Part 12

The arches of the strata are rarely distinctly indicated in the topography, but must be studied where the ground has been partly dissected, as in cliffs, gorges, quarries, etc. They are also, as a rule, far more irregular and complex than they are usually conceived or represented. The wrinkles of our clothing are often better illustrations of rock-folds than the models and diagrams used for that purpose. This becomes self-evident when we reflect that the earth’s crust is exceedingly heterogeneous in composition and structure, and must, therefore, yield unequally to the unequal strains imposed upon it.

The folds or undulations of the strata may be profitably compared with water-waves. In fact, the comparison is so close that they have been not inaptly called rock-waves. Folds, like waves, unless very large, rarely continue for any great distance, but die out and are replaced by others, giving rise to the _en echelon_ or step-like arrangement. The plan of both a wave and a fold is a more or less elongated ellipse, each stratum in a fold being semi-ellipsoidal or boat-shaped. In other words, a normal fold is an elongated mound of concentric strata, being highest at the centre, sloping very gradually toward the ends, and much more abruptly toward the sides.

The imaginary line passing longitudinally through a fold, about which the strata appear to be bent, is the _axis_; and the plane lying midway between the two sides of a fold and including the axis is the _axial plane_. The two principal kinds of folds are the _anticline_ (Fig. 18, _A_), where the strata dip away from the axis; and the _syncline_ (Fig. 18, _B_), where they dip toward the axis. They are commonly, but not always, correlative, like hill and valley.

Rock-folds are of all sizes, from almost microscopic wrinkles to great arches miles in length and breadth, and thousands of feet in height. The smaller folds, or such as may be seen in hand specimens and even in considerable blocks of stone, are commonly called contortions, and it is interesting to observe that they are, in nearly everything except size, precisely like the large folds, so that they answer admirably as geological models. Large folds, however, are almost necessarily curves, but contortions are frequently angular (Fig. 19). With folds, as with waves, the small undulations are borne upon the large ones; but the contortions are not uniformly distributed. An inspection of Fig. 18 shows that when the rocks are folded they must be in a state of tension on the anticlines (_A_), and in a state of compression in the synclines (_B_), and the latter is evidently the normal position of the puckerings or contortions of the strata, as shown in Fig. 20. Contortions are also most commonly found in thin-bedded, flexible rocks, such as shales and schists. And when we find them in hard, rigid rocks, like gneiss and limestone, it must mean either that the structure was developed with extreme slowness, or that the rock was more flexible then and possibly plastic.

It is very interesting to notice the relations of anticlinal and synclinal folds to the agents of erosion. At the time the folds are made, the anticlinals, of course, are ridges, and the synclinals, valleys, and this relation sometimes continues, as shown in Fig. 21; but we have seen that the rocks in the trough of the synclinal are compressed and compacted, _i.e._, made more capable of resisting erosion, while those on the crest of the anticlinal are stretched and broken, _i.e._, made more susceptible of erosion. The consequence is that the anticlinals are usually worn away very much faster than the synclinals; so much faster that in many cases the topographic features are completely transposed, and in place of anticlinal ridges and synclinal valleys (Fig. 21) we find synclinal ridges and anticlinal valleys (Fig. 22).

Besides the anticlinal and synclinal folds already explained, there are folds that slope in only one direction, one-sided or _monoclinal_ folds (Fig. 23). Anticlinal and synclinal folds are _symmetrical_ when the dip or slope of the strata is the same on both sides and the axial plane is vertical. The great majority of folds, however, are _unsymmetrical_, the opposite slopes being unequal, and the axial planes inclined to the vertical (Fig. 24, _A_). This means that the compressing or plicating force has been greater from one side than from the other, as indicated by the arrows. It acted with the greatest intensity on the side of the gentler slope, the tendency evidently having been to crowd or tip the fold over in the direction of the steep slope. When the steep slope approaches the vertical, this tendency is almost unresisted, and when it passes the vertical, gravitation assists in overturning the fold (Fig. 24, _B_). Such highly unsymmetrical folds, including all cases where the two sides of the fold slope in the same direction, are described as _overturned_ or _inverted_, although the latter term is not strictly applicable to the entire fold, but only to the strata composing the under or lee side of it. Fig. 24, _B_, shows that these beds are completely inverted, the older, as the figures indicate, lying conformably upon the newer. This inversion is one of the most important features of folded strata, and it has led to many mistakes in determining their order of succession. In the great mountain-chains, especially, it is exhibited on the grandest scale, great groups of strata being folded over and over each other as we might fold carpets. An inverted stratum is like a flattened S or Z, and may be pierced by a vertical shaft three times, as has actually happened in some coal mines. Folds are _open_ when the sides are not parallel, and _closed_ when they are parallel, the former being represented by a half-open, and the latter by a closed, book. Closed folds are usually inverted, and when the tops have been removed by erosion (Fig. 25), the repetition of the strata may escape detection, and the thickness of the section be, in consequence, greatly overestimated. Thus, a geologist traversing the section in Fig. 25 would see thirty-two strata, all inclined to the left at the same angle, those on the right apparently passing below those on the left, and all forming part of one great fold. The repetition of the strata in reverse order, as indicated by the numbers, and the structure below the surface, show, however, that the section really consists of only four beds involved in a series of four closed folds, the true thickness of the beds in this section being only one-eighth as great as the apparent thickness.

