. 

6136 29 6
68 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

Several of the diabase dikes of this district can be traced into the 

Triassic sediments farther to the north, in Lancaster and Berks 

Counties.

CORRELATION OF ROCKS OF M5CALLS FERRYJQTTARRYVILLE DISTRICT WITH 

THOSE OF OTHER AREAS

The stratigraphic problems in the McCalls Ferry and Quarryville 

quadrangles were also recognized' in Maryland during the areal 

mapping of Baltimore County, and similar problems exist in the 

region northeast of the Quarryville quadrangle as far as Trenton, 

N. J., where the pre-Cambrian and Paleozoic rocks disappear under 

Triassic strata and the sediments of the Coastal Plain. The preCambrian rocks that lie east of the Triassic basin in the New York 

City area are lithologically identical with those of the QuarryvillePhiladelphia belt, and the stratigraphic problems involved in their 

mapping appear to be the same as those in the Pennsylvania region. 

(See fig. 4.) In recent years Berkey 56 has published some conclusions concerning the age and correlation of the pre-Cambrian 

rocks of the West Point quadrangle that make it possible to suggest 

with reasonable certainty a correlation between the pre-Cambrian 

section along Hudson Eiver south of Peekskill and that of southeastern Maryland and Pennsylvania. The probable stratigraphic equivalence is shown in the accompanying generalized correlation table.

The correlation of the pre-Cambrian rocks of the McCalls FerryQuarryville district with those of other areas of the Piedmont province involves (1) the relation of the metamorphosed pre-Cambriaii 

sediments of the Honeybrook and Mine Eidge uplands to the Bal*^

more gneiss of Maryland and southeastern Pennsylvania and to the 

pre-Cambrian gneisses of the Beading and Durham Hills, and (2) 

the relation of the Glenarm series to the early pre-Cambrian Baltimore gneiss and to the Paleozoic rocks that occur in Chester Valley 

and on the flanks of the Mine Eidge uplift.

Correlation of the Baltimore gneiss in Maryland and Pennsylvania 

with the fire-Cambrian schists of the Highlands of New, Jersey and 

New TorM. The pre-Cambrian core of the Mine Eidge upland in the 

Quarryville quadrangle and in the Honeybrook upland is correlated 

with the Baltimore gneiss of Maryland because the schists of the 

Mine Eidge anticline are similar both in stratigraphy and history to 

the rocks that form the core of the anticlinal uplift north of Avondale and Kennett. The rock that occurs in the center of the anticline

M Berkey, C. P., and Rice, Marion, Geology of the West Point quadrangle: New York 

State Mus. Bull. 225-226, pp. 129-140, 1921. Berkey, C. P., and Sanborn, J. F., Engineering geology of the Catskill water supply: Am. Soc. Civil Bng. Trans., vol. 86, p. 7, 

1923.
generalized correlation tuolnjo'f Ihe regwn between tiallimore and Hudson Kiver

System

Ordovician.

Cambrian.

Algonkian.

Archean.

Series

Upper Cambrian.

Middle Cambrian.

Lower Cambrian.

Glenarm.

Maryland - >/ ^

Baltimore County and Harford County

Knopf, E. B., and Jonas, A. I., Am. 

Jour. Sci., 5th ser., vol. 5, pp. 43-44, 

1923; Maryland Geol. Survey, Baltimore County (in press).

Woodstock granite and pegmatite 

(possibly Paleozoic).

Granite and granite pegmatite (possibly Paleozoic).0 

Port Deposit gneiss. 

Metagabbro and serpentine.

Peters Creek schist.

Wissahickon formation: 1 MetaAlbite-culorite schist. Vmorphic 

Oligoclase-mica schist.) facies.

Cockeysville marble.

Setters formation.

Unconformity.

Hartley augen gneiss (intrusive). 

Baltimore gneiss.

Carroll County

Jonas, A. I., Geol. Soc. America Bull., 

vol. 35, pp. 355-304, 1924.

Conestoga [Frederick] limestone. 

(Rests on Harpers phyllite.)

Unconformity.

[Antietam quartz schist.]

Harpers phyllite.

[Chickies formation.]

Unconformity.

Metagabbro and serpentine.

Peters Creek schist.

Wissahickon formation : 1 MetaAlbite-chlorite schist. >morphic 

Oligoclase-mica schist. | facies.

Metabasalt and tuffaceous slates 

iuterbedded %vith Cockeysville marble.

Setters formation.

Unconformity.

Baltimore gneiss.

Pennsylvania

McCalls Ferry-Quarryville district 

This report.

Granite pegmatite.

Conestoga limestone (lower part of 

Chazy? age). (Rests in places on 

Harpers schist.)

S

Unconformity.

3 g Ledger dolomite.

"5 S S Kinzers formation.

> P o

W Vintage dolomite.

Antietam schist.

Harpers schist.

Chickies quartzite, including Hellam 

conglomerate member at base.

Unconformity.

Granite (Paleozoic or older). 

Quartz monzonite and quartz diorite 

(Paleozoic or older). 

Metagabbro and serpentine.

Peach Bottom slate.

"Cardiff conglomerate.

Peters Creek schist.

Wissahickon formation: 1 MetaAlbite-chlorite schist. >morphic 

Oligoclase-mica schist.) facies.

Arnphibolite schist and chlorite 

schist (altered dikes and flows). 

Cockeysville marble.

[Not exposed.]

Baltimore gneiss (graphite schists and 

limestone lenses) in Mine Ridge 

upland.

Lancaster- York area

Stose, G. W., and Jonas, A. I., Washington 

A cad. Sci. Jour., vol. 12, pp. 358-369, 1922; 

Geol. Soc. America Bull., vol. 34, pp. 507- 

524, 1923; idem, vol. 38, pp. 505-536, 1927; 

Pennsylvania Top. and Geol. Survey Geol. 

Atlas, New Holland quadrangle, No. 179, 

pp. 1-15, 1926.

[Granite pegmatite.]

Cocalico shale (lower part 

contains fossils of lower? 

Trenton age). (Rests in 

places on Beekmantown 

Conestoga lime- limestone.)

in places on 

lower forma- Unconformity, 

tions down to

schist.)

Unconformity.

Beekmantown limestone.

Conococheague limestone.

Elbrook limestone.

Ledger dolomite.

Kinzers formation.

Vintage dolomite.

Antietam quartzite.

Harpers phyllite.

Chickies quartzite, including Hellam conglomerate member at base.

Unconformity.

[Granite (Paleozoic or older).] 

[Quartz monzonite and quartz diorite.] 

Gabbro (and anorthosite) in Welsh Mountain 

and Honeybrook upland.

[Absent.]

Metabasalt and aporhyolit'e in Hellam Hills 

and Pigeon Hills. (Cockeysville marble 

absent.)

[Absent.]

Baltimore gneiss (graphitic schists and limestone lenses) in Welsh Mountain and Honeybrook upland.

Trenton, Philadelphia, and Wilmington areas 

Bascom, F., U. S. Geol. Survey Geol. 

Atlas, Folios 167, 1909; 162, 1909; 211, 

1920. 

Bliss, E. F., and Jonas, A. I., U. S. 

Geol. Survey Prof. Paper 98, pp. 

9-34, 1917.

Octoraro schist [equivalent in part to 

Peters Creek schist; the rest to Wissahickon albite-chlorite schist].

Shenandoah limestone.

Chickies quartzite.

Unconformity.

Gabbro gneiss.' 

Granodiorite and granite gneiss.

Wissahickon mica gneiss. (Includes 

part of the Peters Creek schist.)

[Apparently represented in ElktonAVilmington area but mapped as 

Chickies quartzite and Shenandoah 

limestone.]

Unconformity.

Baltimore gneiss. 

Franklin limestone.

New Jersey

Northern New Jersey

Bayley, W. S., Kummel, H. B., and 

Salisburv, R. D., U. S. Geol. Survey 

Geol. Atlas, Folio 191, 1914.

Martinsburg shale ("equivalent of 

Trenton and Utica").

Jacksonburg limestone ("equivalent of 

Lowville, Black River, and lower 

Trenton").

Kittatinny limestone.

Hardyston quartzite.

Unconformity.

d Byram gneiss (predominantly 

 § granite gneiss). 

 g Losee gneiss (predominantly dio- 

| rite gneiss). 

O Pochuck gneiss (probably comi prising an older amphibolite 

£ and a younger metagabbro).

[Absent.]

Franklin limestone and graphitic 

schists.

New York

New York City and Westchester County

Berkey, C. P., and Rjce, Marion, New 

York State Mus. Bull. 225-226, 1921.

[Apparently not represented, although 

some authors place Manhattan schist, 

Inwood limestone, and Lowerre quartzite here.]

Basic dikes. 

Pegmatites. 

Yonkers gneissoid granite. 

Pegmatites. 

Ravenswood granodiorite. 

Injection granites and pegmatites. 

Fordham gneiss (in part injection gneiss). 

Occasional basic injections. 

Serpentines. 

Hornblende schists.

Manhattan schist.

Tnwood limestone.

Lowerre quartzite.

Not recognized.

Interbedded limestone and schist. 

Fordham gneiss (in part) (paragneiss).

West Point quadrangle

Berkey, C. P., and Rice, Marion, New York State 

Mus. Bull. 225-226,- 1921; Fettke, C. R., New York 

Acad. Sci. Annals, vol. 23, pp. 193-2CO, 1914.

Hudson River formation (Trenton to Cincinnatian 

age).

Wappinger limestone (Lower Cambrian to Trenton 

age).

Poughquag quartzite.

Unconformity.

Basic dikes. 

Pegmatites. 

Storm King granite. 

Pegmatite. 

Reservoir granite. 

Canada Hill granite and associated injection gneiss. 

Pegmatites and magnetites. 

Occasional basic injections.

Peekskill diorite injection gneiss. 

Pochuck gneiss (possibly equivalent of hornblende 

schist layers in Baltimore gneiss).

Manhattan schist.

Inwood limestone.

Lowerre quartzite. ^\ "

Grenville metamorphics=mica schists, metaquartzites, metalimestones (Sprout Brook limestone). 

Paragneisses (Fordham gneiss).

f

Cuts the Port Deposit gneiss in Harford County, Md. » North of the Elkton and Wilmington quadrangles a granite is reported to cut the gabbro (Hawkins, A. C., Am. Jour. Sci., 5th ser., vol. 7, p. 360, 1924).

613G 29. (Pace p. 6S.)
CORRELATION" 69

that extends from Avondale to Chadds Ford is known to be Baltimore gneiss, both because it is in line of strike with certain anticlines 

in Maryland that bring the Baltimore gneiss to the surface and because it underlies later pre-Cambrian strata.

The Baltimore gneiss of the Mine Ridge and Honeybrook uplands, 

together with the interbedded limestones, is correlated with the preFIODRB 4. Sketch map showing principal pre-Cambrian areas of the Piedmont 

province and Appalachian Mountains in part of Maryland, Pennsylvania, and New 

Jersey. 1, McCalls Ferry and QuarryvJ'ie quadrangles; 2, Elkton and Wllmington quadrangles (U. S. Geol. Survey Geol. Atlas, Folio 211) ; 3, Tolchester quadrangle (Folio 204) ; 4, Washington quadrangle (Folio 70) ; 5, Philadelphia district 

(Folio 162) ; 6, Trenton quadrangle (Folio 167) ; 7, Raritan quadrangle (Folio 

191) ; 8, Passaic quadrangle (Folio 157) ; 9, Franklin Furnace quadrangle 

(Folio 161) ; 10, New York City district (Folio 83) ; 11, West Point quadrangle 

(New York State Mus. Bull. 225-226)

Cambrian graphitic schists and limestones of the Eeading and Durham Hills, which are lit,hologically similar to the rock in the center 

of the Mine Ridge anticline. Moreover, the pre-Cambrian rocks of 

the Reading and Durham Hills occupy the same, stratigraphic position as the pre-Cambrian core of the Mine Ridge upland in that the}7 

unconformably underlie the Paleozoic rocks and are cut by a series 

of petrographically related igneous rocks that are presumably of
70 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

late pre-Cambrian age. The old pre-Cambrian sedimentary gneisses 

and associated intrusive rocks of the Reading Hills continue northeastward through the Highlands of New Jersey into the Highlands 

of New York, which cross Hudson River north of Peekskill. These 

old pre-Cambrian rocks are the so-called Grenville metamorphics of 

Berkey's section, but the relation of this pre-Cambrian schist in the 

Highlands to the so-called Grenville. schists'of the Adirondacks and 

to the Grenville series of the type locality in Ontario is not yet 

definitely established.

Correlation of the Glenarm series ancD the Manhattan, Inwood, and 

Lowerre formations of New York. The Manhattan, Inwood, and 

Lowerre formations are restricted in their occurrence to the New 

York City area south of the Highlands. They afle considered by 

Berkey to be of Grenville age and to be part of a conformable section whose base is the Fordham gneiss, which he correlates with the 

banded Grenville gneisses of the Highlands. There is a strong 

similarity in lithology and thickness between the Glenarm series and 

the Manhattan, Inwood, and Lowerre formations, although the 

Cardiff conglomerate and Peach Bottom slate which have been provisionally placed in the Glenarm series, are apparently absent in the 

New York section. The rocks of the Glenarm series and underlying 

Baltimore gneiss in Pennsylvania have been driven northward from 

their original position by an overthrust fault, and they are absent 

from the stratigraphic succession in the Reading and Durham Hills, 

the Honeybrook and Mine Ridge uplands. In New York the crystalline schists of the Manhattan-Inwood-Lowerre succession come to 

an abrupt stop on the southern margin of the Highlands, just as the

Glenarm schists cease on the. south side of Chester Valley in Pennsylvania, nowhere reappearing to the north. There is thus a considerable ground for the belief that the Manhattan-Inwood-Lowerre 

succession may prove to be the northeastward continuation of the 

Glenarm series thrust northward over the southern rocks of the 

Highlands.

Apparently the unconformity that marks the base of the Glenarm 

series in Maryland has not been recognized in New York, for, according to Berkey, there is no unconformity between the Lowerre 

quartzite, where present, and the underlying Fordham gneiss, or between the Inwood limestone, where the Lowerre is absent, and the 

Fordham gneiss. However, Berkey 57 makes the point that a selective granitization has invaded and in many places completely transformed the strata underlying the Inwood limestone, while the limestone appears to have shown a remarkable resistance to the action of 

igneous magma. Such granitization may have obscured the relation

67 Berkey, C. P., op. cit., p. 136.
STRUCTURE 71

between the Lowerre and the underlying Fordham gneiss, so that 

an unconformity between the Lowerre and the Fordham is unrecognizable, or, as suggested by Berkey, the Fordham gneiss may be 

really the downward continuation of the Manhattan-Inwood succession where the Lowerre is absent. However, in view of the fact that 

a comprehensive injection of both the,upper beds of the Fordham 

and the lower beds of the Manhattan-Inwood-Lowerre succession 

would easily obliterate evidence of an unconformity, it seems best 

at present to correlate the Fordham gneiss with the Baltimore gneiss 

and the Manhattan, Inwood, and Lowerre formations with the Glenarm series.

c STRUCTURE

The McCalls Ferry-Quarryville district is traversed by the southern part of a compound syncline, a major anticline, a major syncline, 

an overthrust fault of great magnitude, and normal faults that cross 

the northern part of the district.

The major anticline is the Mine Kidge anticline, which with its 

spurs forms a belt 4 to 7 miles wide, extending from the northeast 

corner of the district southwestward nearly to Susquehanna River. 

This anticline lies south of a more gently folded synclinal valley 

which is the southern part of the valley area of the New Holland 

and Lancaster quadrangles. The axis of the Mine Ridge anticline 

crosses the southern upland as the Tucquan anticline. The Mine 

Ridge anticline and i|s southwestern continuation, the Tucquan 

anticline, lie northwest of a wide syncline, the Peach Bottom syncline, 

so called from the youngest rock infolded in it, the Peach Bottom 

slate.

The district is traversed by a low-angle overthrust fault called the 

Martic overthrust, which has carried the. pre-Cambrian rocks of the 

southeastern part over the Paleozoic rocks of the northern and northwestern part. Subsequent normal faulting in the northern part of 

the district has broken into blocks the Paleozoic rocks and the overlying overthrust mass. The relation of these structural features to 

the structure of a larger area may be seen in Figure 5.

FOLDS

The northern part of the district is occupied by a syncline which 

extends from a point just east of Gap through Strasburg, New Danville, and Washingtonboro and across the Susquehanna River north 

of Long Level. It is underlain by the Conestoga limestone and 

contains, in the eastern part, anticlines which bring to the surface 

older Paleozoic rocks. This syncline is the southern part of the 

synclinal valley area which lies south of the Welsh Mountain and
72 McCALLS FERRY-QUABRYVILLE DISTRICT, PA.

Chestnut Hill anticlines of the New Holland and Lancaster quadrangles and north of the Mine Ridge anticline and its outlying folds. 

The Mine Ridge anticline is the southwestern spur of the broad 

Honeybrook upland, which is northeast of the Quarryville quadrangle. The anticline is a double fold which crosses the northern

part of the Quarry ville quadrangle in a direction S. 75° W. It 

exposes a core of pre-Cambrian rocks with overlying Lower Cambrian 

rocks on its flanks, dipping 75° S. on the southeast side and more 

gently northwest on the northwest side. North and northwest of 

the Mine Ridge anticline there are a series of anticlinal folds which 

expose Antietam and Harpers schists in the center and overlying 

limestones on the flanks. Bunker Hill and Oak Hill are formed
STRUCTURE 73

by these anticlines, which extend S. 80° W. to Safe Harbor, where 

their west ends are beveled by overthrust Wissahickon albite-chlorite 

schist. The Shenks Ferry syncline lies south of these anticlinal hills 

from Susquehanna River to Oak Hill.

The Lower Cambrian quartzites and schists on the southwestern 

nose of the Mine Ridge anticline plunge steeply under Conestoga 

limestone of the Quarryville Valley, which in turn is overlain by 

overthrust Wissahickon schist. The folding of the Mine Ridge 

anticline is continued southwestward in the Wissahickon albite 

schist, where it is known as the Tucquan anticline. The southwestern continuation of the southern crest of the Mine Ridge anticline crosses the Susquehanna at Tucquan and extends southwestward 

through York County into Maryland. There it has been recognized 

as far as Woodbine, on the Carroll-Howard County line. The 

northern spur of the anticline crosses the river north of Pequea and 

south of Martic Forge, and it also extends southwestward into Maryland. At a point near Chanceford, York County, it brings to the 

surface a small area of Cockeysville marble, the formation which 

normally underlies the Wissahickon schist.

Railroad cuts along the left bank of Susquehanna River afford 

excellent opportunities for studying the structure of the overthrust 

rocks, continuously from Creswell Station for 20 miles to the southern edge of the district except for three-fourths of a mile near 

Shenks Ferry Station, where rocks of the autochthonous block cross 

the river. The major folds in the Wissahickon albite schist the 

Tucquan and Pequea anticlines are open folds, not overturned, with 

dips of 20° to 40°. The minor folds, however, are much compressed 

and the rocks are finely crumpled with quartz injections along the 

bedding. From Shenks Ferry to a point north of Safe Harbor the 

minor folds are recumbent toward the southeast. A cleavage more or 

less steeply inclined to the bedding occurs throughout the overthrust and autochthonous beds. The Paleozoic rocks of the district 

are closely folded, but the folds are for the most part not overturned. 

The less resistant of them are finely crumpled, and this fine crumpling is especially well developed in the Harpers and Antietam schists 

and the less massive beds of the Conestoga limestone.

The Peach Bottom syncline lies south of the Mine Ridge-Tucquan 

anticline and trends S. 50° W. across the southeastern part of the 

area. This syncline is occupied for the most part by the Peters 

Creek formation, which has an average width of outcrop of 5 to 8 

miles. Cardiff conglomerate and Peach Bottom slate occupy the 

deepest part of the fold from Kings Bridge to Susquehanna River 

and extend southwestward as far as Pylesville, Md., 3^ miles 

southwest of Delta and Cardiff, at the Pennsylvania-Maryland line.
74 McCALLS FERRY-QTJARRYVILLE DISTRICT, PA.

In the area northeast of Susquehanna River the syncline is overturned toward the northwest, and the beds on both limbs dip 

50° to 60° SE. The dip of the cleavage also is southeast. The overturning was accompanied by a break on the northern limb of the 

fold, along which the Cardiff conglomerate is faulted out for 5 

miles along the strike. The Peach Bottom syncline has been traced 

50 miles southwest of this district to Sykesville, Md., where the 

intrusive granite that extends from Sykesville to Washington and 

southward interrupts it.

FAULTS

The principal fault of the region is an overthrust fault which 

carried pre-Cambrian schists of the southeastern Piedmont northwestward over the Paleozoic sediments of the northern part of the 

area. This fault has been named by the writers the Martic overthrust from the prominent line of hills south of the front of the 

fault in Marticville Township, east of Susquehanna River. The 

trace of the fault plane on the surface is the northern boundary of 

the Wissahickon albite schist and the northern area of the Peters 

Creek schist. This boundary follows the south side of the ChesterQuarry ville Valley; just west of Quarry ville it turns northwest for 

5 miles, thence runs southwest again to Susquehanna River at Shenks 

Ferry. North of Shenks Ferry the trace of this fault swings northwestward in a wavy line nearly to Creswell Station and crosses the 

river at Long Level, southwest of which it passes out of the McCalls 

Ferry quadrangle.

All observed exposures of the fault contact are poor, but the best 

outcrop occurs in a cut of the Atglen & Susquehanna branch of the

Pennsylvania Railroad 1 mile west of New Providence. This exposure shows Conestoga limestone in contact with folded Wissahickon albite schist, but the rocks are weathered and their relations 

are not clear. There is no evidence of a great amount of brecciation 

of rock along the plane of movement in any of the observed exposures. Near Quarryville the contact of albite schist and Conestoga 

limestone is a zone of limonite replacement. The limonite has cemented quartz fragments which may be part otf a breccia produced 

along the fault contact. This limonite-quartz rock is best exposed 

in a cut of the Pennsylvania Railroad 2 miles west of Greentree. At 

many places along the fault contact in the vicinity of Quarryville 

limonite has been mined in the past, but the weathered exposures now 

to be seen throw no light on structural problems.

The existence of this extensive overthrust fault has therefore not 

been established by the discordant relations seen in exposures of fault 

contacts. It has rather been inferred >from the abnormal contacts 

of the pre-Cambrian Wissahickon albite schist and other pre-Cam-
STRUCTURE ' ' /'O

brian formations with rocks of Cambrian arid Ordovician age on the 

north and northwest. From the irregular surface outline of the front 

edge produced by erosion and from the occurrence of " windows " or 

 ' fensters " of Conestoga limestone surrounded by Wissahickon albite 

schist near the northern edge otf the overthrust mass west of Quarry - 

ville it is evident that the fault plane has a low dip.

This overthrust is part of a major fracture which has been traced 

from Schuylkill Kiver north of Philadelphia across Pennsylvania 

and Maryland, where it is covered by Triassic sediments north of 

Union Bridge. The rocks of the overthrust mass are for the most 

part pre-Cambrian, comprising the Baltimore gneiss, the Glenarm 

series, and the intrusive and extrusive rocks; but in southern York 

County, Pa., and in Carroll, Frederick, and Montgomery Counties, 

Md., basal Cambrian rocks that overlie these pre-Cambrian crystalline rocks have participated in the movement.

It is not- loiown how far northwestward the overriding block 

moved, but the distance must have been considerable. At present it 

covers the southern and greater part of the syncline south of the 

Mine Ridge anticline and the southwestern extension of the Mine 

Ridge anticline across Pennsylvania and Maryland. From the 

neighborhood of Susquehanna River into York County it overlies 

the anticlines north of Mine Ridge, but these anticlines emerge from 

the thrust in southern York County and Maryland. Southeast of 

these folds which emerge from its edge it covers a large area whose 

structure and rocks are a matter of conjecture.

It is believed that overthrust rocks extended over the Mine Ridge 

anticline and the region north of it, but at present its northwestern 

limit on Susquehanna River is 11 miles north of its front south of 

Quarryville. The Mine Ridge anticline, which has been a rising 

area since pre-Cambrian time, was probably higher at the time of 

thrusting than the region along Susquehanna River and southwestward, and the result of resistance to forward progress of the thrust 

offered by the Mine Ridge anticline was a thinning of the overthrust 

mass in that area. Folding subsequent to thrusting has further elevated the Mine Ridge area and made it easier for erosion to remove 

the overthrust in that area than to the southwest.

The Mine Ridge anticline has been thrust northwestward along a 

fault roughly parallel to the line of the Martic overthrust south of 

Chester Valley. This thrust has been traced northeast of Downingtown, but from structural relations northwest of the Mine Ridge 

anticline it seems to die out in the valley near Martinsville. The 

Mine Ridge anticline has moved across the syncline which lay south 

of Oak Hill and across the eastern ends of the anticlinal folds south 

of London Grove. A minor thrust extends for 5 miles north of 

Conestoga but is cut off by normal faults.
76 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

The northern part of the Quarryville-McCalls Ferry area is cut 

by normal faults trending about S. 70°-80° W. This area lies in 

the southern part of the northwestern Piedmont of Pennsylvania 

and Maryland, which is considerably broken by block faulting, and 

the normal faults which cut it are parts of faults of wider extent.

Welsh Mountain, Barren Hill, and Mine Eidge upland and its 

spurs are broken by a series of more or less parallel faults which 

produce marked effects in both pre-Cambrian and Paleozoic rocks. 

They are readily traceable except in the Conestoga limestone, in 

which it is difficult to recognize individual beds for any distance 

along the strike.

The most northerly fault of the Quarryville area is the fourth of 

the series of faults lying south of Welsh Mountain. It offsets the 

southern spurs of Barren Hill and passes south of Compass and 

westward into the Quarryville quadrangle near Kinzers. It separates the anticline of the Kinzers formation and the Conestoga limestone of the Bellemont area and extends westward into the Conestoga 

limestone, where for 15 miles it can not be certainly traced. It is in 

strike with the fault that runs west of Letort to Creswell Station 

and Long Level, where it cuts off the northern edge of the overthrust Wissahickon of the Turkey Hill area. The most extensive 

normal fault of the area is traceable 23 miles on the north side of 

Mine Ridge westward to a point north of Highville, where it offsets 

the Martic overthrust. In the vicinity of Burnt Mills a series of 

cross faults that strike southwest break up the folds of the Paleozoic 

rocks and offset the Wissahickon schist west of Safe Harbor and at 

Shenks Ferry station. These normal faults trend in the general

direction of the post-Paleozoic folding and also cut the Paleozoic

trends at angles of N. 30°-50° E. The latter direction is about that 

of the strike of the Triassic diabase dikes. Southwest of the McCalls 

Ferry-Quarryville district normal faults with a trend of about N. 

