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A STUDY OF JOINT PATTERNS IN HIGHLY FOLDED AND CRYSTALLINE ROCKS, WITH PARTICULAR REFERENCE TO NORTHERN NEW JERSEY

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ivlW
LD3907
.G7
Appleby, Alfred Noel.
9
1940
A study of joint patterns in highly
.A5
folded and crystalline rocks, with
particular reference to northern Hew
Jersey...
New York, 1940,
9p,l.,106 type-written leaves.
illus.,
f o l d . m a p . ,tables (part fold.) diagrs.
(1 f o l d . ) 29cm.
Thesis (Ph.D.) - New York university,
Graduate school, 1940.
"Selected bibliography": p . 99-106.
A54487
Shelf List
Xerox University Microfilms,
A n n A r b o r,
Michigan 48106
T H IS D IS S E R T A T IO N HAS BEEN M IC R O F IL M E D E X A C T L Y AS REC EIVED .
J.JHSiAKY
N. Y. U niv ,'
A Study of Joint Patterns in Highly
Folded, and Crystalline Hock*, with Par­
ticular Reference to Northern New Jersey*
By
Alfred Noel Appleby
B.S.
College of the City of New York
M.S.
New York University
April 15, 1940.
Thesis submitted in partial fulfill­
ment of the requirements for the degree of
Doctor of Philosophy in Geology in the
Graduate School of New York University.
P LEA S E NOTE:
S o m e p a g e s m a y ha v e
i n d i s t i n c t print .
F i 1m e d as re ce i v e d .
University Microfilms, A Xerox Education Company
POEEWDRD
FOREWORD
The writer has been fortunate to have been the
recipient of many kindnesses from those with whom the
preparation of this thesis has brought him into contact*
He takes this opportunity of acknowledging his gratitude:
To Dr. Ernest R* Lilley, tinder whose guidance
the thesis was written.
He has been unstinting in help­
ful criticisms and pertinent suggestions during its prep­
aration.
To Dr. Howard 0. Bohlin, who, through friend­
ship, accompanied the writer on several field trips.
His
assistance and helpful suggestions were invaluable*
To Professor Peter L. Tea, of the College of the
City of New York, who checked the mathematical investiga­
tion concerning stress relationships, contained in this
thesis*
To Mr. Claude Roberts, who accompanied the
writer on some field trips, and whose knowledge of the
terrane was of great assistance in the preliminary work.
To Mr. Thomas Byrnes, Superintendent of the
Reservoirs of the Pequannock Watershed, who extended per­
mission to the writer to visit outcrops on the posted
land of the Pequannock Watershed, and whose influence
secured for the writer the same permission for the post­
ed land of the Wanaque Reservation*
I
TABLE OF CONTENTS
TABLE OF CONTENTS
FOREWORD
-
- -
TABLE OF CONTENTS
- -
- -
- -
- -
- -
-
Page
I
- -
- -
II
- -
- -
- -
IV
-
l
Baeic Conceptions Concerning the Development of
Joints and Joint Systems
- - - - -
i
INTRODUCTION -
-
-
-
Definitions
Joint
Faul t
Stress
Strain
- -
- -
- -
LIST OF ILLUSTRATIONS
- -
- -
- -
-
-
-
- -
-
-
-
-
-
-
- - - - - - l
.................................. i
- - - - - - - - - - 2
- - - - - - 2
- - - - - - 3
The Effects of Simple Tensile Stress upon
Rocks - - - - -
4
The Effects of Simple Compression upon
Rocks - - - -
5
- -
The Strain Ellipsoid
- - - - - - - -
6
Hartmann* s Law
--
8
-
- -
- -
- -
- -
The Effects of Rotational Stress upon Rocks
-
10
Combined Rotational and Non-Rotational
Stresses - - - - - -
n
Application of the Strain Ellipsoid to Rock
Deformation - - - - -
13
Critical Analyses of Some Typical Studies of Joint
Systems and Mechanics of
Joints - - - - - Summary of Cited Studies
- - -
- -
- -
Reasons for the Selection of the Area Studied
Description of the Area Studied
II
- -
- - - - - - -
14
23
26
28
Page
Background, of Previous Geological
Studies of the Area
Description of the Hocks Gneisses - - Pochuck Gneiss Losee Gneiss
Eyram Gneiss
Franklin Limestone
Hardyston Quartzite Kittatinney Limestone
Green Pond Conglomerate
Shales and Slates
Skunnenrunk Conglomerate
Formational Relationships
Structure
PROCEDURE
-
- -
Field Studies
-
- -
- -
- -
- -
-
-
-
28
- - - - - - - - - - - -
30
30
- - - -
- -
- -
-
-
- -
- -
- -
34
35
36
37
37
39
41
44
- -
- -
-
- -
-
- -
52
- - - - - - - - - - - - -
57
Comparison of Joint Patterns - - - - - - Joint Patterns and Foliation - - - - Joint Patterns and Local Folding and
Faulting
- - - - Joint Patterns and Regional Structure - -
65
65
-
- -
- -
- -
- -
- -
- -
49
52
CONCLUSIONS
- -
-
-
Correlation Studies - - Tabulation of Data
Analysis of Tabulation
- -
32
33
- -
SELECTED BIBLIOGRAPHY - - - - - - - - - -
III
57
51
67
70
-
92
-
99
LIST OF ILLUSTRATIONS
LIST OF ILLUSTRATIONS
Photographs
Fig. 1
Following Page
Typical Pochuck Gneiss, Pochuck Mountain -
H
2 Typical Losee Gneiss, Greenwood Lake
n
3 Losee Gneiss, Wanaque
"
4
-
-
-
33
-
33
- - - - - - - -
Typical Byram Gneiss, Lake Pochuck -
34
- -
-
34
"
5 Byram Gneiss, Denville
"
6
n
7 Franklin Limestone, McAfee
- - - - - -
3?
"
8
Franklin Limestone, McAfee
- - - - - -
37
"
9 Franklin Limestone, McAfee
- - - - - -
37
M
"
"
- - - - - - -
34
- - - - - - -
34
Byram Gneiss, Riverdale
10 Green Pond Conglomerate and Byram Gneiss,
Copperas Mountain
- - - - -
3g
11 Devonian Shale, Greenwood Lake
38
12
- - - - -
Devonian Shale, Oak Ridge Reservoir
- -
-
38
”
13 Devonian Shale, Greenwood Lake
- - - - -
38
"
14 Devonian Shale, Greenwood Lake
- - - - -
38
"
15
Devonian Shale, Greenwood Lake
-
,
- -
-
41
"
15
Devonian Shale, Clinton Reservoir
-
- -
-
41
"
17 Devonian Shale, Clinton Falls
"
18 Skunnemunk Conglomerate, Bearfort Mountain
-
42
"
19 Skunnemunk Conglomerate, Bearfort Mountain
-
42
"
20 Skunnemunk Conglomerate, Bearfort Mountain
-
42
"
21 Byram Gneiss, Riverdale
IV
-
- - - - -
- -
- -
-
42
-
69
Photographs
Fig. 22
Following Paga
Byram Gneiss, Riverdale
- - - - - - - -
"
25
Pochuck Gneiss, Pochuck
Mountain
11
26
Newark Sandstone, Kingsland -
69
- - - - -
- -
- -
86
- -
89
Tables
la to li
2a
to 2j
Joint Strikes and Dips
Shear Plane
Angles -
- -
- - - - - - -
35
- - - -
60
Plates
1
Determination of Shear Plane Relationships
-
2
Map of Area Studied
- - - - - -
Map Envelope
3
Map of Area Studied
- - - - - -
Map Bavelope
Diagrams
Fig. 23
"
24
-
60
Page
Force Diagram
-
Analytical Diagram
V
- -
- -
- -
- -
- -
7?
- -
- -
- -
- -
83
INTRODUCTION
BASIC CONCEPTIONS CONCERNING THE DEVELOPMENT
OF JOINTS AND JOINT SYSTEMS
Definitions
It may be seen that different writers on struc­
tural geology, in making similar reports, have placed dif­
ferent shades of meaning upon certain words.
This is
particularly true of such words as stress, strain, joint,
fault, shear, and torsion.
It is necessary, therefore,
to define some of the terms used before attempting to
explain any theories of rock deformation, or the causes
of such deformation.
JOINT
ways.
The word joint has been defined in many
Nevin (r. 79, p. 138) and Leith (r. 61, p. 29), in
much the same words, say that a joint is a rock fracture or
fissure along which there has been little or no movement.
Willis (r. 104, p. 49) makes a distinction between the terms
fracture and joint.
He states that a fracture is an ir­
regular break, but that a joint is a parting plane which
separates or tends to separate two parts of a once con­
tinuous block.
There is in this definition no statement
as to the amount of movement, but there is an implica­
tion of the type of force causing the joint*
In this paper, the word joint will be used to
indicate a linear break in a once continuous block.
The
word is not intended to include the measure of the move­
ment between the parts on either side of the break, or
its direction, although obviously some movement must have
occurred or there would be no break.
In general, through­
out this paper, the word joint will imply one parting of
a group of parallel partings, called a joint system.
FAULT
A fault is a break between two parts
of a mass along a plane where obvious movement between
the two parts has occurred.
great or small.
This movement may have been
The chief distinction between jointing
and faulting is the obviousness of the movement*
STRESS
When a force is acting on a body, the
body is undergoing stress.
It is common to refer to
the force acting upon a unit area of a body as stress*
Tension or tensile stress is a force applied in such
manner as to tend to pull apart the particles of the
body undergoing stress*
Compressive stress, or com­
pression, is a force applied in such manner as to press
together the particles of the body subjected to stress*
2-
When two forces act upon a body in directions opposite
to each other and in such a manner that the opposing
forces act not in the same plane but in parallel planes,
the stresses are in couples, and the body is said to be
in shear.
The effect of the members of the shear
couple is to tend to rotate the body under shear.
Shear
stress is, therefore, sometimes referred to as rotation­
al stress.
Torsion is the term used for shear stresses
in which two shear couples are acting in different di­
rections at the same time, and in which the forces form­
ing the shear couples act in circular directions about
an axis in the body undergoing the stress.
STRAIN
Strain is the yielding under stress
in a solid body that causes it to deform.
Since no
body is absolutely rigid, all yield or are strained to
some degree, however slight, when subjected to stress.
The ability of a body to return to its original form
upon the removal of stress is called elasticity.
Most
solid materials possess elasticity to some degree.
The ability of a body to withstand the deform­
ing effects of an applied stress is the resistance to
stress.
When a rock possesses this quality to a marked
degree it is said to be competent, and has the ability
to transmit the stress to an adjacent body without
-3-
being itself permanently deformed.
An incompetent rock
is one which cannot transmit stress, but the forces ex­
erted upon it are expended in the process of permanent
deformation.
The words brittle and plastic are usually
associated with the terms competency and incompetency*
It must be understood that the terms may not be used in
an absolute sense; that they are relative; that under
differing conditions of temperature and pressure the
same rock may be competent or incompetent, brittle or
plastic*
The Effects of Simple Tensile Stress upon Bocks
In considering the effects of tensile stresses
upon rocks, a distinction must be made as to the degree
of brittleness of the rocks.
A rock that is highly
brittle when subjected to tension sufficient to cause
failure will break in an approximate plane perpendicular
to the direction of stress.
As an example of this case,
the strain of elongation of a rock bending into an
anticline may be relieved by a series of cracks that
are radial to the fold, or at right angles to the elon­
gation*
Such jointing will of necessity be of a local
type, limited to the confines of single folds*
-4-
It ia said that under conditions of tempera­
ture and pressures such as may be encountered by a rock;
at some depth from the surface, a rock that is brittle
at the surface may act under tension as a mild steel
does*
As shown by Hartmann's experiments (40), a mild
steel subjected to a simple tension develops lines of
strain at acute angles to the direction of stress*
Under
increase of stress the steel will break in the general
direction indicated by these lines of strain.
However,
it may be safely assumed that a rock that forms a con­
siderable mass, beneath a large overburden, is not sub­
jected to tension alone*
The Effects of Simple Compression Upon Rocks
A plastic rock, when subjected to compression,
may fail by flowing; that is, its particles may be
squeezed together and flow in the direction of easiest
relief*
A brittle rock, however, will resist a con­
siderable pressure, and then fracture along planes that
are theoretically at 45 degrees to the direction of
stress.
Actually, under a simple compressive stress
these planes of fracture are at an angle of something
less than 45 degrees.
The planes of fracture are planes
of shear in the material.
-5-
The mechanics of such fracture are as follows:
When a simple compression acts upon a block of brittle
rock, its effsct upon any plane within the rock is broken
up into two components, one normal to the plane and one
parallel to the plane.
The parallel component, moving
along the face of the plane, acts as a shear.
Since
rocks are weaker in shear than in compression, failure
occurs by shear.
The intensity of the shear component is
at a maximum when the plane is at an angle to the com­
pression equal to 45 degrees.
Assuming that the block is
confined by pressures on its sides, of unequal intensities,
two sets of shear planes at 45 degrees are the usual re­
sult, with the intersection of the planes parallel to the
stress of intermediate intensity, and with relief in the
direction of the least stress.
Due to internal friction
in the particles of the rock, the actual angle in a
brittle rock is less than 45 degrees to the direction of
the maximum stress.
The Strain Ellipsoid
It is always possible to resolve the forces
acting upon a rock into three mutually perpendicular
forces.
It is possible that in all three directions
the stresses are of equal intensity; or that any two
are of equal intensity greater or less than the third;
-6-
but it may be assumed that in general these stresses are
all unequal.
The direction of action of the greatest
force will be called the greatest axis of stress; the di­
rection of action of the smallest force is called the
least axis of stress; and the direction of the intermediate
force is called the mean axis of stress*
Let us consider an imaginary sphere of rode
subjected to stresses upon all sides.
If these stresses
are all equal, there can be no change in form, although
dilatation may occur*
In the general case, where the
stresses are not equal to each other, the sphere may be
deformed into an ellipsoid called the strain ellipsoid,
with it8 three principle axes opposed to the three axes
of stress.
The shortest axis is called the least axis
of strain; the longest axis is called the greatest axis
of strain; and the third axis, between the greatest and
least axes, is called the mean axis of strain*
It is possible to pass two planes through the
mean axis of strain of the strain ellipsoid in such a
manner that they cut circles from the ellipsoid.
These
planes have been called by Leith (r* 61, p. 23) the
planes of no distortion*
maximum shear.
They are also the planes of
For a brittle rock, the maximum shear
planes form an acute angle facing the greatest axis of
stress.
7-
Hartmann* s Law
In 1896, Hartmann (r. 40) conducted experiments
on the failure of metals*
From the results of these ex­
periments came the conclusions now known as Hartmann* a Law*
A summary by Bucher (r* 19, p. 712) of Hartmann's Law follows!
a.
Under simple compression the acute
angle formed by shearing planes in brittle
materials is bisected by the axis of
greatest stress, wbile the obtuse angle
is bisected by the axis of least stress*
b*
For each material the angle between
the shear planes is a constant, indepen­
dent of the nature or intensity of the
stresses involved*
c.
The shearing planes do not originate
simultaneously and are not uniformly dis­
tributed.
d.
It follows, then, that a set of
joints may change direction in passing
from one type of rock to another, or,
where there are petrographic differ­
ences, in traversing one rock.
Aside from the effect of internal friction upon
a rock, the angle between the planes of maximum shear may
be acute, facing the direction of applied compressive
stress only if the rock undergoes positive dilatation*
With no increase in volume the shear planes intersect at
an obtuse angle facing the compressive stress*
It has been shown experimentally (Kerman r . 47)
that under confining pressures a rock may change from
-8-
brittle to plastic in its mode of failure*
Karman in­
serted cylinders of rock into an apparatus that could
subject the specimen to circumferential pressure while
applying axial compression.
Under differing controlled
axial and circumferential pressures, the marbles and
sandstones he used failed with an increasing shear angle,
with increase of lateral pressures in addition to increase
of axial pressure.
The amount of axial compression nec­
essary to cause failure also increased as the circumferen­
tial pressure was increased.
Mason (r. 70) also showed that with circumferen­
tial tensile stress applied simultaneously with axial com­
pression, the angles of the shearing planes facing the
compressive force decreased.
