The Martinsburg Formation 

(Middle and Upper Ordovician) 

in the Delaware Valley 

Pennsylvania-New Jersey

By AVERY ALA DRAKE, JR., and JACK B. EPSTEIN

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1244-H

Prepared in cooperation with the 

Pennsylvania Geological Survey

A clarification of the stratigraphy 

of clastic rocks of Middle and L,ate 

Ordovician age in western New Jersey 

and eastern Pennsylvania
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

U.S. GOVERNMENT PRINTING OFFICE 

WASHINGTON : 1S67

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 15 cents (paper cover)
CONTENTS

Page

Abstract __________________________________________ HI

Introduction _______________________________________ 1

Martinsburg Formation _________________________________ 3

Structural relations ________________________________ 6

Bushkill Member __________________________________ 6

Eamseyburg Member _______________________________ 9

Pen Argyl Member _________________________________ 12

Summary __________________________________________ 15

Eeferences cited _____________________________________ 15

ILLUSTEATIONS

Page

FIGURE 1. Chart showing interpretations of Martinsburg stratigraphy by

previous workers and by authors of this report _______ H4 

2. Geologic sketch map showing distribution of the Martinsburg

Formation ______________________________ 5 

3-6. Photographs showing:

3. Typical outcrop of ribbon slate in the Bushkill Member ______________________________ 7

4. Typical exposure of Eamseyburg Member at the type

locality ___________________________ 10

5. Outcrop of Eamseyburg Member showing refracted

cleavage, cross-stratification, and graded bedding _ 11

6. Typical exposure of Pen Argyl Member________ 14

III

CONTRIBUTIONS TO STRATIGRAPHY

THE MARTINSBURG FORMATION (MIDDLE AND

UPPER ORDOVIGIAN) IN THE DELAWARE VALLEY

PENNSYLVANIA-NEW JERSEY

By AVERY ALA DRAKE, Jr., and JACK B. EPSTEIN

Abstract

Detailed mapping of ten 7^ -minute quadrangles in eastern Pennsylvania and 

western New Jersey has shown that the Martinsburg Formation of Middle and 

Late Ordovician age can be divided into three members: a lower thin-bedded 

slate, a middle graywacke-bearing unit, and an upper thick-bedded slate. These 

units are herein named the Bushkill, Eamseyburg, and Pen Argyl Members, 

respectively. The Pen Argyl Member is buried east of the Delaware Eiver and 

west of Kempton, Pa., near the Schuylkill Eiver because of unconf ormable overlap 

by the superjacent Shawangunk Conglomerate of Silurian age. Commercial slate 

is presently quarried only in the Pen Argyl Member and the upper part of the 

Eamseyburg Member.

INTRODUCTION

Pelitic and lesser psammitic rocks of Middle and Late Ordovician 

age crop out along the Appalachian Valley from Tennessee to New 

York, where they are covered by rocks of the Taconic sequence near 

the Hudson River. In recent years these rocks, which lie between the 

Jacksonburg Limestone of Middle Ordovician age and the Shawangunk 

Conglomerate of Silurian age, have been referred to collectively as 

the Martinsburg Shale or the Martinsburg Slate in Pennsylvania and 

New Jersey. The subdivision of the formation, however, has been 

controversial. In this report we divide the Martinsburg into three 

members, call it a formation, and clarify the stratigraphy of this 

complex unit. This intraformation subdivision is of the utmost importance in deciphering the magnitude and extent of the unconformity 

between the Martinsburg and the overlying Shawangunk Conglomerate.

Hi
H2 CONTKIBUTIONS TO STKATIGKAPHY

It clarifies the geometry and mechanics of folding in this region and 

develops a clear picture of the Taconic orogeny. In addition, this 

subdivision is economically important because it fixes the stratigraphic 

position and areal distribution of the commercial slate in the formation.