30° W. are common in addition to those found in this district.

The movement seems to have been largely vertical and was accompanied by brecciation, which is not observed along trace of the Martic 

overthrust fault. A fault 1 mile north of Burnt Mills passes through 

a brecciated zone of Vintage dolomite and Conestoga limestone along 

which argentiferous galena occurs in small veins. Brecciation of 

Vintage dolomite has been noted at other places along these faults. 

Dolomite sheared and made slaty by faulting is well exposed on the 

north side of Pequea Creek at the trolley bridge west of Burnt Mills, 

and limonite replacing limestone occurs along the fault zones north 

of Shenks Ferry Station and just west of Safe Harbor, 1 mile southeast of Highville. Six of these normal faults cut the overthrust 

between Shenks Ferry and Creswell stations. There may be others 

which are not apparent; the most noteworthy extends- from Letort,
STRUCTURE 77

Oeswell, and Long Level south westward in York County, a distance 

of 15 miles. It has dropped down the overthrust rocks and uplifted 

Conestoga limestone which is in contact with them.

AGE OF THE STRUCTURE

The direction of the major folds of this district is in general southwest, parallel to the long folds of the Appalachian Mountains. Because of this parallelism, and because Paleozoic rocks are the latest 

rocks involved in the folding, the age of the dominant folds is considered to be post-Paleozoic, or the same as that of the Appalachian 

structure elsewhere.

Although the rocks of the region have major trends parallel to the 

Appalachian folding, they show also evidence of older folding, both

pre-Cambrian and Paleozoic, as well as a younger structure acquired during Triassic deformation. All these structural disturbances are described below. The later Cretaceous, Tertiary, and 

Pleistocene deformation was in. the nature of simple tilting and 

warping.

PRE-CAMBRIAN FOLDING

The pre-Cambrian rocks show evidence of at least one period of 

pre-Cambrian folding and probably two. In Maryland the rocks 

of the Baltimore gneiss basement have a trend discordant to that of 

the overlying Glenarm series, and hence had undergone folding 

before the younger series was deposited.

It is probable that the Glenarm series also was folded before Cambrian time, but subsequent folding has obscured the amount of late 

pre-Cambrian deformation. In the region of the Mine Ridge upland, 

however, elevation and complete removal of the Glenarm series took 

place before Cambrian deposition if Glenarm sediments were ever 

deposited in that region.

PALEOZOIC FOLDING AND FAULTING

The earliest Paleozoic movement of which there is record in this 

region is the uplift and arching along the Mine Kidge axis and to 

the northeast of it. This uplift occurred after the deposition of the 

Beekmantown limestone and before the Conestoga limestone of Chazy 

age was laid down. A later folding which involved the Conestoga 

limestone and which followed the same direction preceded the overthrust which bevels the folds.

The overthrust and underlying rocks alike were subsequently 

folded into longitudinal folds trending in general southwest, parallel to the folds of the Appalachian region. Although the youngest 

rock in this district that is involved in this folding is the Ordovician
78 McCALLS FEHRY-QUARRYVILLE DISTRICT, PA.

Conestoga limestone. Carboniferous rocks in other parts of the Appalachian folded belt are affected but Triassic rocks are not. .Hence 

this latest folding; is assigned to post-Carboniferous time.

The age of the folding that involved the Conestoga limestone and 

preceded the overthrust and the age of the overthrust fault remain 

to be discussed. Although most of the folding once referred to the 

Taconic revolution 58 has been shown to be the result either of Middle 

Devonian or Appalachian mountain building, yet there is some evidence in the eastern United States of movement at the end of the 

Ordovician period. In southeastern New York, New Jersey, and 

northeastern Pennsylvania the Silurian Shawangunk conglomerate 

overlies unconformably Ordovician shales with a difference in dip 

of about 15°. Berkey 59 describes folding in Ordovician shales of the 

Hudson River area north of Kingston and pictures folded shales 

and an unconformity below the Shawangunk conglomerate in sections drawn along the line of the New York Aqueduct. This contact 

shows evidence of tilting, folding, and erosion at the end of the 

Ordovician in this part of the St. Lawrence trough. The absence 

of Silurian and Devonian beds west of the Green Mountains 60 is 

considered further evidence of folding at the same period in the 

northeastern part of this trough.

The mountain making which preceded the Upper Devonian epoch 

in New Brunswick and New England and in Appalachia northeast 

of the Piedmont of Pennsylvania seems to have been more vigorous 

than that of post-Ordovician time, but whether it was accompanied 

by folding is not known. Although it is not possible to fix the exact 

age of the prethrust folding it is known that the McCalls FerryQuarryville district and the Piedmont Plateau in general were subjected to diastrophism throughout the Paleozoic era. This area was 

part of Appalachia, that old land mass which arose in pre-Cambrian 

time and persisted as a borderland along the east side of the North 

American continent until after the Paleozoic era. The directions of 

folding that were established in pre-Cambrian time were followed by 

the later folding. Thus post-Beekmantown folding followed old 

trends of the pre-Cambrian rocks of the Mine Ridge-Honeybrook 

area, and so did later Paleozoic folding that involved the Conestoga 

limestone. The major folds that were developed in post-Carboniferous time were parallel to these old trends, and hence were posthumous 

folds along lines initiated by pre-Cambrian folding of the southeastern part of Appalachia.

68 Clark, T. H., A review of the evidence for the Taconic revolution : Boston Soc. Nat. 

Hist. Proc., vol. 30, p. 163, 1921.

^Bei-key, C. P., and Sanborn, J. F., Am. Soc. Civil Eng. Trans., vol. 86, pp. 1-9, 1923. 

80 Schuchert, Charles, Geol. Soc. America Bull., vol. 34, pp. 178-179, 1923.
STEUCTUBE 79

/

AGE OF THE MABTIC OVERTHBUST

'; , It is known that the overthrust occurred later than the folding of 

1 the Ordovician Conestoga limestone, which it bevels. It also preceded the latest Appalachian folding, because that folding involves 

both the overthrust and overlying rocks.

The folding in Appalachia was initiated in pre-Cambrian time. 

^ It is assumed as probable that the Baltimore gneiss has been prac- 

/ tically inert since that time; also that the zones of folding in the 

early part of the Paleozoic era lay in the southeastern part of Appalachia and throughout that era moved northwestward into the belts 

j of active sedimentation, while the land to the southeast kept rising 

and became immune to further folding. The McCalls Ferry-Quarry-:

ville district has been in a zone of active folding since the Beekmantown epoch, but it is probable that by post-Mississippian time the 

region to the southeast had become resistant to folding, and pressure, 

found relief there by breaking and thrusting, which moved the rocks 

of the Martic overthrust northwestward, bringing them again into a

' zone where along with the underlying mass they were involved in the 

Appalachian folding.

[ Both southeast and northwest of the Martic overthrust are other

I thrust faults that are traceable for considerable distances. The most 

southeasterly known is the Ruxton overthrust of Baltimore County, 

Md. (See fig. 5.) This extends from Baltimore northeastward to 

the Phoenix anticline, and its probable connection with thrusts in

) Pennsylvania has been indicated on the generalized structure diagram. The Ruxton. thrust and its probable northeastern extension 

in Pennsylvania are entirely in pre-Cambrian rocks; hence the age 

of the faulting is not known. It is possible, however, that the thrusting was later than the metamorphism and folding of these rocks, because the faulting has broken up the folds, especially in Pennsylvania, 

and has shortened the width of the belt of highly. metamorphosed 

rocks in the vicinity of Philadelphia to no more than one-half of its 

width in Baltimore County. The Martic overthrust both in Pennsylvania and in Maryland is paralleled roughly on the north and 

northwest by other thrust faults. West of the Blue Ridge and 25 

miles west of the Martic overthrust in Maryland there are thrust

t faults that extend southward through Virginia, Tennessee, and North 

Carolina and cut Carboniferous rocks. These faults mark the outer 

border of the overthrusts and may be somewhat later than the Ruxton 

and Martic faults. It seems probable that the zone of thrusting has 

moved progressively westward and that if the Martic overthru&t 

belongs to the same period of deformation as the faults west of the 

Blue Ridge, the post-Carboniferous, it occurred in the early part of 

the period.
80 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

\

CLOSE FOLDING OF MINE RIDGE ANTICLINE '

The Paleozoic formations on the flanks of the Mine Ridge anticline^ 

and in the folds lying to the north and west of this anticline are 'l 

closely folded and greatly metamorphosed. This condition represents a locally intense metamorphism in the vicinity of the Mins 

Ridge > anticline which dies out rapidly northward and northeastward. The cause of this* locally strong metamorphism is discussed at 

length on pages 118 to 141, where the conclusion is reached that it may » 

be explained as the result of regional contact influence of a postCambrian batholith.

The one pre-Triassic structure which all the rocks of the district \ 

possess in common is a cleavage, which was probably produced in 

the latest period of post-Carboniferous folding. This cleavage or 

schistosity is well developed in the softer rocks, the Conestoga limestone and the Antietam and Harpers schists.

The Antietam schist in an anticline north of London Grove (see 

pi. 6, A) shows a steep northwesterly schistosity which cuts across 

gently dipping beds of the anticline. Another outcrop in the x 

same formation on the southwest limb of the Oak Hill anticline 

(see pi. 6, B) shows northwest schistosity brought out by weathering ~\ 

across the crumpled bedding outlined by quartz injection, which may < 

be related in origin to the post-Cambrian batholith. V

LOAD

The load under which the overthrusting and subsequent folding { 

took place is not known. The nearest record of late Ordovician, 

Silurian, and Lower and Middle Devonian s'edimentation occurs 

in the Appalachian Valley of Maryland and Pennsylvania, where 

a probable maximum of about 5,000 feet of sediments was deposited. 

There is no evidence of how far southeast these sediments extended, 

but it is believed that they covered a considerable area southeasi 

of their present limits. In Upper Devonian 61 and lower Mississippian time a great thickness of sediments was deposited in central 

Pennsylvania, and although no remnant of them has been found in 

this area the mountain range that furnished them is considered to 

have been situated east of the present western margin of the Coastal 

Plain. The area between these mountains and the Upper Devonian 

delta of central Pennsylvania is thought to have been a piedmont 

gravel plain, while the shallow Upper Devonian sea extended west * 

of the delta.

Late Ordovician, Silurian, and Devonian sediments therefore probably constituted the moderate load under which Appalachian thrusting and folding in this region took place.

61 Barrel!, Joseph, Upper Devonian delta of the Appalachian geosyncllne: Am. Jour. 

Sci., 4th .ser., vol. 37, p. 232, 1914.
STRATIGRAPHIC RECORD 81 

TRIASSIC STRUCTURAL FEATURES

The McCalls Ferry-Quarryville district lies 15 miles south of the 

Triassic area of Pennsylvania and forms part of an area which has 

been considerably broken by normal faults, some of which are traceable into the Triassic rocks. These faults 02 both parallel the long 

Appalachian folds and cut across them. Near Ephrata, 14 miles 

north of the Quarryville quadrangle, blocks of Triassic rocks are 

faulted down in a syncline of Cocalico shale. Triassic faults break 

up the rocks, of the northern part of the McCalls Ferry-Quarryville 

district intq a series of blocks nearly parallel to the strike of the 

Appalachian folding.

Incorrect interpretation of Triassic deformation at some distance 

from Triassic basins has caused many errors in unraveling the structure in the Appalachian region. Recognition of normal faults in the 

areas of the Phoenixville, Honeybrook, New Holland, and Lancaster 

quadrangles, north of the McCalls Ferry-Quarryville district, and 

tracing of these faults across the York and Hanover quadrangles into 

Carroll and Frederick Counties, Md., has been the means of avoiding 

this error in this closely folded region, where thrust faulting rather 

than tension faulting would be expected and where fault outcrops are 

not preserved.

The normal faulting occurred during the crustal deformation 

which set in at the end of the deposition of the Newark group. This 

deformation produced the Palisade mountain system, in general 

parallel to that of the Appalachian Mountains. The stress was tensional, breaking into blocks the rising arch which lay east and southeast of the Pennsylvania-Maryland Triassic basin and west of the 

Connecticut Valley Triassic area. The record of this post-Triassic 

mountain making is left in the rocks of this region, although erosion 

has removed the Triassic sediments which probably were once there.

GEOLOGIC HISTORY

STRATIGRAPHIC RECORD 

PRE-CAMBRIAN HISTORY

The oldest record of pre-Cambrian time in the Piedmont province, 

between Philadelphia and Baltimore, is the sedimentary Baltimore 

gneiss, derived from sands mixed with feldspathic debris and clay. 

In the northeastern part of the Piedmont province, in Pennsylvania, 

these sediments contained a considerable amount of carbonaceous 

matter and numerous calcareous lentils. The Baltimore gneiss ino* Stose, G. W., New type of structure in the Appalachians: Geol. Soc. America Bull., 

vol. 35, pp. 465-480, 1924.
82 McCALLS FERRY-QTJARRYVILLE DISTRICT, PA.

eludes bands of hornblendic composition, rich in iron oxides and 

sphene, from whose presence it may be inferred that the ancient 

sediments were either intruded by sills of basic igneous rock or interlaminated with basaltic flows.

The resultant sedimentary and igneous complex was deeply buried 

and profoundly altered, partly no doubt under the influence of high 

temperature and pressure generated by deep burial, and partly under 

the action of the strong tangential thrust of a period of mountain 

building, by which the sediments and included igneous material were 

folded and welded together into an intensely metamorphosed complex. Doubtless as an accompaniment to the folding, a granite batholith invaded the lower beds of the sedimentary'series, furnishing to 

the upper beds alkalic vapors and a highly fluid magma that penetrated along the bedding planes of the batholithic cover, forming a 

banded gneiss by lit-par-lit injection. The folding, which either 

accompanied or followed the batholithic invasion, has left its mark in 

the intense cataclastic deformation of the granite, which has been 

stretched and crushed into a typical augen gneiss, known in Maryland as the Hartley augen gneiss. The trend of this ancient preCambrian folding can be identified by a distinct divergence in strike 

between the early pre-Cambrian Baltimore gneiss and the overlying 

later pre-Cambrian Glenarm series.

The coarse texture of the granitic intrusions in the Baltimore 

gneiss shows that the folding and metamorphism took place under a 

thick cover. Prolonged erosion then removed this thick cover of 

sediments and exposed the coarsely crystalline core of the granite 

batholith. Upon the eroded surface the basal sediments of the 

Glenarm series were laid down. The irregular distribution and

progressive overlap of Setters, Cockeysville, and Wissahickon deposits across the underlying Baltimore gneiss indicates that the 

Glenarm sediments, if marine, were laid down in a rapidly transgressing sea. The complete and in many places intense metamorphism of the Glenarm series makes it impossible to say with 

certainty whether the conditions of deposition were marine or 

terrestrial.

The deposition or nondeposition of the Glenarm series over the 

area of the Mine Ridge upland is a matter for speculation, because 

no Glenarm rocks occur on the flanks of the uplift between the 

ancient pre-Cambrian complex and the overlying Paleozoic beds. 

It is possible that the full thickness of Glenarm sediments may have 

once covered the area of pre-Cambrian,rocks exposed in the Mine 

Ridge-Honeybrook upland. These beds may have been completely 

removed before the basal Cambrian strata were deposited. This, 

assumption receives some justification from the fact that the pre-
STRATIGRAPHIC RECORD 83

/

Paleozoic plutonic rocks in the Mine Ridge upland, which have been 

.4 tentatively correlated with the post-Glenarm igneous intrusions in 

; Maryland, presuppose a sedimentary cover of considerable thickness. 

The necessary cover might have been furnished by Baltimore gneiss 

that was originally much thicker in the area of the Mine Ridge- 

» Honeybrook upland than over the Avondale-Kennett uplift, farther 

south, where Glenarm sediments overlie the Baltimore gneiss. If 

Glenarm sediments were never present north of Chester Valley the 

stratigraphic horizon of the Baltimore gneiss that is overlain by 

basal Cambrian deposits would have been determined by erosion 

that continued throughout Glenarm time, whereas the Baltimore 

/ gneiss horizon that is overlain by Glenarm schists on the flanks of 

the Avondale-Kennett uplift, although approximately the same 

horizon as the floor of the Paleozoic strata in Mine Ridge, was 

obviously determined by pre-Glenarm erosion. It is an improbable 

coincidence that pre-Glenarm erosion and erosion operating over the 

whole period of Glenarm time should have arrived at the same place 

in the work of denudation, and it seems more probable that Glenarm 

sediments were laid down in both localities and later removed by 

pre-Paleozoic erosion.

'i The disappearance of Glenarm sediments north of Chester Valley

I is probably due to the thinness of sedimentation there as compared

with the area of the Avondale-Kennett uplift, which originally lay

at least 20 miles farther south of Chester Valley, its present position

being due to northward overthrusting.

i The middle beds of the Glenarm series in the western Piedmont 

province of Maryland include several volcanic flows, which have been 

interpreted by Jonas 63 as eruptions probably derived from volcanic 

vents farther west and northwest.

The deposition of the Glenarm series was succeeded by intrusion 

of basic rocks ranging from gabbro to pyroxenite and peridotite. 

Probably after the intrusion of the basic magma there was a second 

! period of granitic invasion, as shown by rocks that range in compov sition from quartz monzonite and granodiorite to hornblende-quartz 

i,.' diorite. These light-colored plutonic rocks occur in large bodies in 

the Glenarm and pre-Glenarm strata of Maryland and southeastern 

Pennsylvania. Their relations to the basic intrusives are nowhere 

^ very clear, but at several localities west of Susquehanna River granodiorite correlated with the Port Deposit gneiss is intruded into 

* hornblende schists and metagabbros that are presumably part of the 

larger gabbro masses with which they are associated. In several

83 Jonas, A. 1., Pre-Cambrian rocks of the western Piedmont of Maryland: Geol. Soc. 

America Bull., vol. 35, pp. 355-364, 1924.

6136 29 7
84 McCALLS FERBY-QUABRYVILLE DISTRICT, PA.

localities both granodiorite and gabbro are cut by a younger granite 

and pegmatite.

It is difficult also to interpret the history of igneous intrusion in 

Pennsylvania north of Chester Valley, because, as in Maryland, there 

is no definite evidence of the relative age of the gabbro and the rocks 

of monzonitic or dioritic composition; but the igneous sequence in 

the McCalls Ferry-Quarryville district appears to have been the same 

as in Maryland. The gabbro petrographically resembles gabbro of 

southeastern Maryland, and the monzonitic rocks are apparently 

intruded into the gabbro.

On the other hand, Bascom 64 describes gabbro dikes in Cecil 

County, Md., that cut the quartz monzonites represented by the Port 

Deposit gneiss. Likewise she describes the granodiorite in eastern 

Pennsylvania and in Delaware 65 as cut by the gabbro. It is probable 

that more detailed petrographic work will show the existence of 

either two periods of gabbroic intrusion, as has been suggested for 

the igneous complex of the Heading and Durham Hills, or else two 

periods of monzonite intrusion, one older and one younger than the 

gabbro. The granite of the Mine Ridge anticline, which may prove 

to be Paleozoic, is probably the same as the granite and pegmatite 

that cut the quartz monzonite in Maryland and the gabbro southwest 

of Wilmington, Del. 66

The igneous intrusions may have produced regional contact metamorphism of the Glenarm sediments, which are profoundly altered 

throughout the eastern Piedmont area, in which igneous rocks are 

abundant. They were probably again accompanied by folding, as 

Jonas1 has found evidence in the western Piedmont belt of Maryland 

of a discordance in structure between the Glenarm rocks and the 

overlying basal Cambrian series.

To summarize, during early pre-Cambrian time there was deposition of sediments with possible intrusion of gabbroic sills or outpouring of basaltic flows. The basement complex was later invaded by 

a granite batholith, and the invasion was accompanied by folding 

and strong metamorphism. A long period of erosion followed, after 

which a series of later pre-Cambrian sediments was laid down and 

basic and acidic lava flows were extruded. A second period of 

igneous invasion and probable folding, followed by uplift and prolonged erosion, closed the pre-Cambrian history of the region.

64 Bascom, F., The crystalline rocks of Cecil County, Md.: Maryland Geol. Survey, Cecil 

County, p. 112, 1902.

65 Bascom, F., U. S. Geol. Survey Geol. Atlas, Philadelphia folio (No. 162), 1909; 

Elkton-Wilmington folio (No. 211), 1920.

89 Hawkins, A. C., Alternative interpretation of some crystalline schists in southeastern 

Pennsylvania: Am. Jour. Sci., 5th ser., vol. 7, p. 360, 1924.
STRATIGRAPHIC RECORD 85

PALEOZOIC HISTORY 

GENERAL OUTLINE

The fact that the conglomerates occurring in the basal Cambrian 

beds on the flanks of the Mine Kidge upland contain only quartz 

pebbles in a matrix of silty origin suggests that the pebbles were 

derived from erosion of a low-lying land with an interior elevated 

tract in which the streams had a gradient sufficient to carry down 

fairly coarse detritus. During its long journey to the sea this 

detritus would lose most of the readily disintegrated material, retaining only the residual quartz pebbles, which were deposited at intervals from flooded streams, in silt derived from the low-lying 

shoreward plains. The fact that there are more than 2,200 feet 

of arenaceous Lower Cambrian deposits indicates prolonged erosion 

of a surface of moderate relief. The succeeding 1,800 feet of Lower 

Cambrian sediments are dominantly calcareous or dolomitic with 

subordinate layers of argillaceous material. As pointed out by Stose 

and Jonas 67 the axis of the Mine Kidge uplift was rising during 

. Lower Cambrian time, and the Lower Cambrian section was probably 

thinner over the crest of the arch.

Calcitic and dolomitic sedimentation prevailed in the areas of 

Lancaster Valley and Chester Valley through Middle and Upper 

Cambrian time, although deposition was probably not continuous 

throughout this whole time. After the deposition of the Ordovician 

Beekmantown limestone slowly accumulated crustal stress found relief in a moderate flexure along the axis of the Mine Kidge anticline. 

This arching brought the strata above sea level long enough to allow 

erosion of the gently bowed sediments, so that when the Chazy sea 

transgressed over the eroded fold the Conestoga limestone was laid 

down unconformably over Upper, Middle, and Lower Cambrian 

strata down to the horizon of the Harpers schist.

After the deposition of the Conestoga limestone crustal stress, 

which had reaccumulated along the eastern border of the sinking 

Appalachian geosyncline, manifested itself in a moderately steep 

folding that followed the general trend line of the pre-Conestoga 

flexure. Subsequently a block of the earth's crust that originally lay 

some distance southeast of the Mine Ridge fold was driven northward across the folded edges of the Conestoga and pre-Conestoga 

strata. It is probable that this overthrust block extended at that 

time over the area occupied by the Mine Ridge and Honeybrook 

uplands.

The occurrence of folding subsequent to the overthrust is shown by 

the fact that the overriding fault block southwest of the Mine Ridge

07 Stose, G. W., and Jonas, A. I., Onlovician overlap in the Piedmont province of Pennsylvania and Maryland: Geol. Soc. America Bull., vol. 33, p. 521, 1923.
8O McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

anticline is folded in the double-crested arch of the Tucquan anticline. This anticline is a structural continuation of the doublecrested Mine Eidge anticline, which must accordingly be later than j 

the over thrust. The south westward pitch of the Mine Eidge anticlinal axis was thus probably acquired or at least strongly intensified 

during the folding that succeeded the overthrust movement.

The only post-Conestoga and pre-Triassic events that are recorded 

in the McCalls Ferry-Quarryville district are, first, a post-Conestoga 

folding; later, extensive overthrusting; and finally, a folding of the 

overriding and underlying blocks. After the deposition of the Conestoga limestone there was a profound metamorphism of the Paleozoic i 

rocks around the Mine Eidge anticline and westward to Susquehanna ' 

Eiver at Shenks Ferry.

AGE OF THE POST-CONESTOGA DEFORMATION AND METAMORPHISM

There are no sedimentary rocks in the McCalls Ferry-Quarryville 

district that are younger than the Conestoga limestone, and the 

geologic evidence of the district shows only that the post-Conestoga 

deformation was pre-Triassic, because the Triassic strata north of the 

district are comparatively unaffected by folding. Therefore any dis- ^ 

cussion as to whether this deformation took place in one or more than j 

one period of folding and as to the probable age of such deformation \ 

is based solely upon inferences drawn from the geologic history of 

adjoining regions and upon inferences as to the probable method 

of development of the observed structure. (

Inferences drawn from the post-Chazy stratigraphy of adjoinmg ' 

regions. The Martinsburg shale, on the northwestern border of the 

Lebanon and Cumberland Valleys in Pennsylvania, probably represents a depth of half a mile of sediments deposited during Trenton, 

Eden, and Maysville time. Such a thickness not more than 40 miles 

northwest of the McCalls Ferry-Quarryville district means that by 

the end of Martinsburg time there was probably a light load of sediments upon the Conestoga limestone in this district. At Lehigh ^ 

Gap, in the Blue Mountain-Kittatinny range of the Appalachian 

Valley ridges, there is an angular unconformity between the Martins- | 

burg and the overlying beds. Such an unconformity would represent the continuation into Pennsylvania of the crustal movement 

recorded at Otisville, N. Y., where Shawangunk conglomerate rests A 

upon the tilted edges of shale of Martinsburg age with a difference ^ 

in dip of 15°. The angular differences between the underlying and 

overlying beds at Lehigh Gap is also only about 13°, and therefore 

the crustal disturbance recorded in these two localities, although distinct, may be slight. A pronounced stratigraphic break at the base
STRATIGEAPHIC EECORD 87

of the Richmond has long been recognized by Ulrich,68 but the 

amount and extent of the folding involving pre-Bichmond beds is 

still a matter of considerable uncertainty. The continental origin 

of the lower Silurian beds that occur in east-central Pennsylvania 

suggests land conditions in the McCalls Ferry-Quarryville district 

until the early part of the Niagaran transgression, which may have 

extended southwestward over a part of the Piedmont province. 

However, if Silurian sediments ever were deposited so far south 

they must have been removed before upper Silurian time, because 

in northern New Jersey there are no strata of Niagaran age beneath 

the Cayugan shales in Green Pond Mountain. The folding in the 

Green Pond syncline involves rocks laid down as part of the Upper 

Devonian gravel plain and is, therefore, post-Devonian. Barrel! C9

has estimated a possible thickness of 2,500 to 3,500 feet of sediments 

embraced in upper Silurian and Lower and Middle Devonian time 

in the same area. He estimated the rate of southward and eastward 

thinning of Upper Devonian sediments at 100 feet to the mile, which 

would give not more than 1,200 feet of Upper Devonian sediments 

as a possible cover upon a piedmont gravel plain in the McCalls 

Fe"rry-Quarryville district, which is 60 miles southeast of the Appalachian Valley ridges, where Upper Devonian sediments attain a 

thickness of 7,200 feet. If the full thickness of upper Silurian and 

Lower and Middle Devonian sediments in northern New Jersey continued southeastward as far as the McCalls Ferry-Quarryville district the probable maximum load upon the Conestoga limestone at 

the time of post-Devonian folding would have been of the order of 

magnitude of 5,000 feet. The actual thickness of sediments may, 

of course, have been much less than this figure, but it was probably 

not much greater.