He used mild steel tubes
subjected to hydrostatic pressure from within, while
applying compression longitudinally.
He also subjected
the tubes to an external hydrostatic pressure while
subjecting them to longitudinal tension.
He found that
the angle between the shear planes of fracture and the
direction of the applied compressive stress was about
50 degrees, before the dilating hydrostatic pressure,
which caused a tensile stress in the specimen, was
applied.
When this tensile stress was applied simulta­
neously with a longitudinal compressive stress, the angle
between the shear planes and the compressive stress be-
-9-
came less than 45 degrees*
It may he inferred that the
material tended to become more brittle under the stresses
applied*
To generalize from these experiments, it can
be seen that, as has long been assumed, a rock that is
brittle at the surface may become plastic under the con­
fining pressures found at depth;, and conversely, a rock
that is somewhat plastic under a covering burden may be­
come brittle as the burden is ramoved*
The Effects of Rotational Stress Upon Rocks
Let us assume that a cubic block of rock cir­
cumscribing a sphere be subjected to stress that results
in a shear couple, acting on two opposite faces of the
cube*
The result of deformation will be
sphere into an ellipsoid*
to strain the
If we consider only the plane
of the shear couple, a section of the deformed block will
become a rhomboid, and the section of the enclosed ellip­
soid will become an ellipse whose major and minor axes
are the axes of greatest and least strain*
The major axis
is in the direction of the long diagonal of the rhomboid.
Let us disregard other forces that might be
present besides the shear forces, and assume that the
third dimension of the original cube has not been affected*
■10-
The effect of the shear in such a case is to set up a
strain ellipsoid of the same sort as might be formed
by pure stresses acting normal to the faces of the cube*
Failure under these assumed conditions of stress will
take place along shear planes of this ellipsoid that are
in the same relation to the strain axes as if the causal
stresses were non-rotational.
The most important difference between the
strain ellipsoid formed by shear and one formed by nonrotational stress is in the relation of the axes of
stress to the axes of strain*
In the case of the pure
or non-rotational stress, the stress and strain axes are
respectively parallel; that is, the greatest stress
axis is parallel to the least strain axis, the least
stress axis is parallel to the greatest strain axis,
and the intermediate stress and strain axes are parallel.
With an ellipsoid caused by shear, the direction of the
application of the rotational stress can only be deter­
mined if we know the amount of deformation that has
taken place and the magnitude of stress necessary to
cause such deformation*
Combined Rotational and Non-rotational Stresses
A rock may be subjected to stresses that are
-11-
not resultant either in simple stresses or shear stresses,
but in a combination of both.
Just as in the cases where
a single type of stress was the cause of deformation, a
strain ellipsoid might be set up.
The axes of greatest
and least strain of this ellipsoid would be in some di­
rection intermediate to the axes that might be set up if
the causal stress were either a pure stress or a rotation­
al stress.
Again the maximum shear planes along which
failure might occur would be referable to the axes of
greatest and least strain in the ellipsoid.
It must be
noted that the position of the axis of greatest strain
will change direction with continued application of the
shear stress, so that its position at the beginning of
deformation may be quite different than its position
when failure occurs*
Since this is not true in regard
to the simple stress, the position of the strain axes
at failure is a function of the relative importance of
the rotational and non-rotational stresses*
It is very difficult, if not impossible, to
determine the directions of the causal stresses when
the rock has been subjected to a combination of pure
and shear stresses*
A rock may be deformed experimen­
tally in the laboratory under known conditions as to
type and amount of stress, and certain conclusions
formed*
It is a far different thing to examine a rode
-12-
t
mass in the field, and from that examination to de­
termine just how a combination of forces may have acted
to produce its deformation*
Application of the Strain Ellipsoid to Rock
Deformation
The use of the ellipsoid in the field is
limited to those cases in which supporting data permit
an assumption as to type of stress causing deformation*
If a simple compression may be assumed to have caused
fracturing along joint planes, it may be possible to
determine the direction of application of the deforming
stress*
It is merely necessary to find the angle be­
tween the two most persistent sets of Joints.
Their
intersections will then be parallel to the axis of mean
strain.
The deforming Btress will then be parallel to
the bisector plane of the acute angle, and to the normal
to the intersection of a pair of joints in the bisector
plane, assuming that the rock failed as a brittle sub­
stance*
However, inasmuch as a torsional stress or a
combined stress of compression and shear may also result
in similar planes of maximum shear being set up in the
-13-
strain ellipsoid, one must be certain that his initial
assumption as to type of stress is correct.
Thus it may
be seen that the use of the strain ellipsoid in deter­
mining local causal conditions of rock deformation is ex­
tremely limited*
Critical Analyses of Some Typical Studies of
Joint Systems and Mechanics of Joints
It is perhaps fitting at this point to submit
short critical reviews of some of the studies that have
been made upon the subject of joints and joint systems.
Among the earlier writers on the subject of joints are
Becker (r. 10, r. 11) and Hoskins (r. 43).
these two men have been widely quoted.
The papers of
Because their
views have influenced the studies of later writers, it is
felt that short summaries of their work must be included in
any discussion of later studies.
In 1892, a paper by G. F. Becker, "Finite
Homogeneous Strain, Flow and Rupture of Rocks", was pre­
sented to the Geological Society of America (r. 10).
The
writer discussed the mechanical principles of rupture of
materials and introduced the strain ellipsoid idea.
He
considered mathematically the conditions under which bodies
fail when subjected to different types of stress.
He show­
ed that according to its rigidity the lines of maximum
14-
tangential strain in a rode approach 45 degrees to the
direction of an applied simple stress*
He stated also
that when a rock undergoes an appreciable amount of
deformation before fracturing, that is, is somewhat plastic,
that the planes of maximum tangential strain or planes of
no distortion of the strain ellipsoid form an obtuse angle
to each other*
In a paper, "The Torsional Theory of Joints"
(r. 11), Becker reviewed the experiments of Daubree
(r.
11
, p. 133) on glass plates subjected to torsion*
He stated that the torsional stresses resulted in tensional strains, and that the body tested really failed because
of a series of tensile stresses (p* 13?)*
In this paper
he again described the strain ellipsoid, but reiterated
the statement that under simple compression the angle of
fracture must be 45 degrees to the axis of stress for a
hard homogeneous body (p* 138)*
It seems to this writer that Becker was some­
what biased in his observations by the mathematical con­
clusions that he had formed*
He did not admit the pos­
sibility that the planes of maximum shear in the strain
ellipsoid might form an acute angle to the direction of
stress*
-15-
Hoalcins (r. 43), in a paper included with the
report of Tan Hise on the pre-Cambrian geology of North
America (r. 100) in the 16th Annual Report of the United
States Geological Survey,
the strain ellipsoid.
restated the principles of
He also brought out the fact that
dilatation must be considered as part of the problem of
rock deformation.
Bucher (r. 19, 20) published in 1920 a study
called "The Mechanical Interpretation of Joints".
He
described the experiments of Hartmann (r. 19, p. 709),
Daubree (r. 19, p. 718), Karman (r. 19, p. 714), and
Mason (r. 20, p. 13) and showed that brittle materials
under compression failed by shear with the shear planes
at less than 45 degrees to the direction of stress.
He
stated that when tension alone was the stress involved,
a rock would fracture in planes, or surfaces almost plane,
normal to the direction of stress; but that when the tension was accompanied by a compression, insufficient to
cacuse failure if acting alone, at right angles to it, the
failure was along shear planes with the obtuse angle be­
tween them facing the tensile stress.
Bucher used Mohr's Theory of Rupture to show
that the shear plane angle is independent of the hard­
ness of a material and also independent of the absolute
16-
amount of deformation of which a substance is capable
below the elastic limit (r. 20, p. 11)*
The controlling
factor in the size of the angle between shear planes of
fracture is the relative brittleness of the substance
under stress*
He restated Hartmann's Law, as given on
page 8 of this report, to include the statement:
"In
ductile materials, the obtuse angle between the shear
planes is bisected by the axis of greatest stress"
(r• 20, p • 17) •
The field examples used by Bucher are for the
most part in gently dipping sedimentary beds*
The ob­
servations he himself made are in a coarse-grained sand­
stone of the Pottsville series, and in thin and even beds
of fine-grained dolomite (Bisher formation)*
Other ex­
amples are cited by Bucher, notably the observations of
Pearl Sheldon near Lake Cayuga and Lake Seneca, and those
of Thwaite on sandstones of the Wisconsin shore of Lake
Superior (Orienta sandstones)*
His citations of joint­
ing in thrust faulted areas are not very convincing.
Mead (r. 71), in "Notes on the Mechanics of
Geologic Structures", described the results of some
laboratory experiments on deformation.
He used a frame
through which force could be applied in various ways.
On this frame he stretched sheets of rubber, then coat-
-17-
ed them with paraffin, and chilled the paraffin until
it became brittle*
Mead's experiments seemed to confirm the strain
ellipsoid theory of failure.
Leith and Nevin.
They have been cited by
His compression experiments produced
fractures at 45 degrees to the direction of shortening*
His experiments with shear stress seem to have been
affected somewhat by the rubber sheet upon which the
paraffin was coated*
Sheldon (r* 95) had earlier made
deformation experiments in which paraffin was used.
Her
results were similar to those of Mead for application of
like stress.
It seems to the writer that paraffin is not
the best medium for experiments in which a comparison
with rocks is to be made.
The homogeneity of the sub­
stance is not likely to be exactly duplicated in any
large rock mass*
Leith (r. 61, pp. 21-27) in his text book,
"Structural Geology", describes the strain ellipsoid
rather fully without entering into a mathematical dis­
cussion of it.
For a mathematical analysis he refers
to Merriman (p. 35).
He gives some importance to his
"wire screen experiments".
In his experiment, a circle
on cardboard is placed between two sheets of wire
-18-
screen.
The three sheets are then pinned together at
the center of the circle, and the circle traced on the
wire mesh.
The screening is then fastened in a hinged
rectangular frame.
By the application of pressure at
opposite comers of the screen, compression or shear
stresses result, according to the direction of applica­
tion.
As a result, the circles painted on the screen
become distorted to ellipses.
At the same time a
comparison with the original form may be made.
There
will be two diameters of the circle unchanged in length
as the wire netting is distorted.
Leith calls this
ellipse a section through the axes of greatest and least
strain of the ellipsoid, with the planes of maximum shear
represented by the unchanged diameters.
He points out
that the planes of maximum shear are "at about 45 degrees
to the pressure" (r. 61, p. 26).
Obviously, however, as a glance at his illus­
trations shows, the angle of these planes is more nearly
50 degrees to the direction of stress than 45 degrees.
The wire netting may be regarded as a ductile material,
and consequently, according to Bucher, it may be expect­
ed that the planes of maximum shear will have the obtuse
angle between them face the direction of greatest short­
ening.
Another obvious factor in the wire netting ex­
periment is the fact that the ellipse is of smaller area
-19
than the original circle.
Assuming no change in the
intermediate axis of strain, this means a considerable
loss in volume in deforming a sphere*
Since this as gump­
tion is untenable, we must grant an increase in the
intermediate axis, which in turn calls for a change in
the directions of the circular sections.
Carrying this
argument to its limits, suppose the wire frame to be
collapsed so that the ellipse approaches a line*
The
third dimension of the ellipsoid must then approach in­
finity, or else the mass of the solid must approach zero*
It is felt by the writer that this experiment has no
value as applied to any but the most plastic materials
or to materials that are highly ductile.
In fact, it is
less than valueless; it is misleading in the conclusions
Leith draws from it*
Another statement of Leith to which exception
might be taken is that the planes of maximum shear in the
strain ellipsoid are planes of no distortion.
This is
only true when the intermediate axis of strain is a
diameter of the sphere from which the ellipsoid was de­
rived.
The writer is in complete agreement with the
following statement by Leith:
"In naming joints, care
should be taken to exclude terms which imply a more
specific knowledge of the stress conditions than we
possess" (p. 39)*
•20-
Willis (r* 104), in the text book "Geologic
Structures", explains the mechanics of fracture in
much the same manner as those writers cited above*
His
explanation of a combined normal and shear stress has
been used by this writer (p.12).
He also shows that in
brittle substances the angle between shear planes is
acute*
Willis suggests that confining pressures at
depth may change the tendency of a rock to break as a
brittle substance*
Nevin (r. 79) also makes use of the strain el­
lipsoid in his explanation of fracture of rocks*
His de­
scription is largely taken from the paper by Hoskins
(r* 43) • His summary of Hartmann’3 Law is largely that of
Bucher (r. 19).
Nevin offers some theories of failure without
trying to draw conclusions from these theories*
His
Stress Theory is that "elastic breakdown begins when the
maximum normal stress reaches a certain value"*
This
statement tells little either quantitatively or qualita­
tively*
The Shear Theory states that "elastic breakdown
begins when the maximum shearing stress reaches a certain
magnitude"*
This must be sufficient to overcome friction.
The resulting angle of fracture is acute toward the max­
imum stress (an implied non-rotational stress)*
21-
An in-
crease in volume is postulated in the rupture of brittle
rocks*
The Strain Theory,as explained by Nevin, is that
"fracturing occurs where the strain is greatest"*
He
ascribes to Becker (p* 25) the statement that the obtuie
angle between sets of fractures faces the direction of nonrotational stress*
He states that this applies to rocks
at depth, as these tend to lose their brittleness due to
confining pressures*
The shear and strain theories seem to the writer
to be the same when the conditions of plasticity or brittle­
ness are taken into consideration*
Further on, in his chapter on cleavage, Nevin
cites as examples of fracturing the joints described by
Sheldon (p* 170).
To these examples he has applied the
strain ellipsoid to determine the direction of the causal
stresses*
He assumes that the stresses causing the joints
were of a non-rotational type*
He treats the joint systems
reported by Parker (r* 83) in a like manner*
-22<
Summary of Cited Studies
The papers by Becker (r. 10, 11) and Hoskins
(r.43 ) are to a great extent philosophical studies based
upon principles of mechanics*
While the phenomena of
jointing had been examined, no specific cases were report­
ed by either writer.
These papers are of great importance
in that they formulate workable theories that may be used
as a basis for the stress and strain relationships in
rocks*
While the mechanical principles expounded by
Becker and Hoskins had long been known by physicists,
these writers were among the first in this country to
apply them to geologic structures.
Bucher (r* 19, SO) drew some conclusions from
the work of others, and showed that laboratory experiments
confirmed the strain ellipsoid theory.
He then applied
these conclusions to some field observations that he had
made, and to those of Sheldon (r* 95) and Thwaite (r. 98)*
Mead (r* 71) used laboratory experiments, as
outlined elsewhere in this thesis, as a basis for the in­
terpretation of deformation of geologic structures.
used no field observations in his paper*
He
His conclusions
were based upon applications of known causal stresses,
and their effects upon his test specimens*
-23-
He made no
attempt to interpret causes of deformation of actual
rock masses*
Leith (r. 61), Willis (r. 104), and Nevin (r. 79,80)
have given summarized accounts of the various theories of
jointing in their texts.
All explain the strain ellipsoid
theory and give instances of types of formations to which it
might apply.
Willis covers the theories of jointing and
their relations to causal stresses in a competent manner,
hut gives no specific instances in the field to which these
theories might be applied.
Leith gives some criteria for the
recognition of types of joints in the field.
Nevin cites
the work of Sheldon (r* 95) and Parker (r. 83), and from
their work derives assumptions as to the causal stresses
that resulted in the jointing they recorded*
There appears somewhere in each of the studies
cited above, and in others, a statement to this effect:
"Great care must be taken in ascribing the deformation
now apparent in a rock mass to a type of stress*
We see
only the result and can but deduce the causal conditions."
It is notable that the field observations used
by the above-mentioned writers have been taken in areas
in which the rocks, except for the jointing, were not
badly deformed.
In such areas, there seems to have been a
-24-
tendency to use the attitudes of the joints as the most
important evidence for the determination of the region­
al stresses that may have been predominant.
Assumptions
as to the mode of application of regional stresses have
been made in the absence of supporting evidence for dis­
turbances other than those which caused the jointing.
Un­
fortunately, all of the conclusions in the theories of
rock fracture are based upon known or controlled applica­
tions of stress, and similar results may eventuate from
the applications of different types of stress.
