CHAPTER V

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_.

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.

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_.

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_).

=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.

=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.

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_.

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_.

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_).

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.

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.

=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

=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.

=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.

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.

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.