The most important features to be noted in observing and describing inclined or folded strata are the _strike_ and _dip_. The strike is the compass bearing or horizontal direction of the strata. It is the direction of the outcrop of the strata where the ground is level. It may also be defined as the direction of a level line on the surface of a stratum, and is usually parallel with the axis of the fold.

The dip is the inclination of the beds to the plane of the horizon, and embraces two elements: (_a_) the direction of the dip, which is always at right angles to the strike, being the line of steepest descent on the surface of the stratum, and (_b_) the amount of the dip, which is the value of the angle between the line of steepest descent and the horizon.

In Fig. 26, _s t_ is the direction of the strike, and _d p_ that of the dip. The strike and direction of the dip are determined with the compass, and the amount of the dip with the clinometer, an instrument for measuring vertical angles.

The strike is much less variable than the dip, being often essentially constant over extensive districts; while the dip, except in very large or closed folds, is constantly changing in direction and amount.

When the dip and surface breadth of a series of strata have been measured, it is a simple problem in trigonometry to determine the true thickness, and the depth below the surface of any particular stratum at any given distance from its outcrop. When the strata are vertical, the surface breadth or traverse measure is equal to the thickness.

By the _outcrop_ of a stratum or formation we ordinarily understand its actual exposure on the surface, where it projects through the soil in ledges or quarries. But the term is also more broadly defined to mean the exposure of the stratum as it would appear if the soil were entirely removed. It is instructive to observe the relations of the outcrop to the form of the surface. Its breadth varies with its inclination to the surface, appearing narrow and showing its true thickness where it is perpendicular to the surface, and broadening out rapidly where the surface cuts it obliquely. The outcrops of horizontal strata form level lines or bands along the sides of hills and valleys, essentially contour lines in the topography; and appear as irregular, sinuous bands bordering the streams and valleys in the map-view of the country. The outcrops of vertical strata, dikes, or veins, on the other hand, are represented by straight lines and bands on the map. While the outcrops of inclined strata are deflected to the right or left in crossing ridges and valleys, according to the direction and amount of their inclination.

A geological map shows the surface distribution of the rocks, _i.e._, gives in one view the forms and arrangement of the outcrops of all the rocks in the district mapped, including the trend or strike of the folded strata. The map may be lithological, each kind of rock, as granite, sandstone, limestone, etc., being represented by a different color; or, it may be historical, each color representing one geological formation, _i.e._, the rocks formed during one period of geological time, without reference to their lithological character. But in the best maps these two methods are combined. The geological section shows the arrangement of the rocks below the surface, revealing the dip of the strata and supplementing the map, both modes of representation, the horizontal and vertical, being required to give a complete idea of the geological structure of a country. For a detailed and satisfactory explanation of the construction and use of geological maps and sections, students are referred to Prof. Geikie’s “Outlines of Field Geology.”

CLEAVAGE STRUCTURE.—This important structure is now known to be, like rock-folds, a direct result of the great horizontal pressure in the earth’s crust. It is entirely distinct in its nature and origin from crystalline cleavage, and may properly be called lithologic cleavage. It is also essentially unlike stratification and joint-structure. It agrees with stratification in dividing the rocks into thin parallel layers, but the cleavage planes are normally vertical instead of horizontal. And the cleavage planes differ from joints in running in only one direction, dividing the rock into layers; while joints, as we shall see, traverse the same mass of rock in various directions, dividing it into blocks.