For in­
stance, it is possible that the same relationship between
a pair of shear planes of fracture may result from a pure
compressive stress or from a rotational stress.
In the
first case, the shear planes have a constant angle to the
causal stress throughout the duration of its application,
and the angle between a pair of planes remains constant.
In the second case, the angle of the shear planes to the
stress changes with the continued application of the
rotational stress, while the angle between a pair of
planes may remain constant.
With the view of overcoming the uncertainties
concerning stress and strain relations, the writer sought
to carry on his studies in an area marked by strong
folding and faulting.
An area of close folding, even to
the point of isoclinal folding, marked by pronounced
-25
parallelism of folds, appeared to offer the greatest
opportunity for avoiding these uncertainties*
An area
of regionally metamorphosed rocks with foliation, and
other metamorphic structures showing clearly the direction
of dynamic thrusting, appeared highly desirable*
It
would, of course, be necessary that the area be one in
which the alignment and characteristics of folds, faults,
and metamorphic structures all point to the same or closely
related causal forces*
Reasons for the Selection of the Area Studied
The Highlands of northern New Jersey is pre­
dominantly a region of highly metamorphosed and crystalline
gneisses, probably of pre-Paleozoic age*
Associated with
the gneisses is the ancient Franklin limestone.
To a
lesser extent, there are Paleozoic shaJes and conglomerates,
somewhat metamorphosed, in the area.
The gneisses have
been faulted and injected by magmas.
They are foliated
and banded, and highly folded.
They lie in parallel,elongated
ridges having a general northeast-eouthwest trend.
The
shales are also highly folded and have all developed cleav­
age to a greater or lesser degree.
The conglomerates,
while not in general so highly folded, are in a crystalline
condition.
-26-
The rocks of this area form part of the
Appalachian Mountain system.
The tight isoclinal fold­
ing has generally been assumed to have occurred at least
in part during the Appalachian Revolution (r. 75, 16, 94,
14, 15, 6, 96 etc.).
It has also been suggested that
some of the folding occurred during the Taconic orogeny
(r. 75, 23, 92, 93).
In each of these times of great
diastrophism, the area of the Highlands of New Jersey may
be assumed to have been acted upon by forces that orig­
inated from land masses that were somewhere to the east.
This region, therefore, seemed well suited to
tho type of investigations proposed.
Description of the Area Studied
The area in which the observations recorded
in this paper were taken lies between the Franklin
Furnace and Passaic Quadrangles in New Jersey*
Some of
the readings were taken in the eastern part of the
Franklin Furnace Quadrangle, and a few in the western
part of the Passaic Quadrangle.
The greater number of
recordings were made in the area covered by Topographic
Atlas Sheets Nos. 22 and 23, as surveyed by the Geology
and Topography Division of the New Jersey State Depart­
ment of Conservation and Development.
In addition, some
readings were taken elsewhere in the State for purposes
of comparison*
Background of Previous Geological Studies of the Area
The rocks and the formations that occur in
the district under consideration have been described by
the writers of the Franklin Furnace Folio (r« 96) and
the Passaic Folio (r. 27) of the United States Geologi­
cal Survey*
No formation was seen in the area studied
in this paper whose general description had not been
made in these two folios*
28-
The only detailed report of the Paleozoic for­
mations in the Green Pond Mountain region is found in
the Annual Report of the State Geologist of New Jersey
for the year 1901, Part 1*
The area studied in this
paper is a strip about four miles wide, extending from
Greenwood Lake on the north to the vicinity of Dover on
the south.
This strip incltides the Bearfort, Kanouse,
Copperas, and Green Pond Mountains and the Milton and
Berkshire Valleys.
The report is by H. B. Kummel and
Stuart Weller (r. 54).
The ancient crystalline rocks
of the area have been described in connection with the
report of the iron mines of New Jersey (r. 9), entitled
"Iron Mines and Mining in New Jersey", by W. S. Bayley
in Vol. 7 of the Final Report Series of the State
Geologist of New Jersey (1910).
In addition to the above descriptions, some
mention has been made of the rocks of the New Jersey
highlands in various other papers.
Van Hise makes
brief mention of the gneisses in his report on the
pre-Cambrian Geology of North America (r. 100, p. 8S6).
C. N. Fenner (r. 35) describes the rodk at the Pompton
Granite Company's quarry at Riverdale, New Jersey (1914).
There is a report by J. V. Lewis on the building stone
of New Jersey in the annual report of the State Geol­
ogist of New Jersey for 1908, in which the gneisses of
-29
I
Pompton, New Jersey, are described.
H. G. Bohlin (r. 17)
described the rocks of the Wanaque district in his Mas­
ter's Degree Essay (1925).
He was guided in this work
by Charles Berkey.
The descriptions of the rocks in the abovementioned papers were made with respect to the mineralogical content, the petrological behavior, the areal
distribution, and the historical correlation.
While
these features are outside the scope of this paper, it
may be well to review the characteristics of the rocks
of the district studied.
In the pages iranediately following, therefore,
the writer has summarized the descriptions of the rocks
as found in the above-mentioned papers, and has added
some further details observed in his field studies.
Description of the Bocks
GNEISSES
The most prominent rocks of the
area are the gneisses.
They lie in ridges having a
northeast-southwest trend.
Usually the southeast faces
of the ridges dip steeply, and the northwest faces dip
more gently.
The gneisses are foliated, although in
many places the foliation is not a prominent feature.
The disposition of the foliation indicates steep fold-
-30-
ing of the isoclinal type*
Exposures showing the crests
or troughiof the folds are rare*
The average strike of
the foliation is, in general, parallel to the trend of
the ridges*
There are, however, many places where the
banding does not acrree with the general trend.
One such
place was seen in crossing the ridge between Wanaque
Reservoir and Greenwood Lake*
Another was found along
the shore of Lake Wawayanda.
In both these localities
the banding had a northwest-southeast strike and a low
dip*
In the vicinity of Erskine Lakes, about a mile
east of Waneque Reservoir, the foliation in the gneiss
is irregular and changes direction frequently in a
distance of a quarter of a mile.
In the vicinity of
Split Rock Pond the banding strikes northwest-southeast,
and then farther along the shore of the pond its strike
returns to the general northeast-southwest direction*
The gneisses exhibit differences of composi­
tion, both chemical and mineralogica!, at different
places*
Because of these differences, the gneisses
have been divided into three main groups.
These are
called the Pochuck gneiss, the Byram gneiss, and the
Losee gneiss.
There is no clean-cut line of separation
of these three types of gneiss*
throughout the area.
All of them occur
The geologic maps of the Franklin
Furnace and the Passaic quadrangles show different areas
-31
of occurrence of these three gneisses, hut the authors
of the folios are careful to explain that these desig­
nations merely indicate which of the three is predominant
in the area*
POCHUCK GNEISS
The Pochuck gneiss is a dark
i
rock of foliated structure and fairly fine-grained tex­
ture.
It contains very little quartz.
The composition
varies, but the minerals commonly found in the Pochuck
gneiss are hornblende, pyroxene, plagioclase, microcline,
and biotite.
Magnetite has been found in the Pochuck
at several places, in sufficient quantities to warrant
mining*
The foliation is caused by the alignment of
the dark minerals such as hornblende, pyroxene or mica,
whose crystals have their long axes parallel to each
other.
This banding is not everywhere apparent, but
where the foliation is not distinct it can be seen that
the dark minerals are in pencil form parallel to the
general foliation.
The rock has been invaded by magmas,
although dikes and intrusive bodies are not so common as
in the Byram and Loses gneisses*
The Pochuck gneiss is not so widely distribu­
ted as the other two gneisses.
The chief areas in which
observations were taken in the Pochuck gneiss were on the
-32-
east side of Copperas Mountain and down into the
Berkshire Valley, near Stockholm, and in the vicinity
of Pochuck Mountain.
LOSEE GNEISS
The Losee gneiss is a light-
colored foliated granitoid rock.
The foliation, while
usually present, is not so easily distinguished as in
the Pochuck.
Its texture is like that of a medium
granite as a whole, hut it is alternatively medium and
coarse-grained in layers that parallel the foliation.
The Losee gneiss differs mineralogically
from the Pochuck gneiss in that it contains consider­
able amounts of quartz, and very little of the dark
hornblende and pyroxenes.
The feldspar in the rock is
oligoclase rather than the microcline of the Pochuck.
While magnetite is present in the rock, it is dissemina­
ted and is nowhere as abundant as in the other two va­
rieties of gneiss.
There has been much igneous injec­
tion in the Losee gneiss.
There are many granitic and
pegmatitic dikes and irregular masses in the gneiss.
While these do not necessarily follow the foliated
structure exactly, they seem to be more or less con­
trolled by it.
The Losee gneiss is found in many places in
the area studied.
Where it is fresh or only super-
-33-
Fig* 1
Fig. 2
Typical Pochuck Gneiss, Pochuck Mountain.
Typical Losee Gneiss, eastern shore of
Greenwood Lake.
ficially weathered, it has very light gray to light
green color*
In some knobs and ledges it is almost
white in color.
Where it is more deeply weathered, it
is stained quite brown*
BYRAM GNEISS
The Byram gneiss displays dif­
ferent facies, in some resembling the Pochuck gneiss
closely, and in others more nearly resembling the Losee
gneiss*
It differs from the Losee gneiss in the preva­
lence of potash feldspars, particularly in the microperthite form, and from the Pochuck gneiss in having
smaller proportions of the hornblende and pyroxene
minerals*
It grades into the Losee where some oligo-
clase is present, and into the Pochuck type where
hornblende and magnetite are more abundant*
In some places the Byram gneiss is distinct­
ly banded, and its foliation shows clearly*
However,
in many exposures the rock has a granitic appearance*
Over a great part of the area in which the Byram gneiss
occurs, the foliation takes the form of a lengthened
arrangement of the mineral particles, so that the darker
minerals show a pencil form, with long axes parallel but
not running in bands or streaks*
As in the Pochuck and Losee gneisses, the
Byram is extensively invaded by igneous material.
-34-
For
Fig* 3
Losee Gneiss, near Waaaque Reservoir*
Fig* 4
typical Byram Gneiss, with joints indicated
only by lines on the surface*
Fig* 4
Fig. 5
Byram Gneiss, near Denville
A
Fig. 6
Byram Gneiss, in Pompton Granite Company*s
quarry at Riverdale
Icylp*b
Fig. 5
Fig. 6
the most part the injections are acidic, but a few
basic dikes are known*
The injections may be in the
form of irregular masses which appear in places to have
digested the country rock*
Granitic masses large enough
to be quarried for building stone occur (Lewis r. 65),
but in most cases the graniti solutions have run along
the foliation planes so as to form a sort of psuedo-banding*
Fenner (r. 35, p* 610) has described such invasion
as "lit-par-llt" injection.
There are varying amounts of
magnetite in the Byram gneiss, and many concentrations
of magnetite have been mined*
Because of the several phases exhibited by the
Byram gneiss, it is rather difficult to define its limits
with respect to the other gneisses.
It is clearly, how­
ever, the most extensive of the gneiss bodies in the area*
FRANKLIN LIMESTONE
The Franklin limestone is
associated with the gneisses of the Highlands region.
i8 a white, highly crystalline limestone*
It
Although ap­
proximately 150 different minerals have been found in the
limestone, it is essentially a carbonate rock, ranging
from an almost pure calcium carbonate to a dolomitic
combination of calcium and magnesium carbonates*
Except
in the places where there are concentrations of zinc min­
erals and magnetite, the carbonates compose approximately
■35-
98$ of the rock*
In places the rock is tinted green,
because of pyroxene minerals in the limestone.
sionally its color is yellow or pink.
Occa­
There are some
siliceous inclusions of a lens-like character.
Many
flakes of graphite are found disseminated through the
rock, flattened in parallel planes and giving the lime­
stone somewhat a gneissic aspect.
The observations re­
corded in this paper were taken at the large quarry at
McAfee.
The Bethlehem Steel Company formerly operated
a magnetite mine along the boundary between the lime­
stone and the gneiss on the northwest side of the quarry.
There is, however, little indication of magnetite in the
quarry itself.
The stone has been removed for a distance
of about three-quarters of a mile in a northeast-southwest direction and to a depth of approximately 150 feet.
The limestone here exhibits a bedded structure and is
highly folded and injected.
The Franklin limestone and the gneisses of
the region are probably of pre-Paleozoic age.
The
other rocks in the region have all been identified as
Paleozoic or younger.
HARDYSTON QUARTZITE
Although the Hardyston
quartzite of lower Cambrian age has been mapped as oc­
curring in the area covered in this paper, the writer
-36-
did not succeed in locating any outcrops of the rock*
As described by Kummel and Weller (r* 54) and by the
authors of the Franklin Furnace Folio (r* 96), the
quartzite is in a comparatively thin bed and is missing
over the greater part of the area studied.
The writer
discovered that the chief outcrops of the Hardyston
quartzite reported by Kummel and Weller (r. 54) have been
since covered by an artificial lake, Pinecliff Lake, and
the real estate development at its south end.
Another oc­
currence reported at Macopln Lake has been covered by the
enlargement of this pond to form the Echo Lake Reservoir*
Since it was not a major formation in the district, the
writer made no further attempt to find it*
KITTATINNEY LIMESTONE
The Kittatinney is a
blue magnesian limestone of some thickness.
It underlies
most of the Wallkill Talley, but it has few outcrops in
the district covered by the writer*
Where seen, it was
interbedded with shale in thin laminations in the lower
part of the outcrops, becoming a purer limestone higher
in the exposures.
It is not so highly crystalline as
the other rocks studied.
It is placed in the lower
Cambrian, next younger to the Hardyston quartzite*
GRIEN POND CONGLOMERATE
The Green Pond
conglomerate forms the greatest part of Green Pond,
•37-
Fig* 7
Franklin Limestone, at McAfee, showing
inclusion*
Fig* 8
Franklin Limestone at McAfee*
.
The limestone
appears to have flowed about the dark
inclusions in the lower part of the
quarry wall.
Fig. 8
Fig* 9
Franklin Limestone, McAfee, showing injection*
Fig. 9
Copperas, Kanouse, and Bowling Green Mountains, running
in a line from northeast to southwest that nearly bisects
the area covered.
glomerate.
It consists of coarse siliceous con­
The pebbles of the conglomerate range from
less than 1/2 inch in diameter up to 2 inches and over,
and are almost entirely of white quartz, although there
are pebbles of almost every hue that quartz may have.
Pink is the most common of the colored varieties.
The
matrix is quartz sand from a dull red to white or gray
in color*
The bedding of the conglomerate does not show
clearly.
However, there is a parallel arrangement of the
pebbles in the conglomerate, with a concentration in lay­
ers that would seem to indicate bedding.
In places the
rocik in the ledges has weathered out along these pebble
streaks.
This would seem to indicate a periodic deposi­
tion and sorting of the pebbles.
This arrangement prob­
ably denotes the original bedding of the rock.
According
to the bedding, as described above, the rock has been
folded so that the beds dip to the west on Copperas
Mountain at varying angles, from almost horizontal to
about 80 degrees.
On Kanouse Mountain the beds are near­
ly vertical, with a steep dip to the west.
In addition,
the pebbles in the conglomerate show a tendency to be
elongated parallel to the bedding, indicating that the
-38-
Fig. 10
Green Pond Conglomerate and Byram Gneiss
on Copperas Mountain*
The conglomerate
is to the right, and the gneiss to the
left.
Actual contact could not he seen*
I
Fig. 11
Devonian Shale, western shore of Greenwood Lake.
Fig* 12
Devonian Shale, Oak Ridge Reservoir, below the
dam»
V
Fig. 11
Fig. 12
Fig. 13
Devonian Shale, western shore of Greenwood
Lake*
The flexure shown is rather un­
usual .
Fig. 14
Devonian Shale, western shore of Greenwood Lake*
Fig. 13
rock hat been subjected to considerable pressure*
The
Green Pond conglomerate has been identified, due to
stratigraphlc relations, as a Silurian formation.
SEALES AMD SLATES
Silurian shales conformable
with the Green Pond conglomerate have been described by
Hummel and Weller (r. 54).
ous in the area*
Their outcrops are not numer­
The writer did not differentiate the
outcrops of Silurian shale from those of Devonian shale,
as the cleavage and jointing in all the shales are found
to be similar.
