THE ARCHITECTURE OF THE FRACTURED SUPERSTRUCTURE
[Illustration:
FIG. 36.—A set of master joints developed in shale upon the shores of
Cayuga Lake near Ithaca, New York (after U. S. G. S.).]
[Illustration:
FIG. 37.—Diagram to show how sets of master joints differing in
direction by half a right angle may abruptly replace each other.]
[Illustration:
FIG. 38.—Diagram to show the different combinations of the series
composing two double sets of master joints, and in _a_, _a_, _a_
additional disorderly fractures.]
=The system of the fractures.=—In referring to experiments made upon
the fracture of solid blocks under compression (p. 41), it was shown
that two series of parallel fractures develop perpendicular to each
free surface of the block, and that these series are each of them
inclined by half of a right angle to the direction of compression, and
thus perpendicular to each other. The fragments into which a block
with one free surface would thus tend to be divided should be square
prisms perpendicular to the free surface. It would be interesting, if
it were practicable, to learn from experiment how these prisms would be
further fractured by a continuation of the compression. From mechanical
considerations involving the resolution of forces with reference to
the ready-formed fractures, it seems probable that the next series of
fractures to form would bisect the angles of the first double series
or set. Wherever rocks are found exposed in their original attitudes,
they are, in fact, seen to be intersected by two parallel series of
fractures which are perpendicular to the earth’s surface and to each
other and are described as _joints_. In many cases more than two series
of such fractures are found, yet even in these cases two more perfectly
developed series are prominent and almost exactly perpendicular to each
other as well as to the earth’s surface. This omnipresent double series
or _set_ of joints is the well-known set of _master joints_, and very
often it is found developed practically alone (Fig. 36). Over large
areas, the direction of the set of master joints may remain practically
constant, or this set may quite suddenly give place to a similar set
which is, however, turned through half a right angle from the first
(Fig. 37). Not infrequently two such sets of master joints are found
together bisecting each other’s angles within the same rocks, and to
them are sometimes added additional though less perfect series of
joint planes.
Studied throughout a considerable district, the various series which
make up these two sets of master joints may be seen locally developed
in different combinations as well as in association with additional
fissure planes which are not easily reduced to any simple law of
arrangement (Fig. 38 _a_, _a_, _a_). Only rarely are regular joint
series observed which do not stand perpendicular to the original
attitude of the rock beds. In a few localities, however, rectangular
joint sets have been discovered which divide the rock into prisms
parallel to the earth’s surface and with the joint series inclined
to it each by half a right angle. Where the rock beds have been much
disturbed, the complex of joints may be such as to defy all attempts
at orderly arrangement.
[Illustration: FIG. 39.—View on the shore at Holstensborg, West
Greenland, to show the subequal spacing of the joints (after Kornerup).]
[Illustration:
FIG. 40.—View of an exposed hillside in Iceland upon which the snow
collected in crannies along the joints brings out to advantage both
the larger and the smaller intervals of the joint system (after
Thoroddsen).]
=The space intervals of joints.=—The same kind of subequal spacing
which characterizes the fractures near the surface of the block in
Daubrée’s experiment (Fig. 19, p. 41) is found simulated by the rock
joints (Fig. 39). Such unit intervals between fractures may be grouped
together into larger units which are separated by fractures of unusual
perfection. We may think of such larger space units as having the
smaller ones superimposed upon them (Fig. 40).
=The displacements upon joints—faults.=—In the vast majority of
cases, the joint fractures when carefully examined betray no evidence
of any appreciable movement of the two walls upon each other. Generally
the rock layers are seen to cross the joints without apparent
displacement. Joints are therefore planes of disjunction only, and not
planes of displacement.
[Illustration:
FIG. 41.—Faulted blocks of basalt divided by joints near Woodbury,
Connecticut. To show the structure of the rock, some of the foliage has
been removed in preparing the sketch from a photograph.]
