STRUCTURAL FEATURES ARISING FROM DISTURBANCE.
Inclination and folding of strata.—The original attitude of beds, whether formed by water or by lava-flows, is normally horizontal, or nearly so. Both kinds of deposits, however, occasionally take place on considerable slopes. Modifications of the original attitude result from earth movements, and the measurement of these modifications is an important feature of field study. It is recorded in terms of dip and strike. The dip is the inclination of the beds referred to a horizontal plane, as illustrated in [Fig. 386], and is usually measured by a clinometer, the principle of which is shown in [Fig. 387]. In measuring the dip, the maximum angle is always taken. In [Fig. 386], for example, the angle would be less if the direction were either to the right or left of that indicated by the arrow. The direction as well as the amount of the dip is always to be noted. This must be determined by the compass, to which the clinometer may be conveniently attached. Dip 40°, S. 20° W. gives the full record of the position of the bed of rock under consideration. The strike is the direction of the horizontal edge of dipping beds, or more generally, the direction of a horizontal line on the surface of the beds. This is illustrated in [Fig. 386]. Since the strike is always at right angles to the dip, the strike need not be recorded if the direction of the dip is. Thus dip 40°, S. 20° W. is the same as dip 40°, strike N. 70° W.
Fig. 386.—Diagram illustrating dip and strike. (Geikie.)
Fig. 387.—The clinometer.
Fig. 388.—Open anticline, near Hancock, Md. (Russell, U. S. Geol. Surv.)
Fig. 389.—Closed anticline, near Levis Station, Quebec. (Walcott, U. S. Geol. Surv.)
Fig. 390.—Inclined anticline. (Van Hise, U. S. Geol. Surv.)
Fig. 391.—Recumbent anticline. (Van Hise, U. S. Geol. Surv.)
Fig. 392.—Syncline, C. & O. canal, 3 miles west of Hancock, Md. The beds are shale and sandstone near base of the Silurian. (Walcott, U. S. Geol. Surv.)
When the beds incline in a single direction, they form a monocline. When beds are arched so as to incline away from one another, they form an up-fold or anticline (Figs. [388] to [391]). The anticline may depart from its simple form, as shown in Figs. [390] and [391]. When beds are curved downward so as to incline towards one another, they form a syncline ([Fig. 392]). When beds assume the position shown in [Fig. 393], the folds are said to be isoclinal. When they are arched so as to form a cone or dome, and incline in all directions from a central point, they are said to have a quaquaversal dip. When considerable tracts are bent so as to form great arches or great troughs with many minor undulations on the flanks of the larger, they are designated as geanticlines, or anticlinoria (Figs. [394] and [395]), and geosynclines or synclinoria (Figs. [396] and [397]). Folding is often accompanied by the development of slaty cleavage ([p. 440]).
Fig. 393.—Isocline. (Van Hise, U. S. Geol. Surv.)
Fig. 394.—Anticlinorium: diagrammatic. (Van Hise, U. S. Geol. Surv.)
Fig. 395.—Anticlinorium. General section in the central massif of the Alps. (Heim.)
Fig. 396.—Synclinorium: diagrammatic. (Van Hise, U. S. Geol. Surv.)
Fig. 397.—Synclinorium, Mt. Greylock, Mass. (Dale, U. S. Geol. Surv.)
Fig. 398.—A series of diagrams illustrating actual field relations in regions of folded strata. Westchester Co., N. Y. (Dana.)
Fig. 399.
Fig. 400.
Fig. 399.—Diagram to show how dip and strike are recorded.
Fig. 400.—Map record of dip and strike, showing synclinal structure.
Fig. 401.
Fig. 402.
Fig. 401.—Map record of dip and strike showing anticlinal structure.
Fig. 402.—The structure of the area shown in Fig. 401, in cross-section.
Fig. 403.—Map record of dip and strike showing plunging (dipping down at ends) anticline.
As found in the field, folds are usually much eroded, and often completely truncated ([Fig. 398]). The determination of anticlinal or synclinal structure is then not based on topography, or even on such sections as shown in Figs. [394] to [397], for such sections are relatively rare. The structure is determined by a careful record of dips and strikes. On the field map, the record may be made as shown in Figs. [399] to [401], where the free ends of the lines with but one free end point in the direction of dip, while the other lines represent the directions of strike. Applying this method, the structure shown in [Fig. 400] represents a syncline, and that in [Fig. 401] an anticline. In cross-section, the structure presented by [Fig. 401] would appear as in [Fig. 402]. [Fig. 403] shows a doubly plunging anticline; that is, an anticline the axis of which dips down at either end. [Fig. 404] shows a combination of synclines and anticlines, and [Fig. 405] a cross-section along the line ab of [Fig. 404]. The outcrops of rock where the dip and strike may be determined may be few and far between, but when they are sufficiently near one another, the structure of the rock, as shown in [Fig. 405], may be worked out, even though the surface be flat.