Devonian shales are quite common in the region
of the conglomerate ridges, and form the greater part
of the floors of the valleys in this belt.
There have
been two shale formations identified, the Monroe shales
and the Bellvale flags.
The line of demarcation between
them is not clear, and is based more on their condition
and textural differences than upon mineralogical differ­
ences.
Both shales contain a Hamilton fauna.
The United
States Geological Survey does not distinguish the two
formations, but calls all the Devonian shale the
Pequannoek shale*
The shales are all sufficiently alike to have
reacted to stresses in much the same way.
They all occur
in the same belt of Paleozoic formations and are approxi-
■39-
mately of the same age*
The shale is everywhere somewhat metamorphosed.
It displays a slaty cleavage in many places, particular­
ly in what might be termed the "Monroe" portions.
Else­
where the cleavage is more of the schistose type, whose
planes do not persist, giving the cleavage a flaky ap­
pearance.
On the west shore of Greenwood Lake the rock
is cleaved in parallel planes, but rather widely spaced.
The shale here is sandy, becoming a graywacke or dark
gray flagstone.
At this place, several vein-like in­
clusions of quartz appear in a crisscross pattern
running transverse to the cleavage (Fig. 15).
Farther south, in Longweod Valley, south of
Oak Ridge Reservoir, the shale has been metamorphosed to
slate, and the same may be said of the shale on the bot­
tom at the west side of Oak Ridge Reservoir.
A similar
pronounced slaty cleavage has developed in places in the
West Milford Valley, between Greenwood Lake and Newfound­
land.
Elsewhere, as below the dam of Clinton Reservoir,
at Clinton Falls, below the dam of Oak Ridge Reservoir,
and at various places between Upper Longwood and Green­
wood Lake, the shale displays the flaky type of cleavage.
It is not possible everywhere to distinguish
-40-
the cleavage from the bedding in the shale*
In a few
places* notably below the dam at Oak Ridge Reservoir,
the bedding may be determined from alternate light and
dark bands in the rock.
it has a gentle dip*
Where the bedding is apparent,
It seems probable that at these
places the cleavage was caused by pressures that did not
disturb the bedding greatly.
At other places the shale
was tightly folded.
Some fossils were found in the shales*
Below
the dam cf Oak Ridge Reservoir some distorted and broken
casts and molds of pelecypods or brachiopods were found.
About three miles north of Newfoundland some casts of
crinoid stems were obtained*
SKUNNEMONK CONGLOMERATE
Bearfort Mountain is
largely composed of a conglomerate somewhat similar to
the Green Pond conglomerate.
The conglomerate has been
mapped as the remains of a synclinal fold, which stands
out above the surrounding shales because of its greater
hardness.
This conglomerate was formerly held to be of
the Green Pond formation.
Darton (r. 31) studied the
conglomerates in this area in 1893, and determined that
there were marked differences in composition and se­
quence between the conglomerates of Green Pond Mountain
and those of Bearfort Mountain.
-41
He called the Bearfort
Pig. 15
Devonian Shale, western shore of Greenwood Lake.
The quartzose injections seem to follow joint
planes.
Fig. 16
One eet cuts across the other.
Devonian Shale, below the dam at Clinton Reservoir.
Fig. 15
Mountain formation the Skunnemunk Conglomerate*
The Skunnemunk conglomerate is typically com­
posed of large pebbles up to six or eight inches in
diameter, enclosed in a dull purple-red matrix*
White
quartz pebbles are the most conspicuous, but there are
many dark red quartzite and sandstone pebbles*
Beds of
red sandstone containing no large pebbles are frequent in
the formation, and occasional lenslike inclusions of red
shale are seen.
These are not necessarily confined to
the lower parts
of the rock, but are at several places
up to the crest
of the mountain. The matrix is fira&y
cemented, and seems to
be of the same materials as the
pebbles but of much finer texture.
There are usually
networks of fine quartz veins, having the appearance of
filled and recemented joints, in the finer-grained por­
tions of the rock.
At the southern tip of the mountain,
near Clinton Reservoir, the conglomerate has the appear­
ance of a graywacke*
The bedding is not distinct, but the evidence
for its position is seen in the lens-like arrangement of
the pebbles and shaly inclusions.
folded.
The beds are highly
The pebbles of the conglomerate have their long
axes oriented parallel to each other, and this orienta­
tion usually has a high angle of dip.
-42-
In some places it
Fig* 17
Devonian Shale, Clinton Falls*
There are many
potholes in the shale at this place*
Fig* 18
Skunnemunk Conglomerate, Bearfort Mountain*
The
conglomerate is in a fine-grained phase, show­
ing some healed joints*
Fig. 17
Fig. 18
Fig* 19
Skunnemunk Conglomerate, Bearfort Mountain,
showing included lens of shale*
Fig* 20
Skunnemunk Conglomerate, south end of Bearfort
Mountain*
The conglomerate here displays
a quartzitic phase*
Fig. 20
I
is parallel to a cleavage that ia displayed in the
fine-grained portions*
-43-
Formations! Relationships
The gneisses and the Franklin limestone are
in their present relationship closely associated.
This
group is the oldest and most highly metamorphosed in the
area studied.
The different gneisses do not occupy any
one area exclusively.
They occur in long narrow belts,
wedging out at the ends and interwoven with each other.
Even in these belts more than one kind of gneiss or more
than one phase of the same gneiss may be found.
The Pochuck gneiss is generally regarded as be­
ing older than either of the other gneisses.
The Franklin
limestone is always found associated with the Pochuck and
it, too, is considered older than the Byram or Losee
gneisses.
In places, Pochuck gneiss interfingers with the
Franklin limestone.
At Franklin Furnace, where it contains
zinc ores, the limestone is in the form of a canoe-shaped
trough of an isoclinal fold in the Pochuck.
The Byram
and Losee gneisses are considered to be nearly contem­
poraneous in formation (r. 9, 15, 17, 33, 96).
The question of the origin of the gneisses of
the New Jersey highlands has not been satisfactorily
answered.
The New Jersey state geologists of the nine­
teenth century, Rogers, Kitchell, Cook, and Smock believed
-44-
that the gneisses had a sedimentary origin, and dated
them as pre-Cambrian.
Bayley (r. 33, p. 5) stated in the
Passaic Folio that there is no evidence to support a
theory of sedimentary origin and that it is more probable
that they are of igneous origin.
This idea is partly
supported by Spencer, in the Franklin Furnace Folio
(r. 96, p. 2).
All are agreed that whether the rocks were
originally sedimentary or igneous, that they have since
been altered greatly by younger intrusives.
For the purposes of this paper, the important
factors concerning the gneisses are that they are now in
a highly crystalline condition, are hard, and resistant
to weathering, so that everywhere fresh rock may be
found near the surface.
In these qualities the gneisses
may be said to be homogeneous.
They appear to have re­
acted to stresses in much the same manner, no matter
what the nature of local phase differences.
Paleozoic formations are not extensive in
the district.
It seems clear that the Highlands in
this vicinity has been emergent, except locally, since
the latter part of the Paleozoic Era.
Over much of the
area there are no traces of any deposits younger than
the gneisses, except the debris of Pleistocene gla­
ciation.
-45-
Where the Hardyston quartzite has been de­
scribed by Kummel and W eller (r. 54), namely, in the
Wallkill Valley and in the valley Just to the east of
Kanouse Mountain, it has been reported to be unconform­
able to the gneiss.
The Kittatinney limestone is con­
formable to the Hardyston (r. 54, 96).
The belt in which the shales and conglomerates
are found is defined on the west side by faults that run
in the direction of the general trend of the ridges.
These faults have been described in the Franklin Furnace
Folio (r. 96), the Passaic Folio (r. 33), and in publica­
tions of the Geological Survey of New Jersey (r. 54, 9).
One
such fault runs from a point some miles north of
the New York-New Jersey state line, along the valley to
the west of Bearfort Mountain, along the west side of
Clinton Beservoir and past the west side of Oak Ridge
Reservoir.
Another fault has bifurcated the northern
end of Green Pond Mountain.
Another extends down the
west side of Berkshire Valley, from a point to the east
of Bowling Green Mountain.
Still another fault separates
Green Pond Mountain and Copperas Mountain.
On the east
side of this Paleozoic belt, the gneiss underlies the
younger formations unconformably.
The faults mentioned above are of the high
-46-
angle type that is common to this region*
They have
been mapped by the Geological Survey of New Jersey.
The writer has followed the lines of these faults for
some miles*
The fault in the Berkshire Valley is
characterized by an abrupt scarp of gneiss about 200
feet in height near Lower Longwood.
On the floor of the
valley there is shale whose cleavage is pronounced*
The
shale is hard, and it does not seem to the writer that
the vertical difference can be ascribed to erosion alone*
Although a covering of lichens on the rock concealb any
possible slickensides that might be present, the face of
the scarp is smooth*
To the west of Oak Ridge Reservoir
and farther north, there is no such abrupt scarp, but
the sudden disappearance of the gneiss marks the line of
the fault.
This line is straight, in a northeast-south-
west direction throughout the length of the Pequannock
watershed*
The actual contacts between the gneisses and the
later H o k e were not found.
There is a mantle of glacial
debris everywhere along these valleys, so that the out­
crops are only occasional in the valleys.
It is felt,
however, that the topographic and stratigraphic relation­
ships in this belt confirm the locations of faults as
they have been mapped in the above-mentioned papers*
■47-
The great fault that hounds the Highlands on
the east, at the edge of the Trlassie hasin, Is inferred
from the sudden cessation of gneiss outcrops*
Places where
outcrops of the Newark beds appear in close juxtaposition
to the gneiss are rare, because of the presence of Quarternary glacial deposits, the mantle of soil, and the rather
dense vegetation*
The entire area studied is overlain by glacial
deposits, of greater or lesser thickness*
While glacia­
tion has laid bare many knobs and ridge crests of rock,
glacial deposits cover the area so completely, especial­
ly in the valleys and well up the stoss sides, usually
the northwest sides of the ridges, that one can, in
places, travel for two or three miles without seeing a
rock outcrop*
Almost every type of glacial deposit
can be found, both sorted and unsorted.
The glacial
deposits have been adequately described in the Franklin
Furnace and Passaic Folios.
These deposits proved to
be somewhat of a hindrance to the aims of this study*
In addition to the deposits directly attributable to
glaciation, all of the valleys have accumulated a con­
siderable mantle of soil*
Dense vegetation covers the
terrane, in the form of grasses, bushes, and trees*
The hills are covered for the most part with young
forest growth, having a dense underbrush.
-48'
Because of
the extensive cover, no actual contacts between forma­
tions were seen*
In several places, however, different
formations were identified within 25 to 50 feet of each
other*
STRUCTURE
The most notable structural feature
of the rocks of this region is their Intense folding.
The gneisses lie in tight isoclinal folds*
folds are high in angle*
The dips of the
The shales have been folded and
have developed cleavage which is, in general, parallel to
the folding in the gneisses.
The conglomerates have also
been folded, although not so intensely as the gneisses*
Except in a few places, such as at the shore of
Split Reck Pond and on the ridge northwest of Wanaque
Reservoir, the positions of the crests of the anticlines
are not evident.
At no place was it possible to identify
definitely the bottom or center of a syncline.
The rocks,
as now seen, appear to be the limbs of tight isoclinal
folds extending over many miles*
The foliation in the gneisses, the cleavage in
the shales, and the bedding in the conglomerates have
the same general trend as the ridges that are formed by
the rocks.
These ridges lie in the general direction of
N. 30 degrees E. - S. 30 degrees W*
At times the axes
of the ridges point more nearly N-S, as seen at Lake
-49-
Waywayanda, at Pochuck Mountain, and in the vicinity of
Wanaque Reservoir*
The major faults in the district are parallel
to the general trend of the folding.
It is very probable
that the faulting accompanied the folding.
Except in
the vicinity of the Paleozoic belt, there are few large
faults in the Highlands.
Minor faults of small extent,
and with throws from a fraction of an inch to a few
feet, abound in the gneiss areas.
These seem to be more
closely allied to the jointing than to the folding.
The forces that caused the folding in these
rocks apparently acted in a more or less east-west
direction.
This is in agreement with the geological
history of North America and the hypotheses that orogenic disturbances in this part of the country emanated
from land masses that were to the east of the present
Appalachian System*
Jointing is a very prominent feature in the
gneisses of the area*
In places the joint sets are
such that the rock is fractured into small pieces, and
at others there are large massive blocks.
are rare.
Single joints
As a rule, at least two systems of parallel
joints are found, and in most places many more than two
sets are seen.
The conglomerates also are well jointed,
-50-
but there are not so many joint systems in them as
there are in the gneisses, nor do the joint sets per­
sist without change in direction as do those in the
gneiss.
The jointing in the shales is not
bo
promi­
nent a feature as its cleavage, but regular joints cut­
ting across the cleavage do appear almost everywhere*
The jointing in the rocks is not usually af­
fected by local conditions*
In the gneisses, joints
cut across intrusive masses regardless of how such in­
trusions interrupt or cut across the foliation*
In the
conglomerates, joint planes cut across the included
pebbles and the matrix alike*
-51-
PROCEDURE
Field Studies
The field studies were undertaken throughout
1939*
Occasional trips were made in the spring.
were largely in the nature of reconnaissances.
These
Some
week-ends were spent in comparing the conditions in the
field with the descriptions previously published.
In
the gneiss areas, outcrops were located and the general
trends of the ridges and valleys noted.
Old hack roads,
some of them merely wagon tracks, were explored.
Arrange­
ments were made for access to posted land, such as the
Pequannock watershed and the Wanaque reservation.
In a
similar manner, the shale and conglomerate belt was
traversed.
The mapped faults (r. 54) were traced and
were considered to have been correctly placed, as far
as indirect evidence could be depended upon.
Daily field trips were made throughout the
entire summer.
Headings were taken at 259 outcrops.
As was stated above, outcrops of bedrock are not too
frequently encountered in the area.
Large boulders
abound in the district, some of which, lying partial­
ly buried in the heavy brush, might easily be mistaken
for outcrops*
-52-
In many places roads have been cut through
the rock.
In these places much of the jointing may have
been caused by the blasting of the road-makers.
Wherever
possible, outcrops away from the roads were examined.
It
was found, however, that in general the dynamite followed
joints or incipient joints that already existed, for joint
planes in undisturbed areas were parallel to those that
were exposed by blasting.
Frequently the joint planes
were only indicated by their lines of intersection with
the exposed surfaces.
At such places the writer broke
away some of the rock so as to expose the joint planes.
At times this entailed considerable labor, particularly
in the gneiss areas.
Joint observations in the shale areas were
comparatively simple.
There were usually not more than
two joint systems present in addition to the cleavage.
However, it was difficult to determine the bedding
planes of the shale except at a few places.
The con­
glomerates have been fractured by more joints than the
shales, but many of the joints in the conglomerates are
not of sets that persist for any distance.
The bedding,
as evidenced by the alignment of the pebbles, is some­
times difficult to trace.
At each outcrop, readings of joint systems and
-53-
foliation or bedding plane9 were taken with a Brunton
compass.
The strike of each plane was recorded by
holding the compass level.
The dip was taken by holding
the side of the compass against the joint face, so that
the plane of the face of the compass was perpendicular
to the level line of the strike reading.
These readings
were taken during the day, so that it was necessary, be­
cause of the possibility of local magnetic disturbance,
to come back at night and check the strike readings
against the pole star.
It was felt sufficient to do this
about every linear mile, for except where there were
magnetite deposits the regional average deviation of
about 10 degrees was found to prevail.
Places where the
magnetic attraction is exceptional have been mapped (r. 9).
These locations were checked for excess of magnetite in
the rock.
Samples taken were reduced to a powder in a
mortar, and then tested with a hand magnet.
At these
places, compass correction readings were taken every
hundred feet.
The writer checked this compass in dif­
ferent areas on three different nights during the summer.
Joint readings were not taken where only a
single joint appeared.
In most cases, the joints were
repeated in parallel planes, and it was these systems
that were considered to be significant.
-54-
In many places
in the gneisses there were minor faults, indicated by
slickensided surfaces and offsets in the banding or in
the dikes.