In Pennsylvania the sequence of rocks now known as the Martinsburg was called the Matinal Series by Eogers (1858) and was thought 

to consist of two units, a lower black slate and an upper shale. Lesley 

(1892), of the Second Pennsylvania Geological Survey, assigned the 

rocks to the Utica and Hudson Eiver Slates, which consisted of an 

upper thick-bedded and lower thin-bedded series. Dale (1906) followed this terminology but recognized a distinct difference between 

the "hard" and "soft" slates. In a later report (Dale and others, 1914, 

p. 95-96), however, he recognized a lower hard thin-bedded shale and 

slate, a middle member of sandstone, and an upper member of soft 

thick-bedded shale and slate. Peck (1908), working in the Lehigh 

Valley, believed the sequence contained three subdivisions, the middle 

of which included sandstone and graywacke. Stose (1910) first used 

the name Martinsburg Shale in Pennsylvania. He thought the formation consisted of a lower black shale and an upper sandstone. Behre 

(1927, 1933), in his classic works on the commercial slate districts, 

divided the Martinsburg into a lower ."hard" slate member, a middle 

sandy member, and an upper "soft" slate member. Stose (1930), working into the area of this report from the southwest, restated his belief 

in a bipartite interpretation of the Martinsburg and felt that Behre's 

upper member was the lower member repeated in the north limb of 

a regional syncline.

Willard and Cleaves (1939) concurred with the bipartite interpretation and named their upper member the Shochary Sandstone. B. L. 

Miller (in Miller and others, 1939), in his report on the Martinsburg 

in Northampton County, Pa. (authored with Behre), accepted Behre's 

tripartite interpretation, but in his later Lehigh County, Pa., report 

(Miller and others, 1941), the two-member interpretation is presented 

by Willard. Willard (1943) later raised the Martinsburg to a group, 

consisting of his Dauphin Shale below and Willard and Cleaves' 

Shochary Sandstone above.

In New Jersey, the sequence of rocks was called the Hudson River 

Slate by Cook (1868, p. 135-145), who recognized both slate and sandstone but made no attempt to subdivide the formation. Weller (1901, 

p. 5-8) followed the same terminology. The name Martinsburg was 

first applied by Bayley, Salisbury, and Kummel (1914); the Martinsburg was thought by Lewis and Kummel (1915) to have lower slaty and 

upper sandy subdivisions.
MAKTINSBURG FOKMATION, DELAWARE VALLEY, PA-N.J. H3

In recent years, most geologists working in the area have accepted 

the name Martinsburg and espoused the bipartite interpretation. Willard's (1943) concept of a Martinsburg Group was never tested by 

mapping and has not been adopted by other geologists. A summary of 

the more important interpretation is given in figure 1.

With our colleagues J. M. Aaron, D. C. Alvord, and R. E. Davis, 

all of the U.S. Geological Survey, we have studied the Martinsburg 

Formation in the course of detailed mapping of the Bangor, Belvidere, 

Bloomsbury, Bushkill, Kunkletown, Nazareth, Portland, Saylorsburg, 

Stroudsburg, and Wind Gap 7^-minute quadrangles (fig. 2). In this 

area, the Martinsburg consists, without question, of three lithologic 

units a lower thin-bedded claystone slate, a middle unit of interbedded 

graywacke and slate, and an upper thick-bedded claystone slate 

much the same as those defined by Behre (1927).

We sincerely thank our colleagues for their generous contributions of 

data, as well as for their stimulating discussions of the problem.

MARTINSBURG FORMATION

The Martinsburg was named for exposures near Martinsburg, W. Va. 

(Keith, 1894), and forms a continuous belt of outcrops along the 

Appalachian Valley. Complications exist between the Susquehanna 

and Lehigh Eivers, where Stose (1946) attributed the presence of varicolored shales and other exotic rock types to a far-traveled Hamburg 

klippe. This interpretation has not gained wide support, but recent 

work suggests that much of the rock in that area may well be allochthonous. In any case, typical Martinsburg lithologies can be traced 

through to the Delaware Valley and into New Jersey. Here the Martinsburg forms a wide belt of outcrops rising in rolling hills 250-300 feet 

above the carbonate valley to the south and lying more than 700 feet 

below Kittatinny and Blue Mountains to the north (fig. 2). In addition to this major belt of outcrops, outlying bodies occur along the 

Musconetcong Valley in New Jersey (Bayley, Salisbury, and Kummel, 

1914; Drake, 1967a).

