CONTORTIONS OF THE STRATA WITHIN THE ZONE OF FLOW
=The zones of fracture and flow.=—It is easy to think of the
atmosphere and the hydrosphere as each sustaining at any point the load
of the superincumbent material. At the sea level the weight of air
upon each square inch of surface is about fifteen pounds, whereas upon
the floor of the hydrosphere in the more profound deeps the load upon
the square inch must be measured in tons. Near the lithosphere surface
the rocks support by their strength the load of rock above them, but
at greater depths they are unable to do this, for the load bears upon
each portion of the rock with a pressure equivalent to the weight
of a rock column which extends upward to the surface. The average
specific gravity of rock is 2.7, and it is thus easy to calculate the
length of the inch square column which has a weight equivalent to the
crushing strength of any given rock. At the depth represented by the
length of such a column, rocks cannot yield to pressure by fracture,
for the opening of a crack implies that the rock upon either side is
strong enough to prevent the walls from closing. At this depth, rock
must therefore yield to pressure not by fracture, as it would at the
surface, but by flow after the manner of a liquid; and so the zone
below this critical level is referred to as the _zone of flow_.
[Illustration:
FIG. 19.—Two intersecting parallel series of fractures produced upon
each free surface of a prismatic block of stiff molders’ wax when
broken by compression from the ends (after Daubrée and Tresca).]
In contrast, the near-surface zone is called the _zone of fracture_.
But different rocks possess different strengths, and these are subject
to modifications from other conditions, such, for example, as the
proximity of an uncooled magma. The zone of flow is therefore joined
to the zone of fracture, not upon a definite surface, but in an
intermediate zone described as the _zone of fracture and flow_.
=Experiments which illustrate the fracture and flow of solid
bodies.=—A prismatic block prepared from stiff molders’ wax, if
crushed between the jaws of a testing machine, yields a system of
intersecting fractures which are perpendicular to the free surfaces
of the block and take two directions each inclined by half of a right
angle to the direction of compression (Fig. 19). This experiment
may illustrate the manner in which fractures are produced by the
compression within the zone of fracture of the lithosphere, as its core
continues to contract.
To reproduce the conditions within the zone of flow, it will be
necessary to load the lateral surfaces of the block instead of leaving
them unconstrained as in the above-described experiment. The experiment
is best devised as in Fig. 20. Here a series of layers having varying
degrees of rigidity is prepared from beeswax as a base, either
stiffened by admixture of varying proportions of plaster of Paris, or
weakened by the use of Venice turpentine. Such a series of layers may
represent rocks of as widely different characters as limestone and
shale. The load which is to represent superincumbent rock is supplied
in the experiment by a deep layer of shot.
[Illustration: FIG. 20.—Apparatus to illustrate the folding of strata
within the zone of flow (after Willis).]
When compression is applied to the layers from the ends, these normally
solid materials, instead of fracturing, are bent into a series of
folds. The stiffer, or more competent, layers are found to be less
contorted than are the weaker layers, particularly if the latter have
been protected under an arch of the more competent layer (pl. 2 A).
=The arches and troughs of the folded strata.=—Every series of folds
is made up of alternating arches and troughs. The arches of the strata
the geologist calls _anticlines_ or _anticlinal folds_, and the troughs
he calls _synclines_ or _synclinal folds_ (Fig. 21). When a stratum is
merely dropped in a bend to a lower level without producing a complete
arch or a complete trough, this half fold is termed a _monocline_.
[Illustration:
FIG. 21.—Diagrams representing _a_, an anticline; _b_, a syncline; and
_c_, a monocline.]
Any flexuring of the strata implies a reduction of their surface area,
or, considering a single section, a shortening. If the arches and
troughs are low and broad, the deformation of the strata is slight,
the shortening is comparatively small, and the folds are described as
_open_ (Fig. 22 _b_). If they be relatively both high and narrow, the
deformation is considerable, a larger amount of crustal shortening has
gone on, and the folds are described as _close_ (Fig. 22 _c_). This
closing up of the folds may continue until their sides have practically
the same slope, in which case they are said to be _isoclinal_ (Fig. 22
_d_).
[Illustration:
FIG. 22.—A comparison of folds to express increasing degrees of
crustal shortening or progressive deformation within the zone of flow:
_a_, stratum before folding; _b_, open folds; _c_, close folds; _d_,
isoclinal folds.]