There is no evidence in the eastern part of Pennsylvania that 

would show the nature of the uplift that occurred along the eastern 

border of the area of the Piedmont province in Devonian time. That 

such an uplift occurred is shown by the character of the Upper 

Devonian delta deposits. The location of the highland, from which 

the clastic deposits were derived, was somewhere in the area now 

covered by the Coastal Plain, as shown by the direction and rate 

of southeastward thinning of the delta deposits. In the Maritime 

Provinces of Canada and in the New England States, where Devonian rocks remain to record the history of the post-Middle Devonian 

movement, elevation was accompanied by folding and granitic intrusion. The amount of folding that accompanied the Devonian uplift

68 Ulrich, E. O., The Ordovician-Silurian boundary: Cong. ge"ol. internal:., 12th sess., 

Compt. rend., pp. 614-615, 1913.

68 Barrell, Joseph, Upper Devonian delta of Pennsylvania and New York: Am. Jour. 

Sci., 4tU ser., vol. 36, pp. 234-239, 1914.
88 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

in eastern Pennsylvania, where Devonian sediments have been removed, can only be inferred.

During the Mississippian, Pennsylvanian, and early Permian time 

the area of the Piedmont province in Pennsylvania and Maryland 

was being eroded. In Virginia, along the western front of the Blue 

Ridge, Carboniferous rocks are folded and covered by an overthrust 

block of pre-Cambrian and Paleozoic rocks that has ridden westward along a nearly horizontal plane. This shows that folding at 

or near the end of the Paleozoic era was of the type that is accompanied by overthrusting. It does not prove that the Martic overthrust fault was formed during the same deformation as the overthrust west of the Blue Ridge, although it is a reasonable inference 

that they may be contemporaneous.

It was pointed out by Ulrich 70 that the folding of the Appalachian 

geosyncline took place in more than one period of orogeny and that 

the relative intensity of the folding in these different periods is hard to 

estimate. The Mine Ridge fold shows the existence of a periodically 

rising anticlinal axis that was initiated in early Paleozoic time or 

perhaps in pre-Cambrian time. Ulrich's suggestion of a westward 

migration of the belt of active folding leaves an open question h6w 

much earlier Paleozoic folds may have been intensified by later 

deformation. The evidence of two periods of metamorphism in 

post-Conestoga time suggests that the McCalls Ferry-Quarryville 

district and the immediately adjoining regions have undergone at 

least two compressive deformations, of which the second was the 

less intense, the inference being that the earlier folds remained more 

or less inert and resistant to the later folding or else that the later 

folding in this region was subordinate in intensity to the earlier.

Inferences drawn from the probable method of development of 

observed structure. The absence of dislocation phenomena in the 

few localities where the thrust plane has been seen and the diminution in metamorphic intensity in the overriding block as compared 

with the metamorphic condition of the overridden strata in the 

McCalls Ferry-Quarryville district suggests that the thrusting took 

place under so-called " deep-seated " conditions. Such conditions are 

furnished not only by deep burial -but also by batholithic intrusion, 

which raises the temperature gradient over wide areas. It has been 

shown that the evidence of adjoining regions does not indicate much 

more than a mile of sedimentary cover under which the rocks of the 

McCalls Ferry-Quarryville district have been folded. Such a load 

is comparable to the cover under which the great overthrusts of the 

western United States have taken place. In the Lewis overthrust, 

along the eastern flank of the Rocky Mountains, the older rocks of

70 Ulricli, E. O., Revision of the Paleozoic systems : Geol. Soc. America Bull., vol. 22, 

pp. 436-442, 1911.
STRATIGEAPHIC BECORD 89

the mountain ranges have been shoved eastward over younger rocks, 

and in the eastward overthrust fault in the Philipsburg quadrangle, 

Montana, which is considered by Calkins to be the probable continuation of the Lewis overthrust, both underlying and overlying blocks 

were ifolded in rather close folds.71 The attitude of the underlying 

beds is in general fairly simple, and the hypothesis is advanced that 

the rocks were not much folded before the overthrust, which followed 

a plane of slight resistance approximately parallel to the bedding. 

Such a movement as this overthrust in the Kocky Mountains is similar to the Martic overthrust in its wide extent, in the general parallelism between bedding and overthrust plane, and in the warping 

of the overthrust plane, which is, however, much more pronounced 

in the Martic overthrust. But a striking dissimilarity between the

two faults is the present condition of the rocks involved. In the 

overthrust in the Kocky Mountains the rocks of the overriding block 

are ripple-marked sandstones, shales, and impure limestone that show 

only a very slight metamorphism, whereas in the Martic overthrust 

they are schists, phyllites, and slates. There is a well-marked brecciation along the base of the overthrust block in the Kocky Mountains, 

but so far as indicated in available exposures brecciation is conspicuously absent in the Martic overthrust. These differences indicate an essential difference in conditions of faulting, the overthrust 

in the Kocky Mountains having apparently taken place under relatively light load where mechanical deformation was dominant. The 

overthrusts of southern Nevada described by Longwell 72 have taken 

place under similar conditions, favorable to strong mechanical deformation, and yet the thickness of load in the Spring Mountain 

region at the time of overthrusting is estimated to be at least 10,000 

feet. It is obvious that the Martic overthrust, at least in the McCalls 

Ferry-Quarryville district, took place under so high a temperature 

that mechanical deformation was at a minimum. Such a temperature could not be attained at depths of little more than a mile on a 

normal geothermal gradient, but it might have been furnished by the 

influence of a rising batholith that has never reached the surface but 

has produced the extensive pneumatolysis and pegmatitization of 

the Paleozoic rocks in the region.

SUMMARY OF PALEOZOIC GEOLOGIC HISTORY

During the earty part of the Paleozoic era the McCalls FerryQuarryville district was accumulating first arenaceous, later calcitic

71 Emmons, W. H., and Calkins, F. C., Geology and ore deposits of the Philipsburg quadrangle, Mont.: U. S. Geol. Survey Prof. Paper 78, pp. 146-149, 1913.

72 Longwell, C. R., Geology of the Muddy Mountains, Nev.: Am. Jour. Scl., 4th ser., 

vol. 50, pp. 39-62, 1921; Thrust faults and flaws in southern Nevada (abstract) : Geol. 

Soc. America Bull., vol. 35, pp. 64-65, 1924; Complex structure in the Spring Mountains, 

Nev. (abstract) : idem, p. 150, 1924.
90 McCALLS FERRY-QTJARRYVILLE DISTRICT, PA.

and dolomitic sediments. Even in Cambrian time, however, a slight 

unrest in the basin of deposition is manifested by the several probable emergences that correspond to breaks in sedimentation in adjoining regions. Such unrest apparently increased, so that before Chazy 

time the Paleozoic and underlying rocks of the district were folded 

into a gentle arch over the site of the Mine Ridge anticline. The 

Conestoga limestone was laid down over the eroded surface of this 

arch probably during Chazy time. The district was then folded in 

a period of deformation whose exact date can not be determined. 

Subsequently a block of the earth's crust that originally lay between 

10 and 20 miles to the south was driven northward and westward, 

truncating the folded edges of Cambrian and Ordovician strata. A 

moderate folding was superimposed upon both overthrust and underlying rocks. The last record of compressive force in the district is 

the development of a schistosity that cuts across the older folding 

and foliation. The date of the folding subsequent to the overthrust 

can not be placed more definitely than pre-Triassic and post-Chazy.

POST-PALEOZOIC STRATIGRAPHIC RECORD

The arkosic conglomerates on the eastern edge of the Triassic basin 

in Pennsylvania and Maryland indicate that the McCalls FerryQuarryville district was a highland during Triassic time. Westwardflowing streams carried the debris from this highland into the basin 

of deposition. All during Triassic time dikes and sills of diabase 

forced their way upward into the sedimentary beds. These intrusions 

extended beyond the belt of sedimentation into the eastern highland 

and are recorded in the narrow dikes that cross the McCalls FerryQuarryville districts. The only other record of Triassic history in 

this area is the extensive normal faulting that is characteristic of 

Triassic deformation. This faulting broke the northern half of the 

district into several well-defined blocks and doubtless extended farther south than it is possible to trace the faults in the pre-Cambrian 

mica schists.

There is no record of Jurassic and early Cretaceous history in this 

district, but the presence of Upper Cretaceous marine deposits in the 

area southeast of the district that was a part of the 'old Appalachian 

highland shows that at some time during Jurassic or early Cretaceous 

time there was an upwarping of the Triassic basin together with a 

foundering of the eastern part of the old Appalachian land mass, 

which initiated the present eastward-flowing drainage of the Atlantic 

slope and the present relations of land and sea. The subsequent history of the region must be read in the physiographic record, which 

furnishes at least a partial epitome of erosion subsequent to the establishment of the eastward-flowing drainage.
McCALLS FERRY-QUARRYVILLE DISTRICT, PA. 91 

PHYSIOGRAPHIC RECORD

Detailed topographic studies of the Piedmont province show that 

there exist in the highlands of the region a number of terracelike 

surfaces, and that these surfaces bevel the upturned underlying rocks. 

Moreover, in the lowlands the plane indicated by flat-topped hill 

summits of accordant altitude also bevels the underlying structure. 

Thus the hilltops are obviously remnants of old surfaces of low relief 

into which the present streams have incised their meandering course. 

The large number of residual erosion surfaces marking cycles and 

subcycles of erosion thus suggested can not be fitted into the idea 

that the topography of this region has been developed in three erosion cycles, as has been advocated by previous workers; nor can they 

be explained by a widespread warping of two or three uplifted peneplains, such as was invoked by Campbell in order to explain the 

occurrence of his uplifted Harrisburg peneplain at different altitudes 

in different places. The succession of numerous flat surfaces, one 

above another, cutting across the underlying rock structure of the highlands, and the appearance of the same level surfaces on both sides 

of the highland areas make the warping hypothesis inadequate to 

reduce the number of erosion cycles, indicated by the successive surfaces, to the three cycles that have been previously recognized. It is 

evident that the region between the Appalachian Valley ridges and 

the Coastal Plain furnishes evidence of numerous interrupted erosion 

cycles of relatively short duration, and level interfluves at various 

altitudes are recognized as remnants of old peneplains cut during 

these different cycles. But the proper correlation and the sequence 

of these remnants of peneplains or partial peneplains 'present a 

tantalizing problem, as the altitude of residual erosion surfaces depends so largely on the relative resistance of different rocks and on 

the distance of each part of the surface from base level as well as 

from the trunk stream of the drainage basin.

The fact that the master streams of the Atlantic slope show a lack 

of accordance with the structure of the underlying rock, combined 

with the extensive stream adjustment of the tributaries, suggests 

the possibility of tracing backward, step by step, the drainage history of the region, thereby determining the sequence of past cycles 

whose record is preserved in present and preexistent drainage channels and establishing the connection between such records of old 

drainage and the so-called peneplain remnants preserved in the level 

surfaces of the region.

EVIDENCE OF FORMER DRAINAGE COURSES

In a region where the drainage has become thoroughly adjusted 

the present streams will retain little evidence of their former courses.
92 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

However, in spite of extensive stream adjustment, successive rejuvenations may be recorded in the drainage of a region in one or more 

of four ways (1) superimposition, (2) modification of normal cross 

profiles} of stream valleys, (3) wind gaps, (4) " knickpoints," or 

points of abrupt change in the longitudinal profile of the stream 

valleys from a gentle concave curve to a curve that is convex 

upward.

Superimposition. 73 Superimposition as the term was employed 

by Powell 74 in 1875 was applied to streams that acquired their courses 

on marine sediments and that have retained their courses after they 

have cut down to the underlying surface on which the sediments 

were deposited. In Powell's definition the idea of a marine cover 

through which streams cut their way into a discordant underlying 

terrane is an essential feature of superimposition, and this usage 

has been widely followed. However, even as far back as 1877 Gilbert 75 recognized and defined three classes of superimposition 

namely, superimposition by sedimentation, or subaqueous deposition; 

superimposition by alluviation, or subaerial deposition; and superimposition by planation. His additions to the analysis of superimposition were twofold. (1) Subaerial erosion as well as marine erosion 

might produce conditions under which streams would be superimposed. He cites, as processes by which these conditions might be 

produced, alluviation and lateral planation, both associated with the 

mature or postmature degradation of a land surface. (2) Superimposition does not necessarily imply the previous existence of covering strata but may take place as a result of planation rather than of 

deposition.

McGee 76 carried the analysis a step farther when he defined superimposition "through base-level degradation so perfect that the old 

waterways were completely obliterated." It was thus gradually 

becoming apparent that the essential feature of a superimposed 

drainage pattern is the fact that the streams have been let down 

from above upon a rock surface, into which they are now incised 

without regard to the underlying rock structure. The attempt to 

explain many such superimposed drainage patterns as an inheritance 

from a cover of marine sediments has led to some unwarranted 

assumptions of the former existence of marine deposits in the east73 The term " superimposition" was apparently first used by George Maw in 1866 

(Notes on the comparative structure of surfaces produced by subaerial and marine denudation: Geol. Mag., vol. 3, pp. 441-445, 1866).

74 Powell, J. W., Exploration of the Colorado River of the West and its tributaries, p. 

166, Washington, 1875.

75 Gilbert, G. K., Report on the geology of the Henry Mountains, 1st ed., p. 144, U. S. 

Geog. and Geol. Survey Rocky Mtn. Region, 1877.

76 McGee, W J, Geology of the head of Chesapeake Bay: U. S. Geol. Survey Seventh 

Ann. Rept., pp. 562-563, 1888.
PHYSIOGEAPHIC RECOED 93

ern Appalachian Highlands in areas where there is no evidence 

of such deposition. Recognition of the fact that streams may be 

let down not only from a sedimentary cover of either marine or 

alluvial origin but also from an almost completely degraded surface 

(a peneplain) thus clears away one fruitful source of unjustifiable 

speculations in interpreting physiographic history.

In nonglaciated regions the essential feature of superimposition is 

that the unadjusted courses of major streams across ridges carved 

out by tributary streams indicate a reduction of the region to a lowlying level surface before the formation of the flat-topped ridges. If 

the original level surface was of marine origin and covered by sediments the drainage is said to be superimposed by sedimentation on 

the underlying hard rock. If the level surface on which the drainage

has established its unadjusted course was formed by subaerial erosion the lack of accord between the major drainage channels and the 

underlying rock shown by the passage of the streams through resistant ridges is due to the fact that, consequent upon uplift of a peneplain, the streams that had been wandering upon the peneplain were 

intrenched and may be said to be superimposed by peneplanation. 

The term superimposed is thus used to denote the fact that the major 

streams which are flowing in courses that are out of accord with the 

underlying structure have originated not as initial consequent streams 

but as resuscitations of a previous drainage system whose courses 

were established on an approximately level surface produced by 

either marine or subaerial denudation. Whether the original level 

surface was of marine origin and once covered by sediments (superimposition by sedimentation), or whether it was of subaerial origin 

(superimposition by peneplanation), does not affect the value of the 

evidence of degradation that is afforded by the superimposition. 

Streams once firmly intrenched in a superimposed course will cut 

down across the hard ridges in valleys that will persist as long as the 

hard rock stands up above the level of the surrounding % country. 

Thus are produced the water gaps that are so conspicuous a feature 

of the superimposed streams of the eastern Atlantic Slope, such as 

Delaware, Lehigh, and Susquehanna Rivers. A superimposed stream 

will record the surface of degradation on which it originally flowed 

as long as the flat top of the ridge remains unreduced, a period which 

may cover many subsequent partial cycles of erosion. In the relatively soft lowlands intervening between the hard ridges, such as the 

Lebanon, Lancaster, and Cumberland Valleys, these subsequent partial cycles of erosion have removed the rock so rapidly that the 

intrenched character of the master streams is not so apparent as in 

their courses athwart the resistant ridges. Therefore, in the stream 

gaps should be sought the evidence of superimposition that dates
94 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

back to the time of formation of the highest flat-topped surfaces on 

both sides of the gap. The only clue to the erosion history of the 

region subsequent to the intrenchment of the old stream will be found 

in the shape of the cross section of the gap and in the longitudinal 

profiles of existing streams.

Gross profiles of stream valleys. The cross profiles of rejuvenated 

stream valleys may show the outlines of wide older valleys in the 

bottom of which the present streams are intrenched. Composite 

valley slopes that indicate by their shape one or two rejuvenations 

in the course of normal erosion have been aptly termed by Matthes 77 

" 2-story valleys" or " 3-story valleys." But, although the evidence of the oldest rejuvenations may be preserved in the composite upper slopes of the valley, the presence of a uniformly steep 

inner gorge does not indicate that the gorge has been formed during 

an uninterrupted erosion cycle. On the contrary, the downcutting 

of the lower gorge may continue through several partial cycles of 

erosion. For even though the progress of eros.ion in the region is 

quickened by an uplift or by successive uplifts, there can be no 

record of such quickenings in a gorge through hard rock, if lateral 

planation has not yet begun to take effect. In the vicinity of a hard 

ridge softer rocks may have been reduced to a local peneplain, not 

once but repeatedly, while a gorge in the hard rocks was simply 

being deepened. \

Wind gaps. The volume of water and the debris carried in the 

main stream of a superimposed drainage system is much greater than 

in the tributaries, therefore downcutting progresses most rapidly 

in the master valley, and the minor streams are one by one diverted 

from their superimposed courses by the subsequent tributaries of the

master stream. The relic valleys of the original transverse minor 

streams are preserved as wind gaps in the hard ridges. Barrell 78 

was the first to call particular attention to the value of wind gaps 

as indicators of former cycles of fluvial erosion. Briefly stated, 

the value of wind gaps in fixing points upon partly obliterated drainage systems lies in the fact that their present outline has been comparatively little modified since the time when water was flowing 

through them, because they are hung up, so to speak, out of reach 

of subsequent erosion.

The stage of erosion at which capture took place may be inferred 

from the form oft the cross section of the wind gap and from the 

altitude of the base of the wind gap compared with the altitudes of 

contemporaneous stream channels recorded elsewhere in the region 

in the upper slopes of composite valleys. The subaerial erosion cycle

" Matthes, F. E., U. S. Geol. Survey Yosemite Valley map, 1922.

78 Barrel!, Joseph, The Piedmont terraces of the northern Appalachians : Am. Jour. Sci., 

4th ser., vol. 49, p. 388, 1920.
PHYSIOGRAPHIC RECORD 95

during which any one group of wind gaps was cut can be fixed only 

by the correlation .of the wind gaps with composite slopes in the 

adjoining stream valleys and with remnants of erosion surfaces whose 

present altitude shows an evident relation to the base of the wind 

gaps.

Longitudinal pro-files of streams. The value of the three evidences 

of past drainage that have been discussed above lies in the fact that 

in resistant rocks all three kinds of evidence will be preserved as 

long as the ridges stand above the surrounding lowland. They will 

thus persist long after all other indications of the original drainage 

course have been destroyed by extensive stream adjustment and by 

repeated planations of the surrounding country. In addition there 

remains, in the existent drainage courses of a region, still a fourth 

evidence of recent breaks in the continuity of erosion namely, the 

knickpoints in the longitudinal stream profiles.

When a region is uplifted a stream whose channel had attained 

the normal concave stream profile 79 will be quickened and will begin 

to cut down its bed. In the lower reaches of the stream, where the 

longitudinal profile that has become nearly flat, has been steepened 

by the uplift, there will be a development of a new concave curve of 

maturity, and inasmuch as erosion works progressively backward 

the reduction of the former stream profile to a new curve will 

progress backward from mouth to source. Obviously at the headward limit of this reduction to a new grade there will be an arch 

'in the curve a change in the stream profile from a concave to a 

convex curve. Therefore a stream that has become intrenched in a 

peneplain and has maintained its course through several partial 

erosion cycles can show, from mouth to source, evidence of successively older periods of uplift and consequent valley cutting by a 

succession of bends or knickpoints in the curve.

RELATIVE PRESERVATION OP SUCCESSIVE DRAINAGE COURSES

In a region that has been repeatedly uplifted we find near the 

seacoast widespread and clear records of the work of recent erosion. 

The dissection of successively emerged continental shelves by extended streams and the rejuvenation of drainage shown in the stream 

profiles of both recent and older stream courses, combined with the 

presence of residual deposits, make it comparatively easy to fix the 

duration and extent of each successive advance and withdrawal of 

the sea. But the amount of erosion accomplished near the seacoast, 

by either subaerial or marine agencies, has been a maximum in every 

cycle; therefore near the present shore line the clearness of the 

record of recent erosion is combined with an obliteration of the

78 Penck, Albrecht, Morphologic der Erdoberflache, vol. 1, pp. 319-341, Stuttgart, 1894.
96 McCALLS FERRY-QTJARRYVILLE DISTRICT, PA.

record of earlier erosion cycles by the formation of new seaward 

peneplains and marine plains in the later cycles. The record of 

earlier erosion must be sought farther inland, near the present headwaters of the streams, where isolated residuals of earlier plane surfaces have been preserved from dissection and where hard ridges that 

have not preserved their original surfaces during the peneplanation 

of adjacent soft rocks retain, in a more or less ephemeral way, the 

records of earlier erosion in the form of wind and water gaps.

Therefore in order to trace the effects of successive rejuvenation it 

is necessary to study the course, of a major drainage system from 

the mouth to a point so remote from present base-level that the 

record of the old drainage channels is still preserved in the cross 

profiles of the valleys. Such an investigation comprises a study of 

interstream areas that are remnants of former peneplains or of incompletely developed peneplains and a study of progressive stream 

adjustment and of the modifications of stream gradients and valley 

profiles during successive cycles of erosion. The method of investigation has been described at length in a previous paper,80 and the 

results here presented are those which apply more particularly to 

the McCalls Ferry-Quarryville district.

STUDY OF INTERSTREAM AREAS

The flat-topped interstream areas were first outlined upon good 

topographic maps of a scale of 1:62,500. The individual levels thus 

estalished were then studied in their extended development across a 

wide stretch of country by the construction of a projected profile 

adjoining the northwest side of a profile projected by Barrell along 

a section plane running N. 35^2° W. from the mouth of Susquehanna 

River at Havre de Grace. The foreground of the profile thus projected extends from the inland edge of the Coastal Plain northwestward as far as the eastern ridges of the Appalachian Valley. The 

projected profile embraces the plane surfaces that are upheld in the 

Blue Mountain-Kittatinny Ridge and Lebanon Valley divisions of 

the Great Appalachian Valley, in the South Mountain and Reading 

and Durham Hills of the Appalachian Mountains, in the Lancaster 

Valley and the Triassic Lowland divisions of the Piedmont Province, 

and in the Piedmont Upland to its junction with the Coastal Plain. 

(See pi. 7, A.)

The complexity of levels shown in such an extended profile is at 

first sight bewildering and is increased by the fact that in passing 

eastward from the Susquehanna Valley certain apparently level 

surfaces show a rise toward the divide. For example, in the Lebanon 

Valley the 400-foot level on limestone hills near the Susquehanna

80 Knopf, B. B., Correlation of residual erosion surfaces in the eastern Appalachian Highlands : Geol. Soc. America Bull., vol. 35, pp. 633-666, 1924.
PHYSIOGRAPHIC RECORD 97

rises to about 500 feet 6 miles northwest of Wernersville. The reason 

for the disappearance of the 400-foot surface near the divide is that 

on existent divides remnants of surfaces cut during previous cycles 

are more or less immune to the destructive agencies of the present 

cycle. The profile of the divide between the Susquehanna and the 

Schuylkill drainage systems (pi. 7, B) shows three conspicuous highlands occupied by the Blue Mountain, South Mountain, and Welsh 

Mountain-Barren Hill ridges. The southernmost highland terminates in Mine Kidge upland at an altitude of aoout 860 feet. These 

highlands are separated by intervening lowlands about 20 miles in 

width. Although Blue Mountain lies 24 miles farther inland than 

South Mountain, the general altitude of the intervening lowland 

between Blue Mountain and South Mountain is more than 100 feet 

lower than that of the lowland that separates South Mountain and 

Welsh Mountain.

The most striking feature of the divide between Susquehanna and 

Schuylkill Rivers is the difference between the portion that extends 

seaward from the Mine Ridge upland and the inland extension 

northward from the Mine Ridge upland. South of the Mine Ridge 

upland the profile of the divide is strikingly similar to Barrell's 

projected profiles of the Piedmont terraces in Maryland.81 The descent from the Mine Ridge upland to sea level comprises a succession 

of seven stepped or terrace-like surfaces that are cut across crystalline 

schists of approximately uniform resistance to erosion. The outline 

of these steps, as shown upon the topographic maps, is not regular 

in its linear extent, as in a flight of stairs, but each successive level 

wraps around the dissected edges of the next higher level,, thereby 

extending head ward up the courses of the major tributaries.

In New England it was such a stairlike arrangement of levels, 

facing seaward, that suggested to Barrell the idea that the rises are 

old sea cliffs. In the Pennsylvania terrace-like surfaces the marine 

origin of the treads is suggested by their gentle seaward slope, the 

lack of monadnocks, and the scarplike descent of each to the next 

lower level. The three lowermost terraces, each carrying a welldefined mantle of unconsolidated deposits, are of Pleistocene age and 

are generally conceded to be of marine origin.

In the absence of marine deposits and of undoubted sea cliffs the 

origin of the four upper terraces must still remain hypothetical, but 

whatever was the erosional process that produced the level terracelike surfaces, the uniformity of resistance in the underlying rock 

leads to the conclusion that they are distinct surfaces of erosion, 

partly dissected, the width of each step being dependent upon the 

duration of the erosion that produced the plane surface.

81 Barrell, Joseph, The Piedmont terraces of the northern Appalachians: Am. Jour. Sci., 

4th ser., vol. 49, pi. 6, 1920.
98 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

The dissected character of the divide north of the Mine Kidge upland 

presents a strong contrast to the terrace-like steps south of the ridge 

and is indicative of subaerial erosion. The divide is notched by 

depressions that fall into two classes. One class, to which belong 

only a few of the numerous depressions, comprises narrow, steepsided transverse trenches, sunk into flat-topped uplands. Their 

shape clearly shows that they were carved by the erosion of former 

streams. The other class, which is represented by most of the depressions, embraces the wide, gently rounded col-like depressions 

that suggest at first a normal lowering of a divide crest by headward 

erosion of opposing streams. But these notches are not depressions 

in a sharp crest such as would be expected if the lowering were due 

to streams contesting a narrow divide. On the contrary, they extend 

with level floors for half a mile or more in a direction at right 

angles to the trend of the divide, and this trend suggests that they 

were originally water gaps, which have been abandoned by their 

streams in a late mature stage of erosion. If these depressions in 

the divide are really the result of stream erosion in previous erosion 

cycles the altitude of the base of the depressions should show a definite relation to that of remnants of erosion surfaces in other parts 

of the region, such as benches, shelves, or hill summits.

