As base-level is approached, the outcrops of hard rock are brought low. When they have been reduced to the level of their surroundings, the streams may flow without regard to the resistance of the rock beneath, for downward cutting has ceased. As this stage of erosion is approached, a readjustment of the drainage may take place, and the waters which had taken long and circuitous courses to avoid hard rock, may change their courses to more direct ones (compare Figs. 130 and 135). Adjustment is, therefore, a relative term, and streams which are adjusted at one stage of erosion, are not necessarily adjusted at another.
It sometimes happens that rocks of unequal resistance are covered by beds of uniform hardness. A consequent stream developed on the latter may find itself out of structural adjustment when it has cut its channel down to the level of the heterogeneous beds below. Such a stream is said to be superimposed (Fig. 136) on the underlying structure. Structural adjustment is likely to follow.
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]
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.
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.
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).
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]
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.
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.
Rise.—If after being base-leveled, or notably reduced by erosion, a region is uplifted so as to increase the gradients and therefore the velocities of the streams which drain it, the streams are said to be rejuvenated, and a new cycle of erosion is begun. If the rise of the area were equal everywhere, while the coast line remained constant in position, there would be an immediate increase in velocity only at the debouchures of the streams flowing directly into the sea. At the debouchures of such streams there would be rapids or falls. Each rapids or falls would promptly recede, and with the recession, the acceleration of velocity resulting from the uplift would be felt farther and farther up-stream, and ultimately to its source. The rejuvenated streams would cut new valleys in the bottoms of their old ones (Figs. 152 and 153). The new valleys would begin where the increase in velocity was first felt, and they would be lengthened by head erosion just as valleys of the first cycle were lengthened.
When the head of the new part of a valley of a rejuvenated stream recedes past the mouth of a tributary adjusted[58] to the gradient of the main stream before rejuvenation, the velocity of the tributary is accelerated at its debouchure, and it begins to excavate a new valley in the bottom of its old one. The new valley commences at the lower end of the old one, and develops headward (a and b, Fig. 153). Good illustrations are furnished by the streams in the west central part of New Jersey. The Delaware has here a sharply defined valley, and its tributaries are essentially as deep as their main at the point of junction. Above this point they have high gradients for a short distance (three to six miles), beyond which they wind sluggishly in wide valleys with low gradients across a relatively high plateau. Their profiles are illustrated by Fig. 154. The flat, though high, surface in which their upper courses lie, appears to have been nearly base-leveled in an earlier cycle, and then to have been elevated. The date of the elevation is fixed, in terms of erosion, by the time necessary for the excavation of the Delaware gorge, and the narrow gorges along the lower courses of its tributaries. It was so recent that the effects of rejuvenation, proceeding from the debouchures of the tributaries toward their heads, have not yet advanced far from the Delaware. Similar relations are found elsewhere (Fig. 1, Pl. XIII, s. c. Col.). Another peculiarity of rejuvenated drainage is shown in Fig. 2, Plate XIII (s. Kan.). Here Elm Creek flows at a level 200 feet below that of Sand Creek, 4 miles distant. The valley of the former appears to have entered upon a new cycle as the result of uplift, while that of the latter, in the area shown on the map, is still unrejuvenated. Farther down-stream, the valley of Sand Creek shows signs of rejuvenation. It may be noted that a tributary of Amber Creek has good opportunity to capture Sand Creek, for the latter flows about 25 miles before reaching the level of Amber Creek at its junction with Elm Creek.
Should the lower end of a tributary valley fail to be degraded as fast as the valley of the main at the point of junction, the tributary is out of topographic adjustment with its main. Falls or rapids may result. When the lower end of a tributary valley is distinctly above the level of its main, the former is called a hanging valley. Hanging valleys developed by stream erosion alone are not common except just after the recession of a falls past the mouth of a tributary. Hanging valleys, as well as the characters and relations illustrated by Figs. 152–154 are criteria of rejuvenation, but they must be applied with discretion. Such profiles, for example, as that shown in Fig. 154 may be developed when the rock of a stream’s bed is unequally resistant, and hanging valleys are generally a result of glaciation (see Chapter V).
Rejuvenated streams sometimes inherit certain peculiarities from their aged ancestors. Thus a rejuvenated stream may intrench the meanders possessed by the old stream which preceded (see Fig. 1, Pl. XIV, near Harrisburg, Pa.), and intrenched meanders are one of the marks of rejuvenated streams. They are not uncommon in the Appalachian Mountain regions, and are known in other parts of the world. The Seine and the Moselle furnish further illustrations.[59]
The history of the new cycle of erosion inaugurated by the uplift would differ from that of the preceding cycle in that the new one would begin with a drainage system already developed. Other things being equal, therefore, the reduction of the land would proceed more rapidly in a subsequent cycle than in the first.