=The elements of folds.=—Folds must always be thought of as having
extension in each of the three dimensions of space (Fig. 23), and not
as properly included within a single plane like the cross sections
which we so often use in illustration. A fold may be conceived of as
divided into equal parts by a plane which passes along the middle of
either the arch or the trough, and is called the _axial plane_. The
line in which this plane intersects the arch or the trough is the
_axis_, which may be called the _crestline_ in an anticline, and the
_troughline_ in a syncline.
In the case of many open folds the axis is practically horizontal, but
in more complexly folded regions this is seldom true. The departure of
the axis from the horizontal is called the _pitch_, and folds of this
type are described as _pitching folds_ or _plunging folds_. The axis
is in reality in these cases thrown into a series of undulations or
“longitudinal folds”, and hence pitch will vary along the axis.
[Illustration: FIG. 23.—Anticlinal and synclinal folds in strata
(after Willis).]
[Illustration:
FIG. 24.—Diagrams to illustrate the different shapes of rock folds.]
=The shapes of rock folds.=—By the axial plane each fold is divided
into two parts which are called its _limbs_, which may have either the
same or different average inclinations. To describe now the shapes
of rock folds and not the degree of compression of the district,
some additional terms are necessary. Anticlines or synclines whose
limbs have about the same inclinations are known as _upright_ or
_symmetrical folds_. The axial plane of the symmetrical fold is
vertical (Fig. 24). If this plane is inclined to the vertical, the
folds are _unsymmetrical_. So soon as the steeper of the two limbs has
passed the vertical position and inclines in the same direction as the
flatter limb, the fold is said to be _overturned_. The departure from
symmetry may go so far that the axial plane of the fold lies at a very
flat angle, and the fold is then said to be _recumbent_. The observant
traveler by train along any of the routes which enter the Alps may from
his car window find illustrations of most of these types of rock folds,
as he may also, though generally less easily, in passing through the
Appalachian Mountains.
[Illustration: FIG. 25.—Secondary and tertiary flexures superimposed
upon the primary ones.]
In regions which have been closely folded the larger flexures of
the strata may be found with folds of a smaller order of magnitude
superimposed upon them, and these in turn may show crumplings of still
lower orders. It has been found that the folds of the smaller orders
of magnitude possess the shapes of the larger flexures, and much is
therefore to be learned from their careful study (Fig. 25). It is also
quite generally discovered that parallel planes of ready parting,
which are described as _rock cleavage_, take their course parallel to
the axial plane within each minor fold. As was long ago shown by the
pioneer British geologists, these planes of cleavage are essentially
parallel and follow the fold axes throughout large areas.
┌────────────────────────────────────────────────────────────────────────┐
│ PLATE 2. │
│ │
│ [Illustration: _A._ Layers compressed in experiments and showing the │
│ effect of a competent layer in the process of folding (after Willis).] │
│ │
│ [Illustration: _B._ Experimental production of a series of parallel │
│ thrusts within closely folded strata (after Willis).] │
│ │
│ [Illustration: _C._ Apparatus to illustrate shearing action within the │
│ overturned limb of a fold.] │
└────────────────────────────────────────────────────────────────────────┘
=The overthrust fold.=—Whenever a stratum is bent, there is a tendency
for its particles to be separated upon the convex side of the bend,
at the same time that those upon the concave side are crowded closer
together—there is tension in the former case and compression in the
latter (Fig. 26). Within an unsymmetrical or an overturned fold, the
peculiar distortions in the different sections of the stratum are less
simple and are best illustrated by pl. 2 C. This apparatus shows two
similar piles of paper sheets, upon the edges of each of which a series
of circles has been drawn. When now one of the piles is bent into an
unsymmetrical fold, it is seen that through an accommodation by the
paper sheets sliding each over its neighbor large distortions of the
circles have occurred. In that steeper limb which with closer folding
will be overturned the circles have been drawn out into long and narrow
ellipses, and this indicates that those rock particles which before the
bending were included in the circle have been moved past each other in
the manner of the blades of a pair of shears. Such extreme “shearing”
action is thus localized in the underturned limb of the fold, and a
time must come with continuation of the compression when the fold
will rupture at this critical place along a plane parallel to the
longest axis of the ellipses or nearly parallel to the axial plane of
the anticline. Such structures probably occur in the zone of combined
fracture and flow, up into which the beds are forced in cases of close
compression. Relief thus being found upon this plane of fracture,
the upper portion of the fold will now ride over the lower, and the
displacement is described as a _thrust_ or _overthrust_.