By a careful study of different localities it is possible to determine, 

for each, the relative position of the erosion surfaces. In the following table the base-levels and erosion surfaces recognized by 

Knopf in the region studied are compared with those previously 

determined bv Barrel! and Bascom.
U. g. GEOLOGICAL STJBVEY BULLETIN 799 PLATE 6

ANTICLINE IN ANTIETAM SCHIST, HOAD CUT 2 MILES WEST OF BELLEMONT 

Shows gently dipping bedding and steeply dipping schistosity cutting it

B. CLOSKLY b'OLDKD \.NTIKTA\1 SCHIST, ROAD CUT ON EAST SIDE OF

OAK HILL

The folded bedding is outlined by quartz veins and cut at a steep angle by schistosity dipping

northwest . 
U. S. GEOLOGICAL SURVEY BULLETIN 799 PLATE 7

BLUE MTN.

J>£K£PLAIN_ SOUTH MTN. "^ILLS*

 _ZT__ __ SCHOOLEY PENEPLAIN .

MIMF otn^f /~\ n r*,. *.- «. I~7T. T ~r\- 

Longitudinal profile of Susquehanna R

WELSH MTN.

.200- __ __ _5CHOOLEY __

MW£_mDG£ PENEPLAIN

7fl° 77° ' Ttf*

A. PROJECTED PROFILES OF AREA BETWEEN BLUE MOUNTAIN AND THE COASTAL PLAIN

B. PROFILE OF DIVIDE BETWEEN SUSQUEHANNA AND SCHUYLKILL RIVER DRAINAGE SLOPES
V

Altitude, in feet, of base-levels and erosion surfaces in App

o 

1-i 

co 

T Fluvial base-levels in northern 

Appalachian Highlands (Joseph 

»o Barrell)

<vi

Kittatinny.

Schooley (?).

(?).

Sunbury.

Harrisburg.

Somerville.

1,600

1,350

950

750

520

400

Piedmont province of Pennsylvania 

(F. Bascom)

Peneplains

Kittatinny.

Schooley.

Honey brook.

Harrisburg.

Early Brandywine. 0

West

1,800

1,300

860

800

500

Terraces

Late Brandywine.6

Sunderland.

Wicomico.

Talbot.

400

300

90

45

East

1, 600-1, 100

1,000-900

800-700

500

400-390

300-200

180-100

45

40-0

Remnants of erosion surfa

Kittatinny.

Schooley.

Mine Ridge.

Honey brook.

Harrisburg.

Bryn Mawr.

Brandywine.

Sunderland. .

Wicomico.

Talbot.

° Now replaced by Bryn Mawr. Bascom, F., Resuscitation of the term Bryn Mawr gravel: U. S. Geol. Survey 

6 Now replaced by Brandy wine. Idem.
100 McCALLS FERRY-QUARRYVILLE DISTEICT, PA.

Comparison shows that the results of the present study bear out to 

a large extent the conclusions of Barrell and Bascom and also suggest the existence of an additional erosion cycle, here called the Mine 

Eidge cycle because the surface then developed is well preserved on 

the summit of Mine Kidge.

STUDY OF DRAINAGE MODIFICATIONS

Mine Ridge cycle of erosion. The highest level surface in the 

summit of Mine Kidge at altitudes ranging from 820 to 880 feet is 

upheld by steeply inclined quartzite beds. The hill summits on the 

two sides of the wind gap at Gap are distinctly lower than the southwest end of Mine Ridge, being only 740 to 760 feet above sea level, 

but the northeastward continuation of Mine Ridge in the area of the 

Coatesville and Honeybrook quadrangles rises to altitudes of 800, 840, 

and 880 feet. That the altitude of this ridge is not determined by 

the relatively high resistance to erosion of the underlying rock is 

shown by the facts that the quartzite ridge has been reduced to the 

above-mentioned wide depression marked by the 760-foot hill summits at Gap and that in this depression there is an old valley preserved in a wind gap, showing that a transverse superimposed stream 

once flowed across a surface of low relief, the remnants of which 

remain in the highest 820 to 880 foot summits of the quartzite ridge 

on the two sides of the gap. Moreover, the same general altitude as 

that in the highest summits of Mine Ridge reappears at Hickory 

School, 10 miles nearer to the trunk stream of the district, where the 

flat-topped ridge at about 900 feet is underlain by micaceous and 

chlorite schists whose resistance to erosion is much less than that of

quartzite.

The identification of the Mine Ridge cycle and its distinction from 

the Schooley cycle rests on the existence of a well-defined level surface 

on South Mountain below the remnants of the Schooley peneplain. 

This lower surface suggests a dissection of the Schooley surface with 

the resultant formation of the present 1,000-foot level. A study of 

the profile of the Blue Mountain front of the Appalachian Valley 

shows the record of this erosion cycle in a group of wind gaps, which 

lie between 1,100 and 1,200 feet. These gaps are approximately 200 

feet lower than the gaps recording the Schooley channels.
PHYSIOGRAPHIC RECORD 101

Altitude of depressions that are pro'ba'bly wnd gaps recording the Mine Ridge 

erosion cycle, during which the Schooley peneplain was dissected

Locality Name

Little Gap. ________________ .

Altitude (feet)

Base

1,180 

1,180 

1,100 

1, 000-1, 080

Adjoining hill 

summits

1,340 

1,340 

1, 440-1, 500 

1,200

Present altitude of valleys cut during the Mine Ridge erosion cycle as shown in 

the restored cross profiles of composite valleys

Locality

Blue Mountain. __ . ............

Name

Manada Gap ___ . _______ .. .....

Altitude (feet)

Base

1,200 

1,200 

1,180

Adjoining 

hill 

summits

1,340 

1,300 

1,440

These gaps indent the Schooley surface preserved upon Blue 

Mountain at altitudes between 1,300 and 1,400 feet and trench the 

mature Schooley valleys. This post-Schooley valley cutting therefore 

coincides with the development of a peneplain or partial peneplain 

south of Blue Mountain whose remnants are lower than the residuals 

of Schooley erosion on Schooley Mountain. A wind-gap record of 

the Mine Ridge erosion cycle is seen in Little Gap, 5 miles northeast 

of Lehigh Water Gap. Little Gap, whose base lies at 1,100 feet, 

records three erosion cycles in its relation to the highest summits of 

Blue Mountain. The oldest cycle is recorded by the Kittatinny surface, whose remnants lie at 1,620 feet 3 miles northeast of Little Gap 

and 3 miles southwest of Lehigh Water Gap. The record of the 

succeeding Schooley cycle appears in the somewhat irregular surface 

between 1,400 and 1,500 feet and in the upper slopes of the composite 

cross profiles of the valleys of Little Gap and Lehigh Gap shown in 

Figure 6. The lower steep gorge of Little Gap records the downcutting produced by quickened erosion in the Mine Ridge cycle.

Therefore, during the erosion cycle that succeeded the Schooley 

cycle Blue Mountain was dissected to a depth now recorded at 1,100 

feet. Local plane surfaces were also cut in the vicinity of South 

Mountain that are recorded in plateau remnants at about 1,000 feet, 

and the region south of South Mountain was reduced to a partial 

peneplain whose remnants are so well preserved at 800 to 900 feet in 

Mine Ridge and in the flat-topped hills southwest of it that this 

erosion cycle is called the Mine Ridge cycle. During the Mine Ridge
.Feet

1,600 -

1,400 -

1,200 -

1,000-

800-

600-

400-

200-

0

Sect fevel

^P

0 12345 

 ''ii________1________1 I____

FIGDBB 6. Projected profile of Blue Mountain showing cross profiles 

3' °> £ CD

| fig,

CD ^ CTQ 

-IP *

u_^ ^ <"": ^ ^

I

O O ca 

 -S KJ S 
PHYSIOGRAPHIC RECORD 103

Honeybrook subcycle of erosion. The rejuvenation that interrupted the continuity of erosion during the Mine Kidge cycle was 

recognized by Bascom 82 in the remnants of erosion surfaces now 

preserved at 720 feet near Honeybrook, Pa. As a result of this 

rejuvenation terraces were cut on the resistant slopes of Mine Ridge 

upland at 720 feet. South of Mine Eidge the seaward erosion of 

the Honeybrook cycle is preserved in the terrace-like surface at 720 

feet. In Blue Mountain streams quickened by the Honeybrook 

uplift cut deep gorges, which were abandoned by their streams in 

 consequence of the next rejuvenation and now stand as the wellknown wind gaps represented by Pen Argyl Gap (980 feet), Culvers 

Gap (920 feet), and Sterrett Gap (920 feet).

Harrisburg subcycle of erosion. The work of the next succeeding cycle has been fully outlined by Barrell 83 for the region between

Sunbury and Harrisburg. He named the resultant surface the Sunbury peneplain because remnants are well preserved at about 850 

feet in the flat hilltops near Sunbury, Pa.

The upland surface underlain by Martinsburg shale north and 

east of Harrisburg shows vestiges of two erosion surfaces. A local 

peneplanation is recorded in the general upland surface at about 540 

feet, and remnants of an earlier planation rise above the 540-foot 

level in isolated monadnocks with an altitude of 660 feet. These 

upper residuals approximate the altitude of the Sunbury peneplain in 

the Harrisburg quadrangle described by Barrell. Campbell, who 

was the first to point out the operation in this region of more than 

two erosion cycles,84 has since decided to restrict the name Harrisburg to the surface that is preserved at altitudes of 600 to 660 feet 

northeast of Harrisburg. Therefore the name Sunbury is abandoned in favor of the name Harrisburg, which has priority of usage. 

It is to be noted, however, that the name Harrisburg as here employed is the equivalent of the name Sunbury employed by Knopf 

in an earlier paper on the subject.85

The following table shows the altitude of several valleys, probably 

wind gaps, that were occupied by stream courses during Harrisburg 

erosion.

82 Bascom, F., U. S. Geol. Survey Geol. Atlas, Elkton-Wilmington folio (No. 211), p. 2, 

1920.

83 Barrell, Joseph, The Piedmont terraces of the northern Appalachians: Am. Jour. Scl., 

4th ser., vol. 49, p. 336, 1920.

84 Campbell, M. R., Geographic development of northern Pennsylvania and southern 

New York: Geol. Soc. America Bull., vol. 14, p. 284, 1903.

85 Knopf, E. B., Correlation of residual erosion surfaces in the eastern Appalachian Highlands : Geol. Soc. America Bull., vol. 35, pp. 653-658, 1924.
104 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

Altitude of probable wind gaps recording the Harrisburg erosion during which 

the Mine Ridge peneplain was dissected

Location

Mine Ridge. _ ... . __ .... ____ . _ . __ ....... __ ...

Name

fStewart Gap... _ . _ . _ . ...

\ Gap north of Port Murray ......

Gap....... ...... ........... __ .

Altitude 

of base 

(feet)

740

740

750

680

Paleophysiographic maps of the region near the end of Honeybrook and Harrisburg erosion (figs. 7 and 8) show that the outlines 

of the present highland areas south of Lebanon Valley were clearly 

delimited and persistent, thus indicating that the normal progress of 

erosion was interrupted before either cycle had attained even approximate completion. The upland of Mine Eidge was initiated in the 

Honeybrook cycle. The shore line at this time probably lay southeast of the Mine Ridge upland, for old drainage courses recorded in 

the highlands north of the Mine Ridge upland suggest a persistence 

of fluvial erosion in that part of the region throughout Honeybrook 

and Harrisburg time. Gravel has been found by the writers at 620 

feet on the Mine Ridge upland about 3 miles northwest of Coatesville.

Bryn Mamy sub cycle of erosion. In the region of Harrisburg below the remnants of the Harrisburg peneplain there are also remnants 

of a lower level surface preserved on the Martinsburg shales at altitudes of 500 to 560 feet. Barrell 86 ascribed to the remnants of this 

erosion surface traced northward from Harrisburg a gradient of 21 

feet to the mile, and this slope projected southward would place 

residual surfaces of the same erosion cycle near Chesapeake Bay 

at an altitude of 440 feet, an estimate which accords remarkably well 

with the 460-foot surface at Woodlawn, northeast of Perryville, Md, 

North of Woodlawn and south of Chester Valley, in the McCalls 

Ferry-Quarryville district, the same surface is well developed at 500 

to 460 feet. At Woodlawn this plane surface bevels the underlying 

Cretaceous formations and gravel formerly mapped as " Lafayette," 8T 

now known as Bryn Mawr gravel.88 As it is probable that the gravel 

associated with the 460-foot surface at Woodlawn is a marine deposit 

upon a wave-cut plane that was formed at the same time as the 

subaerial erosion surface now lying at 540 feet near Harrisburg, the

88 Barrell, Joseph, op. cit., p. 336.

87 Shattuck, G. B., Development of knowledge concerning the physical features of Cecil1 

County: Maryland Qeol. Survey, Cecil County, p. 166, 1902.

88 Bascom, F., The resuscitation of the term Bryn Mawr gravel: U. S. Geol. Survey 

Prof. Paper 132, pp. 117-119, 1924.
PHYSIOGRAPHIC RECORD 105

name Bryn Mawr is used to denote this subcycle of erosion. In an 

earlier paper 89 this subcycle was called the Harrisburg.

Brandywine subcyde of erosion. South of Harrisburg the 560- 

foot level of the shale hills descends abruptly to a wide lowland at 

about 400 feet underlain by calcareous rocks. This lowland, although 

known as Lebanon Valley, is not genetically related to a single stream, 

for Swatara Creek, which occupies the valley during a portion of its 

course, flows out of the limestone into the shale hills, utterly regardless of lithologic control. South of Lebanon Valley Lancaster ValFIODUB 7. Generalized topography of eastern Pennsylvania and Maryland at the 

end of the Honeybrook subcycle of erosion. Stippled areas are uplands; hachured 

lines, escarpments

ley stretches out in a wide, gently undulating surface of low relief, 

underlain by limestone and dolomite. About 130 square miles of 

country east of Susquehanna River maintains an approximate level 

of 380 to 420 feet. Above this level rise the low ridges whose summits are remnants of the dissected Bryn Mawr peneplain. That the 

400-foot level has been produced by erosion and is not due to the 

character of the underlying calcareous rock is established by the fact 

that the surface bevels the folded schist, limestone, and dolomite in 

the McCalls Ferry quadrangle east of Pequea Creek and north of the 

Martic Hills. South of Conestoga the presence of a notch 100 feet

^Knopf, E. B., op. cit, p. 658.
106 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

deep whose base is at an altitude of 400 feet suggests a probable 

wind gap, which therefore fixes a point upon a stream, that dissected 

the Bryn Mawr peneplain.

The remarkable incised meanders of Swatara and Conodoguinet 

Creeks were evidently initiated upon the surface of the local peneplain formed in the Bryn Mawr erosion cycle and cut down during 

the erosion that dissected the Bryn Mawr surface. The lowlands of 

the present topography are therefore due to post-Bryn Mawr erosion,

FIGDRB 8. Generalized topography of eastern Pennsylvania and Maryland at the end 

of the Harrisburg subcycle of erosion. Stippled areas are uplands; hachured lines, 

escarpments

whereas the rolling hills that bound the wide lowlands have been 

formed by erosion during the Bryn Mawr subcycle.

The duration of the subcycle of erosion that dissected the Bryn 

Mawr surface was relatively short, as pointed out by Campbell,90 

who called attention to the rise of the country underlain by limestone to more than 500 feet on the divide between the Susquehanna 

and Schuylkill drainage systems. Similarly on the divide between 

the Schuylkill and the Lehigh the 400-foot level is absent, showing 

that the Brandywine erosion, although relatively extensive where the 

underlying rock is soft, was confined to areas near the master streams

90 Campbell, M. R., Geographic development of northern Pennsylvania and southern New 

York: Geol. Soc. America Bull., vol. 14, p. 283, 1903.
PHYSIOGEAPHIC RECORD 107

and nowhere succeeded in reducing the divide, although it is also 

on soft rock, to the level of Lancaster Valley.

The brevity of the time during which the limestone was being 

eroded is also shown by the fact that south of Mine Kidge the widespread surface at 380 to 400 feet is lacking but is replaced by a 

well-defined terrace-like surface at 320 to 340 feet. This shoulder or 

terrace can be followed up the valleys of the Susquehanna and its 

tributaries for a certain distance and then disappears, being replaced

FIGURE 9. Generalized topography of eastern Pennsylvania and Maryland at the 

end of the Bryu Mawr subcycle of erosion. Stippled areas are uplands; hachured 

lines, escarpments

farther upstream by another terrace at a higher level. The point 

on the stream valley where the 340-foot terrace disappears is marked 

by a knickpoint in the longitudinal stream profile, thus suggesting 

that this point may be the headward limit of rejuvenated erosion 

for the partial cycle that cut the stream terrace at 320 feet. Although 

this partial cycle lasted only long enough to permit stream terracing 

of valleys on resistant rocks in southern Pennsylvania, a local peneplain was developed farther north, across the easily eroded limestone 

of Lancaster Valley. (See figs. 9 and 10.)

The 380 to 420 foot surface in the region east of the Susquehanna 

appears to correspond to what has been called the Somerville plain
108 McCALLS FERRY-QTJARRYVILLE DISTRICT, PA.

by Campbell.91 But the name Somerville has been used to describe 

a low-lying surface that occupies much of the Piedmont area in New 

Jersey and slopes southeastward within the Raritan quadrangle 

from an altitude of 200 to 300 feet near the base of the Highlands 

escarpment to about 100 feet in the southeast corner of the quadrangle.92 This gives a slope of 11 feet to the mile, which is excessive 

as compared with the slopes of the older erosion surfaces and which

FIGURE 10. Generalized topography of eastern Pennsylvania and Maryland at the 

end of the Brandywine subcycle of erosion. Stippled areas are uplands; hachured 

lines, escarpments

undoubtedly indicates the presence of more than one plane of erosion. In the neighborhood of Frankfort the local plane surface at 

140 feet, carrying Pensauken gravel, is probably equivalent to the 

surface having the same altitude around Somerville.

The name Somerville, as originally used, clearly embraces several 

different erosion surfaces, and because the surface around Somerville 

was developed later than the local peneplain preserved in the floor 

of Lancaster Valley the latter can not now be considered Somerville. 

It is possible that the fluvial erosion surfaces preserved in Lancaster 

Valley can be correlated with the marine gravel of the Brandywine

81 Campbell, M. R., Geographic development of northern Pennsylvania and southern New 

York: Geol. Soc. America Bull., vol. 14, p. 283, 1903.

82 Bayley, W. S., Kiimmel, H. B., and Salisbury, R. D., U. S. Geol. Survey Geol. Atlas, 

Raritan folio (No. 191), p. 3, 1914.
j PHYSIOGRAPHIC RECORD 109

!

that lies at an altitude of 230 feet at .the type locality in Maryland. 

This gravel is undoubtedly the equivalent of the low-level gravel 

(Brandywine formation) at an altitude of 200 feet in Elk Neck, on 

the eastern shore of Chesapeake Bay. The relation of the "late

,^ Brandywine " 93 surface to the Sunderland terrace on Elk Neck corresponds so closely to the relation between the 320-foot benches along 

the Susquehanna and the Sunderland benches at 200 to 220 feet that

k -the contemporaneity of these 320-foot terraces and " late Brandywine " erosion is suggested, and the name Brandywine 94 is used to

 denote the subcycle of erosion in which the terraces were cut and the

->- local peneplain was formed in Lancaster Valley.

i Late Pleistocene erosion. The Atlantic coast line is bordered by 

three terraces known as the Sunderland, Wicomico, and Talbot terraces. The oldest of these is the Sunderland, and the deposits of

 each of the two younger terraces wrap around the scarp of the next 

older one and extend up the valleys cut in the older terrace, thus 

> showing that after the formation of each successive terrace the sea 

level was lowered and the terrace underwent a certain amount of 

erosion, after which the sea once more encroached upon the land and

*- the deposits of the succeeding terrace were laid down in the lower 

portions of the stream valleys. This sequence of events was pointed 

out by Shattuck,95 who also called attention to the fact that the 

surface was in many places stripped of one formation before the

- .succeeding deposit was laid down.

\ SIGNIFICANCE OF KNICKPOINTS IN STREAM GRADIENT

J> On the principle that uplift will cause an alteration in the normal

* profile of a stream course and that maturity is first established in the 

lower part of the stream course, the new mature profile after rejuve-

> nation will appear near the mouth and will extend backward as far as

P the stream has succeeded in cutting back its channel before interruption by another uplift. Therefore, the uplifts during which the

V Pleistocene terraces were dissected and partly removed should have 

left some record of their quickening effects upon the stream erosion 

in the cross profiles and longitudinal profiles of the larger streams

 of the region.

However, it must be remembered that abrupt changes in the longitudinal profile may be produced in other ways than by rejuvenation: 

i (1) A steepening of the curve at the point where the river channel 

passes from limestone into quartzite or vice versa might produce a 

sudden steepening of gradient but would obviously have no effect

83 Bascom, F., U. S. Geol. Survey Geol. Atlas, Elkton-Wilmington folio (No. 211), p. 3, 

1920.

°* Knopf, B. B., op. clt., p. 659.

80 Shattuck, G. B., The Pliocene and Pleistocene deposits of Maryland: Maryland Geol. 

T Survey, Pliocene and Pleistocene, pp. 135-136, 1906.
110 McCALLS FERRY-QTJAERYVILLE DISTRICT, PA.

upon the upstream limit of accelerated stream cutting. The effect of 

harder beds athwart a stream would not, however, be seen in a stream 

that has already cut its channel to maturity. A knickpoint that indicates the headward limit of rejuvenated erosion may coincide with 

the interposition of hard layers across the river channel and thus 

may not be caused by the hard layer alone. The stream profile will 

then not show similar irregularities at all such points where hard 

layers occur athwart the stream course. Knickpoints are particularly 

significant as indicating rejuvenation where they occur as gentle 

arches upon an approximately horizontal curve in the profile of a 

stream that has attained maturity, rather than where they occur as 

steps in a steep curve that denotes a stream that is cutting downward 

in a youthful valley. (2) Deviations from the normal stream profile 

may also be produced by climatic factors, such as climatic changes 

during Pleistocene time. But such secular changes in climate would 

influence the eroding power of streams in this region as a whole and 

would not therefore produce knickpoints on the stream gradient. 

(3) Sudden steepening in a prevailing gentle curve may be caused 

by local uplift across the bed of a stream, as pointed out by Campbell 96 in the explanation of Muscle Shoals on Tennessee Eiver. But 

the value of knickpoints as an aid in recognizing regional uplift lies 

in the fact that the headward limit of stream rejuvenation caused by 

such an uplift will -be recognizable not only in the main stream but 

also in the tributaries at points which clearly are not conditioned by 

local factors, such as bedrock, faulting, or local upwarping. Knickpoints can be used as records of physiographic history only after 

many data have been assembled and critically evaluated for the whole 

region. The irregularities in longitudinal stream profile thus determined to be significant must be corroborated in the corresponding 

cross profiles of the stream valleys. The present study of the drainage of that part of the eastern Atlantic slope in which the McCalls 

Ferry-Quarryville district lies is obviously only a small fraction of 

the whole study necessary to establish conclusively the physiographic 

history of the region as a whole. It is presented here as suggestive 

rather than as complete, in the hope that other investigators may be 

tempted to apply it to adjoining regions and that thus the results 

suggested by this investigation may be tested.

LONGITUDINAL PROFILES OF STJSQUEHANNA RIVER AND MINOR STREAMS

o

As far back as 1888 McGee 97 noted the fact that Susquehanna

River, in common with other rivers of the eastern Atlantic slope that

,

98 Campbell, M. R., Drainage modifications and their interpretation: Jour. Geology, vol. 

4, p. 667, 1896.

97 McGee, W J, The geology of the head of Chesapeake Bay: U. S. Geol. Survey Seventh 

Ann. Rept, pp. 548-558, 1888.
PHYSIOGRAPHIC RECORD 111

flow across both the Piedmont Upland and the Coastal Plain, is 

characterized by a mature concave profile at the mouth, which changes 

abruptly, a short distance from the mouth or from the entrance of 

the river into the Coastal Plain, into a convex profile of high declivity characteristic of an actively corrading stream. The available 

map data for Susquehanna River are shown in the following table:

Approximate altitude and distance from the mouth of several points on the 

longitudinal profile of Susquehanna River

Locality

Havre de Grace..-.......... ............

Turkey Hill ....... .....................

York Haven ____ . __ ...... ___ .

Half Falls... ............ ..................

Altitude 

(feet)

0

40 

120 

200 

260 

360 

380 

420

Distance 

from 

mouth 

(miles)

0 

9.5 

25 

39 

56 

93 

100 

118

Distance 

between 

points 

(miles)

9.5 

17 

12 

18.5 

38 

7 

20

Fall between points

Feet

40 

80 

80 

CO 

100 

20 

40

Feet per 

mile

0.4 

4.7 

6.6 

3.24 

2.6 

2.9 

2

A longitudinal profile of Susquehanna River made from the available map data is shown in Figure 11. In this profile a slightly 

concave curve extends almost horizontally for a distance of 3 miles 

from the mouth; thence upstream a steeper curve extends to a point 

about 9 miles from the mouth, where there is a low fall or rapids, 

shown on the topographic map as Amos Falls. From this point 

northwestward to the Holtwood Dam, a distance of 25 miles from 

the mouth, the river flows along a slightly convex immature profile. 

The Holtwood Dam has utilized a natural falls or rapids known as 

Cullys Falls. From the dam northwestward the river gradient maintains a convex curve as far as Turkey Hill, 39 miles from the mouth.

It is a notable fact that these changes in gradient take place in a 

rock bottom that is of approximately homogeneous resistance to 

erosion, and from a point about 3 miles north of Havre de Grace 

to Turkey Hill the river flows in a rocky gorge, whose local widening 

and constrictions show no relation to the steepenings of the longitudinal profile. Thus there are three stretches of the river channel 

between the mouth and Turkey Hill delimited by knickpoints at 

altitudes of about 40 feet and 120 feet. These three stretches suggest 

the influence of three different periods of stream cutting interrupted 

by successive uplifts.

