Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

INFLUENCE OF JOINTS AND FOLDS.

Joints.—Various structural features of rock other than hardness influence its erosion. Apart from the stratification planes, most rock formations are affected by joints or fissures. The joints are often, but not always, nearly vertical. Two sets are generally present, and sometimes more. If but two, they usually meet at a large angle; if more than two, two are likely to be nearly perpendicular to each other, while the third and fourth sets have such directions as to cut the others at large angles. These joints allow the ingress of water, roots, etc., which help to weather and disrupt rocks. Occasionally there is notable sag of the beds of rock along joint planes, but this effect is usually superficial only ([Fig. 137]). Where the jointage planes are frequent and open, the columns bounded by them sometimes topple over on cliff faces, either by undercutting, or by the wedge-work of roots or ice.

The effect of joints on erosion may often be seen along a stream which flows in a rock gorge. In such situations, the outlines of the banks are sometimes angular, and sometimes crenate ([Fig. 138]), the reëntrants being located at the joints. By working into and widening joints, running water sometimes isolates masses of rock as islands ([Fig. 139]). In a region free from mantle rock, or where the mantle rock is meagre, joints often determine the courses of valleys by directing the course of surface drainage. This is shown in many parts of the arid west. In regions where the rocks are notably faulted, the courses of the streams are sometimes controlled by the courses of the fault planes. This is the case, for example, in central Washington.[55]

Fig. 137.—Shows the sagging of beds along joints. The disturbance does not extend far below the surface. Cook’s quarry (Niagara limestone) near La Salle, Niagara Co., N. Y. (Gilbert, U. S. Geol. Surv.)

The jointing of rocks often shows itself distinctly in the weathered faces of cliffs (Figs. [140] and [141]), especially in arid and semi-arid regions, or where the slope is too steep for the accumulation of soil and rock-waste on its surface.

If a stream flowing over jointed rock has falls, the conditions are sometimes afforded for the development of an exceptional and striking scenic feature. If above Niagara Falls, for example, there were an open joint in the bed of the stream (as at b, [Fig. 142]), some portion of the water would descend through it. After reaching a lower level it might find or make a passage through the rock to the river below the falls. If even a little water took such a course, the flow would enlarge its channel, making a passageway between the joint through which the water descended and the valley below the falls (bcde, [Fig. 142]). This passageway might become large enough to accommodate all the water of the river. In this case, the entire fall would be transferred from the position which it previously occupied (f) to the position of the enlarged joint (b). The fall would then recede. The underground channel between the old falls and the new would be bridged by rock (bf″ and f‴, [Fig. 143]), making a natural bridge. The natural bridge near Lexington, Va. ([Fig. 144]), almost 200 feet above the stream which flows beneath it, is believed to have been developed in this way. A similar bridge is now in process of development in Two Medicine River in northwestern Montana ([Fig. 145]). Once in existence, a natural bridge will slowly weather away.

Fig. 138.—Figure showing crenate river bank, the reëntrants being determined by joints. Dells of the Wisconsin River, near Kilbourn, Wis. (Atwood.)

Fig. 139.—Lone Rock. An island isolated by the notable widening of a series of joints. The joints in the rock of the island have themselves been so widened that a rowboat may be taken through it in two directions. Lower Dells of the Wisconsin. (Meyers.)

Fig. 140.—Effect of columnar structure on weathering. Material unconsolidated. Spur of south end of Sheep Mountain. (Lippincott, U. S. Geol. Surv.)

It is not to be understood that all natural bridges have had this history. They are sometimes developed from underground caves when parts of their roofs are destroyed, as well as in various other ways.

Fig. 141.—Effect of columnar structure on weathering. Big Bad Lands, S. D. (Darton, U. S. Geol. Surv.)

Folds.—The erosion of folded strata (anticlines and synclines) leads to the development of distinctive topographic features. So soon as a fold begins to be lifted, it is, by reason of its position, subject to more rapid erosion than its surroundings. For the same reason the crest of the fold is likely to be degraded more rapidly than its lower slopes, and must suffer more degradation before it is brought to base-level. Folds are usually composed of beds of unequal resistance, and as the degradation of a fold proceeds, successive layers are worn from the top, and the alternating hard and soft layers composing it are exposed. So soon as this is accomplished, adjustment of the streams is likely to begin, and the watercourses, and later the valley plains, come to be located on the outcrops of the less resistant layers, while the outcrops of the harder beds become ridges.

If the axis of an eroded anticline were horizontal, a given hard layer, the arch of which has been cut off, would, after erosion, outcrop on both sides of the axis. When the topography was mature these outcrops would constitute parallel ridges, or parallel lines of hills; when the region had been base-leveled, the outcrops would be in parallel belts, though no longer ridges or hills. The lower the plane of truncation, the farther apart would the outcrops be in the anticline, and the nearer together in the syncline (compare ab and cd, [Fig. 133]).

Fig. 142.—A natural bridge in process of development; longitudinal section at the left; transverse section, looking toward e, at the right.

Fig. 143.—The same as [Fig. 142] at a later stage of development.