When such faults were accompanied by joints
that were parallel to the faults, readings were taken.
Some of the outcrops were so extensive that readings
had to be taken at different points along them.
In
consequence, over 800 joint readings were taken from
the outcrops examined.
The taking of joint readings was the most im­
portant part of the field work.
In addition, however,
the rocks were examined for variations in the usual
structure or texture, or for breaks or changes in the
foliation.
The crystal habit was noted, as far as it
could be determined by examination with a hand lens.
All of the hand specimens that were obtained were
broken out of bed rock.
While the shales were being studied, they
were constantly scrutinized for fossil content.
fossils were found, although they are scarce.
Some
This was
to be expected, as all the shale is somewhat metamor­
phosed, and some of it is definitely slaty.
Further field trips were taken in the fall
of 1939 and in the spring of 1940.
-55-
Headings were taken
in the Newark rocks to the east of the district, and
in Newark rocks to the south of the Highlands.
This
was done in order to compare the jointing in the loss
disturbed Triassic rocks with that in the Highlands
district.
-56“
Tables la to lj.
These tables indicate the strikes and
dips of the joint sets as read at the different
outcrops.
The extreme left and right hand col­
umns represent the locations of the several out­
crops, as indicated on Maps 2 and 3 (in map en­
velope).
The asterisks indicate the joints
tested for shear plane angles.
Location
Number
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Correlation Studies
TABULATION OF DATA
When sufficient data had
been collected, the strike and dip readings of the joints
were tabulated.
First, the strike records were corrected
from the compass readings so that true north became the
prime point of reference.
Then a grid was ruled, with
vertical columns for every 5 degrees of the compass, from
west through north to east.
The horizontal lines repre­
sented the locations of the outcrops.
From these tables
it was seen that by far the greatest number of joint strikes
lies in the middle 30 degrees of the northeast and northwest
quadrants.
The readings were then retabulated as shown in
Tables la to li.
The surfaces studied were practically true planes,
regardless of intrusions or inclusions in the rock.
There
was very little, if any, separation of the abutting faces
of the blocks into which the rock fractured.
In many
cases the joint surfaces were slickensided, indicating a
sliding movement, but in most such cases the movement ap­
peared to be slight*
Such movement had too little magni­
tude to be considered faulting, and the writer believes
it merely represents a readjustment in the rocks due to
the increase of volume accompanying the deformation.
The
writer believes, therefore, that in the great majority of
-57
cases the joint planes studied were planes of shear.
As can be seen from the tables, a comparison
of strike readings merely suggests the directions of the
major forces causing deformation.
Since the strike is
only a horizontal line in a plane, it is evident that an
infinite number of planes could have the same strike.
It
is also evident that two joint planes whose strikes are
perpendicular to each other might be nearly parallel to
each other or might be perpendicular to each other.
It is
necessary, therefore, to take the dips of the joints into
consideration as well as the strikes.
The greatest number of joints occurred in such
fashion that their strikes were within 15 degrees of
northeast or within 15 degrees of northwest.
Where joint
systems in both of these groups occurred at one outcrop,
the writer chose joints from each group for further ex­
amination.
In many cases some of the joints seen at an
outcrop did not fit with one or other of these groups.
At some outcrops no joint systems had strikes that fitted
the usual groupings.
Wherever it seemed likely, either be­
cause of marked differences of direction of strike or mark­
ed variations in dips, that the joint planes might repre­
sent conjugate planes of fracture, the writer tested their
relationships.
-58-
The writer selected about 200 pairs of inter­
secting joint planes, and determined the angles between
the planes.
The joint planes were so chosen as to in­
clude pairs from all sections of the area studied.
In
addition, some pairs of planes from outcrops of Newark
rocks to the east and to the south of the area under dis­
cussion were examined, in order that comparisons might be
made.
Pairs of joint planes were taken from most of the
outcrops studied.
All possible combinations by pairs of Joint
planes within each outcrop reading add up to more than
1200 pairs.
The labor involved in testing all such pairs
is tedious, to say the least.
Fortunately, it was seen
that in many places the strikes and dips of pairs of
planes were alike, or very nearly so, and it was felt un­
necessary to examine more pairs than were chosen.
Since strike is measured along a horizontal
line of a plane, and dip along a line of the plane per­
pendicular to the strike, the determination of the angle
between the two intersecting planes is a descriptive
geometry problem.
It is only necessary to set up a pair
of principal planes of projection.
One of these is the
horizontal plane tangent to the surface of the earth at
the point studied; the other is a vertical plane.
-59
Since
compass readings are used in these solutions, this
vertical plane is chosen perpendicular to a north-south
line on the horizontal plane.
The intersection of these
planes, or ground line, is an east-west line.
The strike line now becomes a horizontal trace
of a plane, and the dip is measured along a line of slope
of the plane.
From these known factors, vertical traces
of a pair of joint planes may be located.
If now a plane
is passed so as to be perpendicular to the line of inter­
section of the two joint planes, and its lines of inter­
section with each of the joint planes are found, the
plane angle of the dihedral angle of the two planes will
have been obtained.
Typical examples of this procedure
are shown in Plate 1.
Assuming for the time being that the rocks
sheared under pure stress, and that therefore the de­
forming forces acted in directions bisecting the angles
between the shear planes, the next problem was to find
that direction.
While obtaining the angle between pairs of
joint planes, the bisectors of the angles were also
found.
Then the process was reversed in order to find
the projections of the angle bisectors.
Since these bi­
sector lines are perpendicular to the line of intersection
-60-
Tables 2a to 2j.
These tables contain the results of the
testing of pairs of joint set planes.
Key to rocks (indicated in right hand column):
Gn
Fk
Ck
Sc
Ds
Dc
Tw
Tn
-
Gneiss
Franklin Limestone
Kittatinney Limestone
Green Pond Conglomerate
Devonian Shale
Skunnemunk Conglomerate
Watchung Basalt
Newark Sedimentary Bocks
Table 2a
Reading Joints,
Nos.
No.
Angles
Acute An,gle Bis.
Strike
Dip
Obtuse Anigle Bis.
Strike
Dip
N 84 E
Nil W
1
2,5
78-102
1
4,5
88i-91J
1
1,5
87£-92|
2
1 .2
84-96
N
88
W
30 E
3
1 .2
84-96
N
88
E
11
5
1,3
81-99
6
1 .2
7
8
V
22 N
On
Gn
N 3 W
6
S
N 48 £
37 Sff
Gn
N
9
S
Gn
£
8
£
N 14 E
34 S
Gn
N 89 £
30 E
N 13 E
25 S
Gn
88-93
N
13 S
N 89 £
40 E
Gn
1.3
90-90
N 69 £
40 NE
N
14 S
Gn
12
2,3
87-93
N 83 £
22
N-S
21 N
Gn
12a
1.3
67-113
N 80 £
29 W
N 11 E
37 N
Gn
17
1 .2
86-94
N 77 E
W
N 13 W
27 N
Gn
18
1 ,2
77-103
N 74 W
17 W
N 47 E
28 NE
Ds
21
1.3
89-91
N
2 E
21 N
N
E
13 N
Ds
22
1.3
75-105
N 12 £
7 N
N 81 E
23 W
Ds
23
1,3
78-102
N
24
1.3
81-99
N 85 W
25a
1 .2
87-93
N
V
26
1.3
88-92
N 84 £
29
1.3
85-95
N
30
2,5
70-110
33
1 .2
34
37
8
E
8
W
9 W
86
N
8
W
4 N
Ds
N
6
E
27 N
Ds
18 V
N
3 E
15 S
Dc
7 £
N
8
W
16 N
Do
30 £
N 22 E
23 N
Gn
N 40 W
35 SE
N 40 W
55 NW
Gn
86-94
N 77 E
5 £
N 10 E
34 N
Gn
1 ,2
80-100
N
2 W
61 S
N 81 E
6
W
Gn
1 .2
80-100
N 14 V
26 N
N 85 E
20
E
86
88
86
£
£
30 E
0
1
Gn
Table 2b
Beading Joints,
Nos.
No.
Angles
Acute Axlgle Bis.
Dip
Strike
Obtuse Axigle Bis.
Dip
Strike
26 W
N
60 NW
38
1 ,2
83-97
N
39
1,3
61-119
N 53 W
40
1,3
83-97
N
41
1,3
71-109
44
1,3
51
E
4 E
N
Gn
N 53 W
30 SE
Gn
H S
N
86
W
44 W
Gn
N 87 E
7 E
N
6
w
29 N
Gn
78-102
N 84 W
60 E
N 23 w
26 N
Gn
1 ,2
82-98
N 87 w
N
12* N
Gn
52
2,3
89-91
N 61 E
28 N
Ck
52
3,5
84-96
N 36 W
0
53
2,4
83-97
N 78 E
21
55
1,4
83-97
N 60 E
35 NE
57
1,3
82-98
N
60
2,3*
30-150
N 28 E
61
1,3
66-114
N
10
W
11
62
1,3
6 8 -1 1 2
N
88
W
14 E
N
65
3,7
86-94
N
3 E
38 N
N 89 E
66
1,5
80-100
N 89 V
20
E
N
68
1,3
87-93
N 78 E
23 W
N
71
1,3
83-97
N
E
13 E
N
72
1,3
88-92
N 89 E
17 E
N-S
73
1.3
82-98
N
8
E
32 S
N
74
1,3
79-101
N
9 E
76
1,3
73-107
77
2,3
79-101
86
5 w
21
W
6
W
27 sw
3 E
N 15 W
N 54 E
w
Ck
0
N 29 E
51 N
Gn
N
37 S
Gn
7 E
N 69 E
0
11*
Gn
0
Gn
7 E
4 W
5k
33 N
5k
W
Gn
42 S
Gn
3 E
21* N
Gn
3 W
13* N
Gn
42 S
Gn
E
17 E
Gn
S
N 89 W
36 E
Gn
N 75 E
41 N
N 19 E
21 N
Gn
N 73 W
25 E
N
11 N
Gn
88
10
N
N 74 E
4 E
22
88
12
E
E
6
Tabic 2e
Reading Joints,
Noe.
No.
Angles
Acute Anj;le Bis.
Strike
Dip
Obtuse Angle Bis.
Dip
Strike
78
1 .2
88-92
N 17 E
25 E
N 75 W
5 W
Gn
80
1 .2
75-105
N 26 W
30 NW
N 75 E
6
W
Gn
81
1,4
83-97
N 58 W
13 SE
N 25 E
82
3,4
84-96
N 56 E
12 E
N 34 W
83
1,5
77-103
N-S
23 S
N 72 E
34 V
Gn
84
4,5
87-93
N 82 W
N
E
42 N
Gn
85
2,5
80-100
N 50 E
12 SW
N 79 W
60 W
Gn
85
1,7
88-92
N 32 E
5 NE
N 63 W
21 W
Gn
86
1,3*
23-157
N 16 W
41 N
N 81 E
8
E
Gn
92
1 .2
87-93
N 10 E
17 N
N
6
W
Gn
93
2.4
87-93
N 17 W
3 S
N 82 E
44 E
Gn
95
1.7
82-98
N 78 W
16 W
N 18 E
9 N
Gn
96
2,3
78-102
N 80 W
13 E
N
7 W
41 N
Gn
97
1 ,2
83-97
N 27 E
16 NE
N 64 W
5 E
Gn
97
2 ,6
70-110
N 76 W
7 E
10 N
Gn
98
2,4
81-99
N 82 W
8
7 E
25 N
Gn
99
1 ,2
75-105
N 14 E
N 74 W
10 E
Gn
100
1,3
86-94
N
N
40 S
Gn
101
2,3
87-93
N 28 E
20 NE
N 61 W
101
1,5
40-140
N
4 W
31 N
E-W
103
1 ,2
74-106
N 89 W
15 W
N
104
1 ,2
56-124
N
26 N
N 61 W
106
1 ,2
89-91
N 70 W
86
8
E
E
0
E
19 S
2 W
0
8
86
V
N-S
N
2 E
1 E
N 20 E
33 NE
Gn
0
8
Gn
SE
Gn
10 E
Gn
S
Ds
15 W
Ds
8
0
Gn
Table 2d
Heading
No.
Joints,
Nos.
Angles
Acute An gle Bis.
Strike
Dip
Obtuse Ajogle Bis.
Dip
Strike
107
1 .2
88-92
N 61 E
E
N 30 W
38 NW
Gn
109
1 .8
83-97
N
13 S
N 78 E
10 E
Gn
110
2,3
50-130
N 80 W
8
7 E
5 N
Gn
113
1 .8
85-95
N 34 E
9 SW
N 57 W
37 NW
Gn
114
1 .8
72-108
N 23 E
25 NE
N 74 ff
10 E
115
1,3
67-113
N 85 W
45 E
N
12
E
7 S
Gn
115
2,4
88-92
N 62 W
23 SE
N
20
E
21 N
Gn
116
1,3
78-102
N
10
w
13 S
N 72 E
27 E
Gn
118
1 ,2
86-94
N
20
E
10
119
1,4
87-93
120
1 ,2
121
3 W
6
E
N
Gn
N
N
86
V
5 E
Gn
N 82 E
12 E
N
8
W
0
Gn
72-108
N 82 E
28 ff
N 27 ff
1 ,2
63-117
N
W
30 ff
N
E
0
123
1,3
70-110
N 35 E
25 NE
N 60 W
8
124
1,4
78-102
N 28 ff
38 SE
124
1 ,8
82-98
N
7 E
22
126
1,3
87-93
N 85 E
10
129
1.3
82-98
N 45 E
130
1 ,2
84-96
132
1.3
133
88
11
40 S
Gn
Gn
NW
Gn
N 58 E
48 SW
Gn
S
N 80 W
7 E
Gn
ff
N
w
4 S
Gn
26 NE
N 78 w
42 V
Sc
N 15 E
45 N
N 89 ff
12 E
Sc
90-90
N 76 E
23 E
N
40 S
Sc
1 ,2
88-92
N 14 E
47 N
E-ff
21 E
Sc
135
1,5
87-93
N 81 I
26
136
2,4
72-108
N
137
1 .2
73-107
N 67 E
86
W
6
7 E
N 45 ff
12 SE
Gn
3 W
N
9 E
13 N
Gn
ff
N
7 W
20 N
Gn
20
E
Table 2e
Reading
No.
Joints,
Nos.
Angles
Acute An*;le Bis*
Dip
Strike
139
1,4
34-146
N 76 £
139
4,5
50-130
N
88
£
140
2,3
62-118
N
11
£
142
2,3
67-113
143
1 ,2
145
42 ff
Obtuse Anigle Bis.
Dip
Strike
N 15 £
23 N
Gn
N -S
21
N
Gn
S
N 48 ff
70 NW
Gn
N 69 £
14 £
N 16 ff
13 S
Gn
65-115
N 13 ff
5 N
N 78 E
£
Gn
1,4
52-128
N 76 W
42 £
N 19 W
30 N
Gn
147
2,3
87-93
N
w
22
£
N 17 E
37 S
Gn
148
1,3
83-97
N 61 £
11
W
N 17 £
65 N
Gn
149
1 ,2
47-133
N 75 £
11
ff
N 17 £
65 N
Gn
152
2,3
88-92
N
86
£
7 £
N-•S
29 £
Ds
154
1 ,2
6 8 -1 1 2
N
88
ff
5 £
N-■S
33 N
Gn
155
1 ,2
73-107
N 80 £
12
W
N
12
ff
9 N
Ds
156
1 ,2
73-107
N 84 £
17 £
N
10
ff
2
156
2,4
77-103
N 74 £
8
ff
N 14 ff
21
157
1 ,2
83-97
N
2
N
N 85 £
16 ff
Ds
158
2,3
88-92
N 50 £
N
5 £
33 N
Gn
159
1 .2
78-102
N
N 73 £
13 £
Gn
160
2,3
86-94
N 52 £
32 SV
N-S
45 N
Gn
161
2,3
86-94
N 67 £
16 Sff
N 13 W
30 N
Gn
161
4.5
77-103
N
15 N
N 73 £
43 ff
Gn
162
4,7
85-95
N
10
£
26 N
N 72 E
42 ff
Gn
162
6,7
71-109
N
86
£
18 E
N
5 £
23 S
Gn
163
1,4
77-103
N
4 E
30 S
N 87 E
£
Gn
88
8
22
£
ff
5 ff
9 ff
20
50 SW
3 N
6
12
ff
Ds
N
Ds
Table 2f
Beading
No.