North of Turkey Hill the river channel passes into limestone and 

dolomite. There is a suggestion that the flattening of the profile 

north of Turkey Hill is not caused by change in the.underlying rock, 

because for a distance of 18 miles from Turkey Hill to the vicinity 

of Yorkhaven the river maintains an even and relatively low gradient over limestone, quartzite, shale, and sandstone. Although not
f

o 

t> 

<S.

e 

o< 5' 

£- 

 a

rocks Granitic 

FALLS AMOS 

schist Sericite 

rocks Granitic 
PHYSIOGRAPHIC RECORD 113

shown in the profile constructed from map data (see fig. 11) the 

natural fall of the river near Yorkhaven is steep, amounting to 23 

feet in three-quarters of a mile at Conewago Falls.98 This steepening is conspicuously shown in the Susquehanna River profile published in the report by Hoyt and Anderson and might be ascribed 

to the presence of diabase in the river bed at this point, but between 

Yorkhaven and Half Falls, a distance of about 38 miles, the river 

flows across Triassic strata, diabase, and Paleozoic limestone and 

shale along a uniform gradient which is once more gentle, with a 

total fall of 60 feet. Therefore the fact that Conewago Falls occurs 

at a point where the channel is in diabase does not appear to explain 

the knickpoint. Between Half Falls, at 340 feet, and a point 2 miles 

north of Selinsgrove, where the altitude is 420 feet, the river falls 

60 feet in 20 miles, and above Selinsgrove the gradient is almost horizontal for a long distance up the main branch of the Susquehanna.

There are therefore knickpoints on the profile of the Susquehanna 

River at 40, 120, 200, 260, 340, and 420 feet. To test the significance 

of these knickpoints a number of longitudinal profiles were constructed for tributaries of the Susquehanna and also for two streams 

draining directly into the head of Cheasapeake Bay. In order to 

remove the subjective element as far as possible the profiles of each 

stream were constructed on more than one scale using different 

vertical exaggeration. The lithologic character of the bedrock was 

platted on each profile, and the relation between stream cutting and 

the resistance of the eroded rock was thus taken into account in 

estimating the significance of knickpoints. Some of the profiles are 

shown in Figure 11.

The following table shows the knickpoints that were recognized 

in nine profiles and that were regarded as significant.

Altitude, in feet, of significant knickpoints on longitudinal profiles of Susquehanna River and its tributaries

Point

Mouth.. 

1.. ...

2... ..

3........

4........

5.. ......

Susquehanna 

River

0

40 

120 

200

260 

360

Principio 

Creek

0

40 

120 

180

260 

380 

400

Blythedale 

Creek «

0 

40 

120 

200

380 

420

Mill 

Creek

20

60

220

360

Rock 

Run

20

160

240 

380 

440

Octoraro Creek

40 

60 

120 

200

East 

branch

420 

560

West 

branch

380 

780

Basin 

Run

60

200 

280

380

Conowingo 

Creek

50

120 

220

300 

420 

800

Fishing 

Creek

90

120 

200

260 

420 

750

 This stream, which is not named on the map of the Havre de Grace quadrangle, flows through the 

village of Blythedale.

88 Hoyt, J. C., and Anderson, R. H., Hydrography of the Susquehanna River drainage 

basin : U. S. Geol. Survey Water-Supply Paper 109, p. 199, 1905.
114 McCALLS FERRY-QTJARRYVILLE DISTRICT, PA.

From the table it is apparent that three out of the six streams 

whose mouths lie at altitudes of 40 feet or less show knickpoints at 

40 feet and two show knickpoints at 60 feet. From-40 feet to the 

mouth Principio Creek and Blythedale Creek have cut through the 

Talbot terrace, showing that this stretch of the stream profiles was 

formed during the post-Talbot erosion and suggesting that the headward limit of post-Talbot erosion in the streams whose mouths lie 

above sea level is 60 feet. Above the knickpoint at 40 feet Blythedale 

Creek and Principio Creek are cutting into the Wicomico terrace. 

The Wicomico deposits extend up to 80 feet, thereby suggesting that 

the knickpoints at 120 feet indicate the headward limit of postWicomico erosion. It is probable that the knickpoint at 160 feet 

on Rock Run has the same significance, although the profile of 

Rock Run shows no change in gradient at 40 feet or at 120 feet. 

There is a knickpoint at 160 feet in the profile of Octoraro Creek 

which coincides with the presence in the stream bed of a pegmatite 

ledge shown in the geologic map of Cecil County. This knickpoint 

is therefore disregarded, as being due to rock control. A knickpoint 

at 80 feet on the profile of Principio Creek is disregarded, because 

it does not appear on the other eight profiles. Every one of the 

nine profiles shows a knickpoint at elevations ranging from 160 to 

220 feet. On the profile of Blythedale Creek the knickpoint occurs 

at 200 feet and on Principio Creek at 180 feet. These streams have 

both cut through a Sunderland cover that attains an altitude of 

180 feet, thus suggesting that 200 feet is the headward limit of postSunderland erosion.

The knickpoint at 260 feet that coincides with Conewago Falls on 

the Susquehanna is duplicated in five out of the other eight profiles. It does not show in the bed of Octoraro Creek, a fact which 

niay possibly be explained by confluence of the two main branches 

of Octoraro Creek at 240 feet. Figure 12 shows cross profiles of 

the Octoraro Valley at various places and of the wind gap against 

which the Octoraro heads. The cross section near Porter Bridge 

(fig. 12, C) shows the 3-story valley formed by the entrenchment of a valley superimposed upon the surface that carries Bryn 

Mawr gravel at Woodlawn. abed shows the outline of the postmature valley cut in the Brandywine erosion cycle, during which 

a rock bench at 300 to 320 feet was developed by lateral planation. 

Within this valley of Brandywine age a shallow valley at cdfg 

is associated with a similar bench at 220 feet. These 220-foot benches 

can be traced down Susquehanna River to the mouth, where they 

appear to merge into the level occupied by remnants of Sunderland 

deposition. This valley at cdfg is probably a Sunderland valley, 

and the streams of the region that formed the 220-foot benches were
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116 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

probably cutting along the gradient above the 200-foot knickpoint 

during the time that a marine plain was formed and covered with 

Sunderland deposits, which now lie at altitudes from 100 to 180 

feet. The 260-foot knickpoint thus probably represents the headward limit of stream cutting during Sunderland marine deposition.. 

In eaqh stream whose source lies above 360 feet there is a knickpoint at an altitude between 360 and 420 feet, in four at 380 feet,, 

and in three at 420 feet. It is probable, therefore, that during the 

erosion cycle that cut the 300 to 340 foot benches which are conspicuous along Susquehanna River and Octoraro Creek for a certain 

distance the headward limit of stream erosion had attained approximately 400 feet, which thus marks the limit of Brandywine 

rejuvenation.

Above 420 feet on the longitudinal profile of the East Branch of 

Octoraro Creek the cross profiles just below the wind gap show 

shallow valleys in the wide Harrisburg erosion surfaces at 600 -to 

620 feet. Therefore from 420 feet to the source, at 560 feet, a 

stream was cutting into the Harrisburg surface. Directly above the 

source of the East Branch the wind gap at Gap records the course 

during Harrisburg time, when the ancestral Octoraro Creek, which 

flowed through the gap, still had its source north of Mine Ridge. 

During the Bryn Mawr subcycle the stream was diverted and the 

Harrisburg valley was hung up as a wind gap.

Mine Ridge is a local divide, and above the level of the wind gap 

the record of present stream courses fails, because the streams that 

flow northward from the divide are of more recent age than the 

summit divide. The evidence of previous cycles lies in the superimposition of the ancestral Octoraro Creek across the Mine Ridge 

surface and in the shape of the composite valley of the wind gap 

(fig. 12, A), which records in its gentle upper slope the intrenchment 

of the superimposed stream accompanied by extensive lateral planation during the Honeybrook subcycle and in its steep inner gorge 

the downcutting of the Harrisburg valley from which the Octoraro 

Creek was diverted, probably in the early stages of Bryn Mawr 

rejuvenation.

The profiles of Octoraro Creek and of the gap above its source, 

therefore, suggest-the physiographic history of the region from the 

period of valley cutting that dissected the Wicomico marine terrace 

as far back as the Mine Ridge cycle.

SEQUENCE AND CORRELATION OF EROSION RECORDS

It has been possible, therefore, by an analysis of present and past 

drainage to suggest that eight successive uncompleted erosion cycles 

are recorded in the region between Chesapeake Bay and Mine Ridge..
PHYSIOGRAPHIC KECOED 117

North of Mine Eidge vestiges of two older cycles are preserved in 

remnants of former erosion surfaces occupying the summits of South 

Mountain.

AMOUNT AND DURATION OF EROSION

The relative amount and duration of erosion comprised in each of 

the 10 complete or partial cycles of erosion, whose existence has been 

indicated, are impossible to estimate, because there is no means of 

determining the maximum area undergoing erosion during the older 

cycles, the agents by which the work of these cycles was accomplished, or the rate at which they acted. During the Kittatinny cycle 

the region between South Mountain and Blue Mountain was reduced 

to a plane surface of low relief that was cut across the most resistant 

formation, giving for the region lying between Delaware and Susquehanna Rivers a minimum area undergoing erosion amounting to 

2,000 square miles, which is undoubtedly only a fraction of the original extent. In the succeeding three cycles the corresponding eroded 

area amounted to at least 5,000 square miles, but the reduction was 

much less complete than in the previous smaller area. It is evident 

that a large part of the 5,000 square miles was undergoing fluvial 

erosion, which probably progresses more slowly than marine planation; and the failure to reduce the region completely during all 

post-Schooley erosion may be due to the fact that the Kittatinny,and 

Schooley peneplains were formed by marine erosion, which reduces 

alternating belts of hard and soft rock with variable rapidity, but 

with equal completeness. If, on the other hand, the 2,000 square 

miles that was eroded during the Kittatinny and Schooley erosion 

cycles was degraded under subaerial conditions the length of these 

two cycles must have been appreciably longer than that of any 

succeeding partial cycle.

PERIODICITY OF UPLIFT

Whatever the duration of the earliest cycles it must be emphasized 

that the present study of the evolution of drainage in the region 

between the Blue Mountain range of the Appalachian Kidge and 

Valley province and the Coastal Plain clearly disproves the idea of 

numerous successive peneplanations of the area and at the same 

time equally clearly establishes the recurrence of numerous slight 

uplifts that caused repeated interruptions to the continuity of baseleveling. The three youngest uplifts were oscillatory in character 

along the inland margin of the present Coastal Plain. Whether thp 

older uplifts were accompanied by similar oscillations in level is 

not clear, but the net result of coastal movement has been an intermittent but progressive elevation that has raised the Blue Mountain-
118 McCALLS FERBY-QUARETVILLE DISTRICT, PA.

Kittatinny range at least 1,600 feet in a saltatory series of slight 

uplifts. Such a succession of slight uplifts operating throughout a 

long time suggests a periodic relief of a steadily accumulating vertical 

strain. It is an interesting speculation whether such periodic movements record the periodic establishment of isostatic equilibrium along 

the border of an area of deposition, rather than the continuous 

maintenance of isostatic balance demanded by the extreme isostasists.

AGE OF EROSION CYCLES

The effect of such repeated uplift is to postpone indefinitely the 

total degradation of a region by constant addition to the area undergoing reduction and by setting back the progress of the work, so to 

speak, after each interruption. It is therefore idle to speculate as to 

the age of the oldest surface in the region on the basis of the probable 

time required to reduce such a surface to sea level.

The evidence on the age of erosion cycles that is based upon the 

 correlation of the present slope of the old fluvial gradient with the 

present slope of buried peneplains is perhaps more reliable, and 

according to thi,s criterion the gentle seaward slope of the old fluvial gradients suggests that no erosion cycle recorded in the region 

between Blue Mountain and the Coastal Plain is older than late 

Tertiary. The gaps in the geologic record over late Cretaceous and 

early Tertiary time may eventually be filled, at least partly, by a 

physiographic study of the Atlantic drainage slope in areas more 

remote from present base-level than those covered by the present 

study.

ORIGIN OF METAMORPHISM IN THE CRYSTALLINE

SCHISTS OF THE PIEDMONT PROVINCE

VARIABLE METAMOEPHIC INTENSITY IN THE McCALLS FERRYQTTARRYVILLE DISTRICT

The most striking feature of the crystalline schists of the McCalls 

Ferry-Quarryville district is the relatively intense metamorphism 

of the autochthonous Paleozoic strata in comparison with that of the 

pre-Cambrian rocks in the overlying thrust block. The argillaceous 

Cambrian sediments on the flanks of the Mine Ridge anticline have 

been metamorphosed to garnetiferous biotite schists, and impure 

Ordovician limestone in the Chester and Quarryville Valleys has been 

altered to micaceous marble that contains numerous beds of mica 

schist. The metamorphism of the pre-Cambrian rocks that have been 

thrust northward is most intense in the cores of the double-crested 

Tucquan anticline, which is the southwestward continuation of the
METAMOKPHISM 119

downward plunging anticlinal fold underlying the Mine Eidge upland. But in the Mine Kidge anticline, where the pitch of the anticline brings up the Paleozoic rocks that underlie the thrust plane, 

the overriden Paleozoic schists show an intensity of metamorphism 

that exceeds even the maximum alteration of the older overthrust 

rocks. As the metamorphic intensity diminishes on both sides of the 

Mine Eidge-Tucquan anticlinal axis the maximum metamorphism 

may be said to coincide with the core of the double-crested arch.

PREVIOUS IDEAS ON METAMORPHISM OF THE PIEDMONT

PROVINCE

A pronounced variation in intensity of metamorphism throughout 

the Piedmont province has long been recognized. Williams" in 1891 

first called attention to the fact that the rocks of the Piedmont 

Plateau consist of an eastern area of strongly crystalline rocks and a 

western area of less metamorphosed rocks, which he called semicrystalline. His generalizations from the facts then available were 

comprehensive and striking, but he drew the erroneous conclusion 

that the highly crystalline eastern rocks are older than the less 

metamorphosed series on the west. In 1904 Mathews 1 pointed out 

that the rocks of the eastern and western areas are probably contemporaneous and that the difference in metamorphism is caused by 

the presence of large bodies of plutonic rocks in the eastern area, 

whereas the rocks of the western area inclose only smaller bodies of 

surface volcanic rocks.

The results of the present more extended and detailed field study 

of the Piedmont province has shown that the variation in intensity of 

metamorphism does not correspond to any simple generalization 

of a western area of semicrystalline rocks and an eastern area of 

thoroughly recrystallized rocks. This field study combined with a 

detailed petrographic examination of the rocks both in hand specimen 

and in thin section has furnished data for a fairly detailed and 

accurate generalization of relative metamorphic intensity. This 

generalization is shown in Figure 13, which is a diagrammatic representation of the region between meridians 75° and 77° and parallels 

40° 15' and 39°. Three stages of metamorphic intensity are represented by three different patterns. These three stages have been 

determined by applying certain definite criteria for measuring the 

degree of metamorphism.

00 Williams, G. H., The petrography and structure of the Piedmont Plateau in Maryland: 

Geol. Soc. America Bull., vol. 2, pp. 301-317, 1891.

1 Mathews, E. B., The structure of the Piedmont Plateau as shown in Maryland : Am. 

Jour. Sci., 4th ser., vol. 17, p. 358, 1904.
120 McCALLS FEKE.Y-QUABRYVILLE DISTEICT, PA.

CBITEKIA FOB DETERMINATION OF METAMOBPHIC INTENSITY

As the chemical reactions of metamorphism depend mainly upon 

temperature and upon unequal pressure (shearing stress),2 it follows 

that the higher the temperature and the greater the shearing stress 

the more intense will be the metamorphism. Two indices of the 

conditions under which metamorphism takes place are the texture 

and the mineral constituents of the resultant schist or gneiss. The

FIGORH 13. Diagrammatic map showing distribution of metamorphic intensity in the 

crystalline schists of the Piedmont province of portions of Pennsylvania and 

Maryland

significance of these indices, of course, depends upon the character 

of the rock that has been altered. The same intensity of metamorphism will produce from an argillite a mica schist, in which the 

original constituents are completely obliterated, and from a conglomerate a schistose conglomerate, in which most of the original 

constituents are recognizable in a more or less mechanically deformed 

condition.

For purposes of comparison, therefore, the original rocks in different localities must have been of approximately the same susceptia Johnston, John, and Nlggli, Paul, The general principles underlying metamorphic 

processes : Jour. Geology, vol. 21, pp. 482-516, 588-624, 1913. Johnston, John, Pressure 

as a factor in the formation of rocks and minerals: Idem, vol. 23, pp. 730-747, 1915.
METAMORPHISM 121

bility to metamorphosing influences, or else the application of criteria 

for metamorphic intensity must be accompanied by a certain correction for relative degree of susceptibility. Thus the character of the 

original rock from which the schist has been derived will be considered in the discussion of criteria of metamorphic intensity.

METAMORPHIC TEXTURES

Metamorphic textures fall into two main groups cataclastic texture and crystalloblastic texture.

Cataclastic texture. The cataclastic texture is associated with 

mechanical deformation in the upper zone of metamorphism and is 

regarded as a metamorphism of low-grade intensity. It is characterized by all stages of mechanical breaking down, from strain shadows,

granulation, and mortar structure in the brittle constituents, such as 

quartz and feldspar, to slicing and twisting of the fibrous and platy 

minerals, such as hornblende and mica. The end result of cataclastic 

deformation is a mylonite in which the original constituents have 

been rolled out and sheared to such an extent that they are recognizable only with difficulty.

Such mechanical deformation is usually combined with the development of new minerals, owing to the fact that some rock constituents readily undergo a rearrangement into new minerals whose 

shape conforms to the metamorphic requirements, while other more 

resistant constituents as, for example, quartz show only the mechanical effects of stress. Thus low-grade metamorphism will produce new minerals and parallel arrangement in a fine-grained rock 

such as a shale, whose constituents yield readily to rearrangement. 

In such a rock the low intensity of the metamorphism is indicated 

not by the cataclastic texture but by the fine grain of the constituents 

in the resultant schist. Under the same intensity of deformation a 

rock that is coarser grained than a shale will show only mechanical 

strain in the original constituents. Therefore while cataclastic texture is always indicative of metamorphism of low-grade intensity 

the reverse relation is not necessarily true that is, low-grade metamorphism doe's not always produce cataclastic texture.

Crystalloblastic texture. A complete recrystallization of the original constituents of the rock produces the so-called crystalloblastic 

texture, which differs from igneous texture in the order or rather 

in the lack of definite order of crystallization. Each mineral in a 

crystalloblastic rock has been formed at nearly the same time, idiomorphism of form being conditioned not by the laws that govern 

crystallization from a rock melt but by the relative ease with which 

certain minerals break down and form new constituents and by the
122 McCALLS FER.RY-QUABRYVILLE DISTRICT, PA.

power of crystallization, by virtue of which the newly formed 

mineral can maintain its boundaries against its associates.

Crystalloblastic texture is an index of complete recrystallization 

and is therefore the result of high-grade metamorphic intensity, 

which is characteristic of the deep-seated zone of metamorphism. 

But, once more, the converse is not true, for Crystalloblastic texture 

may also result from a complete recrystallization of the original constituents under conditions of temperature and pressure that are characteristic of the upper zones of metamorphism and therefore 

indicative of fairly mild metamorphic intensity.

METAMORPHIC MINERALS

Fortunately, although texture is in many respects of negative 

value in estimating metamorphic intensity, we have an additional 

criterion in the nature of the metamorphic minerals that are formed. 

The formation of new minerals by recrystallization is dependent 

upon three factors namely, cubical pressure, temperature, and 

shearing stress. The most influential factor is the temperature. 

Under normal conditions the temperature gradient increases with 

depth, but under such conditions as batholithic intrusion into the 

upper levels of the earth's crust the temperature even at comparatively shallow depths may be abnormally high, and the regional 

contact metamorphism produced by such an intrusion may be of a 

strong order of intensity. Therefore minerals that are stable at 

high temperatures, such as sillimanite, biotite, and oligoclase, are 

indices of high-grade metamorphic intensity but do not necessarily 

indicate the depth at which alteration took place. On the other

hand, such low-temperature minerals as albite, chlorite, and sericite

are indices of a relatively low metamorphic intensity, which is never 

associated with great depth.

The character of the new minerals formed during recrystallization 

will often furnish a clue to the conditions under which metamorphism 

took place and thus evaluate relative degrees of metamorphic intensity in rocks whose original character was such that cataclastic alteration predominates over the formation of new minerals. For example, 

a quartz conglomerate whose matrix is argillaceous will alter to a 

schistose conglomerate in which the stretched or mashed pebbles are 

embedded in a mica schist. Such a micaceous matrix in which biotite 

is the dominant mica indicates the operation of more intense 

metamorphism than is shown by a matrix composed of muscovite, 

chlorite, or chloritoid.

PROGRESSIVE METAMORPHISM

The application of the criteria that have just been outlined shows 

the wide range in the effects of metamorphism, from simple undula-

.-!
METAMORPHISM 123

j

tory extinction in quartz caused by a slight mechanical deformation

' to the complete transformation of the original constituents of a rock

into minerals that are stable under conditions of high temperature

and pressure. The complete process of metamorphism may be illusjx trated by the passage of a shale into a coarse-grained mica schist 

under conditions of steadily increasing metamorphic stress. When 

the normal process is arrested before completion we find a slate, 

phyllite, or fine-grained mica schist, according to the stage reached. 

The first attempt to classify metamorphic rocks according to their 

stage of development at the moment of arrest was made by Sedery holm, who grouped them into rocks altered in an upper zone of metamorphism and rocks altered in a lower zone. Later Becke and Grubemnann amplified the scheme of classification so as to embrace rocks

formed in three metamorphic zones that correspond to upper, interi mediate, and lower depth.

In the present regional study the relative intensity of metay morphism is represented upon the map by three degrees, as follows:

First degree, low metamorphism. The original texture is easily recognizable. 

Cataclastic deformation is marked. Certain new minerals have been formed, 

V chiefly sericite. There is incipient dimensional parallelism.

Second degree,-moderate metamorphism. , .Crystalloblastic texture is complete 

or nearly so. Relict structures are absent or rare. There is strong dimensional 

parallelism. New minerals have been formed that are characteristic of low temvvj perature, such as albite, chlorite, and muscovite.

Third degree, high metamorphism. Crystalloblastic texture is complete.

There is strong dimensional parallelism. New minerals have been formed that

; are characteristic of high temperature, such as biotite, oligoclase, and sillimanite.

^ These classes are called degrees rather than stages, because they do 

not necessarily represent completeness of metamorphism, although 

' * the first degree is obviously associated with partly advanced alteration. Both the last two degrees are associated with the end products 

^ of the metamorphic process under different conditions. Second-de1 gree intensity is often produced at the end of alteration in the upper 

^ or intermediate zones, under conditions of moderate metamorphic 

intensity, and third-degree intensity is the final result of alteration 

* under conditions of very strong metamorphism, characteristic of the 

deep zone. Thus a rock that has been completely reconstituted in the 

upper metamorphic zone may be subjected to increased metamorphic 

J intensity by deeper burial or as a consequence of batholithic intrusion, and it will therefore renew the process of chemical interaction 

and crystallization in an attempt to fit itself to the new environment. 

The sum total of all such changes may be considered progressive 

metamorphism, because the ultimate product of the most intense 

metamorphism is a rock that has undergone great condensation of 

volume and increase in density. Such a rock may be considered the
124 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

end product of anamorphism. Certain changes in its physical environment may result in its further alteration by refusion into a ' 

liquid state, but as a rock it can undergo no further change except ;i 

reversal of metamorphism.

RETROGRESSIVE METAMORPHISM

In order to appreciate the complex metamorphic history of rocks 

like the pre-Cambrian schists that have undergone repeated alterations it must be remembered that the effects of repeated metamorphism are not necessarily cumulative and that later deformation often 

results in the breaking down of textures and constituents that have lbeen built up during an earlier metamorphic process. Such a break- V 

ing down may be recognized by the superimposition of cataclastic 

effects and low-grade metamorphism upon a high-grade metamorphism, the result being the formation of diaphthoritic schists such as ^ 

those of the Wissahickon formation. (See pp. 27-28.) The original intense metamorphism by which the schist was formed is disclosed 

by certain relict minerals that are characteristic of deep-zone meta- , 

morphism. For example, garnets formed in an earlier intense metamorphism have later been converted, at least partly, to chlorite. 

Other minerals, such as biotite, which are stable under deep-seated 

conditions, have recrystallized in the upper zone as chlorite. In many 

chlorites the titanium content of the original biotite has been freed as "^ 

rutile. The question how far earlier metamorphism may be masked < 

or even obliterated during a later deformation thus adds an increased 

complexity to the already obscure history of rocks that have passed ' 

through more than one erogenic period. *

REGIONAL VARIATION IN METAMORPHIC INTENSITY *

In the preparation of the map shown in Figure 13 field study over * 

a large part of the region was supplemented by a careful study of \ 

the published petrographic (descriptions of the rocks of Cecil County, 

Md.,3 and the southeastern portion of Pennsylvania.4 A summary * 

of the data for the construction of the map is given below.

EARLY PRE-CAMBRIAN PARAGNEISSES (BALTIMORE GNEISS) * .

The number of thin sections studied was1 25. All show crystalloblastic texture 

and minerals characteristic of the deep zone of metamorphism. Two show 

superimposed mechanical deformation in a zone of overthrust faulting on the * 

east side of the Woodstock anticline. 3 miles north of Ellicott City, Md.

_____________; : \

3 Bascom, F., The crystalline rocks of Cecil County: Maryland Geol. Survey, CecU 

County, pp. 83-143, 1902. <

* Bascom, F., U. S. Geol. Survey Geol. Atlas, Philadelphia folio (No. 162), 1909; ElktonWilmington folio (No. 211), 1920.
METAMORPHISM 125 

LATER PRE-CAMBRIAN PARAGNEISSES (GLENARM SERIES)

Setters formation. Seven thin sections were studied. All show crystallohllistic texture developed by an intense metamorphism, as shown by the nature 

of the mineral constituents. Four show superimposed mechanical deformation 

in areas where there has been igneous invasion of the Setters by the postGleuarm Gunpowder granite.

Wissahickon formation (oliffoclase-mica schist fades). The number of 

thiri sections of Maryland rocks that were studied was 25. All show crystalloblastic texture and minerals characteristic of the intermediate or of the deepseated zone of metamorphism. The metamorphic intensity represented by theserocks ranges from the second to the third degree. Three show superimposed', 

mechanical deformation. The thin sections of Pennsylvania rocks that were 

studied were 11 in number. The rocks are sillimanite-bearing biotite gneisses. 

Five show intense crystalloblastic metamorphism superimposed by intense 

cataclastic deformation. One specimen is completely mylonitized. Six sections show crystalloblastic schists whose metamorphic intensity lies between 

the second and third degree.

Wissahickon formation (a-lbite-chlorite scMst fades). Three thin sections 

of Maryland rocks were studied. All show the crystalloblastic texture of the 

upper zone of metamorphism. The number of thin sections of Pennsylvania 

rocks that were examined was 10. They show a metamorphism similar to 

the albite schists of Maryland. Two show superimposed mechanical deformation. On Susquehanna River between Conestoga Creek and Pequea Creek there 

is a strong local intensification of metamorphism shown by the presence of 

biotite instead of chlorite or in combination with chlorite.