The principal characteristics of lithologic cleavage are: (1) It is rare, except in fine-grained, soft rocks, having its best development in the slates, roofing slates and school slates affording typical examples. Hence it is commonly known as _slaty cleavage_. (2) The cleavage planes are highly inclined or vertical, very constant in dip and strike, and quite independent of stratification. (3) It is usually associated with folded strata, and often with distorted nodules or fossils. The more important of these characteristics are illustrated by Fig. 27. This represents a block of contorted strata in which the dark layers are slate with very perfect cleavage parallel to the left-hand shaded side of the block; while the white layers are sandstone and quite destitute of cleavage. Many explanations of this interesting structure have been proposed, but that first advanced by Sharpe may be regarded as fully established. He said that _slaty cleavage is always due to powerful pressure at right angles to the planes of cleavage_. All the characteristics of cleavage noted above are in harmony with this theory. Cleavage is limited to fine-grained or soft rocks, because these alone can be modified internally by pressure, without rupture. Harder and more rigid rocks may be bent or broken, but they appear insusceptible of minute wrinkling or other change of structure affecting every particle of the mass. Since the cleavage planes are normally vertical, the pressure, according to the theory, must be horizontal. That this horizontal pressure exists and is adequate in direction and amount, is proved by the folds and contortions of the cleaved strata; for, as shown in Fig. 27, the cleavage planes coincide with the strike of the foldings, and are thus perpendicular to the pressure horizontally as well as vertically. The distortion of the fossils in cleaved slates is plainly due to pressure at right angles to the cleavage, for they are compressed or shortened in that direction, and extended or flattened out in the planes of cleavage. Again, Tyndall has shown that the magnetism of cleaved slate proves that it has been powerfully compressed perpendicularly to the cleavage. And, finally, repeated experiments by Sorby and others have proved that a very perfect cleavage may be developed in clay (unconsolidated slate) by compression, the planes of cleavage being at right angles to the line of pressure. When, however, Sharpe’s theory had been thus fully demonstrated, the question as to _how_ pressure produces cleavage still remained unanswered. Sorby held that clay contains foreign particles with unequal axes, such as mica-scales, etc., and that these are turned by the pressure so as to lie in parallel planes perpendicular to its line of action, thus producing easy splitting or cleavage in those planes. And he proved by experiments that a mixture of clay and mica-scales does behave in this way. But Tyndall showed that the cleavage is more perfect just in proportion as the clay is free from foreign particles, and in such a perfectly homogeneous substance as beeswax, he developed a more perfect cleavage than is possible in clay. His theory, which is now universally accepted, is, that the clay itself is composed of grains which are flattened by pressure, the granular structure with irregular fracture in all directions, changing to a scaly structure with very easy and plane fracture or splitting in one definite direction.

Observations on distorted fossils and nodules have shown that when slaty cleavage is developed, the rock is, on the average, reduced in the direction of the pressure to two-fifths of its original extent, and correspondingly extended in the vertical direction. Thus, whether rocks yield to the horizontal pressure in the earth’s crust, by folding and corrugation, or by the flattening of their constituent particles, they are alike shortened horizontally and extended vertically; and it is impossible to overestimate the importance of these facts in the formation of mountains.

FAULTS OR DISPLACEMENTS.—We may readily conceive that the forces which were adequate to elevate, corrugate, and even crush vast masses of solid rock were also sufficient to crack and break them; and since the fractures indicate that the strains have been applied unequally, it will be seen that unequal movements of the parts must often result. If this unequal movement takes place, _i.e._, if the rocks on opposite sides of a fracture of the earth’s crust do not move together, but slip over each other, a _fault_ is produced. The two sides may move in opposite directions, or in the same direction but unequally, or one side may remain stationary while the other moves up or down. It is simply essential that the movement should be unequal in direction, or amount, or both; that there should be an actual slip, so that strata that were once continuous no longer correspond in position, but lie at different levels on opposite sides of the fracture. The vertical difference in movement is known as the _throw_, _slip_, or _displacement_ of the fault. Fault-fractures rarely approach the horizontal direction, but are usually highly inclined or approximately vertical. When the fault is inclined, as in Fig. 28, the actual slipping in the plane of the fault exceeds the vertical throw, for the movement is then partly horizontal, the beds being pulled apart endwise. The inclination of faults, as of veins and dikes, should be measured from the vertical and called the _hade_. Faults are sometimes hundreds of miles in length; and the throw may vary from a fraction of an inch to thousands of feet.