[Illustration:
FIG. 26.—A bent stratum to illustrate tension upon the convex and
compression upon the concave side (after Van Hise).]
In the long series of experiments conducted by Mr. Bailey Willis of the
United States Geological Survey, all the stages between the overturned
fold and the overthrust fold were reproduced. Where a series of folds
was closely compressed, a parallel series of thrusts developed (pl. 2
B), so that a series of slices cutting across neighboring strata was
slid in succession, each over the other, like the scales upon a fish
or the shingles upon a roof. Quite remarkable structures of this kind
have been discovered in rocks of such closely folded districts as the
Northwest Highlands of Scotland, where the overriding is measured in
miles. Near the thrust planes the rocks show a crushing of the grains,
and the planes themselves are sometimes corrugated and polished by the
movement.
=Restoration of mutilated folds.=—Since flexuring of the rocks takes
place within the zone of flow at a distance of several miles below the
earth’s surface, it is quite obvious that the results of the process
can be studied only after some thousands of feet of superincumbent
strata have been removed. We are a little later to see by what
processes this lowering of the surface is accomplished, but for the
present it may be sufficient to accept the fact, realizing that before
foldings in the strata can reach the surface, they must have passed
through the upper zone of fracture.
It might perhaps be supposed that the anticlines would appear as
the mountains upon the surface, and occasionally this is true; as,
for example, in the folded Jura Mountains of western Europe. More
generally, the mountains have a synclinal structure and the valleys an
anticlinal one; but as no general rule can be applied, it is necessary
to make a restoration of the truncated folds in each district before
their character can be known.
=The geological map and section.=—The earth’s surface is in most
regions in large part covered with soil or with other incoherent rock
material, so that over considerable areas the hard rocks are hidden
from view. Each locality at which the rock is found at the earth’s
surface “in place” is described as an _outcropping_ or _exposure_. In
a study of the region each such exposure must be examined to determine
the nature of the rock, especially for the purpose of correlation
with neighboring exposures, and, in addition, both the probable
direction in which it is continued along the surface—the _strike_—and
the inclination of its beds—the _dip_. If the outcroppings are
sufficiently numerous, and rock type, strike and dip, may all be
determined, the folds of the district may be restored with almost as
much accuracy as though their curves were everywhere exposed to view.
A cross section through the surface which represents the observed
outcrops with their inclinations and the assumed intermediate strata
in their probable attitudes is described as a _geological section_
(Fig. 27). A map upon which the data have been entered in their correct
locations, either with or without assumptions concerning the covered
areas, is known as a _geological map_.
[Illustration: FIG. 27.—A geological section based upon observations
at outcrops, but with the truncated arches restored.]
If the axes of folds are absolutely horizontal, and the surface of
the earth be represented as a plain, the lines of intersection of the
truncated strata with the ground, or with any horizontal surface, will
give the directions of continuation of the individual strata. This
strike direction is usually determined at each exposure by use of a
compass provided with a spirit level. When that edge of the leveled
compass which is parallel to the north-south line upon the dial is held
against the sloping rock stratum, the angle of strike is measured in
degrees by the compass needle. If the cardinal directions have been
placed in their correct positions upon the compass dial, the needle
will point to the northwest when the strike is northeast, and _vice
versa_ (Fig. 28 _a_). Upon the geologist’s compass it is therefore
customary to reverse the initials which represent the east and west
directions, in order that the correct strike may be read directly from
the dial (Fig. 28 _b_).
[Illustration:
FIG. 28.—Diagram to illustrate the manner of determining the strike
of rock beds at an outcropping. _a_, a compass which has the cardinal
directions in their natural positions; _b_, a compass with the east
and west initials reversed upon the dial; _c_, home-made clinometer in
position to determine the dip.]
By the dip is meant the inclination of the stratum at any exposure, and
this must obviously be measured in a vertical plane along the steepest
line in the bedding plane. The dip angle is always referred to a
horizontal plane, and hence vertical beds have a dip of 90°. The device
for measuring this angle of dip, the _clinometer_, is merely a simple
pendulum which serves as an indicator and is centered at the corner of
a graduated quadrant. A home-made variety is easily constructed from a
square piece of board and an attached paper quadrant (Fig. 28 _c_), but
the geologist’s compass is always provided with a clinometer attachment
to the dial.