Fig. 404.—Map record of dip and strike showing complex structure.
Fig. 405.—Cross-section of [Fig. 404] along the line ab.
Fig. 406.—Complex folding. Section across the Alps from the neighborhood of Zürich toward Como: about 110 miles. (Heim and Prestwich.)
Fig. 407.—Generalized fan fold of the central massif of the Alps. (Heim.)
Fig. 408.—Intimate crumpling of beds near head of Sperry glacier, Mont. (Meyers.)
Fig. 409.—Intimate crumpling in detail, accompanied by faulting. Jasper Hill, Ishpeming, Mich. (Meyers.)
Much the larger portion of the earth’s surface is occupied by beds that retain nearly their original horizontal attitude; but in mountainous regions the beds have usually suffered bending, folding, crumpling, and crushing, in various degrees, in the course of the deformations that gave rise to the mountains. Distortion is on the whole most intense and characteristic in the most ancient rocks known, the Archean, in which a distorted condition is nearly universal, so far as observation goes. Distortion is assigned chiefly to lateral thrust arising from the shrinkage of the earth, as explained in the chapter on Earth Movements. The simpler, and some rather complex forms of deformation, are shown in the preceding figures, but the folding is sometimes much more complex ([Fig. 406]), the folds sometimes “fan” ([Fig. 407]), and the beds of which they are composed are sometimes intricately crumpled (Figs. [408] to [410]). Among these various phases of deformation there are all gradations and combinations. Overturned folds reverse the order of the strata in the under limb of the fold. After such folds have been greatly eroded, so that their outer form is lost and their relations have become obscure, the reversed beds are likely to be interpreted as though they lay in natural order. In such a case as that represented in [Fig. 411], a complex structure may be interpreted as a simple one. Thus the strata of [Fig. 411] may have the structure shown in [Fig. 412], [413], or [414], so far as dip and strike show.
Fig. 410.—Plicated layers of thin-bedded chert in limestone, etched by erosion. Lower Cambrian (?), two miles southwest of Big Pine, Inyo Co., Cal. (Walcott, U. S. Geol. Surv.)
Joints.—The surface rocks of the earth are almost universally traversed by deep cracks called joints (Figs. [415], [138] and [140]). In most regions there are at least two systems of joints, the crevices of each system being roughly parallel to one another, while those of the two systems, where there are two, are approximately at right angles. In regions of great disturbance, the number of sets of joints is often three, four, or even more. The joints of each set may be many yards apart, or in exceptional cases, but a few inches, or even a fraction of an inch.
Generally speaking, there are more systems of joints, and more frequent joints in each system, where the rocks are much deformed than where they have been but little disturbed. In undisturbed rocks the joints approach verticality, but in regions where the rocks have been notably deformed, the joint planes may have any position. Not rarely they simulate bedding planes, especially in igneous and metamorphic rocks ([Fig. 416]). In the latter case especially, the cleavage due to jointing is often mistaken for bedding. They do not ordinarily show themselves at the surface in regions where there is much mantle rock, but they are readily seen in the faces of cliffs, in quarries, and, in general, wherever rock is exposed (Figs. [138] and [140]). Though some of them extend to greater depths than rock has ever been penetrated, joints are, after all, superficial phenomena. They must be limited to the zone of fracture, and most of them are probably much more narrowly limited. Joints frequently end at the plane of contact of two sorts of rock. Thus a joint extending down through limestone may end where shale is reached. Joints are frequently offset at the contact of layers or formations, and a single joint sometimes gives place to many smaller ones. All these phenomena are to be explained on the basis of the different constitution and elasticity of various sorts of rock. Generally speaking, rigid rock is more readily jointed than that which is more yielding.
Joints may remain closed, or they may gap. In the latter case, they may be widened by solution, weathering, etc., but they are quite as likely to be filled by detritus from above, or by material deposited from solution (veins). It is along joint-planes that many rich ore-veins are developed ([pp. 478–484]).