Transverse sections, such as are represented by Fig. 28 and many specimens and models, do not give the complete plan or idea of a fault; but this is seen more perfectly in Fig. 30. We learn from this that a typical fault is a fracture along which the strata have _sagged_ or settled down unequally. The most important point to be observed here is that the strata do not drop bodily, but are merely bent, the throw being greatest at the middle of the fault and gradually diminishing toward the ends. In other words, every simple fault must die out gradually; for we cannot conceive of a fault as ending abruptly, except where it turns upon itself so as to completely enclose a block of the strata, which may drop down bodily; but the fault is then really endless. A fault may be represented on a map by a line; if a simple fault, by a single straight line. But faults are often compound, and are represented by branching lines; that is, the earth’s crust has been broken irregularly, and the parts adjoining the fracture have sagged or risen unequally.

The rock above an inclined fault, vein, or dike (Fig. 28) is called the _hanging wall_, and that below the _foot wall_. Now inclined faults are divided into two classes, according to the relative movements of the two walls. Usually, the hanging wall slips down and the foot wall slips up, as in Fig. 28. Faults on this plan are so nearly the universal rule that they are called _normal_ faults. They indicate that the strata were in a state of tension, for their broken ends are pulled apart horizontally, so that a vertical line may cross the plane of a stratum without touching it.

A few important faults have been observed, however, in which the foot-wall[**no hyphen before] has fallen and the hanging-wall[**] has risen (Fig. 29). These are known as _reversed_ faults; and they indicate that the strata were in a state of lateral compression, the broken ends of the beds having been pushed horizontally past each other, so that a vertical line or shaft may intersect the same bed twice, as has been actually demonstrated in the case of some beds of coal.

The usual explanation of normal faults is given in Fig. 31. The inclined fractures of the earth’s crust must often be converging, bounding, or enclosing large V-shaped blocks (_A_, _B_). If now, through any cause, as the folding of the strata, they are brought into a state of tension, so that the fractures are widened, the V-shaped masses, being unsupported, settle down, the fractures bounding them becoming normal faults, as is seen by tracing the bed _X_ through the dislocations. The single fracture below the block _A_ is inclined, and the stretching has been accomplished by slipping along it and faulting the bed _Z_ as well as _X_, the entire section to the right of this fracture being part of a much larger V-shaped block the right-hand boundary of which is not seen. But the united fracture below the block _B_ being vertical, any horizontal movement must widen it into a fissure, which is kept open by the great wedge above and may become the seat of a dike or mineral vein. The beds below the V may, in this case, escape dislocation, as is seen by tracing the bed _Z_ across the fissure. These pairs of converging normal faults are called _trough_ faults; and this is the only way in which we can conceive of important faults as terminating at moderate depths below the surface, and not affecting the entire thickness of the earth’s crust.

Important reversed faults are believed to occur chiefly along the axes of overturned anticlines (Fig. 24) where the strata have been broken by the unequal strains, and those on the upper side shoved bodily over those on the lower or inverted side.

An extensive displacement of the strata is sometimes accomplished by short slips along each of a series of parallel fractures, producing a _step_ fault.

Faults cutting inclined or folded strata are divided into two classes, according as they are approximately parallel with the direction of the dip or of the strike. The first are known as _transverse_ or _dip_ faults, and the second as _longitudinal_ or _strike_ faults. The chief interest of either class consists in their effect upon the outcrops of the faulted strata, after erosion has removed the escarpment produced by the dislocation.

Dip faults cause a lateral shift or displacement of the outcrops, as shown in Fig. 32, which represents a plan or map-view of the strata traversed by the fault _b b_, the down throw being on the right and the up throw on the left. The dip of the strata is indicated by the small arrows and the accompanying figures; and it will be observed on tracing the outcrop of any stratum, _a a_, across the fault that it is shifted to the right. If the throw of the fault were reversed, the displacement of the outcrop would be reversed, also. Strike faults are of two kinds, according as they incline in the same direction as the strata, or in the contrary direction. The effect of the first kind is to conceal some of the beds, as shown in Fig. 33, in which beds 5 and 6 do not outcrop, but we pass on the surface abruptly from 4 to 7. The apparent thickness of the section is thus less than the real thickness. When the fault inclines against the strata, on the other hand (Fig. 34), the outcrops of certain strata are repeated on the surface; and a number of parallel faults of this kind, a step fault, will, like a series of closed folds (Fig. 25), cause the apparent thickness of the section to greatly exceed the real thickness. Repetition of the strata by faulting is distinguished from repetition by folding by being in the same instead of the reverse order.

Folds and faults are really closely related. In the former the strata are disturbed and displaced by bending; in the latter by breaking and slipping; and the displacement which is accomplished by a fold may gradually change to a fracture and slip. This relation is especially noticeable with monoclinal folds (Fig. 23), in which the tendency to shear or break the beds is often very marked.