[Illustration:
FIG. 29.—Diagram to show the use of T symbols to indicate the dip and
strike of outcroppings.]
Since the strike is the intersection of the bedding plane with
a horizontal surface, and the dip is the intersection with that
particular vertical plane which gives the steepest inclination, the
strike and dip are perpendicular to each other. To represent them upon
maps, it is more or less customary to use the so-called T symbols, the
top of the T giving the direction of the strike and the shank that of
the dip. If meridians are drawn upon the map, the direction or attitude
of the T can be found by the use of a simple protractor; and when
entered upon the map, the exact angle of the strike may be supplied by
a figure near the top of the T, and the dip angle by a figure at the
end of the shank. It is the custom, also, to make the length of the
shank inversely proportional to the steepness of the dip, so that in
a broad way the attitudes of the strata may be taken in at a glance
(Fig. 29). It is further of advantage to make the top of the T a double
line, so that some symbol or color may show the correlations of the
different exposures. To illustrate, in Fig. 29, the symbol marked _a_
represents an outcrop of limestone, the strike of which is 50° east
of north (N. 50° E.), and the dip of which is 45° southeast. In the
same figure _b_ represents a shale outcrop in horizontal beds, which
have in consequence a universal strike and a dip of 0°. An exposure of
limestone in vertical beds which strike N. 60° E. is shown at _c_, etc.
[Illustration:
FIG. 30.—Diagram to show how the thickness of a formation may be
obtained from the angle of the dip and the width of the exposures.]
=Measurement of the thickness of formations.=—When formations still
lie in horizontal beds, we may sometimes learn their thickness
directly either from the depth of borings to the underlying rock, or
by measurements upon steep cañon walls. If the beds stand vertically,
the matter is exceedingly simple, for in this case the thickness is the
width of the outcrops of the formation between the beds which bound it
upon either side. In the general case, in which the beds are neither
horizontal nor vertical, the thickness must be obtained indirectly from
the width of the exposures and the angle of the dip. The factor by
which the exposure width must be multiplied is known as the sine of the
dip angle (Fig. 30), which is given with sufficient accuracy for most
purposes in the following table. It is obvious that in order to obtain
the full thickness of a formation it is necessary to measure from the
contact with the adjacent formation upon the one side to a similar
contact with the nearest formation upon the other.
_Natural Sines_
0° .00 35° .57 70° .94
5° .09 40° .64 75° .97
10° .17 45° .71 80° .98
15° .26 50° .77 85° 1.00
20° .34 55° .82 90° 1.00
25° .42 60° .87
30° .50 65° .91
[Illustration: FIG. 31.—Combined surface and sectional views of a
plunging anticline (after Willis).]
[Illustration: FIG. 32.—Combined surface and sectional views of a
plunging syncline (after Willis).]
=The detection of plunging folds.=—When the axis of a fold is
horizontal, its outcrops upon a plain will continue to have the same
strike until the formation comes to an end. Upon a generally level
surface, therefore, any regular progressive variation in the strike
direction is an indication that the folds have a plunging or pitching
character. Many serious mistakes of interpretation have been made
because of a failure to recognize this evidence of plunging folds. The
way in which the strikes are progressively modified will be made clear
by the diagrams of Figs. 31 and 32, the first representing a pitching
anticline and the second a pitching syncline. In both these reciprocal
cases the strikes of the beds undergo the same changes, and the dip
directions serve to distinguish which of the two structures is present
in a given case. There is, however, one further difference in that the
hard layers of the plunging anticline, where they disappear below
the surface in the axis, will present a domed surface sloping forward
like the back of a whale as it rises above the surface of the sea.
Plunging folds in series will thus appear in the topography as a series
of sharply zigzagging ranges at those localities where the harder
layers intersect the surface. Such features are encountered in eastern
Pennsylvania, where the hard formations of the Appalachian Mountain
system plunge northeastward under the later formations. The pitch of
the larger fold is often disclosed by that of the minor puckerings
superimposed upon it.
[Illustration:
FIG. 33.—Unconformity between a lower and an upper series of beds upon
the coast of California. Note how the hard layer stands in relief upon
the connecting surface (after Fairbanks).]