Within many districts, however, a displacement may be seen to have
occurred upon certain of the joint planes, and these are then described
as _faults_. Such displacements of necessity imply a differential
movement of sections or blocks of the earth’s crust, the so-called
_orographic blocks_, which are bounded by the joint planes and play
individual rôles in the movement. A simple case of such displacements
in rocks intersected by a single set of master joints is represented
in the model of plate 4 C. The most prominent fault represented by
this model runs lengthwise through the middle, and the displacement
which is measured upon it not only varies between wide limits, but is
marked by abrupt changes at the margins of the larger blocks. This
vertical displacement upon the fault is called its _throw_. Though not
illustrated by the model, horizontal displacements may likewise occur,
and these will be more fully discussed when the subject of earthquakes
is considered in the following chapter. An actual example of blocks
displaced by vertical adjustment is represented in Fig. 41, a simple
type of faulting which has taken place in rocks but slightly disturbed
from their original attitude, but intersected by a relatively simple
system of master joints. In those regions where the beds have been
folded and perhaps overthrust before their elevation into the zone
of fracture, and which are further intersected by disorderly fissure
planes, the results are far more complex. In such cases the planes of
individual displacement may not be vertical, though they are generally
steeper than 45°. For their description it is necessary to make use
of additional technical terms (Fig. 42). The inclination of a sloping
fault plane measured against the vertical is called the _hade_ of
the fault. The _total displacement_ is measured along the plane of
the fault from a point upon one limb to the point from which it was
separated in the other. The additional terms are made sufficiently
clear by the diagram.
[Illustration:
FIG. 42.—A fault in previously disturbed strata. _AB_, displacement;
_AC_, throw; _BD_, stratigraphic throw; _BC_, heave; angle _CAB_, hade.]
=Methods of detecting faults.=—The first effect of a fault is
usually to produce a crack at the surface of the earth; and, provided
there is a vertical displacement or throw, an escarpment which rises
upon the upthrown side of the fault. In general it may be said that
escarpments which appear at the earth’s surface as plane surfaces
probably represent planes of fracture, though not necessarily planes
of faulting. In many cases the actual displacements lie buried under
loose rock débris near to and paralleling the escarpment, and in some
cases as a result of the erosional processes working upon alternately
hard and soft layers of rock, the escarpment may later appear upon the
downthrown side or limb of the fault (Fig. 43). As an illustration of a
fault escarpment, the façade of El Capitan and many other rock faces of
the Yosemite valley may be instanced.
[Illustration:
FIG. 43.—Diagrams to show how an escarpment originally on the upthrown
side of the fault may, through erosion, appear upon the downthrown
side.]
[Illustration:
FIG. 44.—A fault plane exhibiting “drag.” The opening is artificial
(after Scott).]
When we have further studied the erosional processes at the earth’s
surface, it will be appreciated that faults tend to quickly bury
themselves from sight, whereas fold structures will long remain in
evidence. Many faults will thus be overlooked, and too great weight
is likely to be ascribed to the folds in accounting for the existing
attitudes and positions of the rock masses. Faults must therefore be
sought out if mistakes of interpretation are to be avoided.
The most satisfactory evidence of a fault is the discovery of a
rock bed which may be easily identified, and which is actually seen
displaced on a plane of fracture which intersects it (Fig. 42, p. 59).
When such an easily recognizable layer is not to be found, the plane
of displacement may perhaps be discovered as a narrow zone composed of
angular fragments of the rock cemented together by minerals which form
out of solution in water. Such a fractured rock zone which follows a
plane of faulting is a _fault breccia_. If the fault breccia, or vein
rock, is much stronger than the rock on either side, it may eventually
stand in relief at the surface like a dike or wall. At other times the
displacement produces little fracture of the walls, but they slide over
each other in such a manner as to yield either a smoothly corrugated or
an evenly polished surface which is described as “slickensides.” It may
be, however, that during the movement either one or both of the walls
have “dragged”, and so are curled back in the immediate neighborhood of
the fault plane (Fig. 44).
When, as is quite generally the case, the actual plane of displacement
of a fault is not open to inspection, the movement may be proven by
the observation of abrupt, as contrasted with gradual, changes in the
strikes and dips of neighboring exposures (Fig. 45); or by noting
that some easily recognized formation has been sharply offset in its
outcrops (Fig. 46).
[Illustration:
FIG. 45.—Map to show how a fault may be indicated in abrupt changes of
the strike and dip of neighboring exposures.]
[Illustration:
FIG. 46.—A series of parallel faults indicated by successive offsets
in the course of an easily recognizable rock formation.]