Fig. 411.—This diagram might represent either isoclinal or monoclinal structure. In the former case the strata might have the structure shown in any one of the following Figures, 412 to 414, so far as dip and strike show. (Dana.)
Fig. 412.
Fig. 413.
Fig. 414.
Fig. 412.—A possible interpretation of [Fig. 411]. (Dana.)
Fig. 413.—A possible interpretation of [Fig. 411]. (Dana.)
Fig. 414.—A possible interpretation of [Fig. 411]. (Dana.) ]
Fig. 415.—Jointed rocks. Cayuga Lake, N. Y. (Hall.)
Joints have been referred to various causes, among which tension, torsion,[214] earthquakes,[215] and shearing[216] are the most important. Most of them may probably be referred to the tension or compression developed during crustal movements.[217] In the formation of a simple fold, for example, tension-joints parallel with the fold will be developed, if tension goes beyond the limit of elasticity of the rock involved. If the axis of a fold is not horizontal, that is, if it “plunges,” as it commonly does, a second set of joints roughly perpendicular to the first will be developed. If the uplift be dome-shaped and sufficient to develop joints, they will radiate from the center. It is true that joints affect regions where the rocks have not been folded, and where they have been deformed but little, but deformation to some extent is well-nigh universal.
Fig. 416.—Jointing in granite. The surface of the rock is a joint plane. Northwest boundary of the United States. The edges of other joint planes normal to the surface are also shown. (Ransome, U. S. Geol. Surv.)
Fig. 417.—Sandstone dike. Northern California. (Diller, U. S. Geol. Surv.)
A minor cause of tension-jointing is shrinkage, due (1) to cooling, as in the development of the columnar structure of certain lavas, and (2) to dessication, as shown by the cracks developed in mud when it dries. These causes, however, are not believed to affect rock structures to any considerable depth. Torsional joints and joints due to earthquake vibrations appear to be special phases of tension-joints.
Two or more sets of joints may also be produced by compression, the number being dependent on the complexity of the folding. Many compression-joints correspond in direction with planes of shearing. They are often associated with minor faulting and with slaty cleavage.
Tension-joints appear to be much more widely distributed than compression-joints.
Sandstone dikes.—Exceptionally, open joints are filled by the intrusion of sedimentary material from beneath. Thus have arisen the remarkable sandstone dikes[218] of the West, especially of California ([Fig. 417]). Such dikes are sometimes several miles (nine at least) in length. The sand of these dikes was forced up from beneath either by earthquake movements or by hydrostatic pressure.
Fig. 418.—Diagram of a normal fault.
Faults.—The beds on one side of a joint-plane or fissure are sometimes elevated or depressed relative to those on the opposite side, and the displacement is known as a fault (Figs. 418 and 419). The joint-planes may have any position, and hence fault-planes may vary from verticality to approximate horizontality. The angle by which the fault-plane departs from a vertical position is known as the hade (bac, [Fig. 418]). The vertical displacement (ac) is the throw and the horizontal displacement (bc) the heave. The heave and the throw are to be distinguished from the displacement, which is the amount of movement along the fault-plane (ab, [Fig. 418]).
The cliff above the edge of the downthrow side is a fault-scarp. In many, probably in most cases, the scarp has been destroyed, or at any rate greatly obscured by erosion; but occasionally fault-scarps of mountainous heights, as along the east face of the Sierras and along many of the basin ranges of Utah, Nevada, etc., are found though much modified by erosion ([Fig. 419]).
Faults sometimes arise from over-intense folding ([Fig. 420]). A deformation which at one point results merely in a bending of the beds, may occasion a fault at another. Faults may pass into folds either vertically (Figs. [421] and [422]) or horizontally ([Fig. 423]). In such cases, thickening and thinning, and stretching and shortening of the beds is often involved (see Figs. [421] and [422]). Faults are often due to the greater settling of the beds on one side of a fissure than on the other, without special disposition to fold.
Fig. 419.—A fault-scarp; the triangular faces rising abruptly above the plain at the ends of the spurs. (Davis.)
Fig. 420.—Diagrams showing relations of faults and folds.
Fig. 421.
Fig. 422.
Fig. 421.—The fault above grades into a fold below. Thickening and thinning of layers next the fault-plane evident. Based on experimental results of Willis (13th Ann. Rept., U. S. Geol. Surv.)