In the Delaware Valley, the southernmost outcrops, which are the 

oldest parts of the Martinsburg, are composed of thin-bedded claystone 

slate which contains a few thin interbeds of dolomitic siltstone near 

the base. This unit is succeeded to the north by an interbedded 

sequence of claystone slate, graywacke and graywacke siltstone that 

underlies hills rising 200-300 feet above the slate valleys to the southeast and northwest. These rocks are overlain by a series of thick-bedded 

claystone slate and scattered thin beds of graywacke. The basal slate 

intervals of the graywacke-bearing sequence are typical of the underlying unit, and those above are like the overlying unit. Because of
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6,000

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Upper 

sandy 

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Lower 

shaly 

member

3,000

Stose(1930)

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Lower shaly 

member

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Upper sandy 

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Willard (1943)

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Dauphin Shale

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Pen Argyl 

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Ramseyburg 

Member

Bushkill 

Member

9,800-12,800

FIGURE 1. Interpretations of Martinsburg stratigraphy by previous workers and by authors of this report.
75"00'

41*00'-

52'30'

40'45'

EXPLANATION

I ^ 1 

Rocks of Silurian and Devonian age

Pen Argyl Member

Ramseyburg Member

Carbonate rocks of Cambrian and 

Ordovician age

Precambrian crystalline rocks, including the Hardyston Quartzite 

of Cambrian age

Contact

Inclined Overturned 

Contact of core and inverted limb 

of Musconetcong nappe 

Barb mi core sirfe ofcontn.ct

Thrust fault 

T, on upper plate

Numbers refer to quadrangles: 1 

Bushkill, 2 Saylorsburg, 3 Stroudsburg, 4 Portland, 5 Kunkletown, 6 Wind Gap, 7 Bangor, 8 Belvidere, 9 Nazareth, 10 Easton, and 

11 Bloomsbury

FIGURE 2. Geologic sketch map showing the distribution of the Martinsburg Formation in the Delaware Valley, Pa.~N.J.

a
H6

these different lithologies and because the Martinsburg is not a shale 

in this area, we herein change its lithic modifier from shale to formation.

STRUCTURAL RELATIONS

An appreciation of the; regional structural geology is critical to an 

understanding of the Martinsburg Formation. Behre (1927, 1933) 

visualized the Martinsburg as occupying a regional northwestwarddipping monocline complicated byj numerous overturned, more commonly recumbent folds, some of whose axial surfaces have been rotated 

past the horizontal. Stose (1930) and Willard (1943) on the other 

hand, interpreted the Martinsburg as occupying a regional upright 

syncline keeled by the sandstone-bearing unit. On their sections, they 

also show a number of smaller upright anticlines and synclines, whose 

axial surfaces show no relation to the gently dipping regional penetrative, axial-plane slaty cleavage.

Current detailed mapping (Drake, McLaughlin, and Davis, 1961, 

1967; Drake, 1967a, 1967b; Davis, Drake, and Epstein, 1967) has 

shown that the Martinsburg Formation and older rocks are involved 

in a highly complicated, refolded, crystalline-cored nappe de recouvrement the Musconetcong nappe. The Martinsburg in this area lies 

mainly in th;e normal limb and imbricated brow of this nappe and has 

the. appearance of a northwestward-dipping monocline, as visualized 

by Behre (1927, 1933). Stratigraphic tops are generally evident from 

cro&sbedding and graded bedding, as well as from cleavage-bedding 

relations. Progressively younger rocks are exposed from south to 

north across the outcrop belt, but locally the sequence is overturned, 

especially in the Wind Gap-Pen Argyl area, Pennsylvania. Our mapping everywhere shows that the graywacke-bearing unit underlies the 

thick-bedded "soft" slate and overlies the thin-bedded "hard" slate 

and that the regional upright syncline postulated by Stose (1930) and 

Willard (1943) does not exist.