There are in addition many indications rather than proofs of the
presence of faults, which must be taken account of in every general
study of the geology of a district. Thus the outcrops of all
neighboring formations may terminate abruptly upon a straight line
which intersects all alike. Deep-seated fissure springs may be aligned
in a striking manner, and so indicate the course of a prominent
fracture, though not necessarily of a fault. Much the same may be said
of the dikes of cooled magma which have been injected along preëxisting
fractures.
=The base of the geological map.=—Modern topographic maps form an
important part of the library of the serious student of physiography;
they are the gazetteer of this branch of science. Every civilized
nation has to-day either completed a topographic atlas of its
territory, or it is vigorously prosecuting a survey to furnish maps
which represent the relief with some detail, and publishing the results
in the form of an atlas of quadrangles. Thus a relief map will erelong
be obtainable of any part of the civilized world, and may be purchased
in separate sections. Nowhere is this work being taken up with greater
vigor than in the United States, where a vast domain representing
every type of topographic peculiarity is being attacked from many
centers. Here and elsewhere the relief of the land is being expressed
by so-called contours or lines of equal altitude upon the earth’s
surface. It is as though a series of horizontal planes, separated by
uniform intervals of 20 or 40 or 100 feet, had been made to intersect
the surface, and the intersection curves, after consecutive numeration,
had been dropped into a single plane for printing.
Where the slopes are steep, the contour lines in the topographic map
will appear crowded together and so produce a deep shade upon the map;
whereas with relatively flat surfaces white patches will stand out
prominently upon the map. More and more the topographic map is coming
into use, and for the student of nature in particular it is important
to acquire facility in interpreting the relief from the topographic
map. To further this end, a special model has been devised, and its use
is described in appendix C. Usually before any satisfactory geological
map can be prepared, a contoured topographic map of the district to be
studied must be available.
=The field map and the areal geological map.=—As the atlas of
topographic maps is the physiographic gazetteer, so geological maps
together constitute the reference dictionary of descriptive geology.
Not only are topographic maps of many districts now generally
available, but more and more it has become the policy of governments to
supply geological maps in the same quadrangle form which is the unit
of the topographic map. The geological map is, however, a complex of
so many conventional symbols, that without some practical experience
in the actual preparation of one, it is exceedingly difficult for
the student to comprehend its significance. A modern geological map
is usually a rectangular sheet printed in color, upon which are many
irregular areas of individual hue joined to each other like the parts
of a child’s picture puzzle.
The colored areas upon the geological map are each supposed to indicate
where a certain rock type or formation lies immediately below the
surface, and this distribution represents the best judgment of the
geologist who, after a study of the district, has prepared the map.
Unfortunately the conventions in use are such that his observation and
his theory have been hopelessly intermingled in the finished product.
Armed with the geological map, the student who visits the district
finds spread out before him, it may be, a landscape of hill and valley,
of green forest and brown farming land, which is as different as may be
from the colored puzzle which he holds in his hand. Hidden under the
farm vegetation or masked by the woods are scattered outcroppings of
rock which have been the basis of the geologist’s judgment in preparing
the map. Experience shows that in order to bridge the wide gap between
the geology in the landscape and the patches of color upon the map
something more than mere examination of the colored sheet is necessary.
We shall therefore describe, with the aid of laboratory models, the
various stages necessary to the preparation of a geological map, and
every student should be advised to follow this by practical study of
some small area where rocks are found in outcrop.
Though the published _areal geological map_ represents both fact and
theory, the map maker retains an unpublished _field map_ or map of
observations, upon which the final map has been based. This field map
shows the location of each outcrop that has been studied, with a record
of the kind of rock and of such observations as strike, dip, and pitch.
Our task will therefore be to prepare: (1) a field map; (2) an areal
geological map; and (3) some typical geological sections.
=Laboratory models for the study of geological maps.=—In order to
represent in the laboratory the disposition of rock outcrops in the
field, special laboratory tables are prepared with removable covers
and with fixed tops, which are divided into squares numbered like the
township sections of the national domain (Fig. 47). To represent the
rock outcrops, blocks are prepared which may be fixed in any desired
position by fitting a pin into a small augur hole bored through the
table. The outcrop blocks for the sedimentary rock types are so
constructed as to show the strike and dip of the beds. (See Appendix D.)