Joints,
Nos.
Angles
163
5,8
164
Acute Angle Bis.
Strike
1 Dip
Obtuse
Strike
Dip
60-120
N 17 E
45 N
N 87 W
12 W
On
3,4
52-138
N 4 W
17 N
N 77 E
16 W
Gn
165
3,7
89-91
N
5 E
26 S
N 82 E
25 £
Gn
166
5,6
80-100
N
E
36 E
N 73 E
9 S
Gn
167
2,4
78-102
N 18 W
24 N
N
E
15 E
Gn
168
2,4
86-94
N 80 E
3 E
N
7 W
24 N
Gn
170
2,5
87-93
N 56 E
43 SW
N
5 W
29 N
Fk
172
1 ,2
76-104
N
34 E
N 27 w
174
2,4
69-111
N 84 E
176
1 ,2
83-97
N
177
4,5
82-98
N
178
5,6
84-96
N 31 w
179
1 ,2
6 8 -1 1 2
N 79 w
180
1 ,2
71-109
N 80 E
12
14
2,3
80-100
N 64 W
17
3,4
61-119
N 48 w
42
2,3
55-125
N 19 w
48
2,4
48-132
N 87 w
183
2,3
86-94
N
184
1 ,2
87-93
N 55 w
187
5,6
79-101
N 87 w
17 E
188
1 ,2
87-93
N 30 E
5 E
186
3,4
80-100
N 71 W
11
68
E
66
6
NW
Fk
W
N
12
w
27 S
Gn
3 W
17 S
N
88
E
3 E
Ds
W
4 E
N-S
35 N
Dc
4 SE
N 59 E
24 SW
Tw
24 W
N
4 E
16 S
Tw
W
N
W
1 S
Tw
2
NW
N 46 E
75 NE
Gn
10
NW
N 16 E
59 S
Gn
4 S
N 50 E
79 NE
Gn
17 w
N 37 E
46 NE
Gn
N
N 76 E
8
W
Tw
N 37 E
7 SW
Tw
37 NE
Gn
N 61 W
18 W
Gn
N
14 N
Gn
86
12
w
11
8
0
21
E
N
10
20
12
E
E
Table 2g
Reading
No.
Joints,
Nos*
Angles
Acute Anj;le Bis*
Strike
Dip
Obtuse Azigle Bis*
Strike
Dip
189
1 ,2
87-93
N 15 £
19 N
N 78 ff
22
ff
Gn
190
1 ,2
81-99
N -S
35 N
N 87 W
11
W
Gn
192
3,4
87-93
N 76 £
25 ff
N
22
N
Gn
195
1,3
89-91
N 71 W
212
1.4
86-94
N 82 £
27 £
N 43 ff
17 SE
Gn
213
3,4
85-95
N 69 £
34 NE
N
17 S
Gn
214
1 ,2
87-93
N 14 E
47 N
E-ff
21
W
Gn
215
2,4
74-106
N 83 ff
16 ff
N
7 E
22
N
Gn
216
1 ,2
75-105
N 65 £
20
N
8
20
N
Gn
218
1 ,2
89-91
N 59 ff
220
2,3
84-96
N 81 ff
11
221
1,3
87-93
N
222
3,4
80-100
224
2,3
225
3 ff
N 19 £
0
Sff
3 SE
9 W
ff
Gn
0
N 58 W
63 Nff
Gn
E
N
3 E
31 N
Gn
41 S
N
£
17 E
Gn
N 44 £
26 NE
N 44 £
65 SW
Gn
86-94
N 80 £
3 E
N
7 W
24 N
Gn
1 ,2
87-93
N 54 £
32 Sff
N
2
45 N
Gn
228
1,3
50-130
N 40 £
5 NE
N 58 ff
5 ff
Tn
229
4,5
58-122
N 46 ff
9 Nff
N 40 £
33 NE
Tn
230
1 ,2
80-100
N 69 £
1
W
7 S
Tn
231
1,3
51 -129
N 69 ff
7 SE
N 18 E
13 N
Tn
231
2,3
83-97
N 59 ff
18 SE
N 48 E
23 NE
Tn
232
1 ,2
70-110
N 53 ff
13 Nff
N
28 N
Tn
233
1 ,2
60-120
N
11
234
3,4
72-108
N 41 £
3 W
68
E
£
N
86
20
10
ff
£
Sff
N 30 ff
16 SE
Tn
26 Sff
N 48 ff
20
Nff
Tn
Table 2h
Beading
No.
Joints,
Noa.
Angles
Acute Ang;le Bie.
Dip
Strike
Obtuse A ngle Bis.
Dip
Strike
3 SW
N 34 W
16 NW
Tn
235
1 ,2
81-99
N 65 E
236
2,3
88-92
N
W
13 SE
N 19 E
19 NE
Tn
23?
2,4
83-97
N 17 W
14 SE
N 79 E
20
SW
Tn
238
1 ,2
89-91
N 13 W
8
N 79 E
21
SW
Tn
239
1,3
81-99
N 89 w
0
240
1,3
53-127
N 73 w
241
1,4
75-105
N 72 w
0
N 27 E
242
1 ,2
75-105
N
4 W
N-S
243
1 ,2
90-90
E-W
5 W
244
2,3
70-110
N 31 w
245
1 ,2
82-98
246
3,4
247
68
8?
E
S
3 E
8 S
Tn
N 18 E
14 s
Tn
N
14 NW
0
Tn
N
Tn
N-S
5 N
Tn
14 SE
N 52 E
6
NE
Tn
N 38 E
9 SW
N 55 W
8
E
Tn
66-114
N 34 E
7 NE
N 57 W
9 SE
Tn
1,3
37-143
N 7? E
7 SW
N
W
16 NW
Tn
4
1,3
81-99
N 44 E
27 NE
N 45 E
65 SW
Gn
5
2,4
84-96
N 59 E
10
SW
N 32 W
31 NW
Gn
8
1 .2
6 8 -1 1 2
N 80 W
14 E
9
1 ,2
80-100
N 24 W
3 E
27
1 ,2
70-110
N 60 E
42 SW
N
30
1,3
57-123
N 72 V
25 SE
31
1 ,2
71-109
N 77 V
31 W
32
1 ,2
58-122
N 58 E
10
35
2,4
58-122
N 71 E
12
40
1 ,2
80-100
N 65 E
0
22
11
7 E
12
E
Gn
N 65 E
22
NE
Gn
N
27 N
Dc
N 27 E
32 SW
Gn
N 28 E
18 NE
Gn
SW
N 29 W
50 NW
Gn
E
N 17 W
35 SE
Gn
N 26 w
55 NW
Gn
10
W
Table 21
Reading
No.
Joints,
Nos.
Angles
Acute Angle Bis*
Strike
Dip
Obtuse Angle Bis.
Strike
Dip
1.3
77-103
N 39 E
Sff
N 43 W
28 NW
1,2
85-95
N
12 NE
N 40 E
31 NE
2.4
88-92
N 17 E
20 N
N 81 ff
a v
1,6
35-145
N 26 W
14 SE
N 24 E
69 NE
1.3
70-110
N 16 E
15 S
N 89 ff
32 E
1.3
69-111
N
E
15 N
N 87 ff
1.3
87-93
N 82 W
24 E
N 19 E
44 S
1.3
72-108
N 22 E
5 NE
N 67 ff
a
4.5
62-118
N 50 W
16 SE
N 21 E
30 NE
1,2
90-90
N 72 W
15 W
N 22 E
3 NE
1 ,2
79-101
N 67 E
35 NE
N 26 ff
0
4.5
55-125
N 38 E
40 NE
N 71 ff
1.5
83-97
N 42 E
7 NE
N 49 ff
3.4
85-95
N
1.3
56-124
N 87 W
22 ff
N 43 E
50 NE
2.3
84-96
N 73 E
37 E
N
W
15 S
1.3
60-120
N 26 E
31 Sff
N 80 E
46 E
2.3
82-98
N 11 E
38 S
N 83 E
20 E
8
8
a
W
IS
8
8
Sff
N
68
8
ff
6
W
E
18 W
5 Nff
10 ff
Table 2j
Reading Joints,
No.
Noe.
Angles
Acate An gle Bis.
Dip
Strike
248
1.3
88-92
N 29 W
248
3,4
27-153
N 16 E
249
1 ,2
85-95
N
251
1 ,2
252
Obtuse Angle Bis.
Dip
Strike
N 63 E
4 NE
Tn
3 N
N 74 ff
2
W
Tn
E
16 Sff
N 28 ff
1
Nff
Tn
84-96
N 56 E
31 NE
N 38 ff
12
Nff
Tn
2,3
82-98
N 85 E
4 ff
N
253
1 .8
83-97
N 16 E
7 N
N 74 W
256
1 ,2
78-102
N 25 ff
3 SE
N
259
1 ,2
90-90
N 65 E
66
22
NW
16 W
4 W
68
E
N 23 ff
5 N
Tn
5 ff
Tn
E
Tn
9 N
Tn
10
US A e a r Can rsfenr Res
147 Near Cenv/He.
Cre/ss-J^/ats"/and"A
6/s?7‘- Str A82'C d/p/sT
G neiss - J o in ts * Z a n d *3
A ngle. 8 7 ’- 3 3 °
Bis. 8 7 "- 3 tr. N88°W, dip22’E
0 s 93“ - s tr A S 'd d p / ’A '
Bn. 3 3 ’ - S ir N !7'£. ig 3 7 3
Ang/e 0 7 - 3 3 °
\
<b
N38W
131 On Copperas Mountain
G reer Pane! Conglomerate Joints *2and "3
Angle 8 2 ~ 9 3 "
3/5 8 2 ° - S ir N tC L . d ir 3 8 ’5
B n . 9 8 ° - S tr 7/8. ‘ °C d /p 2 0 °L
2 4 A t west shore o f Greenwood Lake
Devonian Shale
Angle. &
- 99°
B/s 31°-Str.N83°Rd/p O’/S'
Bn.99’-Str Nb’E dip 27°N
147 Near Cenv/He
Gneiss • Joints *2and *3.
Angle. 8 7 -3 3 °
Bis 87°- Str N88°WdipN’8
Bis. 3 3 '- Sir N!7’lc. 3/{ 37'S.
2 4 A1 westshore ofGreenwoodLake
Devonian Shale
Anqle, 3 /° - 39°
Bis 81° - Str. N8S°W.d/p O'/S'
Bis. 39’- Str Nb°D chp 27°N
37 Ateastshi
,
\
Gneiss - Joint.
Anq/e, 83°- 3
Bis 8 3 '-S tr
Bis. 37° - Str .
20. On Bean
SkunnemundCm
Angle. <38‘-'.
Bis.86°- Str A
B/s 32° -Str 7v
PLATE I
3 7 A t eastshore of GreenwoodLake
Gneiss - Joints * ! and *2
Anq/e, 3 3 -3 7 °
Bis 8 3 ' - Str. N B /T .d /p M
Bis. 37° - Str NM'W.clip ST
26. Or? B earfo r t Meanta/n
Skunneman/fCongton era/e - Jo// "s 3and’3
Anq/e. <S B °- 9 2 °
Bis. 8 8 °- Str N84E. d/p 7°B.
3/s 3 2 ° - Str /,■ 8 ’ld. d /p 7J°M
of the planes of the joints and also lie in^the plane
bisecting the angles between the joints, they indicate
the direction of stress causing the deformation.
The di­
rection of these lines of force with respect to the points
of the compass may be read directly from the horizontal
projections (Plate l), and the dips of the lines of force
obtained by a simple geometrical rotation.
are shown in Tables 2a to
The results
2 j«
ANALYSIS OF TABULATIONS
Comparatively few of
the joint planes in the area had a low angle of dip.
666
In
cases out of 952 examined, the angle of dip was found
to be more than 45 degrees.
number tabulated.
This is about 70# of the
From this it was apparent that the high
angles of the joint planes in the district had some signi­
ficance as indicative of the regional forces to which the
rocks had been subjected.
In 84# of the joint pairs ex­
amined, the bisectors of the plane angles of the dihedral
angles dipped at 30 degrees or less*
Since these bisectors
give the directions of the forces resulting in the shear
planes, it would seem that the greatest amount of force
was exerted in a nearly horizontal direction.
It can
also be seen from the percentage of high angle joint
planes that, to a great extent, the relief was in a
lateral direction.
This is also true in all the cases
-61-
of Joint planes of low angle dip checked by the writer.
SHEAR PLANE ANGLES
Let us now examine the
angles between Joint planes as shown in Tables
2a
to 2J*
These tabulations cover all the types of rock examined,
but of course the greatest number of pairs of Joints shown
is in the gneisses.
In the gneisses,70$ of the
acute angles of 75 degrees
Joint pairs formed
or more to each other. Of this
percentage, there was 82$ of pairs forming acute angles
of 80 degrees or more to each other, and 45$ forming
acute angles of 85 degrees
or more to each other. In ten
cases the angle between shear
planes was within
2 degrees
of 90 degrees*
The Skunnemunk and Green Pond Conglomerates
fractured at angles of very much the same magnitude.
All of the Joint pairs examined fractured at acute angles
of more than 80 degrees to each other.
In the shale 8 , 38-^$ of the pairs fractured at
acute angles of 60 degrees or more, and 92$ fractured at
acute angles of 70 degrees or more.
Only two pairs of Joint planes in the Kittatinney limestone were tested.
In one case the acute angle
was 89 degrees, and in the other the acute angle was 84
-62-
degrees*
In the Franklin limestone joint planes ex­
amined, the acute angle between fractures ranged from
66 degrees to 87 degrees, with a fairly even distribu­
tion in that range*
For purposes of comparison, some readings were
taken in the Triaesic Newark rocks to the east of the
Highlands, opposite the chief area studied.
A few obser­
vations were made in the basalt at Little Falls*
Of the 5
pairs of joint planes tested, taken from readings at 5 dif­
ferent places along the basalt gorge at Little Falls, 3
pairs had an acute angle between joint planes of more than
84 degrees*
At the other 2 places, the acute angle between
pairs of shear planes was about 70 degrees*
The indicated
direction of force facing the acute angle was different
for each case, with a range from almost south-north to
almost east-west*
More observations were made in the Triassic
sandstones and shales in the Newark basin.
Unfortunately,
this part of the Newark basin is almost completely covered
by the products of Pleistocene glacial deposition, so that
outcrops of the underlying Triassic sedimentary rocks are
few and scattered*
However, the writer studied outcrops
-63'
of Newark sandstone and shale at Woodcllff Lake, Saddle
Biver, Paterson, Woodridge, Kingsland, and Arlington,
over a distance of about 20 miles.
The rocks at all these
places were in much the same condition.
The manner of jointing in the Triassic sedimentary
rocks is strikingly different than that in the Highlands*
The joints in the Newark beds do not persist vertically,
but form systems that are interrupted by the nearly hori­
zontal bedding planes*
Joints at various levels in a hori­
zon may be parallel to each other, but single joints do not
pass through consecutive layers; they terminate at the bedding
planes.
Many of the joints in the shale are of the open type,
with irregular surfaces and of a shattered appearance*
The evidence seems to indicate that the Newark
sedimentary rocks fractured under the tensile stresses that
are set up during a process of gentle folding or warping,
which caused the various layers of rock to slide along their
bedding planes.
The joints were examined by pairs just as
were those of the Highlands rocks, but it was found that
there was no tendency toward parallelism in the indicated
directions of force*
In this respect the sedimentary New­
ark rocks are like the basalt.
The angles between joint
planes faced directions that varied through a range from
almost south-north to almost east-west (Table 2j).
-64-
COMPARISON OF JOINT PATTERNS
It is significant that joint pairs taken from
major joint sets throughout the entire area studied inter­
sect with each other at approximately the same angles.
This
holds true whether the rock is gneiss, conglomerate, shale,
or limestone.
Exceptions are as likely to he found in one
type of rock as in another.
Where variations from the usual
wide angle of intersection are found in the gneisses, the
same amount of variation may be observed in any of the other
types of rocks.