Peters Creek schist. Of 18 thin sections studied 15 are crystalloblastic schists 

of the second degree of metamorphic intensity. They show textures that are 

characteristic of the upper zone of metamorphism. Three are cataclastic schists 

in the lowest degree of metamorphism containing relicts indicative of an original graywacke. Three of the 15 crystalloblastic schists contain some biotite, 

thus indicating a transition from moderate to strong metamorphic intensity.

Cardiff conglomerate. Two thin sections were studied. They indicate cataclastic deformation of an arenaceous sediment composed of variably coarse constituents. Field study shows cataclastically deformed pebbles in a crystalloblastic matrix composed of muscovite, chlorite, and chloritoid, all minerals that 

indicate moderate metamorphic intensity.

Peach Bottom slate. One thin section was studied. The data were obtained 

chiefly from field study and from microscopic descriptions of the slate by Dale.5 

The rock shows abundant fine-grained platy minerals with strong dimensional 

parallelism producing slaty cleavage, which indicates low-grade metamorphism 

of a shale.

PALEOZOIC ROCKS OF SEDIMENTARY ORIGIN 

LOWER CAMBRIAN ROCKS

Hellam conglomerate member of the Chtekies quartzite as exposed on the 

Mine Ridye upland. No thin sections were studied. In the field the rocks, 

which are mica schists and conglomerates, clearly indicate strong differential 

metamorphism. The matrix of the schistose conglomerate is highly foliated. 

The quartz pebbles are cataclastically deformed (stretched) and show varying

6 Dale, T. N., Slate in the United States : U. S. Geol. Survey Bull. 589, pp. 111-113, 1914.
126 McCALLS FEERY-QUAR.RYVILLE DISTRICT, PA.

I 

degrees of granulation. Such rocks as the conglomerates in the Hellam member

of the Chickies are poor indices of metamorphism, on account of their uneven 

.grain. The intensity may be fairly inferred from the degree of metamorphism 

of the mica schists that underlie and are interbedded with the conglomerate, v 

Irom the overlying beds, which are crystalloblastic quartzites, and from the 

prevalence of biotite in the matrix of the quartz pebbles. The Hellam conglomerate is lithologically very similar to the Cardiff conglomerate, being composed of quartz pebbles in a micaceous matrix; but the relatively low-grade 

metamorphism of the Cardiff is shown by the fine-grained matrix of minute k 

muscovite, chlorite, and chloritoid flakes contrasted with the fairly coarse 

biotite schist matrix of the Hellam. Another clue to the relatively mild ±f 

metamorphism of the Cardiff is the underlying rock, which is a schistose 

graywacke, and the overlying rock, which is a slate both in a low stage of 4 

metamorphism. ,

Quartzite member of Chickies quartzite. The thin sections studied were eight 

in number. They indicate a crystalloblastic metamorphism of a relatively pure 

sandstone. The purity of the original sand makes the rock an unsatisfactory 

index of metamorphic intensity. ,

Harpers schist or phyllite. Seven thin sections were studied. Six of these 

sections show fine-grained phyllites of first-degree metamorphic intensity. One 

thin section cut from a specimen collected on the west flank of the Mine Ridge 

anticline shows complete recrystallization and a strong dimensional parallelism > 

owing to the dominance of micaceous minerals, chiefly biotite. The metamorphism in this locality is strong, between second and third degree intensity.

Antietain schist or phyllite. The thin sections studied were 18 in number. 

They show extreme variability of metamorphic intensity, dependent ,on. geographic location. Increase in intensity from north to south is shown by grada- ^ 

tion from fine-grained biotitic phyllites on the eastward continuation of the 

Hellam Hills, north of Columbia, to fairly coarse-grained mica schist on the 4 

flanks of the Mine Ridge anticline.

Vintage dolomite. No thin sections were studied. The massive beds of this v 

formation are not readily susceptible to metamorphism, but the schistose beds 

sliow' the development of a considerable amount of muscovite. The lower beds

that grade into the underlying Antietam are doloraitic schists that indicate a ^

strong, metamorphism in the Paleozoic-area west and northwest of the Mine

Ridge upland, where the schists contain a considerable amount of biotite. ^

ORDOVICIAN HOOKS , ' ;

Conestoga limestone. Ten thin sections were studied. This formation is a 

valuable index of metamorphic intensity because the calcareous beds in the 

original sediment contained much argillaceous matter. As in the Antietam, the 

intensity of metamorphism in the Conestoga increases from north to south. 

Near Lancaster, where the formation is an impure banded limestone with 

graphitic laminae, the metamorphism of the Conestoga, like that of the adjoining arenaceous sediments, is of low intensity. This is shown by the fine grain 

of the crystallization in the impure beds and by the formation of graphite, a 

mineral that may indicate a low metamorphic intensity where present in 

calcareous rocks. Under conditions of high temperature 6 the partial pressure 

of carbon dioxide will be increased, and it is possible that a certain amount of 

carbon dioxide may combine with the carbon, thereby eliminating the graphite

a Johnston, John, The general principles underlying metamorphic processes: Jour. Geology, vol. 21, p. 511, 1913.
METAMOBPHISM 127

)

from the rock. Caution must be exercised in the use of graphite as an index of 

metamorphism, because it occurs in some highly metamorphosed marbles, and 

in pelitic sediments it will persist in the most intensely altered rocks, such as 

garnetiferous mica schists. The strong metamorphism of the Conestoga formation in the Quarryville and Chester Valleys is shown by the coarse grain of the

> marble and by the abundant development of mica in the impure argillaceous 

layers.

OETHOGNEISSES

' One of the chief factors in determining the alteration of igneous rocks is 

chemical composition. Basic igneous rocks are ready victims to metamorphic

'* changes and alter with surprising readiness to chlorite schists, amphibolite 

schists, and numerous other schists collectively designated by the widely em-

' bracing term "greenstones." Siliceous plutonic rocks, on the other hand,

) show a truly marvelous resistance to metamorphic action. The condition of 

the granite gneiss at Hartley, in Baltimore County, Md., is an excellent exampleof the stability of granitic rocks as contrasted with the. susceptibility of sedi-- 

ments under the same intensity of metamorphism. This granite, which was

** intruded into the Baltimore gneiss before the deposition of the Glenamr series, 

has been altered into a typical cataclastie gneiss (augen gneiss), representing

* a low degree of metamorphism. It is unconformably overlain by highly meta- 

, morphosed sedimentary gneisses of lower Glenarm age. Here the younger 

sediments have been completely reconstructed by the action of metamorphism 

.\ while the older igneous rocks have only been mechanically deformed.

Moreover, the granodiorite gneiss at Port Deposit, although younger than 

the Hartley gneiss, shows by the inverse zoning of its plagioclase that its constituents have been recrystallized. The only explanation for the greater sus-

-* ceptibility of the Port Deposit gneiss is that the calcic feldspars of the"granodiorite have yielded to molecular rearrangement while the potash feldspars 

of the granite have been merely granulated or crushed. Igneous rocks, therej fore, are not valuable as specific indices of metamorphic intensity, except where 

rocks of approximately the same composition show marked difference in con-

) dition of metamorphism, and they are unreliable interpreters of metamorphic

- history except when used in combination with other facts of broader significance.

s For example, the Woodstock granite in Maryland, which is a quartz monzonite

containing about 40 per cent of plagioclase, is practically unaltered, but r.o.ck

9" near by, which was originally a granodiorite containing the same percentage of 

slightly more calcic feldspar, has become the Port Deposit gneiss. The similarity 

in composition between the Woodstock and the Port Deposit is sufficient to 

justify the inference that the lack of metamorphism in the Woodstock is due 

to the fact that it was intruded during a later period of igneous action than 

the Port Deposit, thereby escaping most of the deformation that has left itsmark upon the Port Deposit gneiss. The occurrence of the Woodstock, surrounded as it is on all sides by strongly deformed rocks into which it is 

intrusive, precludes the idea that it owes its comparative immunity from 

metamorphism to a fortunate position in which it has been protected from 

deforming influences.

On the other hand, the relatively slight metamorphism of the igneous complex 

of the Reading and Durham Hills is due to the fact that these rocks were not 

exposed to the stress that affected rocks of the same age in other localities. 

In the Reading Hills the igneous rocks as a whole are notably free from 

any evidence of stress except a moderate amount of mechanical deformation 

that is in many places concentrated along recognizable zones of faulting and1
128 McCALLS FEBBY-QUARRYVILLE DISTRICT, PA.

brecciation. The igneous rocks intrude pre-Cambrian crystalline schists that 

have been formed under conditions of intense metamorphism. The fact that 

they are unconformably overlain by Cambrian strata makes it impossible to 

explain their lack of alteration by the assumption that they are younger than 

the lithologically similar pre-Cambrian igneous rocks of southern Pennsylvania 

and Maryland. The alternative hypothesis seems to be that since the intrusion 

of the igneous rocks the Reading and Durham Hills have been but slightly 

affected by the influence that was so effective in producing the Paleozoic 

metamorphism farther south. On the whole, the irregular distribution and 

widespread variation in intensity of the regional metamorphism is the most 

striking1 fact brought out by the study of the Piedmont metamorphism.

CAUSES OF REGIONAL VARIATION IN METAMORPHIC INTENSITY

A glance at the map that represents the distribution of metamorphic intensity in^he area studied (fig. 13) shows three parallel 

belts of strong, weak, and again strong metamorphism in the Piedmont province north and northeast of Baltimore. The southern belt 

of strongly metamorphosed rocks extends northeastward from the 

neighborhood of Baltimore beyond the limits of the map (fig. 13) as 

far as Trenton. It attains its maximum width of about 18 miles 

north of Baltimore and becomes somewhat narrower toward Philadelphia, where it is scarcely more than 10 miles wide. In this belt 

of strong metamorphism there are certain isolated areas where the 

intensity reaches a maximum namely, in the anticlinal uplifts that 

bring to the surface the oldest pre-Cambrian rocks. Northwest of 

this belt of great intensity there lies another narrower belt in which 

the metamorphism, though distinct, is of low intensity and even in 

some localities incomplete for example, in the area underlain by 

Peach Bottom slate, which occupies the middle of the belt. Still

farther northwest metamorphic intensity once more increases, attaining a maximum along a line that corresponds to the axis of the Mine 

Ridge-Tucquan anticline.

These three belts of differing intensity may be designated for 

convenience the Baltimore-Philadelphia belt, the Peach Bottom belt, 

and the Mine Eidge belt. The metamorphism varies considerably 

in the Mine Eidge belt along the direction of the northeast strike. 

Metamorphism along the strike of the Tucquan anticline in Maryland and in Pennsylvania as far northeast as the Quarryville Valley 

is complete but of moderate intensity. But from Quarryville northeastward along the axial plane of the Mine Eidge anticline the metamorphism, even in the youngest Paleozoic rocks, is once more intense 

and of a grade comparable to the metamorphism in the oldest rocks 

in the Baltimore-Philadelphia belt. An exhaustive and detailed 

metamorphic study will be required, both in the region comprised 

in the compound anticline that underlies the Honeybrook upland 

and in the region that lies west of the Tucquan anticline in southern
METAMORPHISM 129

Pennsylvania and in Maryland, before the local variations in metamorphic intensity can be platted. But the data now available justify 

the generalization that north and west of the Mine Ridge upland 

the metamorphism wanes at first rapidly and then gradually until 

the metamorphosed Palepzoic strata disappear under practically 

unaltered Triassic rocks.

Where Paleozoic and pre-Cambrian rocks reappear in the area 

of the Reading and Durham Hills, north, of the Triassic belt, intense 

metamorphism is confined to the oldest pre-Cambrian rocks. The 

moderately metamorphosed Paleozoic strata of this area nowhere attain the degree of alteration shown by the Paleozoic schists of the 

Mine Ridge upland, nor do the pre-Cambrian intrusive rocks of this 

area show as .much metamorphism as the pre-Cambrian intrusive 

rocks of the Mine Ridge upland.

CAUSES OF REGIONAL METAMORPHISM

Regional metamorphism may be grouped under three heads according to the dominant causal factor dynamic metamorphism, in 

which the dominant influence is nonuniform shearing stress; load 

metamorphism, in which the causal factor is static pressure under 

conditions of high temperature; and regional contact metamorphism, 

produced under the influence of increased temperature caused by 

batholithic intrusion. . Apparently the regional metamorphism of the Piedmont province 

does not show a consistent causal relation to either the structure or 

the age of the rocks involved. In. some places rocks on the limbs of 

anticlines are highly metamorphosed, as might be expected from 

their structural position; in other places rocks occupying the limbs 

of folds are only moderately or incompletely metamorphosed, and 

rocks in the center of an appressed syncline have not advanced beyond the condition of slates, whereas rocks in the arches of open 

anticlines are completely metamorphosed with a moderate or strong 

degree of intensity. Likewise in some placesi pre-Cambrian rocks are 

phyllites or slates, and in others Ordovician rocks are calcareous 

mica schists. Rocks that are equally susceptible to metamorphism 

and that have been exposed to equal intensity of dynamic deformation show more metamorphism in one place than in another. Rocks 

of similar composition, supposedly deformed under the same load, 

are more intensely altered in one place than in another.

These peculiar localizations of metamorphic intensity are at first 

sight extremely puzzling. Regional contact metamorphism is suggested in certain areas because rocks that have been intruded by 

igneous material are intensely metamorphosed, whereas in areas 

where igneous rocks are not exposed there is a decrease in intensity.
130 McCALLS FERRY-QUARK YVILLE DISTRICT, FA.

However, in certain other areas where no igneous rocks are exposed 

the intensity of metamorphism is fully as great as in areas exhibiting large igneous intrusions. Therefore, if regional contact metamorphism is to be invoked to explain the intense metamorphism in 

an area that is lacking in igneous outcrops, it is necessary to fall back 

upon the hypothesis that the area is underlain by an unexposed 

batholith of sufficient size to produce the observed result, an idea that 

has been prevalent in geologic literature since the days of Michel 

Levy and Termier.

Evidently the puzzling variations in metamorphism in the Piedmont province can not be explained by any one alone of the three 

dominant causes of metamorphism. However, regional metamorphism 

has taken place not once but repeatedly and therefore under the 

influence of different factors or combinations of factors at different 

times. The solution to the problem lies in the correct interpretation 

of the whole sequence of metamorphic cJianges during "the preCambrian and Paleozoic history of the region.

DYNAMIC AND CONTACT METAMORPHISM IN THE OLDEST PKE-CAMBRIAN

ROCKS

One generalization may be made in respect to stratigraphic horizon. 

Wherever the oldest pre-Cambrian^basement gneiss is exposed it 

shows the maximum intensity of metamorphism. It is probable 

that the alteration of the Baltimore gneiss has been produced by a 

combination of both contact and dynamic action, because the presence 

of injection gneisses and of granitic intrusions in the Baltimore 

gneiss shows the influence of an active magma and because the 

structural discordance between the Baltimore gneiss and the overlying Setters formation shows that folding had taken place before 

the deposition of the Setters. It is impossible to tell how far the 

pre-Glenarm alteration of the Baltimore may have been intensified 

by later anamorphism. The strong metamorphic intensity in these 

oldest basement rocks suggests a possible cumulative effect of later 

metamorphism, but in view of the fact that repeated metamorphism 

is often retrogressive rather than cumulative in its. effects, and also 

in view of the fact that the microscope shows the Baltimore gneiss 

to have yielded in some degree to a cataclastic action which seems 

to have been connected with the pre-Setters igneous activity and 

whose effects have not been obliterated by subsequent regional metamorphisms, it is possible that the Baltimore gneiss remained an 

inert mass so far as later metamorphosing'influences were concerned.

The intrusion of an active magma in the Baltimore gneiss ; would 

insure a reconstruction of the rock in connection with the folding,
METAMORPHISM 131

although the absence of intense metamorphism in certain strongly 

folded regions like the Appalachian shows that folding alone is not 

necessarily sufficient to cause crystalloblastic metamorphism.

LOAD METAMOKl'HISM AMD REGIONAL CONTACT METAMORPHISM IN THE

GLEN ARM SERIES

It has been pointed out that the metamorphism of the Glenarm 

series shows a relation to the folding that is exactly the reverse of 

what would be expected if the alteration had been caused by the 

folding alone. The cores of open anticlines show a metamorphic 

intensity that is not attained on the limbs and that far surpasses the 

intensity in the trough of a tightly compressed syncline.

On the other hand, the relation of the metamorphism to the 

stratigraphic horizon in the Glenarm series is such as might be ex- 

-pected if the load had-been the* causal "factor, for the-lowermost 

formation of the series is a biotite schist and the uppermost beds are 

slates. Thus in so far as the pre-Cambrian rocks of the region are 

concerned the stratigraphic horizon might be construed as the dominant factor in their transformation into schists. On the hypothesis 

of load metamorphism, however, it is not obvious why the Wissahickon formation should occur in two metamorphic facies that differ 

so widely in intensity as the albite-chlorite schist and the oligoclasemica schist. A possible' explanation of 'this 'difference in mineralogic 

facies may be the presence of large bodies of plutonic rocks along the 

southeastern border of the Piedmont province, where the oligoclasemica schist occurs, and the absence of similar rocks in the area underlain by albite-chlorite schist. The contact effects of these intrusions 

on the oligoclase-mica schist facies of the Wissahickon must have 

been appreciable and may have amounted to regional contact metamorphism.

If, as seems probable, the post-Glenarm igneous intrusions of 

Maryland are pre-Paleozoic, such regional contact metamorphism 

must have antedated the overthrust of the Glenarm series upon the 

Paleozoic rocks. Therefore, if the combined influence of load and 

batholithic intrusion was sufficient to produce metamorphism of the 

Glenarm series as we now see it. that metamorphism must have been 

pre-Paleozoic and earlier than the overthrusting. However, the area 

underlain by Glenarm schists was affected by a Paleozoic metamorphism, because the igneous intrusions invoked as a possible cause 

of regional contact metamorphism in the Glenarm series are them- 

'selveS metamorphosed in'areas where the Paleozoic strata are schists 

and only slightly altered in areas where the'Paleozoic rocks are less 

6136- 29 10
132 McCALLS FERRY-QTJARRYVILLE DISTRICT, PA.

metamorphosed, thus suggesting that the metamorphism of the 

Paleozoic beds was contemporaneous with the alteration of postGlenarm igneous rocks.

*

DOUBLE METAMOKi'HISM IN THE GLENARM SERIES

The Glenarm schists in the overthrust block show two foliations 

an older, more strongly developed foliation that is parallel to the 

bedding and a younger schistosity that makes an angle with the 

bedding. Likewise, the Glenarm schists are cut by two systems of 

quartz veins, and the older veins, which are parallel to the bedding, 

are folded. The relations of these two metamorphisms to the metamorphism in the Paleozoic rocks should throw some light upon the 

age of the metamorphism in the Glenarm series.

VARIATION IN METAMORPHIC INTENSITY IN THE PALEOZOIC ROCKS

The intense local metamorphism of the Lower Cambrian sediments that flank the Mine Ridge anticline is the most surprising 

feature of the metamorphic history of the region. Metamorphism 

due to the normal load of the overlying stratigraphic series can 

hardly be invoked as a cause, because on the slopes of Welsh Mountain, north of the Mine Ridge upland, there are Lower Cambrian 

rocks at the same stratigraphic horizon as the strata in the Mine 

Ridge upland, yet the Paleozoic beds of Welsh Mountain are much 

less metamorphosed than the Paleozoic beds of the Mine Ridge upland, although the load of the normal stratigraphic series in both 

places must have been approximately the same. Dynamic metamorphism operative over a large area is at first sight equally inadequate to account for this peculiar localization of intensity, for any 

regional deformation under strong tangential stress that would have 

reconstructed Paleozoic rocks in the Mine Ridge metamorphic belt 

could hardly have left closely folded rocks farther southeast, in the 

Peach Bottom belt, in the condition of slates, Q

A possible-explanation for .the .puzzling localization of metamorphism is suggested by the idea that intense metamorphism is 

produced by frustrated movement rather than by free movement of 

the rock masses involved, an idea that was employed by Greenly to 

explain the regional metamorphism on the island of Anglesey,7 

where there are three superimposed recumbent fojds of great amplitude. The metamorphism in this tectonic series shows two maxima 

which are not related to the. recumbent folding, because the conditions necessary for molecular readjustment do not exist in freedom 

of movement. They are ascribed rather to a frustration of move7 Greenly, Edward, Geology of Anglesey: Great Britain Geol. Survey Mem., p. 908, 1S19.
METAMORPHISM 133

ment, which takes place when the limit of free horizontal movement 

is reached in any one recumbent fold and which is expressed in 

a budding of the recumbent folds into close secondary folds. If 

the core of the Mine Ridge anticline represents a buttress of preCambrian igneous rocks and crystalline schists it is possible that 

there was a buckling of the Paleozoic sediments that was locally 

intensified and concentrated around the resistant buttress, waning 

gradually as the distance from the unyielding core became greater. 

A serious objection to this hypothesis is the fact that the preCambrian buttress underlies the whole Honeybrook upland and is 

not confined to the Mine Ridge upland. The metamorphism-of the 

Paleozoic strata, on the other hand, is confined to the Mine Ridge 

upland and is not uniform around the entire resistant core.

DOUBLE METAMORPHISM IK THE PALEOZOIC KOCKS

The Paleozoic rocks, like the Glenarm schists, show the influence 

of two metamorphisms. On the east flank of Bunker Hill the dominant metamorphism of the rock corresponds to the older schistosity, 

which is parallel to the folded bedding. A younger metamorphism 

whose effects are distinctly subordinate to the first is shown in a 

cleavage that cuts across the older schistosity. (See pi. 8, A.) The 

older schistosity and folding is cut across by a system of younger 

quartz veinlets that are presumably contemporaneous ,with the 

younger schistosity. In addition to these younger quartz veinlets 

there is an older vein system that may be genetically associated with 

the pegmatites which are so widespread throughout the Paleozoic 

rocks in the region. A vein of blue quartz, cutting the injected hornblende schist north of Coatesville, is folded along with the including 

rock, thereby showing the influence of an erogenic period that was 

later than the igneous epoch, during which the vein was formed. The 

Paleozoic pegmatites themselves show evidence of strong dynamic 

deformation. The plagioclases are saussuritized, the potash feldspar 

is microcline, and the quartz is granulated. The effects of this second deformation are particularly conspicuous in the anticlines south 

of Columbia that bring Lower Cambrian argillaceous beds to the surface. Jonas has found that the banded slates in this locality show 

a well-developed cleavage that crosses the bedding. There is also a 

distinct but weak metamorphism that has produced an earlier foliation parallel to the gently folded bedding. (See pi. 8, B.)

In the older of these two metamorphisms the intensity has waned 

perceptibly in a distance of 3 to 4 miles across the strike of the 

formation. The intensity of the younger metamorphism is practically constant in the same distance and has only produced a slaty 

cleavage. But the younger foliation is less conspicuous in the strongly
134 McCALLS FERRY-QUARRY VILLE DISTRICT, PA.

V

metamorphosed schist from Bunker Hill than in the less altered 

rock near Columbia, thereby illustrating the fact that a second metamorphism is comparatively ineffectual when superimposed upon an 

already highly metamorphosed rock.

i

DEVELOPMENT OF OLDER META'MOR'PHISM IN 'THE GLENARM SERIES 'AND

IN PALEOZOIC ROCKS

As noted above the marked characteristic of the later metamorphism in both Glenarm schists and Paleozoic strata is its uniform *' 

and low intensity over a wide area. In the same area the earlier . metamorphism shows a wide range in intensity. The intensity of 

this older metamorphism is at a maximum in the Paleozoic schists on N 

the flanks of Mine Ridge anticline and in the immediately surrounding neighborhood. It wanes northward very rapidly in the Paleozoic 

-rocks and wanes south ward, less .-rapidly but perceptibly in the pre- * 

Cambrian schists of the overthrust block. The puzzling feature of 

the metamorphic history of the area is the age of the earlier metamorphism and the cause of its variation in intensity.

The variation in metamorphic intensity in the Glenarm series has 

already been discussed, and it is obvious that there are logical objections to ascribing the metamorphism of the series as a whole to 

dynamic metamorphism accomplished under the influence of non- ^ 

uniform pressure or to load metamorphism under the influence 

of uniform pressure. The influence of large igneous intrusions fur- -^ 

nishes a possible explanation of the concentration of metamorphic 

intensity in some areas, and it is possible that the part of the Glenarm section that was beyond the thermal influence of these intrusions 

may have acquired a certain amount of load metamorphism. ^

The metamorphic gradient in the overriden and overriding blocks 

strongly suggests that the metamorphism now seen in the Glenarm '* 

section along Susquehanna Eiver was .not .altogether developed before * 

the thrusting. A waning of intensity between the overridden Paleozoic rocks and the overthrust pre-Cambrian beds is a marked contrast 

to the metamorphic gradient in many overthrusts involving pre- ^ 

Cambrian rocks where highly altered older rocks have been shoved 

over slightly metamorphosed younger formations.

An a priori reason for assuming that a metamorphism of the > 

faulted block accompanied or succeeded the thrusting is the notable 

absence, so far as observed, of strong brecciation along the thrust 

plane. The inference is that the thrusting took place under condi- < 

tions favorable for molecular reconstitution during and subsequent 

to the movement. *
METAMORPHISM 135

The question then arises, under what conditions could such molecular rearrangement take place ? The work of Adams and Bancroft 8 

suggests that " decken " are probably formed only in the upper part 

of the zone of flow, beneath the zone of fracture, where adjustment 

takes place by means of faults and overthrusts. Great movements of 

adjustment, therefore, must take place at or comparatively near the 

surface. The experimental work of Adams 9 would suggest that 

granite rocks do not flow at depths of less than 11 miles below the 

surface of the'earth's crust and that even limestones do not yield 

to deformation at depths of less than 9 miles, provided the temperatures at these depths are such as would be expected from the normal 

temperature gradient. Nevertheless, plastic deformation has frequently taken place under a load of sediments that is much less than 

the load that would furnish the pressure required by the experiments 

of Adams.

The intense Paleozoic metamorphism in the Quarryville district is 

apparently another illustration of the difficulty in reconciling the 

results of experimental work with the available geologic evidence. 