[Illustration: FIG. 47.—Field map prepared from a laboratory table.]
=The method of preparing the map.=—To prepare the map, use is made of
a geological compass with clinometer attachment, a protractor, and a
map base divided into sections like the top of the table, and on the
scale of one inch to the foot. Each exposure represented upon the table
is “visited” and then located upon the base map in its proper position
and attitude. The result is the field map (Fig. 47), which thus
represents the facts only, unless there have been uncertainties in the
correlation of exposures or in determining the position of the bedding
plane.
[Illustration: FIG. 48.—Areal geological map constructed from the
field map of Fig. 47, with two selected geological sections.]
To prepare the areal geological map from the field map, it is first
necessary to fix the _boundaries_ which separate formations at the
surface; and now perhaps for the first time it is realized how large
an element of uncertainty may enter if the exposures were widely
separated. It is clear that no two persons will draw these lines in
the same positions throughout, though certain portions of them—where
the facts are more nearly adequate—may correspond. In Fig. 48 is
represented the areal geological map constructed from the field map,
with the doubtful area at one side left blank.
Some conclusions from this map may now be profitably considered. The
complexly folded sandstone formation at the left of the map appears
as the oldest member represented, since its area has been cut through
by the intrusive granite which does not intrude other formations,
and is unconformably overlaid by the limestone and its basal layer
of conglomerate. The limestone in turn is unconformably overlaid by
the merely tilted sandstone beds at the right of the map. These three
sedimentary formations clearly represent decreasing amounts of close
folding, from which it is clear that each earlier formation has passed
through an episode not shared by that of next younger age. Of the
other intrusive rocks, the dike of porphyry is younger than all the
other formations, with the possible exception of the upper sandstone.
Offsetting of the formations has disclosed the course of a fault, and
from its relations to the dikes we may learn that of these the porphyry
is younger and the basalt older than the date of the faulting.
The dashed lines upon the map (_AB_ and _CD_) have been selected as
appropriate lines along which to construct geological sections (Fig.
48, below map), and from these sections the _exposed_ thicknesses of
the different formations may be calculated. In one instance only,
that of the conglomerate, can we be sure that this exposed thickness
measures the entire formation.
=Fold _versus_ fault topography.=—The more resistant or “stronger”
rock beds, as regards attacks of the atmosphere, in the course of time
come to stand in relief, separated by depressions which overlie the
“weaker” formations. Simple open folds which are not plunging exercise
an influence upon topography by producing generally long and straight
ridges. More complex flexures, since they generally plunge, make
themselves apparent by features which in the map are represented by
curves. Fracture structures, and especially block displacements, are
differentiated from these curving features by the dominance of straight
or nearly rectilinear lines upon the map. The effect of erosion is to
reduce the asperity of features and to mold them with flowing curves.
The fracture structures are for this reason much more likely to be
overlooked, and if they are not to elude the observer, they must be
sought out with care. Fold and fracture structures may both be revealed
upon the same map.
READING REFERENCES TO CHAPTER VI
Joint systems:—
JOHN PHILLIPS. Observations made in the Neighborhood of Ferrybridge in
the Years 1826-1828, Phil. Mag., 2d ser., vol. 4, 1828, pp. 401-409;
Illustrations of the geology of Yorkshire, Pt. II, The Limestone
District. London, 1836, pp. 90-98.
SAMUEL HAUGHTON. On the Physical Structure of the Old Red Sandstone of
the County of Waterford, considered with reference to cleavage, joint
surfaces, and faults, Trans. Roy. Soc. London, vol. 148, 1858, pp.
333-348.
W. C. BRÖGGER. Spaltenverwerfungen in der Gegend Langesund-Skien, Nyt
Magazin for Naturvidernskaberne, vol. 28, 1884, pp. 253-419.
WM. H. HOBBS. The Newark System of the Pomperaug Valley, Connecticut,
21st Ann. Rept. U. S. Geol. Surv., Pt. III, 1901, pp. 85-143.
Geological map:—
WM. H. HOBBS. The Interpretation of Geological Maps, School Science
and Mathematics, vol. 9, 1909, pp. 644-653.