Joint Patterns and Foliation
At a large number of the gneiss outcrops, the
foliation of the gneiss was not distinct.
While the
gneisses may, on the whole, be classified as banded
gneisses, the banding is not a prominent feature through­
out the entire gneiss area.
At perhaps half of the out­
crops visited, the gneissic structure was apparent only in
the orientation of the darker minerals.
Where the gneiss
was badly weathered, even this much could not be determined.
At many places, however, the foliation was distinct.
From
observations of gneissic foliation at 70 outcrops, the
following relationships to joint planes were determined:
-65-
In 10 casas the rock had jointed along
the foliation planes.
In 9 cases there was a joint system
whose strike was approximately perpendicular
to foliation.
In 6 cases there was a joint system with
the same strike as the foliation, but with
a different dip.
In one case there was a joint system
whose strike was perpendicular to the folia­
tion, and one whose strike was parallel to
the foliation. In no other case was there
more than one of the conditions described
above.
It is felt that similar conditions obtain where
the banding wa3 not readily distinguished.
As can be seen
from the above, the terms "strike set" and "dip set", as
suggested by Nevin (r. 79) for joints parallel respective­
ly to the strike and dip of the beds, cannot be used in
designating the joints in the gneisses.
The writer feels
that the foliation in the gneisses exerted little or no
control over the joint systems.
The joint planes were
persistent in their parallelism, regardless of the de­
velopment of banding or of igneous intrusions.
The relationships between jointing and bedding
in the Green Pond conglomerate are very much like those
between jointing and foliation in the gneiss.
At only
one of the outcrops studied were the bedding planes
coincident with a joint system.
-66-
Here again the jointing
does not seem to be closely related to bedding planes*
In the Skunnemunk conglomerate, however, the case is
different.
At all but one of the outcrops recorded, one
of the Joint systems was coincident with the bedding*
It
might be mentioned at this time that the Skunnemunk con­
glomerate is the youngest of the formations studied*
It was difficult to determine the relationship
between the bedding and the jointing in the shales*
In
only a few places was it possible to say that the bedding
is distinct from the cleavage*
In a few places the
cleavage seemed to be definitely the same as the bedding*
At all other places it was impossible to distinguish the
bedding*
Joint Patterns and Local Folding and Faulting
As has been stated earlier in this paper, the
entire region has been subjected to tight isoclinal fold­
ing, with the axes of the folds trending generally northeast-southwest*
Aside from the regional folding, there
are many minor folds throughout the gneisses and in the
Franklin limestone.
At many places in the gneisses no
strike or dip reading of the foliation was taken because
of the irregularity of the banding due to minor folding*
67-
At only a few places in the gneiss areas were
there any evidences of tension joints or sheet joints
such as might accompany folding*
These were of rather
minor importance compared to the great number of shear
joints.
It must be remembered, however, that in the
region as a whole the crests of the anticlines have been
eroded, and the troughs of the synclines have not been
uncovered.
It is also true that there has been a great
quantity of magmatic material injected into the rocks.
Probably the rocks have been invaded or reinvaded since
folding, as evidenced by the dikes that cut across the
foliation.
If this is so, such open fractures as might
have existed would have acted as channels for the
solutions.
In the shales there are very few places where
local folding is recognizable.
The cleavage is uniform
where it is of the slaty type.
The slight flexure in the
cleavage shown in Fig. 13 is exceptional.
It is apparent
that at this place the flexure formed after the cleavage
had been developed.
Fig. 15, taken at the same outcrop,
shows some pegmatitic intrusions that seem to follow
what may have been the original bedding.
A second series
of intrusions, probably from the same magma but invading
at a somewhat later time, seems to have followed along a
-68
joint set.
These intrusions cut across the cleavage,
and consequently it may be said that they occurred
after the disturbance that caused the metamorphism of
the shale*
In the conglomerates, the writer found no
places where purely local folding occurred*
The strike
and dip of the bedding followed the general trend for the
region at all the outcrops where the bedding was recognized.
The major faults in the district are those asso­
ciated with the Paleozoic shale and conglomerate belt.
There are many places where the rocks have faulted locally.
A few of these faults have throws of from 10 to 50 feet.
There is a fault scarp at the West Brook fork of the
Wanaque Reservoir that has an almost vertical face and a
throw of about ?5 feet.
The fault is approximately a
quarter of a mile long.
One joint system parallels this
fault.
There is a set of parallel faults of varying
throw, from a few inches to several feet, at the Pompton
Granite Company's quarry at Riverd&le (Fig. 21).
have a dip of about 50 degrees.
These
The displacement is
clearly shown, due to the banded character of the gneiss
at this place.
This is the location of which Fenner
(r. 35) wrote in describing the granitic type of injec­
tion in the gneiss.
Here the faulting seems to be inti-
-69-
Fig. 21
Byram Gneiss, Riverdale.
There is a fault here
of an approximate throw of 8 feet* parallel
to one of the joint sets.
Fig* 22
Byrern Gneiss, Biverdale#
A series of minor
faults is parallel to one of the joint
sets*
mat el y associated, with the jointing, as in many places
joint sets parallel to the faulting appear throughout
the quarry (Fig* 22)•
There are many other places where faults of
slight throw appear.
Sometimes they are single faults
with a narrow crush zone, and sometimes there are many
parallel faults of very small apparent throw and no
brecciation along the fault planes*
These last are
usually identified by slickensides, with perhaps some
chloritization at the surface*
In each of these cases,
the faults were parallel to one of the joint systems
of the rock.
It is felt, therefore, that the minor
faulting in the region is the result of the same diastrophism as caused the jointing*
Joint Patterns and Regional Structure
It may be safely assumed that the forces
which cause shear planes of fracture act in directions
bisecting the angle between pairs of such shear planes.
As has been stated earlier in this paper, these di­
rections were obtained in connection with the problem
of determining the angles between shear planes*
Let us assume that the deforming force faces
the acute angle between shear planes*
-70-
As was stated
earlier, the greatest number of the bisectors of these
acute angles, or lines of force, dipped at 30 degrees or
less.
In 57# of the pairs tested, these bisectors had
strikes within 30 degrees of the east-west line.
In
other words, the lines of stress were dominantly from the
east toward the west.
About 33# of the acute angle bisectors had
strikes within 30 degrees of the north-south line.
This
would indicate a substantial amount of thrusting from
north to south, or from south to north.
However, the man­
ner in which the rocks were folded, in long parallel
ridges running north-northeast and south-southwest, and
the absence of east-west thrust faults, seems to deny the
implication that any considerable forces acted along a
north-south line.
In this group, 56# of the pairs of planes inter­
sected at angles so close to 90 degrees to each other that
a slight unbalance of forces may have caused the angle
facing the dominant stress to become obtuse, even though
compression from the east was the major deforming stress
of the region.
As can be seen from the accompanying maps
(Plates 2 and 3), there is no suggestion of areal group­
ing of the lines of force according to the percentages
-71-
given above.
A pair of shear planes which indicates
an east-west direction for the deforming force may be
quite near a pair of shear planes that indicate a northsouth direction for the deforming force.
Comparisons
for parallelism of these lines of force may be made any­
where in the area studied.
The strikes of the obtuse angle bisectors, as
a rule, make angles of about 90 degrees to the strikes of
the corresponding acute angle bisectors.
Deviations from
90 degrees are caused by the fact that although the bi­
sectors make that angle to each other, they are inclined
to the horizontal.
If the assumption as to
the direction
of the deforming force is correct, these obtuse angle
bisectors should indicate the direction of easiest relief
for the rock under strain.
The fact that similar arrangements of shear
planes and parallel
directions for lines offorce or
lines of relief may be found in any part of
the area
brings up some important questions.
1.
Did the rocks fracture because
of local conditions of stress such that
the deformation in one place occurred in­
dependently, though in a similar manner,
to that in a nearby place?
-72-
2*
Was a force of regional scope
broken up into vectors that acted in vary­
ing directions to deform the rocks?
3*
Is the character of rocks in a
highly crystalline and highly folded condi­
tion such that they react differently to
regional stress than those of simpler
structure?
4.
Do confining pressures have an
effect upon the angle of intersection of
the shear planes, so that perhaps the de­
forming force faces the obtuse instead of
the acute angle?
5.
What regional disturbances may
have caused the widespread jointing in the
already highly folded rocks?
In answer to the first question, the writer feels
that it is extremely unlikely that the rocks of the district
could have been stressed in such a manner as to have frac­
tured independently in adjacent small areas.
This would
i*ean that forces of limited scope, whose centers originated
within the gneis3, were generated at various places.
In
other words, it means that the deforming forces must have
accompanied the magmatic invasion so common in the gneisses.
This idea is not tenable, because the igneous injected
matter has been fractured as well a3 the host rock.
In
addition, the uninvaded or only slightly invaded Paleozo­
ic rocks and the gneisses have been jointed similarly.
As to the second question, rotational stresses
resultant from a regional stress are postulated.
-73-
The
writer has found no evidence of conditions that might
cause such a dispersion of forces.
The gneisses have
all approximately the same degree of competency, and
should react everywhere in much the same way to a
regional stress.
There is no evidence that joint
planes have changed direction in the younger rocks
because of proximity to the gneisses.
The third question carries with it implications
that are far more complex than do the first two questions.
In the first place, highly crystalline and highly folded
rocks, such as the gneisses of the district studied, are
probably much more brittle than those of regions of
gently dipping sediments, such as have been reported by
Sheldon (r. 95), Bucher (r. 19), and Parker (r. 83).
In the second place, the gneisses must have suffered un­
equal strains while being folded, to an extent unlikely
in rocks of simpler structure.
This inequality of
strain has probably been to a great extent relieved by
recrystallization.
However, it is likely that a greater
departure from normal reaction to a stress may be ex­
pected in the gneisses and other highly folded rocks of
the district than in rocks of gently dipping sediments.
This results in a greater complexity of minor deforma­
tion and a greater difficulty of interpretation.
-74-
As
far as major joint sets or shear planes are concerned,
there is no reason to believe that there should be any
difference in the reactions of the rocks of the two
types of region postulated, other than that caused by
differences of plasticity or brittleness.
The fourth question implies the possibility that
the strain ellipsoid idea must be more carefully examined
than if the deforming force is considered to be a simple
or pure stress, or if it is a rotational stress in one
plane.
Nevin (r. 79), Willis (r. 104), and Leith (r. 61),
among others, ascribe in part the position of the shear
planes of a fractured rode to internal friction.
Merriman
(r. 72, p. 377) uses the idea of internal friction to ex­
plain the departure from the expected angle of 45 degrees
for shear planes to direction of stress.
It is, however,
perhaps significant that the more modern investigators,
such as Nadai (r. 78) and Timoshenko (r. 99), make no
mention of internal friction as affecting the directions
of shear planes of fracture.
According to the experiments of Mason (r. 20,
p. 13), mentioned earlier in this paper, confining
pressures have the tendency to change the angle between
-75-
shear planes resultant from compression.
His experiments
were made "by using hollow tubes of steel, which were sub­
jected to hydrostatic pressures coincidentally with the ap­
plication of axial pure stresses.
It was felt that these
experiments could not quite fit conditions for rock fail­
ure.
For one thing, the mild steel used has not the same
physical properties as a crystalline rock.
For another,
the hydrostatic pressure was applied either on the inside
or the outside of the tubes, and consequently the direction
of easiest relief was under some control.
Mason's experiments suggest the possibility that
confining pressures might change the effects of a deform­
ing stress upon rocks.
It seemed, therefore, that at this
point the suggestion might be investigated mathematically.
In order tc discuss the failure of rocks as a mathemat­
ical problem, we must first assume a condition of homo­
geneity.
While it is true that the rocks under dis­
cussion are probably not homogeneous, the gneisses in
particular seem not to have been greatly influenced by
differences in mineralogic composition.
Consequently,
we may disregard the amount of error arising from a
probable inhomogeneity, and assume a general isotropism
in the reaction of the gneisses to stress.
76-
Let tis consider a section of a rectangular
block of isotropic material under a pure uniform com­
pressive stress P,• At the instant of fracture, lines
will show in the section at an angle aC
the direction of Py
be
s' 3-!r
'
angle
4-
(Fig. 23).
For tension
to
will
• The
varies some­
what with the nature of
the surface or surfaces
transmitting the load­
ing.
Some arrangements
permit more or less
lateral plastic flow of
the block at the top
and bottom surfaces
(Morley r. 77, p. 539).
F q .2 3
Assuming that a plane,whose trace in the block
is AB,is of unit area and making any angle
direction of the uniform stress P ;
block (Fig. 23), then:
-77'
to the
compressing the
P / sin eC =
total force acting on the top
and bottom of the block.
P I sin eC coa
= total transmitted shear
component along AB. _____ _
(1)
S a q — the shear stress on AE, since
the surface AB is of unit area.
The value of eC
rendering (l) a maximum is obtained from
(3)
A
This same result (3) is obtained for tension, since
the shear component (l) will be the same,but changed
in direction.
Note that jC
is independent of P,
The foregoing is known as Navier's Theory.
Navier calculated the stress transmitted
normal
to AB.
He considered that this normal com­
ponent to AB introduces a friction force acting in
a direction opposite to the transmitted shear.
The
appearance of lines (Lauder* s lines) in test specimens would indicate shear motion before rupture.
The friction would add to the strength of the speci­
men along AB
in shear.
-78
The normal stress on AB is
P, sin2/
_______________________________ (4)
and the friction stress on AB is
P ( sin2*C
tan dp ________________________ (5)
where <f> is the angle of repose of the material of the
block, and tan dp -jn , the coefficient of friction.
When Pj is a compression, the net trans- '
mitted shear to the plane AB is
s P^ sin^ c o s ^ - P / tan <p sin^xf _______ (6)
If P f is a tension, the transmitted normal
component to AB is a tension, and it would seem that
friction does not operate in such case.
Navier pic­
tured molecular forces as holding the material to­
gether, and in the plane AB these forces would be
normal to AB.
Hence the transmitted normal compon­
ent of Pj to the surface AB would act to reduce the
molecular forces and reduce friction, whereas if
is
a compression, the molecular forces are increased
by a compression component.
Of course, the reaction
to shear motion and rupture along AB is supplied by
the molecular forces, but (6) indicates the trans­
mitted shear stresa from P, for compression and (7)
the same for P i acting in tension
SAB s P( sin oC cob of + P(tan^sin e£_______ (7)
-79-
Note that (6) and. (7) are the same as (l),
except that a shear stress due to friction is sub­
tracted in (6) or added in (7)*
The angle
of the parallel planes receiving
the maximum net transmitted shear results from (6) for
compression:
(9)
For tension, from (?)
.(10)
Apparently, for the sirnplp cases of single
uniform stress of compression and of tension, the re­
sults agree with experiments, due regard being given to
the method of loading*
According to this theory of Navier, the angle
of maximum shear stress is independent of Pj . The writer,
in his studies of the literature, does not find any record
of change in the angle of the markings, either before or
at rupture.
It appears, then, that Navier’s theory is
correct up to rupture.
-80-
For an added compressive stress Pz at right
angles to
there occurs a disconcerting discontinuity,
which induces the conclusion that Navier’s theory cannot
hold for two stresses, and therefore cannot be used with
assurance in any case*
In Fig* 23 let us consider
angles to P,.
acting at right
The transmitted shear component stress
along AB is in a direction opposite to the transmitted
shear of P(
For
SAB
?/> P z ,
«(?,-Pp) sin/.cos,(- t a n f ^ , s i n V + ^ c o s ‘2,C7_________ (11)
Note that the normal components are in the
same direction, increasing the molecular forces*
For
sA B - ” (p i
P, < P2
) s i n *^ c o s ^
-
tan(fjp, s i n V + P ^ c o s V J
_________________( 1 2 )
For (11)
d
AB -(P - P, ) rcos*/ - sin f 7 - tand/p . 2sin £ cos / ' -2 l
j
^
P^cos/sin/J * 0
and except where P( -P=0,
- tan ( K
tan 2o(scot
^
|
For (12), and when
- 9)
' I
(13)
P/ - P^ ^ O
tan 2((-- cot <p = tan ( H + (p'j
»C *
-81
J
+ -f
<14)
There is a discontinuity in ^
in Pig. 24.