The geologic evidence indicates a load upon the Paleozoic rocks in 

the McCalls Ferry-Quarryville district that did not exceed 1 or 2 

miles at the time that the sediments were folded. This load was 

certainly less than the weight of sediments under which the Cambrian 

and Ordovician rocks of the Lebanon Valley must have been deformed, because the strata in the Lebanon Valley were deposited in 

a deeper part of the geosyncline than the rocks of the Quarryville 

Valley. And yet the metamorphism in the Lebanon Valley is very 

slight in contrast to the Paleozoic metamorphism around the Mine 

Ridge upland. Moreover, the overthrusts of southern Nevada, which 

have taken place under a load that is of the order of magnitude 

of 2 miles, show all the effects of shattering and brecciation that 

would be associated with a light load, whereas the Martic overthrust, 

which is estimated to have taken place under a maximum load of 

less than 2 miles, shows effects that would be attributed to formation 

at great depth. Of course, if the Paleozoic metamorphism took 

place in the course of the earth movement which accompanied or 

followed the overthrusts and during which -the thrust plane was 

folded, the load under which the alteration was accomplished would 

have been locally increased by an unknown thickness in the overriding block.

According to the experimental indications that the zone of flow lies 

at a depth of 11 miles, the overriding block must have been at least

8 Adams, F. D., and Bancroft, J. A., On the amount of internal friction developed in 

rocks during deformation: and on the relative plasticity of different types of rocks: Jour. 

Geology, vol. 25, p. 635, 1917.

8 Adama, P. D.,, An experimental contribution to the question of the depth of the zone 

of flow in the earth's crust: Jour. Geology, vol. 20, pp. 97-118, 1912.
136 McCALLS FERRY-QTJARRYVILLE DISTRICT, PA.

9 miles thick in order to furnish conditions of molecular rearrangement along the plane of the overthrust. At such a depth rock 

transfer would be expected to take place by the rolling out of great 

recumbent folds or " decken " rather than by a translation of a large 

block of the earth's crust along a fracture plane.

Moreover, Lawson 10 has calculated the limiting value for the 

length of the overthrust prism along different inclinations of the 

fault plane under different assumed coefficients of friction. Figure 

14 is his diagram of a cross section through a wedge-shaped thrust 

block. <2i5 represents the length of the overthrust block, ac the maximum depth of the overthrust block, and &<? the horizontal displacement of the thrust. The angle oibc is the angle made by the plane 

of the overthrust with the surface. Lawson's table is given below 

showing the values of ab that he calculated for several assumed values 

of abc and of the coefficient of friction (/).

FIGURE 14. Cross section through a wedge-shaped thrust block. (After 

A. C. Lawson)

Limiting value's for length of overthrust Mocfc, aib, vn miles

abc

2°

5°

10°

20°

/=0.5

20

18

16

12

/=0.4

25

22

19

14

/=0.3

32

26

23

16

The corresponding values for the depth of the overthrust block 

have been calculated and are given in the table below.

Lvmiting values for the depth of the overthrust Mock, ac, m miles

ofic

2°

5°

10°

20°

/=0.5

0.7

1.5

2.8

4.4

7=0.4

0.9

1.9

3.4

5.5

/=0.3

1.1

2.3

4.5

5.6

This table would indicate that the maximum depth of the overthrust 

block along such a thrust as the Martic overthrust, the angle of 

which was doubtless less than 10°, would not amount to 6 miles and

10 Lawson, A. C., Isostatic compensation considered as a cause of thrusting: Geol. Soc. 

America Bull., vol. 32, p. 341, 1922,
' METAMOEPHISM 137

probably not even to 3 miles. Therefore it is hardly likely that 

the added load of the overriding block in itself was sufficient to 

cause deformation under conditions of flowage without the introduction of some other factor of molecular reconstruction. It would 

) seem more probable that the deformation originated as a fracture, 

along which the displaced block was driven toward the northwest, 

and that, either during the movement or "immediately subsequent to 

it, some factor other than depth changed the environment of the 

fault from the zone of fracture to that of flowage.

There remain two factors that would affect the depth at which the 

) observed deformation and folding may have taken place namely, 

time and temperature. It is undoubtedly possible that a comparatively low pressure exerted continuously over a long period of time 

may furnish conditions of deformation strictly analogous to that 

reproduced in Adams's laboratory under enormously higher pressure. 

A long-continued and relatively slight pressure might have produced 

j, the high metamorphism of the Mine Ridge belt, but it would not 

account for the weak metamorphism of the near-by rocks.

The temperature factor thus appears to be the only variable by 

t 1 which differences in intensity can be explained, and the raising of 

the geothermal gradient by batholithic invasion seems to be the only 

possible explanation for so peculiar a localization of high-tempera- 

> ture effects. If it is assumed that the metamorphism of the Paleozoic 

strata and possibly of the overthrust pre-Cambrian beds took place 

at or near the end of the Paleozoic era, the load under which a batholith might have been intruded would be represented by about 8,000 

/ feet of Paleozoic strata in the Lancaster Valley plus a possible addi- 

.; tional overthrust cover of 1 to 2 miles in thickness. At this depth 

> of about 3 miles the normal temperature gradient would give a temperature of about 170° C., which is presumably inadequate to account 

for a coarse-grained crystalloblastic metamorphism of a high order 

' of intensity.

But the presence of highly heated igneous bodies at the comparatively shallow depth of 2 miles below the surface would easily raise 

M the temperature gradient to an amount that would be sufficient to 

account for all the puzzling features of the peculiarly localized variations in metamorphic intensity.

Such a hypothesis is theoretically possible and therefore attractive, but it is peculiarly dangerous in that it is inherently insusceptible of proof, the igneous rock that is supposed to cause the effect 

being unexposed at the surface and therefore a matter of speculative 

origin. The theory was suggested by Barrell" to account for the 

metamorphism in the highlands of western Connecticut, where segre11 Barrel], Joseph, Relation of subjacent igneous invasion to regional inetamorphism: 

Am. Jour. Sci., 5th ser., vol. 1, pp. 1-19, 174-264, 1921.
138 McCALLS FEKRY-QUARRYVILLE DISTRICT, PA.

gation seams and lenses of tourmaliniferous quartz and feldspar are 

abundant. He also accounted for the lack of metamorphism in the 

Lebanon Valley of Pennsylvania, as compared with high metamorphism in the Connecticut highlands, by the absence of evidences of 

igneous intrusion in the Pennsylvania locality. There is some evi- i 

dence °f or the presence of underlying batholithic intrusion along the 

Mine Ridge metamoi:phic oelt, because the Paleozoic rocks are penetrated by pegmatites, many of which were accompanied by orebearing solutions. Tourmaline is of abundant and widespread occurrence even as high in the section as the Conestoga limestone. Many 

of the tourmaline crystals have been broken and pulled apart along ^ 

the foliation planes. (See pi. 8, 6r.) The spaces between the 

stretched fragments are filled with quartz that ha,s apparently 

accompanied the boron vapors from which the tourmaline was 

deposited. As pointed out by Barrell, the presence of tourmaline 

is not an infallible evidence of igneous activity, yet its widespread 

occurrence, even in the calcareous rocks, in rosettes and nests that 

are clearly of epigenetic origin is a reasonable indication that the 

Paleozoic rocks in the Chester, Lancaster, and York Valleys have 

been penetrated. by emanations from .an underlying batholithic in- > 

trusion. Whether the presence of such a batholith is sufficient to 

account for the strong metamorphism of the Mine Ridge belt is a 

pure speculation unless it proves possible to work out in the field some > 

relationship between the intensity and extent of the visible effects of 

igneous activity, such as pegmatitization and tourmalinization, and 

the amount and intensity of metamorphism.

RELATIVE AGE OF DOUBLE METAMORPHISM IN THE GLENARM SERIES AND

IN THE PALEOZOIC ROCKS ^

There is a striking parallelism between the double metamorphism ^ 

that is found in the Glenarnvand the'Paleozoic schists, but a most * 

exhaustive study of the metamorphism in the two areas will be 

required in order to prove that they are contemporaneous. The 

earlier metamorphism of the Glenarm series may be pre-Cambrian, 

and the younger cleavage may be the only record in the Glenarm *" 

schists of the first Paleozoic metamorphism. Metamorphism would ^ 

doubtless register its effects with great intensity upon the unaltered 

Paleozoic rocks but would not necessarily be so effective in altering 

the Glenarm rocks if they had been already transformed to schists in 

pre-Cambrian time.

On the other hand, if the pre-Cambrian metamorphism in the 

Glenarm series was only slight or local and confined to the neighbor- { 

hood of the epi-Glenarm intrusions, the metamorphism in the Wissahickon albite-chlorite schist may be largely Paleozoic and of the same
U. S. GEOLOGICAL SURVEY BULLETIN 799 PLATE 8

A. SPECIMEN OF HARPERS SCHIST

Shows older folded schislosity parallel to bedding cut by younger cleavage inclined to

bedding

B. SPECIMEN OF CHICKIES BANDED SLATE

Shows older folded schistosity parallel to bedding cut by younger cleavage 

inclined to bedding

C. SPECIMEN OF MICA SCHIST FROM UPPER BEDS OF CHICKIES QUARTZITE 

Shows stretched epigenetic tourmaline ". , . .,
METAMORPHISM 139

age as the earlier metamorphism that is so strongly developed in the 

Harpers schist around the Mine Ridge uplift. If the earlier metamorphism in the Paleozoic and pre-Cambrian section is of the same 

age and was associated with or subsequent to the overthrusting, the 

abnormal metamorphic gradient in the area of the overthrust becomes 

intelligible, for whether the cause of the metamorphism is uniform 

pressure or the thermal influence of an underlying batholith, the 

rocks that lie the deepest in the earth's crust would be the most 

intensely altered.

SUMMARY OF CONCLUSIONS

The variable metamorphic intensity in the rocks of the Piedmont 

province is the result of a variety of causes that have operated a,t 

different times. The relative intensity of metamorphism of different formations is no clue to their stratigraphic horizon, and there is 

no simple relation between the degree of metamorphism and geographic position.

The pre-Cambrian basement complex was doubtless formed during 

the pre-Glenarm folding as a result of dynamic metamorphism intensified by the thermal influence of batholithic invasion. The metamorphism of the early pre-Cambrian sediments is everywhere 

intense but shows no evidence that it is a cumulative effect produced 

during superposed metamorphisms. On the contrary, it suggests 

that the resultant crystalline schists have remained more or less inert 

since their formation, owing doubtless to their prolonged deep-seated 

position, which has preserved them from retrogressive metamorphism.

The later: pre-Cambrian formations (Grlenarm series) show a 

strong degree of metamorphic intensity in the schists that form the 

base of the section and a progressive waning of intensity toward the 

slates at the top. Load metamorphism is. however, inadequate to 

explain the occurrence of the same formation in two metamorphic 

facies of different intensity, because the facies were presumably at 

the same depth when they were formed. The rocks of the intensely 

metamorphosed facies have been invaded by numerous widespread 

plutonic intrusions, and these intrusions may have produced regional 

contact metamorphism.

The metamorphism of the Paleozoic sediments in general is of low 

to moderate intensity, but the Paleozoic strata that emerge from 

under the overriding fault block in the neighborhood of the Mine 

Ridge anticline show a puzzling development of abnormally strong 

intensity. The metamorphic gradient in the underlying and overlying blocks wanes from bottom to top, and this abnormal relation 

of strong metamorphic intensity in the Paleozoic rocks, contrasted
140 McCALLS FERRY-QUABK.YVILLE DISTRICT, PA.

with lower intensity in the pre-Cambrian thrust block, suggests that 

the dominant metamorphism in both Paleozoic and Glenarm strata 

was subsequent to the overthrust. This idea is confirmed by the 

character of the thrust plane so far as observation of it has been possible. The thrusting appears to have taken place under conditions 

that were favorable to molecular rearrangement and metamorphism.

The strong degree of metamorphic intensity localized in the autochthonous Paleozoic beds near the surface emergence of the fault 

plane suggests that the increased load produced by the weight of the 

thrust block may have had some influence in the metamorphism. 

But the conditions for crystalloblastic alteration could hardly have 

been produced by depth of burial alone. The normal load of sediments upon the Conestoga formation at the time of metamorphism 

could hardly have amounted to as much as 10,000 feet. Even if this 

normal load were considerably increased by the weight of the thrust 

block the additional load could not have been sufficient to cause anything but a cataclastic metamorphism in the autochthonous Paleozoic 

sediments or in the displaced thrust block.

The only feasible explanation for a folding and metamorphism of 

the thrust block that would inhibit or obliterate the effects of cataclastic metamorphism is a rise of geothermal gradient. A localized 

rise of geothermal gradient suggests the influence of highly heated 

underlying igneous rock, and such thermal influence combined with 

the locally increased load of the faulted rock would doubtless account 

for the peculiar concentration of intense metamorphism in the overridden Paleozoic rocks. The hypothesis is alluring, but has been 

insusceptible of proof except in so far as the extensive tourmalinization and pegmatitization of the Paleozoic rocks indicate the passage

of magmatic emanations from an underlying igneous intrusion.

There has been a double metamorphism in both the Glenarm series 

and the Paleozoic strata, which is shown by foliation that is parallel 

to the bedding and cut across by a younger slaty cleavage. The older 

foliation has a considerable range in intensity in the Paleozoic rocks, 

being developed most strongly near the Mine Kidge anticline. A 

short distance north of the anticlinal axis of the Mine Ridge uplift 

the intensity of the older metamorphism diminishes and the younger 

cleavage becomes more noticeable. The younger metamorphism was 

subordinate in degree to the older, and is therefore conspicuous 

only when the older metamorphism was at a minimum. The older 

foliation was post-Conestoga and presumably later than the overthrust or possibly contemporaneous with the overthrusting, because 

of the absence of cataclastic effects along the thrust plane and in the 

overriding block. The younger foliation may have coincided with 

the last Paleozoic deformation of the region, but so far it is im-
ECONOMIC GEOLOGY 141

possible to fix the dates of these two periods of metamorphism more 

closely than to say that they occurred in the Paleozoic era after 

Chazy time.

ECONOMIC GEOLOGY

NICKEL ORE 

GAP MINE

The Gap nickel mine is on Mine Ridge, about 12 miles southeast 

of Lancaster and 8^2 miles south of Kinzers on the Philadelphia 

division of the Pennsylvania Railroad. The nickel ore occurs in 

places along the border of a small gabbro mass that intrudes the 

early pre-Cambrian schists. Pyrrhotite and chalcopyrite with a small

amount of pyrite make up the bulk of the metallic sulphides. The 

ore minerals are pentlandite, which is intergrown with pyrrhotite, 

and millerite. The millerite has been formed by downward enrichv ment of the primary sulphides.

The presence of ore was noted apparently as early as 1718 in the 

copper-bearing waters of a mineral spring that issued from the

T ground at the place where the mine was afterward established. For 

80 to 90 years the Gap mine was worked at intervals for copper, 

but the operations proved unsuccessful, and the mine had stood idle

s for about 30 years, when it was reopened in 1849, still as a coppermining enterprise. At that time the nickel ore was being discarded

r on the dump as refuse, called by the miners " mundic," the Cornish 

word for pyrite. In 1852 a miner named Doble, who afterward 

became superintendent of the mine, suggested that some other mineral

, than mundic was present in the discarded material. An analysis

i> by Genth finally showed the presence of nickel in the so-called 

mundic, and the Gap Mining Co. then devoted its attention to the

* mining of nickel ore. The ore was smelted in a preliminary way 

about a mile north of the mine. The operations were not financially 

successful until 1862, when the mine and smelter were purchased by 

a Philadelphian named Joseph A. Wharton, who established a nickel 

refinery, known as the American Nickel Works, in Camden, N. J., 

where the matte produced in the Gap smelter was refined and manufactured. The ore as it left the mines contained from 1 to 3 per cent 

of nickel.12

In 18TT the average annual production of the Gap mine was estimated at 7,200 short tons of ore, carrying from 1 to 3 per cent of 

nickel.13 At that time the largest producer of nickel in the world

13 This account is taken from a mine report prepared by Charles Doble, superintendent 

of the Gap nickel mines, quoted by Frazer, Persifor, jr., The geology of Lancaster County : 

Pennsylvania Second Geol. Survey Rept. CCC, pp. 163-168, 1880.

13 Idem, p. 174.
142 McCALLS FERRY-QUARRYVILLE DISTRICT, PA.

was Norway, whose maximum annual output in 1876 was 42,500 tons

of ore, or 360 tons of nickel. The Gap mine at this time produced

about one-sixth of the total annual nickel output of the world. But

in 1877, when New Caledonia exported 8,000 tons of ore averaging

from 8 to 10 per cent of nickel, the Pennsylvania nickel industry £

received a severe blow, and the advent of the Sudbury nickel ores in

1887 proved a death stroke to the Gap mine, which was closed in 1893.

COMPARISON WITH SUDBURY ORES

4

The Gap nickel ores are similar in character and occurrence to the . 

Sudbury ores, and the problem of their genesis, presents the same 

puzzling features that have caused a geologic controversy of many ' 

years' standing in the Sudbury district. Four theories have been 

advanced to account for the nickeliferous pyrrhotite deposits of 

Ontario (1) that the ores have originated in a laccolithic chamber H 

as magmatic segregations of heavy molten sulphides from a silicate 

melt; 14 (2) that the ores have been introduced by thermal waters 

that have no necessary genetic relationship to the norite mass in 

which the pyrrhotite occurs; 15 (3) that the ores were produced by 

magmatic differentiation into fluid sulphides but that the differentiation took place in the magma reservoir, not in the laccolithic chamber, 

and that the resultant liquid sulphides were injected upward into ^ 

the laccolithic chamber; 1G (4) that the ores are of magmatic origin 

but that their consolidation has taken place at a late magmatic stage, V 

when the temperature was so far reduced that the sulphides could 

not have existed in the fluid condition unless mineralizers were present in such large proportions that " the characteristics of the mixture ! 

are those of a high-temperature solution and not of a melt." 17 *

Nevertheless, after almost 40 years of work on the origin of these 

ores, the question still remains to a large extent open. The answer 1 

is probably, as suggested by Bateman, th,at the ores as they exist * 

to-day have been formed by a combination of magmatic segregation, 

injection of molten sulphides, and hydrothermal action.18 Even so 

confirmed a believer in the segregated origin of the nickel ores as ^

14 Coleman, A. P., The Sudbury nickel field : Ontario Bur. Mines Rept., vol. 14, pt. 3, , 

1905; The Sudbury laccolithic sheet: Jour. Geology, vol. 15, pp. 759-782, 1907. ^

16 Dickson, C. W., The ore deposits of Sudbury, Ontario: Am. Inst. Min. Eng. Trans., 

vol. 34, pp. 1-65, 1903. Campbell, William, and Knight, C. W., On the microstructure of 

nickeliferous pyrrhotites: Econ. Geology, vol. 2, pp. 350-366, 1907.

16 Howe, Ernest, Petrographical notes on the Sudbury nickel deposits: Econ. Geology, 

vol. 9, pp. 505-522, 1914.

17 Tolman, C. F., and Rogers, A. F., A study of the magmatic sulfld ores : Leland Stan- * 

ford, Jr., Univ. Pub., Univ. series, p. 16, 1916.

18 Bateman, A. M., Magmatic ore deposits, Sudbury, Ontario: Econ. Geology, vol. 12, ; 

pp. 391-426, 1917. V
ECONOMIC GEOLOGY 143

Coleman 19 states that these different processes have all operated in 

the Sudbury region. But on the assumption that all these processes 

were involved in the formation of the ores, the extent to which each 

one contributed to the formation of the valuable ore bodies is still a 

matter of doubt. Coleman takes exception to Bateman's idea that 

the injection of molten sulphides was responsible for the greater part 

of the ores and states that in the main they are due to magmatic 

segregation in place.20

OCCURRENCE OF THE GAP ORE

The discussions on the genesis of the Sudbury ores are, of course, 

greatly aided by the fact that the surface outcrops of the ore bodies 

and of their associated rocks have been carefully mapped in much 

detail. Moreover, the underground development of the numerous 

mines has given and still gives ample opportunity to study the structural relations of the ore body to the surrounding rock. On the other 

hand, students of the Gap ore are forced to rely entirely upon the old 

mine reports for the structure and field relations of the ore body, because the workings have been for years filled with water. It is not 

even possible to map with accuracy the intrusive mass with which the 

ore is associated, because the area in which it occurs is notably lacking 

in surface outcrops. Therefore, there is practically nothing new to 

be gained by a field study of the Gap ore body, and hypotheses for its 

genesis must be based upon field observations made in former years, 

which are scanty, and upon petrographic study of the ores and to a 

considerable extent upon a general knowledge of the stratigraphic 

and structural relations of the ores and the including rocks. The 

only geologic description of the ore body was given by Kemp.,^ who 

visited the mine in April, 1894, shortly after it had been closed\ ' The 

map and sections of the ore body shown in Figure 15 have been taken 

from Kemp's paper, which, together with the old mine .reports,22 has 

furnished the data for the following description.

The ore occurs in places around the eastern, southeastern, and 

northeastern margins of a more or less lenticular mass of metamorphosed gabbro or norite. The gabbro mass is approximately 2,400 

feet in length and between 300 and 500 feet in width, with a sharp

18 Coleman, A. P., Magmas and sulphide ores : Econ. Geology, vol. 12, pp. 427-434, 1917; 

Geology of the Sudbury nickel deposits: Idem, vol. 19, pp. 570, 574, 1924.

20 Coleman, A. P., Geology of the Sudbury nickel deposits : Econ. Geology, vol. 19, p. 574, 

1924.

21 Kemp, J. F., The nickel mine at Lancaster Gap, Pa., and the pyrrhotite deposit at 

Anthony's Nose, on the Hudson: Am. Inst. Min. Eng. Trans., vol. 24, pp. 620-633, 883- 

888, 1895.

22 Frazer, Persifor, jr., Geology of Lancascer County: Pennsylvania Second Geol. Survey 

Rept. CCC, pp. 163-176, 1880.
FIGUBB 15. Geologic map and cross sections of the Gap nickel mine. (From Kemp, J. P.
ECONOMIC GEOLOGY 145

constriction toward the east end. The ore body ranged in thickness 

from 4 to 35 feet. It was apparently confined to the east end of the 

gabbro intrusion and occurred along the contact of the gabbro and 

the inclosing mica schist. The least depth at which ore was found is 

14 feet below the surface, and in many places the barren and much 

decomposed upper zone extended down to a depth of 60 feet. The 

ore body, which was nearly A^ertical, was worked to a depth of 250 

feet before mining ceased. Kemp 23 states tha,t the ore occurred to 

some extent in bunches within the lens, but most of the productive 

material was near the contact. Frazer notes the presence of detached 

masses of hornblende (the local name for the basic intrusion) within 

the ore, and Hess 24 mentions the occurrence of orbicules of hornblende in the pyrrhotite masses. 

The available field evidence does not show whether the lenticular

intrusion is a dike or a laccolith. The dip of the contact between 

the intrusion and the inclosing rock was apparently fairly steep on 

both the north and the south sides of the intrusion. Therefore, if the 

intrusion was laccolithic it must have been sharply tilted subsequent 

to its formation.

The igneous intrusive rock with which the ore is associated has 

been described by Kemp as an amphibolite in which the green hornblende is secondary, indicating that the rock is derived from a gabbro, norite, or pyroxenite. He describes the change, to hornblende as 

so complete that a careful search was required to reveal any trace of 

the original pyroxene. Recently Phemister 25 described the intrusive 

rock as a gabbro containing large amounts of pyrogenetic hornblende 

and secondary biotite with subordinate amounts of quartz, 

and plagioclase.

Several specimens that were collected by the writers from the 

mine dump show both a massive and a foliated basic rock. The 

massive variety is an equigranular rock of igneous texture composed 

of hornblende and feldspar with,a small amount, of metallic sulphides 

scattered throughout. Under the microscope it is seen to consist 

roughly half of feldspar and half of hornblende and biotite. The 

feldspar, which was apparently a fairly calcic plagioclase, has been 

so thoroughly saussuritized that it is difficult to determine the original species. Float from a locality about 3 miles east of the nickel 

mine, which is apparently identical with the rock at the mine, shows 

a less altered feldspar, which was determined as Ab04An.,6 . A large 

part of the green hornblende shows absolutely no trace of secondary

23 Ken?p, J. F., op. ctt., p. 627.

M Hess, F. L., U. S. Geol. Survey Mineral Resources, 1915, pt. 1. p. 756, 1917. 

83 Phemister, T. C., A note on the Lancaster Gap mine, Pa. : Jour. Geology, vol. 32, pp. 

500-502, 1924.
146 McCALLS FEREY-QUAEEYVILLE DISTRICT, PA.

origin and may well be pyrogenetic or of magmatic origin. The 

biotite, which is almost equal to the hornblende in amount, is of a 

peculiar reddish-brown color and is in some places intergrown with 

a deep-green clinochlore. A most striking feature of both horn- < 

blende and biotite is the prevalence of large and particularly strong ^ 

pleochroic haloes that surround zircons and prismatic crystals of a 

deep-brown mineral that appears to be allanite. The biotite shows 

none of the evidences of secondary origin that were mentioned by 

Phemister 26 as proofs of the hydrothermal origin of the biotite in 

the rocks that he studied. The individual crystals are fairly large, ^ 

about 0.5 to 1 millimeter in lengthj and are of approximately uniform grain. The direction of the amphibole cleavage does not appear to have determined the orientation of the biotite flakes, which 

is distinctly random in respect to both the hornblende and the other 

biotite individuals. Therefore there is a strong indication that at least 

a large part of the biotite in this gabbro is of pyrogenetic origin. The -^ 

rock may be called a biotite-hornblende gabbro, Titaniferous magnetite altering to leucoxene is of frequent occurrence, as are also apatite and to a lesser extent zircon. Tourmaline is a common accessory. 

Quartz is subordinate in amount.

The laminated variety is a dense, finely foliated schist, which 

shows under the microscope practically the same constituents as the 

massive rock. The tourmaline lies parallel to the foliation in the 

rocks, having been apparently directed by the planes of foliation. 

The metallic sulphides have a tendency to form stringers in the > 

direction of the foliation, and there is a suggestion in some places 

that they may replace the biotite. In one place an irregular mass 

of sulphides is entirely surrounded by biotite. As pointed out earlier 

in this paper, there is no proof that this foliated amphibolite is really 

an integral part of.the intrusive body associated with the nickel ore; 

but its close petrographic resemblance to the coarse-grained gabbro ^ 

and the prevalence of metallic sulphides, which are more abundant l , 

in the foliated than in the massive variety, together with the fact I 

that there is marked laminated structure at the west end of the gabbro ^ 

intrusion,27 suggest that this schist is really the border f acies, which 

has been severely squeezed along the walls.

A light-colored rock of which several specimens were collected < 

from the dump appears to be identical with the altered quartz monzonite or granodiorite that injects the pre-Cambrian sedimentary 

series. These specimens, however, show some interesting features. ' 

The rock is of granitic texture and is composed almost entirely of 4 

plagioclase with a variable amount of quartz. Under the micro20 Phemister, T. C.,. op. cit., p. 502. 