Fig. 422.—Fault below grading into fold above. Stretching and thinning, and shortening and thickening of beds under pressure is involved. Based on experimental results of Willis.
Fig. 423.—Diagram showing a fault grading into a monocline horizontally.
Fig. 424.—Slickenside surface. (Prestwich.)
The rock on either side of a fault-plane is often smoothed as the result of the friction of movement. Such surfaces are slickensides ([Fig. 424]). A slickenside surface has some resemblance to a glaciated surface, but generally gives evidence of greater rigidity between the moving surfaces.
Faults are of two general classes, normal and reversed. In the normal fault ([Fig. 418]) the overhanging side is the downthrow side, i.e., the downthrow is on the side towards which the fault-plane inclines, as though the overhanging beds had slidden down the slope. Normal faults, as a rule, indicate an extension of strata, this being necessary to permit the dissevered blocks to settle downwards. In the reversed fault, the overhanging beds appear to have moved up the slope of the fault-plane, as though the displacement took place under lateral pressure. This is clearly shown to be the case where an overfold passes into a reversed fault ([Fig. 420]). Reversed faults are further illustrated by Figs. [425], [426], and [427]. Where the plane of the reversed fault approaches horizontality, the fault is often called a thrust-fault, or an overthrust. In such cases the throw is to be distinguished from the stratigraphic throw (see [Fig. 426]). In thrust-faults, the heave is often great. The eastern face of the Rocky Mountains near the boundary-line between the United States and Canada has been pushed over the strata of the bordering plains to a distance of at least eight miles.[219] Overthrusts of like gigantic displacement have been detected in British Columbia,[220] Scotland,[221] and elsewhere.
Fig. 425.—Perspective view and vertical section of a thrust-fault. (Willis, U. S. Geol. Surv.)
Fig. 426.—Diagram of a thrust-fault illustrating the several terms used in describing faults. The distinctions between heave and displacement, and between throw and stratigraphic throw, are to be especially noted. (Willis, U. S. Geol. Surv.)
Fig. 427.—Step-fold showing (in 1) break in the massive limestone bed which determines the plane of the break-thrust along which the displacement shown in 2 takes place. (Willis, U. S. Geol. Surv.)
Sometimes a fault branches ([Fig. 428]) and sometimes the faulting is distributed among a series of parallel planes at short distances from one another,[222] instead of being concentrated along a single plane, thus giving rise to a distributive fault ([Fig. 429]). This is perhaps more common in normal than in reversed faulting.
Fig. 428.
Fig. 429.
Fig. 428.—Branching-fault. (Powell.)
Fig. 429.—Diagram showing a series of small faults—distributive faulting.
Fig. 430.—Fault in Gering series. Near Rutland Siding, near Crawford, Neb. (Darton, U. S. Geol. Surv.)
The amount of throw occasionally reaches several thousand feet. Occasionally faults of incredible dimensions are reported, but these are perhaps misinterpretations. Faults are observed to die out gradually when traced horizontally, sometimes by passing into monoclinal folds, and sometimes without connection with folding. In depth they probably die out in similar ways in most cases. Where the throw is great, they probably give place to folds below ([Fig. 421]). Other phenomena of faulting are illustrated by Figs. [430–435]. A fault of thousands, or even hundreds of feet is probably the sum of numerous smaller slippings distributed through long intervals of time. Faulting is probably one of the common causes of earthquakes.
Fig. 431.—Contorted and faulted laminated rock. Cook Inlet. (Gilbert, U. S. Geol. Surv.)
Fig. 432.—Faulting shown in a cobblestone. The fault-planes have become veins by deposition from solution. The figure shows how the relative ages of crossing-faults may be determined. (Schrader, U. S. Geol. Surv.)
Fig. 433.—Figure showing minute faulting. The length of the specimen is 8 inches. The number of faults is nearly 100. (Photo. by Church.)
Fig. 434.
Fig. 435.
Fig. 434.—Diagram illustrating common phenomena of a faulted region. (Dana.)
Fig. 435.—Diagram showing a fault, the plane of which forms an open fissure and has been filled with débris from above. (Powell.)