BUSHKILL MEMBER

The lower thin-bedded claystone slate unit of the Martinsburg is 

here named the Bushkill Member for representative outcrops along 

the Bushkill and Little Bushkill Creeks in the Wind Gap quadrangle, 

Northampton County, Pa. Excellent exposures, although of less complete sections, are also present on both sides of the Delaware Kiver 

north of Belvidere, Belvidere quadrangle, Warren County, N. J., and 

south of Columbia, Portland quadrangle, Warren County, N. J. Good 

exposures are also present along Martins Creek and Oughoughton 

Creek, Bangor quadrangle, Northampton County, Pa. Outlying exposures are present in the Musconetcong Valley, Bloomsbury quadrangle, Hunterdon and Warren Counties, N. J., where the Bushkill
MARTINSBURG FORMATION, DELAWARE VALLEY, PA.-N.J. H7

was mapped as the lower claystone member of the Martinsburg Shale 

by J^rake (1967a), AS defined herein, the member embraces about the 

same sequence of rock called the lower "hard" slate member by Behre 

(192.7). . ",

The Bushkill Member consists of thin-bedded, dark-gray to darkmedium-gray claystone slate that weathers medium gray to very light 

gray to yellowish-brown. The slate contains thin to very thin interbeds 

of quartzose and graywacke siltstone and carbonaceous slate. Beds 

range in thickness from less than % inch to as much as 3 or 4 inches; 

about 75-80 percent of the beds are probably less than half an inch 

thick. On Bushkill Creek, seven dolomite beds, ranging from 6 to 12 

inches in thickness, lie 20-250 feet above the Martinsburg-Jacksonburg 

contact. Slaty cleavage is the dominant planar feature in the Bushkill

FIGURE 3. Typical outcrop of ribbon slate in the Bushkill Member along unnumbered county road in Belvidere quadrangle, Northhampton County, Pa., about 1.4 

miles S. 63° W. from Manunka Chunk, N. J. Bedding dips gently northwest, 

cleavage dips gently southeast, intersection plunges 10° toward the viewer.
H8 CONTRIBUTIONS TO STEATIGBAPHY

Member, although bedding, usually seen as color and composition 

bands on cleavage surfaces, is readily apparent at most places. Many 

of the color bands are formed by siltstone, claystone, and carbonaceous 

claystone that occur in cycles. Such rock has been called ribbon slate 

and is characteristic of this member (fig. 3).

Mineralogically, the slate consists largely of quartz and lesser 

amounts of sericite and chlorite. Minor amounts of calcite are present 

in the lower part of the member, and some beds of siltstone and claystone will effervesce very weakly in dilute hydrochloric acid. Most of 

the mineral grains are of clay size, although as much as 25 percent of 

the quartz is silt size.

The strong slaty cleavage is well marked by a strong directional 

orientation of the micaceous minerals. In the area north of Belvidere, 

N. J., and south of Portland, Pa., the slaty cleavage has been folded, 

and a conspicuous slip cleavage has formed. Slip cleavage is also locally 

conspicuous in the area to the west. Locally, sericite and chlorite are 

recrystallized parallel to this cleavage. In the outlying mass in the 

Musconetcong Valley, an area of extreme deformation, two slaty cleavages have formed.

The Bushkill Member is about 4,000 feet thick, an estimate based 

on the construction of geologic sections. This estimate is less than the 

5,000 feet figured by Behre (1933, p. 139) for the lower member but 

is greater than the 2,250 estimated by Willard (1943, p. 1081-1082) 

for the same sequence of rocks. Our estimate may be high because of 

repetition by unrecognized folds and thrust faults.

In this area, the member is in conformable and gradational contact 

with the underlying cement-rock facies of the Jacksonburg Limestone. 

The transition is one of upward decrease in calcium carbonate content 

through an interval that is 3 feet thick on Bushkill Creek northwest 

of Stockertown, Pa. This relation can also be seen in core samples 

taken across the contact elsewhere (Sherwood, 1964, p. 19-20). The 

contact is easily placed in mapping because Bushkill bedding surfaces 

become color banded upon weathering and those of the Jacksonburg 

do not. In addition, slate beds only rarely effervesce weakly in dilute 

hydrochloric acid, whereas the cement rock effervesces slowly and 

evenly.

The contact with the overlying Ramseyburg Member is transitional 

and is placed beneath the lowest prominent graywacke bed, which in 

most places is more than 1 foot thick.