27 Kemp, J. F., op. cit., p. 626.
ECONOMIC GEOLOGY 147

scope it is seen that tfo-e plagioclase is of two kinds. One, a saussuritic albite, is clearly derived from a more calcic plagioclase. The 

other, which is less abundant, has the optical properties of albite 

and shows the peculiar " chessboard " texture that is formed when 

a crystal of microcline is replaced by homoaxial albite.28 The percentage of albite in the rock is extremely variable, but in those sections that show the most "chessboard" albite the microcline is 

scanty. A little microperthite is visible. The quartz, where present,, 

is granulated, and the rock shows a fairly strong cataclastic deformation. Dark minerals are conspicuously absent, but there are 

small amounts of biotite, muscovite, and chlorite, which rather 

strongly suggest by their method of formation the action of hightemperature solutions. The chlorite occurs in radiating fibrous aggregates, in fibrous veins, or in stringers that traverse the rock 

irregularly. One specimen in which the " chessboard " albite is so

abundant that the rock is practically an albitite contains an irregular 

mass of metallic sulphides of considerable size. This rock is probably identical with the granite shown on Kemp's map (fig. 15).

The ore minerals are pentlandite, mixed with pyrrhotite, and millerite. The millerite was found as incrustations on cracks and is 

evidently, as pointed out by Kemp 29 and Hess,30 a product of downward enrichment from the leached and barren upper zone. The pentlandite is intimately intermixed with pyrrhotite, from which it can 

be separated only by use of the reflecting microscope. Buddington 81 

has called attention to the fact that the pentlandite of the Gap nickel 

ores is replaced by a weakly magnetic secondary nickel mineral, 

which he called " nickel mineral X" to emphasize the fact that it 

may prove to be bravoite, polydymite, or violarite, or even a mixture 

of more than one mineral. Pyrrhotite is associated with chalcopyrite, the order of formation being pyrrhotite, pentlandite, and 

chalcopyrite.

GENESIS OF THE GAP ORE

The first theory to explain the origin of the Gap nickel ores was 

proposed by Kemp. In 1893 he advanced the idea that these nickel 

sulphides were integral parts of the magma that formed the basic 

igneous intrusion. The ores were separated from the magma as it 

cooled and concentrated along the walls in the coolest part of the 

solution. In this explanation of the ores Kemp applied the theory

28 Becke, Priederich, Zur Physiographic der Germengtheile der Krystallinen Schiefer: 

K. Akad. Wiss., Math.-naturwiss. Klasse, Denkschr., vol. 75, p. 125, 1913. 

89 Kemp, J. F., op. clt., p. 626. 

30 Hess, F. L., op. clt., p. 756. 

81 Buddington, A. F., Alaskan nickel minerals: Econ. Geology, vol. 19, pp. 529-531, 1924.

6136 29 11
148 McCALLS FERRY-QTJARRYVILLE DISTRICT, PA.

that had just been outlined by Vogt 32 in his discussion of ores 

formed by magmatic segregation. Both Kemp and Vogt invoke the 

Soret principle, that the concentration of the dissolved substances is 

greatest in the coolest part of the solution, as the probable explanation of the segregation of the sulphides along the walls of the 

intrusion. Vogt makes it very clear, however, that the consolidation 

of these sulphides does not take place until after the silicates have 

crystallized,33 owing to the fact that the melting point of the sulphides is much lower than that of the silicate minerals. Petrographic study of nickeliferous pyrrhotite ores from different localities has shown that the sulphides have formed after the silicates, 

and the metallic sulphides of the Gap ores conform to this rule, 

having clearly been formed later than the silicates around which the 

sulphides have molded themselves. This easily recognized relation 

of sulphides to silicates has recently been emphasized by Phemister, 

who believes that the Gap ores are of hydrothermal origin. In 

fact, he goes so far as to state 34 that this relation is the insurmountable difficulty to the igneous view of the ore genesis. The late 

consolidation of the sulphides is, however, no obstacle to the theory 

that they have been differentiated in place in the fluid condition. 

The pyrrhotite, pentlandite, and chalcopyrite nowhere show relations to the silicates that could not be adequately explained by the 

hypothesis that the sulphides were present to a late stage of crystallization as molten patches in the midst of earlier consolidated silicates. Even their presence in the mineralized schist on the north 

wall of the intrusion, which is no-ted by Phemister,35 might be 

 explained by the theory that in certain places the molten sulphide 

:material penetrated for a short distance into the adjoining heated 

^wall rock. A much more serious difficulty in the way of explaining 

these ores as magmatic segregations is their position along both 

walls of a steep-sided intrusion. Since it has been recognized that 

the difference in concentration between different parts of a cooling 

magma is not sufficient to produce an important segregation of ore 

the localization of sulphide ores has been ascribed chiefly to the 

action of gravity. In order to explain the Gap ores as a gravitative 

differentiate it is necessary to assume that the igneous intrusion is a 

small and closely folded laccolith. In other words, if the ores have 

assumed their present position under the influence of gravity they 

must have been intensely deformed after their formation.

The relation of the ores to the deformation is not, however, easy 

to make out. The positive proof adduced by Phemister in support

12 Vogt, J. H. L., Bildung \oa Erzlagerstatten durch Differentiations-proceisse iu basischen 

Eruptivmagmata: Zeltschr. prakt. Geologic, vol. 1, pp. 4-11, 125-143, 1893. 

M Idem, p. 139.

s* Phemister, T. C., op. cit, p. 508. 

* Idem, p. 501.
ECONOMIC GEOLOGY 149

of the secondary character of the ores is the fact that the sulphides 

replace not only hornblende but biotite, which he considers to be 

of hydrothermal origin. If this biotite. which is surrounded by 

sulphides, is really secondary, there seems to be no escape from the 

fact that at least a part of the Gap ores have been introduced by 

highly heated solutions.

The somewhat cursory investigation of these ores made during 

the geologic study of the region would suggest, although it can 

not be said to prove, that the greater part of the biotite is pyrogenetic rather than hydrothermal. It is an interesting fact that the 

gabbros of Maryland, which are in many respects petrographically 

similar to the gabbros of Mine Ridge, are notably free from biotite. 

Biotite as a subordinate constituent is present in the gabbros that 

intrude the Glenarm schists in the West Chester quadrangle; and it 

is an accessory constituent in the gabbros of the Philadelphia belt.36

It is of universal occurrence in the gabbros and hornblende schists 

throughout the Mine Eidge anticline. This leads to the conclusion 

that either the gabbros of the Mine Ridge anticline differ essentially from the Maryland gabbros in the presence of magmatic biotite or else if the biotite is largely due to hydrothermal replacement the mineralization is not confined to the igneous rocks of the 

Gap mine.

There has been undoubtedly a widespread mineralization in the 

region, as indicated by the prevalence of tourmaline and pyrite 

throughout rocks of widely different ages, by the presence of magnetite and hematite in the pegmatites, and by the occurrence of 

pyrrhotite here and there in the Conestoga limestone. But whether 

all this mineralization was confined to one metallogenetic .epoch is 

not yet clearly established. The probable sequence of events after 

the intrusion of the gabbro mass of the Gap mine is as follows: 

(1) Intrusion of a gabbroic magma; (2) consolidation of pyroxene; 

(3) consolidation of magmatic hornblende; (4) consolidation of 

pyrogenetic biotite; (5) intrusion of granodiorite; (6) formation 

of chessboard albite in the granodiorite during the latest stages of 

<:onsolidation; (7) intrusion of albitic pegmatites that are satellitic 

followers of the granodiorite intrusion; (8) a strong dynamic deformation that affected both the gabbro, as shown by the twisting and 

bending of the biotite and the uralitization of the pyroxene, and the 

granodiorite and pegmatites, as shown by the saussuritization of the 

feldspar; (9) intrusion of granite, perhaps contemporaneous with 

the dynamic action; (10) intrusion of microcline pegmatites accompanied by boron vapors and ore-bearing solutions carrying magnetite, 

hematite, pyrrhotite, pyrite, chlorite, and sericite.

M Bascom, F., U. S. Geol. Survey Geol. Atlas, Philadelphia folio (No. 162), p. 6, 1909.
150 McCALLS FERRY-QUARR.YVILLE DISTRICT, PA.

The sulphides that occur in the Conestoga limestone are in many 

places sharply defined crystals, with no evidence of deformation, and 

therefore can not be ascribed to the earlier pegmatites that have been 

dynamically deformed. In some places the pyrite is flattened and 

slickensided on planes of movement. This movement was doubtless 

Triassic and local, thus affecting only the pre-Triassic sulphides in 

certain places near which Triassic faulting took place. If the Gap 

nickel pyrrhotites have been manifestly deformed they can hardly be 

the equivalent of the undeformed sulphides in the limestones. If 

they have been deformed there are two possibilities that they may 

be magmatic segregations from the gabbro magma, or that they may 

have been introduced by emanations from the granodiorite magma. 

So far petrographic study of the ore has failed to reveal any marked 

evidence* of cataclastic action.

The sulphides that occur in the massive variety of the dynamically 

deformed and saussuritized gabbros are notably free from any evidence of cracking and stretching and are molded as large blebs around 

the silicates. But sulphides, which are quasi plastic under stress that 

breaks down silicate minerals, would not register evidences of cataclastic deformation of the rock. It is true that the sulphides occurring in the laminated schist are arranged parallel to the foliatio^ 

but it is impossible to prove whether they have been dragged out 

along the foliation planes by the shearing action of the deforming 

force or whether they were introduced into the rock subsequent to 

the foliation and their position determined by foliation. As previously pointed out, it is impossible even to tell whether this schist 

belongs to the nickel-bearing intrusion or to the invaded schists. 

Therefore it seems absolutely impossible to come to any definite conclusion in regard to the magmatic or hydrothermal origin of the 

ores unless their relation to the metamorphism can be established. If 

they are later than the deformation they can not be magmatic. The 

country rock has been subjected to pneumatolytic action on a largo 

scale, but whether this pneumatolysis produced the nickel ores is not 

known. If the nickel ores were not produced during the regional 

pneumatolysis and if they were formed before the metamorphism 

of the gabbro, it is highly probable that they are of magmatic origin.

IRON ORE

Limonitic ore that occurs in limestone has been mined to some extent at several localities in the district. The largest workings were 

the C. B. Grubb ore banks in Conestoga limestone, half a mile east 

of Shenks Ferry station. They were worked from 1840 to 1850. The 

ore was a concretionary, somewhat manganiferous limonite. which
ECONOMIC GEOLOGY 151

was mined by open cuts and then washed. An analysis of this ore 

showed 53 per cent of metallic iron. 37

Limonite was also mined at many places in Providence, Eden, and 

Bart Townships. The ore was the same in character and geologic 

occurrence at all these localities, being a concretionary or geodiferous 

limonite deposit that has replaced limestone, in many places along 

the contact of the limestone and the overlying schist. The ore has 

been formed through the action of downward-moving water carrying 

iron in solution and depositing it as limonite most abundantly along 

contacts or fault surfaces. The high-grade ore appears to have been 

removed, and the remaining deposits are believed to be small; at 

least present conditions do not warrant any attempt to mine the

known deposits.

In 1893 an unsuccessful attempt was made to extract magnetite 

from the Cambrian biotite schist on Pequea Creek iy2 miles northwest of Marticville.

CHROME ORE

About 3 miles east of Susquehanna River, in Fulton Township 

near Lyles (locally known as Texas), there is an area of chromiferous 

serpentine. This area lies 2 miles north of the so-called " State 

Line" serpentine belt of southern Pennsylvania and Maryland. 88 

Chromite occurs at intervals along the northern boundary of this 

belt and is reported to occur also in the southern part of the serpentine area near Lyles. The Brown pit, east of Little Conowingo Creek, 

is the only mine known to have been opened in this area, and it has 

been long abandoned and filled up. The ore occurs in three ways 

as massive, compact chromite in nodules and pockets of variable size 

in a serpentine gangue; as a disseminated ore, locally known as 

spotted ore or birdseye, in which the grains of chromite are rather 

evenly disseminated through the serpentine; and as placer deposits. 

The Pennsylvania ore has in general proved to be of good grade 

and superior in quality to the California ores. The chromite of 

Maryland and Pennsylvania was the world's only source of chromium 

from the beginning of the industry until the Civil War. At that 

time, on account of the lowered prices of chromite caused by the 

importation of Turkish and other foreign ores, the Pennsylvania 

deposits were relegated to the position of a reserve supply. The 

increased demand for chrome ore during the World War led to a 

revival of mining in the chromite deposits of the eastern United 

States, but the life of the industry, which was dependent upon the 

abnormal demand, ceased with the termination of the war.

37 Frazer, Perslfor, jr., Geology of Lancaster County: Pennsylvania Second Geol. Survey 

Kept. CCC, p. 221, 1880.

38 Bliss, E. P., Chrome ores of southeastern Pennsylvania and Maryland: U. S. Geol. 

Survey Bull. 725, pp. 85-98, 1921.
152 McCALLS FEBBY-QUARRYViLLE DISTRICT, PA.

SILVER ORE

There is an abandoned silver mine on Silver Mine Run in the 

McCalls Ferry quadrangle, about 1 mile east of Conestoga. It was 

worked about 125 years ago, when a shaft was sunk that connected 

with a tunnel running 400 feet into the hill. The ore occurred in 

small isolated lenses, and no large body was discovered. The ore 

mineral is argentiferous galena in a quartz vein. The country rock 

is Vintage dolomite overlain by the Conestoga formation. In the 

much weathered open cuts, which are all that remain of the old 

workings, the quartz veins appear to have had their maximum 

development in the Vintage dolomite.

ROOFING SLATE

A high-grade black roofing slate occurs in the belt of Peach Bottom slate, which extends northeastward from Susquehanna River 

for a distance of 3 miles and which also extends southwestward 6 

miles into Maryland, where it is extensively quarried on the Maryland-Pennsylvania State line at the towns of Cardiff and Delta. 

The slate belt along Susquehanna River is half a mile wide, but it 

pinches out toward the northeast. The trend of the slate outcrops 

along the river is N. 50° E.

Most of the slate quarrying has been done along the trend of 

Slate Hill, which follows the Maryland and Pennsylvania State line 

for several miles on the west side of Susquehanna River. There 

are two quarries on the east side of the river a quarter of a mile 

northwest of Peach Bottom station. Work was begun here about

1W and continued for 100 years. In 1910 the upper quarry was 

reopened by Gorsuch Bros., who operated it on a small scale for 

local trade. The depth of the quarry is more than 80 feet. The 

good slate, which stands nearly vertical, alternates with nonproductive layers, and in following the valuable layers downward along 

the steep dip the working became too deep to operate profitably. 

There is much waste in connection with the quarrying and the refuse 

was dumped over workable beds. When the Pennsylvania Water 

Power Co. constructed a dam at Holtwood in 1910 the Columbia & 

Port Deposit branch of the Pennsylvania Railroad was obliged to 

relocate its tracks north of Peach Bottom station. The dump from 

the lower quarry was then used as ballast upon the new line. In 

1920 the Gorsuch Bros. Co. built a mill for crushing the waste slate 

into granules to be used for making slate-surfaced prepared roofing. 

The Peach Bottom slates are lustrous, blue-black, unfading slates, 

very sonorous.39 They surpass the slates of Northampton and Lehigh

39 Dale, T. N., Slate in the United States : U. S. Geol. Survey Bull. 586, pp. 110-113, 

1914.
ECONOMIC GEOLOGY 153

Counties in strength, toughness, and impact tests. They are extensively used for roofing and to a lesser extent as gravestones, and 

the excellent condition of slate roofs and gravestones that have been 

J in existence for over 100 years testifies to their durability and quality.

LIMESTONE

The limestone belt in the northern part of the district contains

* valuable stone that has a wide range in use for building stone, ballast, and road metal and for making lime and cement. The largest 

quarry now in operation is at Kinzers, on the north side of the 

Lincoln Highway. This is the E. L. Slaymaker quarry in Vintage 

dolomite that is worked for crushed stone. The rock is a heavybedded, dolomite dipping gently north and quarried with the dip.

Zittle Brothers' quarry, west of Strasburg, near Pequea Creek, 

is operated intermittently for crushed stone, and the Aldus Zittle

v quarry, northeast of Strasburg, is worked for the same purpose. 

These quarries are in the Conestoga blue limestone. The quarry 

of the Quarryville Lime & Stone Co., north of Quarryville, is in the

' micaceous marble facies of the Conestoga limestone. It is known 

as the Orchard quarry and was opened in 1826. The stone is burned 

for lime, and considerable pure marble is available in the quarry.

> Many quarries throughout the Lancaster and Quarryville Valleys 

are now abandoned or worked only to a small extent for local use. 

Many of them are in the marble conglomerate of the Conestoga 

formation. The J. P. Mcllvain quarries, at Bellemont, are among 

the larger ones. Lime was burned there for many years, but the 

kilns are no longer used and the quarries are for the most part idle.

\ There are many old quarries around London Grove, Wheatland, and 

New Providence, from which much limestone has been taken in 

former years. At present the cheapness and availability of lime 

produced in large plants has caused a decline in local lime burning, 

but crushed limestone is in considerable demand for making roads, 

and most of the limestone now quarried is used for that purpose. 

' *

SAND

'*

The Chickies quartzite on the southwest side of Mine Eidge has 

disintegrated into white sand, which is dug for building and furnace sand. A sand pit north of Quarryville was opened by Leander 

Kunkel in 1921 and was being operated in a small way in 1922.

INDEX

Acknowledgments for aid_..._______ 3

Amphibolite schist, occurrences of_____ 62

Antietam schist, plates showing.. _____ SO, 98

Appalachian Mountains, geology of.. __ 10-13

Avondale, Pa., section near_________ 22

Bailey, R. K., analyses by..-............_ 31,49

Baltimore gneiss, correlation of, with preCambrian schists of the Highlands of New Jersey and New 

York............................ 68-70

features of___:...___________ 15 

geology of. . .- 18-24

study of metamorphism in......_..... 124

Bibliography of the district_. _______ 5-6 

Bioite gneiss, in the Reading Hills, features

Of--- . 11-12

Biotitic amphibolite schists, age of.. ..... 60

composition and occurrence of..___.. 59 

Blue Mountain, record of erosion on...._ 100-101

Blythedale Creek, significance of knickpoints

on............................... 114

Booth, Garrett & Blair, analysis by...__.. 40 

Brandywine subcycle of erosion, record of. 105-109 

Brown, Glenn V., analyses by___.____ 37,49 

Bryn Mawr subcycle of erosion, traces of.. 104-105

5 Cardiff conglomerate, description of.__ 37-39,12

Cataclastic texture, nature of____ _._ 12 1 

Catoctin Mountain, features of..------------ ! *

Chickies banded slate, plate showing __... 138 

Chickies quartzite, description of.----------- 42-4*>

mica schist from, plate showing__,. _ 13^ 

Cblorito schist, occurrence of_------_..-. 6^

Chromium ore, occurrence of..._...--.-.-. 151 

Climate of the district........___....'__ 9

Cockeysville marble, features of..._....... 25

Conestoga limestone, age and correlation of.. 57-58 

distribution of..._________.__ 54-55 

lithology of.....__..__.---____- 55-56

metamorphism in.._---..---.-.--,-- 126-127 

plates showing.. ____________. 50,51 

utilization of-. 56,153

Correlation of rocks of the district with those

of other areas.__.......-.-..- 68-71

Crystalloblastic textuie, nature of.-__.. 121-122

"Deeps" of Susquehanna River, occurrence

and origin of_... _ - 8-9

Diabase dikes, features of--___---____ 67-68

Diaphthorite, definition of__________ 27

Drainage of the district, description of.-_-- 8-9

evidence of modifications of_-_----- 100-109

former courses of, physiographic history

recorded in..---------..--------- 91-95

successive courses of, relative preservation

of-------.-...--.-----.---------- 95-96

Erosion, area and duration of _______ 117 

late Pleistocene, traces of......__.... 109

Erosion cycles, age of....._ _.___... 118 

number of... .....__ ....__. 116-117

Erosion surfaces, altitude of - - ____ 99

Faults in the district, description of- __ 74-77 

Field work, record of....__ ..._.. 3

Folding, age of----........- .- 77-78

load on...__...__._........__ 80'

Folds in the district, description of..____ 71-74

Gap mine, description of-... . 141-142 

Genth, P. A., analyses by....._______ 31,45

Glenarm series, correlation of, and the Manhattan, Inwood, and Lowerre 

formations of New York_.__ 70-71 

description of..... __..._...... 24-41

development of older metamorphism

in. . . 134-138 

double metamorphism in_--..... 132,138-139

features of....--.-.- --------------- 15,17-18

load metamorphism and regional contact

metamorphism in...___.... 131-132

study of metamorphism in......_ ... 125

Granitic pegmatite, age of. .. . 65-67 

occuirence of.... . 64-65

Harpeis schist, plates showing...____... 50,138 

Harpers and Antietam schists, age of....-..- 50

areal distribution of..._ ...... ... 46-47

chemical composition of -------- __..... 48-49

general features of-.-..--- 45-46 

lithologic character of... ___... __ 47-48,126 

metamorphism in... ------ 49

thickness of----__-__- ---_ --- 49

Harrisburg subcycle of erosion, record of.. 103-104 

Hawkins, A. C., quoted....---.-........- 21,22,23

Hellam conglomerate member of the Chickies

quartzite, description of. 43-44,125-126 

Hillebrand, W. F., analysis by..___..-__ .- 31 

History, pre-Cambrian.__. .... 81-84

Paleozoic era of-.-.--.------------- 85-86,89-90

physiographic record of_.-.--_.-_...- 91-118 

post-Paleozoic periods of.---------------- 90

Honey brook subcycle of erosion, traces of. 103

Igneous intrusion, sequence of-.-- -- 58-59 

Interstream areas, evidence of physiographic

history in.-. 96-100 

Iron ore, occurrence of .. 150-151

Kinzers formation, nature and distribution of. 52-53 

plates showing_ . 50

Knickpoints, significance of....- ... 109-110 

significance of altitude of, on Susquehanna

River and its tributaries___ 113-116.

155
156 INDEX

Page 

Lawson, A. C., quoted--..-.------.----..... 136

Ledger dolomite, -features of. ---------- __.. 53-54

Limestone, quarrying of...____._._.. 153 

Little Gap, erosion cycles recorded in... __ 101 

Location of the district...................... 1

McCreath, A. S., analysis by................ 40

Map, geologic, and structure sections of the

quadrangles.................In pocket.

Martic overthrust fault, age of.- ..-..-.-.- 79 

description of..-------------------------- 74-75

Metagabbro, composition and occurrence of- 59-60 

Metamorphism in the schists of the Piedmont

province, degrees of _-__ 122-124

intensities of, criteria for comparison of. 120-121

differences in..-...-.--.-------.-.. 118-119

regional differences in._....-..--. 128-129

causes of...---.--.--------..-.. 129-130

minerals produced by..._____..... 122

retrogressive effects of----....._._-.-.- 124

summary of conclusions on............ 139-141

textures produced by...-.-.... __... 121-122

Mine Ridge anticline, close folding on.------ 80

description of__----------_-.._._ 72-73

Mine Ridge cycle of erosion, record of- - - - - 100-102

Mine Ridge upland, Paleozoic rocks in...---- 42

section near............................. 22

Nickel ore, occurrence of---...---.-.-.. 141,143-147 

origin of.......---------- --- 142-143,147-150

Octoraro Creek, physiographic record shown

by profiles of-- - 114-116 

"Octoraro schist," former use of the term.... 34 

Orthogneisses, metamorphismin...__... 127-128 

Outline of the report........__ T _.__ ix-xm

Paleozoic rocks, development of older metamorphism in. ..... ... 134-138

differences in metamorphic intensity in. 132-133

double metamorphism in-. . 133-134,138-139

Peach Bottom slate, description of 39-41,125

Peach Bottom syncline, description of-.... 73-74

Penniman & Browne, analysis by... - - 31

Peters Creek schist, description of..__ 35-37,125

Piedmont province, general features of_ 13-14

general geology of, in Maryland... .... 15-18

in Pennsylvania.--.-.__---_.._. 14-15 

Post-Conestoga deformation and metamorphism, age of--------------___ 86-89

Pre-Cambrian rocks, oldest, metamorphism

in..... .......------------ 130-131

Principio Creek, significance of knickpoints

on-.------------.--. --- 114

Prior work in the district.-.----------------- 3-5

Profile of divide between Susquehanna and 

Schuylkill River drainage slopes, 

plate showing. - . .. 99 

Profiles, cross, of stream valleys, history

recorded in..-----.-.........._ 94

Page

Profiles, longitudinal, of stream channels,

history recorded in.. --------.. 95

longitudinal, of stream channels, physiographic record in.......-. ._ 110-116

projected, of area between Blue Mountain 

and the Coastal Plain, plate 

showing__ ___ 99

Ptygmatic folding, definition of______ 64" 

Publications on the district..--------..-...... 5-6

Quartz diorite, occurrences of._............ 64

Quartz monzonite, age of 65-67

occuri ence and composition of----------- 62-63

Quartz monzonite pegmatite and aplite, occurrence and composition of _ 63-64

Quartzite member of the Chickies quartzite,

description of ......_.. - 44-45,126

Rail and water transportation in the district.. 1-3

Reading Hills, geology of------___-..._ 11-13

Ruxton overthrust, features of _......... 79

Sand, use of---------... ------------------- 153

Serpentine, features of-.__ .....____. 61

Setters formation, texture of.... ...__ 125

Silver ore, occurience of--__--------__.. 152

Slate, roofing, occurience of 152-153

South Mountain near Reading, Pa., location

of.-- ... 11,13

South Mountain, Pa.-Md., features of___ 11,13 

Structuie, age of_ ...-- . --- _ 77

chief features of 71

Triassic features of ---------_ __ 81

Sunbury peneplain, disuse of name . 103 

Superimposition, definitions of--- ..-. . 92

history recorded by__ 92-94

Susquehanna River, features of ...___ 8-9

longitudinal profile of_.......___ 110-113

Thrusting, load on. _______._-____ 80

Topography of the district_---------------- 6-8

Transportation in the district..._--------- 1-3

Tucquan anticline, features of. . _ - 73

Uplift, peiioflicity of.-- 117-118

Vintage dolomite, nature and distribution of. 50-52

12£ 

plates showing.--.-.-- --------- -. - 50

Volcanic flows, altered, possible occurrences

of.-- ' 62

Welsh Mountain, fault south of 76

Paleozoic rocks in...-------------------- 41-42

Wind gaps, diversion of streams recorded in.. 94-95 

erosion of Mine Ridge recorded in.. 100-102

Wissahickon formation, age of. 33-35

albite-chlorite schist facies of _.... 28-30

composition of._ .. 30-32

diaphthoritic schists in. _.... _.. ___ 27-28

metamorphism in._.__ .... 32-33,125

oligoclase-mica schist facies of.. _.. ___ 25-27

-O * 3