The significance of faults.[223]—Faults afford a valuable indication of the conditions of stress to which a region has been subjected, but some caution must be exercised in their interpretation. Normal faults usually indicate an extension of the surface sufficient to permit the fault-blocks to settle down unequally. Reversed faults usually signify a compression of the surface which requires the blocks to overlap one another more than they did before the faulting. In other words, normal faulting usually implies tensional stress, and reversed faulting compressional stress. It is not difficult to see, however, that in an intensely compressed and folded region there might be cases of normal faulting on the crests of folds where local stretching took place, and that reversed faults might occur even in regions of tension. But such cases must usually be local, and capable of detection and elimination by a study of the phenomena of the surrounding region. These exceptional cases aside, the general inference from prevailing normal faults is that the regions where they occur have undergone stretching, while the inference from the less widely distributed reversed faults is that the surface where they occur has undergone compression.
In view of the current opinion that the crust of the earth has been subjected to great lateral thrust as a result of cooling, it is well to make especial note of the fact that the faults which imply stretching are called normal because they are the more abundant; and that the faults which imply thrust are less common, and are styled reversed. The numerical ratio of normal to reversed faults has never been closely determined, but normal faults very greatly preponderate, and are estimated by some writers to embrace 90-odd per cent. of the whole. The testimony of normal faults is supported by the prevalence of gaping crevices, and of veins which are but crevices that stood open until they were filled by deposition. All these phenomena seem to testify to a stretched condition of the larger part of the surface of the continents. This will again claim attention in the study of Earth Movements.
Fig. 436.—Diagram showing an area of rock with monoclinal structure. One layer notably unlike the others.
Effect of faulting on outcrops.—Faulting may bring about numerous complications in the outcrop of rock formations. In a series of formations having a monoclinal structure ([Fig. 436]), many changes may be introduced. Let it be supposed in the following cases that, after faulting, the surface has been reduced to planeness by erosion. If the fault-plane be parallel to the strike of the beds (ab, [Fig. 436]), and hence a strike fault, the outcrop of a given layer may be duplicated (H, [Fig. 437]), or it may be eliminated altogether ([Fig. 438]). If the fault-plane be parallel to the direction of dip (cd, [Fig. 436]), a dip fault, the layer H will outcrop, as in [Fig. 439], if the downthrow was on the far side, or as in [Fig. 440] if the downthrow was on the opposite side. In both cases the outcrop H is offset, the amount of the offset decreasing with increasing angle of dip and increasing with increasing throw of the fault. If the fault be oblique to the direction of dip and strike (ef, [Fig. 436]), an oblique fault, the outcrop of such a layer as H will have the relations shown in [Fig. 441] if the downthrow was to the left, and that shown in [Fig. 442] if the downthrow was to the right. In the former case, it is said that there is offset with overlap; in the latter, offset with gap. The amount of the overlap and gap, respectively, increases with the increase of throw and hade, and decreases with increase of dip. In all cases the outcrop (after the degradation of the upthrow side) is shifted down dip.
Fig. 437.—Same as [Fig. 436], after (1) displacement by a strike fault and (2) base-leveling. The outcrops of certain beds are repeated.
Fig. 438.—Diagram illustrating how a strike fault in such a structure as that shown in [Fig. 436] may cause the outcrop of certain beds to disappear.
Fig. 439.—Diagram illustrating how a dip fault in the structure shown in [Fig. 436] affects the outcrop when the downthrow was on the farther side of the fault-plane.
Fig. 440.—Same as [Fig. 439], except that the downthrow was on the opposite side.
Fig. 441.—Oblique fault in the structure shown in [Fig. 436]. The downthrow was on the left side. The outcrop of layer H is offset with overlap.
Fig. 442.—Same as [Fig. 441], except that the downthrow was on the right side, and the offset is with a gap instead of an overlap.
Fig. 443.—Diagram showing effect of faulting on the outcrops of synclinal beds.
Fig. 444.—Diagram showing effect of faulting on outcrops of anticlinal beds.
Fig. 445.—Diagram illustrating the effect of diminishing throw on outcrops in regions of folded rocks.
If a fault crosses folds at right angles to their axes, the effect is to change the distance between the outcrops of a given bed on opposite sides of the fault, after the truncation of the folded beds. The distance is decreased on the upthrow side of a syncline ([Fig. 443]) and increased on the upthrow side of an anticline ([Fig. 444]). If the throw of a fault in tilted beds diminishes in one direction, it may cause beds to outcrop, as shown in [Fig. 445]. Various other complications arise under other circumstances. Since faults rarely show themselves in the topography of the surface, except under special circumstances (see [p. 151]), their detection and measurement is usually based on the study of the relations of the beds involved, as illustrated by Figs. [436–445].