No fossils have been found in the Bushkill Member in this area. The 

member is younger than the Jacksonburg Limestone, which most recently was considered to be of early or middle Trenton age (Cooper, 

1956). Thus, the Bushkill Member must date from the middle or late 

Trenton, as it is gradational with the Jacksonburg. Its deposition
MAKTINSBUKG FORMATION, DELAWAKE VALLEY, PA.-N.J. H9

continued until Eden time, as equivalent rocks to the west bear fossils 

of that age (Behre, 1933; Willard, 1943).

"Hard" slate has been quarried from the Bushkill Member in the 

areas near Portland, Flicksville, Belfast, and Bath, Pa. This material 

is inferior to the slate taken from the overlying members and has been 

used almost entirely for roofing. In 1966 no quarries were active in 

this member.

RAMSEYBURG MEMBER

The middle graywacke-bearing member of the Martinsburg Formation is here named the Eamseyburg Member for representative outcrops 

along U.S. Highway 46 and the Erie Lackawanna Eailroad near 

Eamseyburg, Portland quadrangle, Warren County, N. J. Good exposures of a fairly complete section are also present between Slateford, 

Pa., and the Delaware Water Gap in the Portland quadrangle and 

along the Bushkill and Little Bushkill Creeks in the Wind Gap quadrangle, Northampton County, Pa. As defined herein, the member 

includes the same sequence of rocks called the middle sandy member 

by Behre (1927) and in addition includes the unit he (1927, p. 105- 

106) called the Bangor Beds. These rocks are placed in the Eamseyburg 

Member because they include a prominent interval of graywacke at 

the top which provides an easily mappable upper contact in areas 

where exposures are otherwise poor.

The Eamseyburg Member is a sequence of alternating beds of claystone slate and light-gray to medium-gray, light-brown- to yellowishbrown-weathering, thin- to thick-bedded graywacke and graywacke 

siltstone. The slate in the lower cycles is identical with that of the 

underlying Bushkill Member in the lower 200 feet. Higher in the unit, 

the slate becomes progressively thicker bedded and terminates in slate 

"runs" like those of the overlying Pen Argyl Member, although thinbedded ribbon slate is common throughout. Graywacke constitutes an 

estimated 20-30 percent of the unit.

The graywacke occurs in beds that range from less than 1 inch to 

more than 4 feet in thickness and average about 1 foot. The beds tend 

to be lenticular and of uneven thickness. The rock is rhythmically 

bedded (graywacke at the base passes up into medium-gray slate, 

which in turn passes into grayish-black carbonaceous slate), each bed 

normally representing one cycle; some cycles, however, are incomplete. 

At most places the beds also are cross-stratified, and convoluted bedding is common. Texturally, most of the unit is fine grained, but 

medium-grained sandstone and siltstone are also common. Fine conglomeratic beds have been noted locally. The areas of graywacke are 

characteristically littered with angular fragments several inches long. 

A typical exposure of the Ramseyburg Member at the type locality is 

shown in figure 4.
H10 OONTEIBUTIONS TO STRATIGKAPHY

FIGURE 4. Typical exposure of Kamseyburg Member at the type locality. Bedding 

dips 45° NW., slaty cleavage dips more gently northwest, and fracture cleavage 

dips 60° SE. Note the range in thickness of the graywacke beds and of the 

sedimentary cycles.
MAETINSBUKG FOEMATION, DELAWAKE VALLEY, PA.-N.J. Hll

The average composition of the graywaeke is 35 percent very fine 

grained detritus, 34 percent quartz, 16 percent rock fragments (mostly 

shale), 8 percent authigenic calcite, 5 percent feldspar (mostly microcline), and 2 percent accessory minerals. Part of the detritus has been 

recrystallized to recognizable muscovite and chlorite, chlorite being 

the more abundant. These minerals penetrate and partly to completely 

replace feldspar grains and are clearly metamorphic. Most grains are 

subangular to angular and ragged, although some are subrounded.

Claystone intervals within the member have the same well-developed, 

penetrative slaty cleavage so characteristic of the formation as a whole. 

The graywaeke, in addition, displays a poor-to-good fracture cleavage 

that in areas of more intense deformation passes into a flow cleavage 

marked by alined sericite and chlorite. Characteristically, the cleavage 

is refracted where it passes from slate into graywaeke (fig. 5).