8^own
We should expect a continuity indicated by
the dotted line in Fig. 24.
VShy should the value of •£.
be constant (13) for any value of P^
=P,
at
except at P; 5
from 0 up to
, and suddenly change to (14)
for any value, no matter how minute when F2 /^ ?/ ?
p,
F i q .
2 4 .
-82-
For Py 2
the normal component stresses add to
P, [ain1^ + cos V 7 = P
,
__________________ (15)
indicating a circular distribution, i.e., uniform com­
pression with zero transmitted shear for all values of
. The value of eC for maximum shear, which is zero,
would be either (14) or (15) according to Navier.
In Fig. 24, the curve for P, and P^
positive^
is half the hyperbola
P;P2 ^ C
_
(16)
where C is a constant of any value and merely indicates
the shift in the value of <>C for maximum transmitted
<
shear for conditions, P = P • On the 45 degree line
P( =?2 an<i on the
C is negative.
degree line P^ = |p2ff>is negative and
Symmetrical discontinuities obtain for
other combinations of P, and P2 where one is tension and
the other compression, or where they are tension and
tension.
For "P a tension, and P^ y jP2|
where P^ is
a compression,
S^g-(P(tP^ )
sin£. coa/ -
sin it( - P2 cos V"_/
f -f
4
1 ______________________ (17)
For P C|P2| , we obtain eC z
83-
*£
______________ (18)
It was hoped that continuity might he expect­
ed and values of <C for transmitted net maximum shear
in terms of P, and P^ in the form
j
*, Z)
,
which we could use to study the P / and P2 values resulting
from the ^
values observed.
It is, of course, highly probable that internal
friction plays an important part in the fracturing of
rocks along shear planes.
The foregoing investigation
merely suggests that the function of internal friction
cannot be simply expressed.
It is felt that this
mathematical analysis does show that the confining
pressures do not affect the angle of shear, since it has
been shown that this angle is independent of the magnitude
of an applied stress.
In other words, the deforming
pressure must exceed the confining pressures sufficiently
to cause fracture.
Before this condition is reached, the
confining pressures may only have the effect of causing
the rock to act as a less brittle substance than its
composition at the surface indicates.
As far as the
rocks of the region studied are concerned, there is no
evidence that the rocks have changed their condition from
plastic to brittle since fracturing.
Parker (r. 83), vA10 studied the jointing in the
-84-
gently dipping sediments of central New York, concluded
that the systems of joints he examined were due to a
combination of a compression normal to the regional trend
of the flexures of the district and an active tension in
the direction of the regional strike*
To this action he
ascribes the fact that the dip joints are more open
fractures than the strike joints.
It is highly probable that the combination of
tension and compression postulated by Parker has caused
the jointing in the area he studied.
It is difficult
to see, since shear planes due to a tension or a com­
pression occur in pairs, how tension affects only a
single set of joints and compression affects only the
conjugate set.
There is little doubt that in the area of this
study the jointing was made possible by a general stretch
of perhaps only a few feet to the mile in the direction
of the trend of the folding.
As has been mentioned
earlier, relief occurred in a more or less horizontal
direction, and there must have been a general horizontal
elongation along at least one strain axis.
Considering the gneisses alone, the great
majority of joint sets were within 10 degrees of being
-85-
at right angles to each other.
It is felt that a region­
al compressive stress was accompanied hy a regional ten­
sile stress at approximately right angles to it, and hoth
stresses acted horizontally.
If this is so, local varia­
tions in mass or structure might so affect the regional
stresses that shear fractures due to tension might occur
in places, although shear fractures due to compression
are the dominant jointing phenomena of the region.
This
would account for the fact that some of the angles be­
tween shear planes are obtuse, facing the same direction
to which the majority of planes form acute angles.
There were a few places in the gneisses where
the angles between the joint sets varied from 90 degrees
much more than was the usual case.
One such pair of
joints was observed at the shore of Split Rock Pond,
where the angle between planes was 24 degrees and 146
degrees.
The directions of stress indicated hy the bi­
sectors were roughly east-west for the acute angle, and
north-south for the obtuse angle.
.Another such case
was found near Glenwood, in the Pochuck gneiss (Fig. 25).
Here the shear planes formed angles of 30 degrees and
150 degrees to each other.
The joints were almost
vertical, and the indicated force directions were
N. 28 degrees £• for the acute angle and N. 62 degrees
V. for the obtuse angle.
■86-
Fig* 25
Joints intersecting at wide angles in Pochack
gneiss, Pochack Mountain.
These joints may
have heen caused hy tension*
Fig. 25
Judging by the angles between fractures at
these and a few other places, tension was the chief stress
involved.
These cases seem to bear out the idea that ten­
sion may be locally dominant in a region that has, in the
main, undergone compressive stresses.
Any answer to the fifth Question must he large­
ly conjectural.
While it is highly probable that the
gneisses and Franklin limestone are of pre-Cambrian age,
there is no evidence to date these formations exactly,
and it is not known just how old the rocks were at the
beginning of the Paleozoic Era*
It seems very likely
that these rocks had undergone a considerable amount
of diastrophism prior to Cambrian time.
Judging by the
general absence of igneous intrusion in the rocks of
known Paleozoic age, the greater part of the injections
probably occurred in pre-Cambrian time.
These magmatic
invasions must have been accompanied hy some folding and
faulting.
Judging also by the granitic nature of much
of the intrusive material, the rocks were buried under
a great cover for a large part of the time.
An unconformity between the earliest of the
Paleozoics, the Hardyston quartzite, and the pre-Cambrian
rocks has been reported (r. 96, p. 3 and r. 54, p. 8).
The sediments from which the gneisses derived had been
-87-
folded before the Bardyston deposition, as shown by
the large angle of this unconformity*
It is not thought
that the jointing as seen in the gneisses to-day is as­
sociated with the pre-Cambrian folding.
The Taconic orogeny is evidenced in this district
by a lack of Ordovician sediments.
There is no trace of
the Martinsburg shale in the Paleozoic belt of the area.
The presence of Hudson Biver slates is questioned by Kummel
and Weller (r. 54), and the writer found no evidence of
them.
It is believed that the area was quite generally
uplifted during the Taconic disturbance, and whatever
Ordovician sediments may have formed, together with a
large part of the Cambrian sediments, were removed follow­
ing this uplift.
The greatest patt of the folding in the area
was probably caused by the Appalachian revolution.
The
shales and conglomerates were affected, as well as the
gneisses.
The slaty cleavage runs parallel to the fold­
ing of both the Paleozoic rocks and the gneisses.
The
jointing may have occurred toward the end of the Appalach­
ian orogeny, after the folding and attendant metamorphism
had been accomplished.
The Palisade disturbance occurred toward the
end of the Triassic period, with general uplift accom-
-88'
Fig. 26
Newark Sandstone and interbedded Shale, King siand.
The Joints do not persist vertically, but com­
parable joints may be seen in different layers.
Fig. 26
panted by some faulting.
The major faults that bound
the belt of Paleozoic rocks on the west may have occurred
at this time, with some tilting of the blocks to the west*
This would account for the presence of the Paleozoic rocks
in the area, which may be the remnants of much more exten­
sive sedimentation.
The belt may represent a depressed
trough, thus enabling the Paleozoic sediments to be pre­
served.
The major joints in the basalt at Little Falls
do not seem to agree with the pattern seen in the gneisses.
While the angles between the shear planes are comparable
to those of the gneisses, the indicated directions of
force are different for each pair of planes, running
from almost south-north to almost east-west.
While not
enough joints were examined in the basalt to be conclusive,
the same thing seems to be borne out by the joint pattern
in the Triassic sandstones and shales in the Newark basin,
to the east of the area studied.
Here a good percentage
of the joint planes form large angles with each other, but
the indicated directions of forces vary widely for the
pairs of planes examined.
The joints do not persist vertically, but are
confined to layers defined by bedding planes.
Comparable
joints may be seen in the various layers, indicating, per-
-89-
hapa, that vertical lines of weakness have been shifted.
The dips of the beds, as recorded, are gentle and vary
in direction.
The jointing is probably due to tension
and torsion that accompanied gentle warping, rather than
to any great compressive force such as seems to have af­
fected the gneisses of the Highlands.
It seems evident
that the jointing in the Highlands area studied antedates
the Mesozoic or later disturbances that affected the
Triassic rocks.
The Appalachian revolution set up forces that
moved in a general direction from southeast to northwest.
However, the direction of the major forces causing the
jointing in the area studied operated from east to west,
if these forces are to be considered purely compressive.
This discrepancy is rather glaring, but it is felt that
it can be explained.
A glance at the topographic sheets used in
this study, or at the geologic map of New Jersey, seems
to indicate a swing in the general trend of the ridges
from northeast-southwest in the more southerly part
of the area, to more nearly north-south in the northerly
part.
The center of this rotation is apparently in the
viclinity of Newfoundland.
The forces, therefore, must
have acted more nearly from east to west in the northern
part of the area.
-90-
It may be pointed out, that in any great
regional tectonic disturbance, there must be some
local diversity in the directions of the forces ris­
ing from a major thrust.
A major stress exerted from
a southeast to a northwest direction, for example,
may encounter local obstacles causing it to change
direction somewhat.
If there then should be a lesser
bulk of resisting rock to the northeast than to the
southwest, the major compressive stress might change
direction so as to be effective from the east.
-91-
CONCLUSIONS
1.
The average of the positions of the
shear planes of fracture in the joints of the re­
gion seems to indicate that the stress of regional
character that caused the jointing came essentially
from the east.
This statement is made on the assump­
tion that the deforming force was non-rotational.
The writer feels that this is a fair assumption, due
to the fact that similar attitudes of the joint planes
were found at both the north and south extremes of the
area studied.
This would indicate that the lines of
force moved parallel to each other, rather than about
a center.
As shown by the general trend of the folds
of the Appalachian mountain system, and by the evi­
dence advanced by other writers, the forces arising
from tectonic changes in ancient Appalachia came from
the southeast and moved in a northwesterly direction.
A glance at the geologic map of New Jersey, and at
the topographic sheets of the area studied, shows that
the trend of the ridges runs approximately N. 30 de­
grees E. in the southern part of the region, and changes
to a more nearly north-south direction north of New­
foundland.
This bears out the statement above, based
upon the joint plane attitudes, that the deforming
forces acted from the east.
-92-
2.
A pair of intersecting shear planes
of fracture correspond to the planes of maximum shear
of the strain ellipsoid.
The line of intersection of
a pair of shear joints corresponds to the intermediate
axis of the strain ellipsoid.
The direction of easiest
relief in the strain ellipsoid is in a plane perpen­
dicular to the intermediate axis.
In the main, the joint sets in this region
are characterized by high dip angles.
The lines of
intersection of pairs of joint sets of high angle dip
pitch very steeply.
It can be seen, therefore, that
the rocks of the region are jointed in a manner indi­
cating that, in general, the easiest relief was in a
more or less horizontal direction.
The implication
may be drawn that when the jointing developed, there
was a burden above the rocks now found at the surface
great enough to prevent relief upward.
3.
According to the analysis made in this
paper, there is no direct connection between the folia­
tion or bedding of the rocks and the jointing.
The
terms "strike set" and "dip set" may not be used in de­
scribing these joints, unless one permits a very loose
interpretation of the words parallelism and perpendicu.
larity.
This leads to the conclusion that, in general,
-93-
foliation or bedding did not control the jointing.
The jointing in the Skrunnemunk conglomerate
appears to form an exception to this generalization.
At
several places one joint system was parallel to the bed­
ding.
However, inasmuch as the joint patterns in the
Skunnemunk conglomerate parallel the patterns in the older
adjacent formations, it is not possible to consider the
joint systems in that formation as being developed sep­
arately and subsequently to those in the older formations.
It is concluded that the parallelism indicated is fortu­
itous.
4.
The minor faults that were observed were
always found to be parallel to one of the major joint
sets.
These faults all had dips of high angle.
None
of them could be classified as low angle thrust faults.
It is concluded that the local faulting in the area ac­
companied the jointing, and that both phenomena were
caused by the same forces.
The manner of the local
faulting may be considered as further evidence of the
difficulty of affording relief from horizontal stress
by vertical movement.
5.
The gneisses contain numerous injections.
It is probable that they were invaded by magmas at var­
-94-
ious times.
No evidence has been found that would indi­
cate any relationship between jointing and igneous in­
jection in the area.
The joint planes everywhere cut
the injections and the parent rock without change of di­
rection.
The invasions did not utilize planes of weak­
ness of joints that are now in existence as avenues of
escape.
The stresses developing out of the magmas did
not give rise to joints sufficiently strong to survive
subsequent diastrophic action.
The jointing, therefore,
was not associated directly with the magmatic invasions
of the area.
The jointing that is everywhere in the
gneisses is probably due to later disturbances than those
which found expression in the igneous injection of the
district.
6.
The attitudes of the joints as observed
in the slates bear out the statements of Bucher (r.19),
concerning jointing in brittle rocks.
The parent shales
from which the slates developed could not be classified
as brittle.
It is concluded that the shale had been
metamorphosed into slate before the jointing took place.
7.
Mathematical studies indicate that con­
fining pressures cannot of themselves affect the angle
between shear planes of fracture that face a deforming
compressive stress, unless the confining pressures are
-95-
so great as to cause the rock to become plastic.
In the
area under investigation, the evidence indicates that while
the rocks were deeply buried at the time of jointing, this
plastic stage was not attained.
It was also determined mathematically that the
accepted formulae concerning internal friction in rocks
cannot be applied when more than one stress is consider­
ed.
It is suggested that further study upon the subject
of internal friction is necessary before attempting to de­
termine the part it plays in the fracture of rocks.
In 1935, Parker (r. 83) wrote of the jointing
in central New York.
He proposed the theory that wide­
spread systematic high angle jointing was the result
of a, combination of compressive stress and tensile
stress acting at right angles to each other.
The writer
agrees with this theory as applying to the region studied,
provided an active but minor role is assigned to the ten­
sile stress.
The elongated pattern of the folds in the
district indicates some longitudinal stretching at right
angles to the forces that caused the folding.
The amount
of elongation in any one square mile was probably slight,
but was sufficient to provide for the increased volume
of the rock masses as they fractured.
-96-
8*
The gneisses and the Franklin limestone
of the region had undergone some folding in pre-Cam­
brian time, as evidenced by an angular unconformity
between the Hardyston quartzite and the underlying
older rocks.
It is also probable that the rocks
underwent further deformation during the Taconic orog­
eny.
The forces involved in these orogenies cannot be
looked upon as causes of the joint systems now visible,
as these systems include joints in rocks of Devonian
age.
It is hardly conceivable that the stresses that
caused the intense metamorphism of the pre-Silurian rocks
would have failed to develop some joints.
However, field
observations indicate that such joints must have been
obliterated in subsequent periods of deformation, or sub­
sequent joints have followed the earlier pattern to such
an extent that there is no recognizable difference between
joints in pre-Silurian, Silurian, and Devonian rocks.
The jointing very clearly developed at some
time following the development of the pronounced folds
and the partial metamorphism of the Devonian rocks.
Metamorphism and folding in this region are commonly
considered to have occurred during the Appalachian
Devolution at the close of Paleozoic time.
-97-
It is
logical, then, to consider the jointing and probably
the related faults as having occurred not earlier than
the closing stages of the Appalachian Devolution.
Be­
cause of the absence of late Paleozoic sediments in
this area, we must look beyond it for support of this
conclusion.
The fact that roughly similar structural
conditions, involving Mississippian and Pennsylvanian
strata, occur in the anthracite regions, supports but
does not entirely prove this conclusion.
9.
While no detailed study of the rocks of
the Newark basin was made, sufficient data were accumu­
lated to indicate that the patterns formed by the joints
in these rocks are in no way comparable to those of the
Highlands area.
The joints in the Newark sedimentary
rocks owe their origin to the tension set up by the
gentle warping and settling to which the rocks have been
subjected.
The crystalline rocks of the Highlands con­
tain no substantial number of joints that either through
fracture characteristics or alignment can be considered
as related to those of the sedimentary rocks of the
Newark series.
From this, it is concluded that the
greater proportion of the joints now visible in the
crystalline rocks of the Highlands were fully developed
before Newark (Triassic) time.
-98-
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