=The meaning of an unconformity.=—The rock beds, which are deposited
one above the other during a transgression of the sea, are usually
parallel and thus represent a continuous process of deposition. Such
beds are said to be _conformable_. Where, upon the other hand, two
series of deposits which are not parallel to each other are separated
by a break, they are said to form _unconformable_ series, and the break
or surface of junction is an _unconformity_ (Fig. 33).
Here it is evident that the sediments which compose the lower series
of beds have been folded in the zone of flow, though the upper series
has evidently escaped this vicissitude. Furthermore, the surface which
delimits the lower series from the upper is somewhat irregular and
shows a hard layer standing in relief, as it would if it had opposed
greater resistance to the attacks of the atmosphere upon it.
[Illustration:
FIG. 34.—Series of diagrams to illustrate in succession the episodes
involved in the historical development of an angular unconformity. The
vertical arrows indicate direction of movement of the land, and the
horizontal arrows the direction of shore migration.]
In reality, an unconformity between formations must be interpreted to
mean that the lower series is not only older than the upper, as shown
by the order of superposition, but that the time of its deposition was
separated from that of the upper by a hiatus in which important changes
took place in the lower series. The stages or episodes in the history
of the beds represented in Fig. 33 may be read as follows (see Fig. 34
_a-e_):—
(_a_) Deposition of the lower series during a transgression of the sea.
(_b_) Continued subsidence and burial of the lower series beneath
overlying sediments, and flexuring in the zone of flow.
(_c_) Elevation of the combined deposits to and far above sea level and
removal by erosion of vast thicknesses of the upper sediments.
(_d_) A new subsidence of the truncated lower series and deposition of
the upper series across its eroded surface.
(_e_) A new elevation of the double series to its present position
above sea level.
[Illustration:
FIG. 35.—Types of deceptive or erosional unconformities.]
From this succession of episodes it is seen that a break of this
kind between two series of deposits involves a double oscillation of
subsidence followed by elevation—a large depression followed by a
large elevation, a smaller subsidence followed by elevation. The time
interval which must have been represented by these repeated operations
is so vast as at first to stagger the mind in contemplating it. When,
as in this instance, the dips of the lower series of beds differ from
those of the upper, we have to do with an _angular unconformity_.
It may be, however, that the lower series was not so far depressed
as to enter the zone of flow, and that its beds meet those of the
upper series with apparent conformity. Such an unconformity is often
extremely difficult to recognize, and it is described as a _deceptive_
or _erosional unconformity_.
With a deceptive unconformity the clew to its real nature is usually
some fact which indicates that the lower series of sediments had been
raised above the level of the sea before the upper series was deposited
upon it. This may be apparent either in the irregularity of the surface
on which the two series are joined, in some evidence of the action of
waves such as would be furnished by a basal conglomerate in the upper
series, or some indication of different resistance of different rocks
of the lower series to attacks of the atmosphere upon them (Figs. 33
and 35 _a-c_).
In most cases, at least, the lowest member of the upper series will
be a different type of rock from the uppermost member of the lower
series, hence the frequent occurrence of the discordant cross bedding
in sandstone should not deceive even the novice into the assumption of
an unconformity.
READING REFERENCES TO CHAPTER V
The zones of fracture and flow:—
C. R. VAN HISE. Principles of North American Precambrian Geology, 16th
Ann. Rept. U.S. Geol. Surv., 1895, Pt. I, pp. 581-603.
BAILEY WILLIS. Mechanics of Appalachian Structure, 13th Ann. Rept.
U.S. Geol. Surv., 1893, Pt. II, pp. 217-253.
A. DAUBRÉE. Études Synthétiques de Géologie Expérimentale. Paris,
1879, pp. 306-328, pl. II.
W. PRINZ. Quelques remarques générales à propos de l’essai de carte
tectonique de la belgique, etc., Bull. Soc. Belge Geol., vol. 18,
1904, p. 143, pl. V.
Analysis of folds:—
VAN HISE and WILLIS as above; DE MARGERIE et HEIM; Les dislocations de
l’écorce terrestre (in French and German languages). Zurich, 1888.
Geological maps:—
WM. H. HOBBS. The Mapping of the Crystalline Schists, Jour. Geol.,
vol. 10, 1902, pp. 780-792, 858-890.