If, on the other hand, the axis of the anticline or syncline to be eroded was not horizontal, that is, if it plunged, the topographic result would be somewhat different. Suppose a plunging anticline to be truncated at base-level. If either end of the fold plunged below the plane of truncation, the outcrops of a given layer on opposite sides of the axis would converge in the direction of plunge, and come together at the end. At a stage of erosion antedating planation (say late maturity) there would have been a ridge, or a succession of hills, in the position corresponding to the outcrop of a hard layer, with a canoe-shaped valley within. If two hard layers were involved, instead of one, there would be two encircling ridges, with a curved valley between them, and a canoe-shaped valley within the innermost ([Fig. 146]). If the anticline plunged both ways, the valley enclosed by the hard-layer ridge would be canoe-shaped at both ends ([Fig. 147]). In such a case there would be likely to be a low gap (water-gap) in the rim of the valley through which the drainage which degraded the surface escaped, but there would be likely to be but one, for if two or more streams had drained the area of the valley at an early stage of erosion, one would be likely to have captured the others (see [p. 138]) before late maturity. A succession of doubly-plunging anticlines and synclines might give rise to a very complex series of ridges and valleys. Illustrations of the above phenomena are found at various points in the Appalachian Mountains, especially in eastern Pennsylvania.[56]

Fig. 144.—The Natural Bridge of Virginia, from the southeast (Walcott, U. S. Geol. Surv.)

Fig. 145.—A natural bridge in development. Two Medicine River, Mont. Corresponds to the stage represented by [Fig. 142], and the view corresponds to that shown diagrammatically at the right-hand end of the figure. (Whitney.)

In the structural adjustment which goes with the erosion of folds, it often happens that the valleys come to be located on the anticlines, while the outcrops of the hard layers on the flanks of the anticlines, or even in the original synclines, become the mountains. The adjustments by which valleys come to be located on anticlines are somewhat as follows:[57] [Fig. 148] represents two doubly-plunging anticlines with a syncline between, the relative elevations being shown by contour lines. At the outset, the drainage of such a region must have followed the structural valley, and its initial course, consequent on the slope, must have been down the axial trough. Drainage from the anticlines into the synclines would have promptly developed valleys, and the valleys would soon have acquired streams.

Fig. 146.—A canoe-shaped valley bordered by a ridge formed by the outcrop of a hard layer in a plunging syncline. The ridge bounding the canoe-valley is separated from an outer ridge by a curved valley underlain by relatively weak rock. (After Willis.)

Fig. 147.—A diagram to illustrate the effects of erosion on a doubly-plunging anticline made up of beds of unequal hardness.

Fig. 148–51.—Diagrams to illustrate the shifting of rivers from a synclinal to an anticlinal position. (After Davis.)

The anticlines and synclines under consideration are assumed to have a thick hard layer at the surface, and softer beds below. This is shown in the cross-section introduced in the figure, the upper hard stratum (m) being indicated by the dots, while the softer one (n) is white. The line oo represents base-level, which is below the hard layer both in the syncline and anticline, but much farther below in the latter position than in the former. Because of their higher gradients, and because of the greater fracturing to which the region they drain was presumably subject at the time of folding, the tributary streams might cut through the hard layer sooner than the main stream which they join. This done, they would enlarge their valleys rapidly in the softer rock beneath, and secondary tributaries would be developed ([Fig. 149]). When the condition of things represented in [Fig. 149] is reached, the streams c and d, tributary to the synclinal stream, come into competition. The former has the advantage over the latter, because it joins the main stream at a lower level. Stream c will therefore be likely to capture d. The incipient stages of the capture are stealthy, and the later bold. At first the divide between their head waters is shifted northward inch by inch, because the gradient toward g is higher than that toward e. The capture of the head waters of e is as slow as the migration of the divide, until the divide reaches the point where e joins f. The stream f is then diverted promptly into the valley of g, and is at once led away to c (see [Fig. 150]). Strengthened by its increased volume, the stream c ([Fig. 150]) lowers its valley across the hard layer more rapidly than before, and so holds the advantage it has gained. Not only this, but the beheaded stream d ([Fig. 150]), because of its diminished volume, sinks its valley into the hard layer less rapidly than before, and its decrease in power also works to the advantage of the stream leading to c. The result is that the divide between fg and d does not remain constant, but is driven back step by step toward a.

Similarly a tributary to the main stream at b ([Fig. 150]), may by means of its tributary h, capture the waters of fg, and lead them to the synclinal valley at b (compare Figs. 150 and 151). Deprived of its main source of supply (at c) the synclinal stream is greatly diminished above b, and cuts more and more slowly, while the stream fgh ([Fig. 151]), having greater volume and working mainly in softer rock, sinks its channel faster than the stream in the synclinal axis. Under these circumstances, the stream at f may cut its valley below the valley in the synclinal axis a ([Fig. 150]). In this event, the divide between f and a ([Fig. 150]) may be pushed back until the synclinal stream is beheaded at a and carried out of the syncline and over into the anticlinal valley ([Fig. 151]). Thus, the old anticlinal axis comes to be the course of the main stream. Similarly the stream entering the syncline at b ([Fig. 151]) might later be captured by i, thus lengthening its anticlinal course.

It is not to be understood that this sequence of events will take place in the degradation of every anticline, but the principles here set forth will always be operative. The result specified will be accomplished wherever hard and soft layers have the relations indicated in the diagrams; that is, where the stream in the syncline finds itself on a resistant layer as it approaches base-level, while at the same time the (original) tributary streams are working in softer beds. It is not to be understood, therefore, that streams migrate from synclines to anticlines for the sake of getting out of the former positions into the latter. If they shift their courses it is to find easier ones.

That these changes are not fanciful is shown by the fact that the adjustment described corresponds with that shown in many parts of the Appalachian Mountains, and in other mountains of similar structure.

If in a later stage of its history, the new main stream, fh, were to cut its bed down to a lower hard layer, while the original stream, ab, reached a softer bed beneath the hard one above, the latter would again have an advantage, and a new series of adjustments would be inaugurated which might result in re-establishing the main stream in its original synclinal position.