FIGURE 5. Outcrop of Kamseyburg Member showing refracted cleavage, crossstratification, and graded bedding. Bedding dips steeply southeast (overturned), 

and cleavage dips gently southeast. Light-colored beds are graywacke, and darkcolored beds are slate.
H12 CONTRIBUTIONS TO STRATIGRAPHY

The Ramseyburg Member is about 2,800 feet thick, an estimate 

based on the construction of geologic sections. This estimate is considerably less than the 4,200 feet figured by Behre (1933, p. 140) even 

without the inclusion of his Bangor Beds in our unit. Graywacke beds, 

however, seem to be lower in the section to the southwest, and the 

member thickens slightly in that direction. The member is much 

thicker than Willard's (1943) estimate of 750 feet. Differences in 

reported thicknesses almost certainly reflect different regional structural interpretations.

The unit grades into the overlying Pen Argyl Member and decreases 

in coarse clastic content. Graywacke beds become progressively thinner 

and are replaced by thin beds of quartzose slate or subgraywacke siltstone in the Pen Argyl cycles. The contact is placed at the top of the 

highest prominent graywacke interval. East of the Delaware River, 

the member is unconformably overlain by the Shawangunk Conglomerate of Silurian age.

No fossils other than brachiopod debris have been found in the member in the Delaware Valley. Several collections, however, have been 

made from graywacke outcrops to the west (Behre, 1933; Willard, 

1943). These collections suggest that the graywacke there is Maysville 

(Pulaski) in age. Some of the dated rock is coextensive with the 

Ramseyburg outcrop belt in our area, so it is possible that the Ramseyburg is Maysville in age.

Slate intervals at the top of the member have been widely exploited 

for commercial purposes, especially in the Bangor and Slateford areas. 

The material yielded is similar to that of the overlying member, although the beds tend to be thinner and somewhat more quartzose.

PEN ARGYL MEMBER

The upper thick-bedded claystone slate unit of the Martinsburg 

Formation is here named the Pen Argyl Member for typical exposures 

in the slate quarries in and near Pen Argyl, Northampton County, Pa. 

The unit underlies a 2- to 4-mile wide belt south of Kittatinny and 

Blue Mountains but is rarely exposed in natural outcrop because of 

heavy cover by glacial drift. The unit can be seen, however, in quarries 

both northeast and southwest of the type locality, as well as in quarries 

south of Lehigh Gap, just west of the area covered in this report. As 

defined herein, the Pen Argyl Member includes about the same sequence 

of rock that was called the Pen Argyl Beds by Behre (1927).

The Pen Argyl Member consists of thick-bedded, dark-gray to 

grayish-black, medium-gray- to yellowish-brown-weathering, evenly 

bedded claystone slate rhythmically intercalated with thin beds of 

quartzose slate or subgraywacke and grayish-black to nearly black 

carbonaceous claystone slate. A typical cycle starts with quartzose
MAKTINSBUEG FOKMATION, DELAWAEE VALLEY, PA.-N.J. HIS

slate or subgraywacke that is followed by thick-bedded slate and finally 

by carbonaceous slate. Individual members of a typical cycle are not 

always present. Slate beds are commonly 5 feet thick, but some beds 

are as much as 15 feet thick. Coarse clastic beds rarely exceed a foot 

in thickness, but some are as much as 3 feet thick. Figure 6 is a typical 

exposure of the unit.

The slate consists primarily of about equal amounts of sericite and 

chlorite, with lesser amount of calcite, quartz, and groundmass detritus. 

The lighter colored beds appear to have more chlorite than sericite. 

The carbonaceous beds owe their dark color to graphite and contain 

noticeable amounts of pyrite. They contain only minor quantities of 

chlorite. Most of the coarse clastic beds are subgraywacke similar to 

those beds of the underlying Eamseyburg Member, but a few beds 

are quartz-rich phases of the slate.

Slaty cleavage is extremely well developed in the unit and is marked 

by a strong parallel orientation of the micaceous minerals. A secondgeneration slip cleavage is locally common in the western part of the 

study area.

The Pen Argyl Member has a minimum thickness of 3,000 feet, an 

estimate based on the construction of geologic sections. It may be as 

much as 6,000 feet thick, however, as the area underlain by the unit is 

wide enough to accommodate this additional thickness, and an unknown 

thickness may be covered unconformably by the Shawangunk Conglomerate. Adequate structural reconstructions are difficult, if not impossible, in the critical areas because of the heavy cover of glacial deposits. 

Our minimum estimate of 3,000 feet is somewhat higher than that 

figured by Behre (1933) for Northampton and eastern Lehigh Counties, although he estimated a thickness of 4,415 feet for the sections 

along the Little Bushkill Creek.

The Pen Argyl Member is unconformably overlain by the Shawangunk Conglomerate of Silurian age. This unconformity is well shown 

by regional stratigraphic relations, by the difference in strike between 

the Shawangunk and Martinsburg in the Delaware Water Gap area 

(fig. 2), and regionally by two distinct topographic trends that can be 

seen on both topographic maps and aerial photographs, particularly 

high-altitude aerial photographs. This unconformable contact is probably also a decollement zone above which the overlying beds folded 

semi-independently of rocks in the Martinsburg. Structures in the 

Martinsburg believed to be of Taconic age have different tectonicstyles than those formed in the overlying strata during post-Taconic 

deformation (Drake, Davis, and Alvord, 1960). Therefore, some of 

the discordance in strike at the contact may be a result of interformational slippage. The Pen Argyl Member disappears along the unconformity just west of the Delaware River (fig. 2), and no exposures
H14 CONTRIBUTIONS TO STRATIGRAPHY

FIGURE 6. Typical exposure of the Pen Argyl Member in the southwest wall of 

the Albion Vein Slate Co. quarry, Pen Argyl, Pa. Beds exposed are as much as 

8 feet thick and lie in the inverted limb of a recumbent syncline, the axis of 

which lies about 400 feet below the surface. Bedding dips 45° SE. (left) at the 

surface and is almost vertical at the bottom of the photograph. Cleavage dips 

about 10° SE., almost parallel to the axial surface of the fold.
MAETINSBUKG FOEMATION, DELAWAEE VALLEY, PA.-N.J. H15

are known in those parts of New Jersey that we have visited. Behre 

(1927, 1933) long ago reported that the upper part of the Martinsburg 

is likewise unconformably overlapped by Silurian rocks west of Kempton, Pa. We have not carried our detailed mapping that far west, but 

topographic trends and reconnaissance support Behre's interpretations. 

In addition, G. H. Wood, Jr. (oral commun., 1966), reports that pelitic 

rocks, similar to those in the Bushkill Member, are in contact with 

rocks of Silurian age at the Schuylkill Gap. This information suggests 

that even the graywacke-bearing Ramseyburg Member is partly covered 

in that area.

No fossils have been found in the Pen Argyl Member, so there is no 

direct evidence of its age. If the Ramseyburg Member is Maysville in 

age, the Pen Argyl is post-Maysville and pre-Silurian in age.

The Pen Argyl Member has yielded vast quantities of slate that has 

been used for a wide variety of commercial purposes. The exceptionally 

thick beds in the unit are especially suited for use as blackboards, 

billiard-table tops, and other such products.

SUMMARY

Detailed mapping in the Delaware Valley of Pennsylvania and New 

Jersey has shown that the Martinsburg Formation is about 9,800- 

12,800 feet thick and can be divided into three members: a lower 

thin-bedded claystone slate, a middle graywacke-bearing slate, and an 

upper thick-bedded claystone slate, herein named the Bushkill, Ramseyburg, and Pen Argyl Members, respectively. This subdivision is contrary to the two-fold interpretation as originally defined by Stose 

(1930) but is approximately the same as that of Behre (1927).

REFERENCES CITED

Bayley, W. S., Salisbury, K. D., and Kiimmel, H. B., 1914, Earitan, N.J.: U.S.

Geol. Survey Geol. Atlas, Folio 191. 

Behre, C. H., Jr., 1927, Slate in Northampton County, Pa.: Pennsylvania Geol.

Survey, 4th ser., Bull. M9, 308 p. 

 1933, Slate in Pennsylvania: Pennsylvania Geol. Survey, 4th ser., Bull.

M16, 400 p. 

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