The recognition of different cycles of erosion, separated by uplifts, is often easy. The principles involved are illustrated by Fig. 155 which represents an ideal profile of considerable length (say 50 miles). The points a, a′, and a″ reach a common level. Below them there are areas b, b′, and b″ which have a nearly common elevation, below which are the sharp valleys d, d′, and d″. The points a, a′, and a″ represent the cross-sections of ridges formed by the outcrops of layers of hard rock. If the crests of the ridges are level, the points a, a′, and a″ must represent remnants of an old base-level, since at no time after a ridge of hard rock becomes deeply notched does it acquire an even crest, until it is base-leveled.[60] At all earlier stages its crest is uneven. After the cycle represented by the remnants a, a′, and a″ was completed, the region suffered uplift. A new cycle represented by the plain b, b′, and b″ was well advanced, though not completed, when the region was again elevated, and the rejuvenated streams began to cut their valleys d, d′, and d″ in the plain of the previous incomplete cycle. The elevations, c and c′ (intermediate in elevation between a, a′, and a″, and b, b′, and b″) may represent either remnants of the first base-level plain which were lowered, but not obliterated, while the plane b, b′, b″ was developing; or they may represent a cycle intermediate between that during which a, a′, a″ and b, b′, b″ were developed. If the intermediate elevations (c, c′) have a common height and level crests, the presumption would be in favor of the latter interpretation. If they be numerous and of varying heights, as is possible, they may in the field obscure the planes (a, a′, a″ and b, b′, b″) developed in the different cycles, which, in the figure, are distinct.
If the strata involved be horizontal the determination of cycles is sometimes less easy. Thus in Fig. 156, it is not possible to say whether a and a′ represent remnants of an old base-level, or whether they represent the original surface from which degradation started. So, too, the various benches below a, such as b, b′, and b″ may readily be the result of the superior hardness of the beds at this level. For the determination of successive uplifts in the field it is necessary to consider areas of considerable size, and to eliminate the topographic effects of inequalities of hardness, and of certain other factors to be mentioned presently.
The inequalities in the depths of the young valleys in Figs. 155 and 156 may be explained on the supposition that the deeper ones belong to main streams, and the shallower ones to tributaries. Such a valley as that shown at e, Fig. 155, suggests rejuvenation at this point; but farther up the stream which occupies this valley, rejuvenation might not be apparent. In this case, the main streams might be flowing in new valleys, d, d′, etc., while the heads of their tributaries are still flowing in the older valleys of the preceding cycle (compare Fig. 154 and Fig. 1, Pl. XIII).
It is by the application of the preceding principles that it is known that the Appalachian Mountains, after being folded, were reduced to a peneplain (p. 76), throughout their whole extent from the Hudson River to Alabama. The peneplain level is indicated by the level crests of the Appalachian ridges, shown in cross profile by the high points of Fig. 157. The system was then uplifted, and in the cycle of erosion which followed, broad plains were developed at a new and lower level, corresponding in a general way to the plains b, b′, and b″ of Fig. 155. The plains were located, for the most part, where the less resistant strata come to the surface. Above them rose even-crested ridges, the outcrops of the resistant layers, which had been isolated by the degradation of the softer beds between. They constitute the present mountain ridges (the high points of Fig. 157). The evenness of their crests, testifying to the completeness of the first peneplanation, is shown in Fig. 158, which represents, diagrammatically, a longitudinal profile of an Appalachian Mountain ridge. The evenness of the crest is interrupted by (1) notches (b, c, etc., Fig. 158) cut by the streams in later cycles, and (2) by occasional elevations above the common level (monadnocks, a, a′, Fig. 158). The monadnocks are generally rather inconspicuous, but there is a notable group of them in North Carolina and Tennessee. Mount Mitchell and Roane Mountain are examples. When long distances are considered, the ridge crests depart somewhat from horizontality. This is believed to be due, in part at least, to deformations of the old peneplain during the uplift which inaugurated the second cycle of erosion.
The extent to which the second cycle of erosion recorded in the present topography had proceeded before its interruption by uplift, is indicated by the extent of the valley plains (Fig. 157) below the mountain ridges. While these plains were being developed on the weak rocks, narrow valleys only (Fig. 158) were cut in the resistant rocks which now stood out as ridges. In Fig. 158 some of these valleys are shallow (c, c′, c″, etc.), and but one of them deep. The former may be either (1) the valleys of streams which crossed the hard layer at the beginning of the cycle, and which were diverted before their valleys became deep; or (2) they may represent the heads of valleys now working back into the ridges. The deep valley (b) represents the work of a stream which has held its course across the hard layer while the latter was being isolated as a mountain ridge (compare Figs. 131 and 132). Deep narrows of this sort are often called water-gaps. Similar valleys, whether shallow or deep, from which drainage has been diverted, are sometimes called wind-gaps. The second cycle of erosion, while still far from complete, was interrupted by uplift (relative or absolute), and a new cycle inaugurated. This event was so recent that the new (third) cycle has not yet advanced far.
Recently it has been urged that another cycle, intermediate between the first and second, is to be recognized.[61]
Some of the features just described are illustrated by Fig. 159. The even mountain crest in the background is the Kittatinny Mountain of New Jersey and its continuation in Pennsylvania. In common with other corresponding crests it represents the oldest recorded base-level (or peneplain) of the region. The great gap in the mountain is the Delaware Water-Gap. Below the mountain crest there is another plain, developed in a subsequent cycle of erosion, while the valley plain in the foreground represents the work of a still later cycle.
The oldest erosion plain of the Appalachian Mountains, the results of which are seen in the even-crested ridges so characteristic of the system, is sometimes called the Kittatinny base-level.[62] It was completed early in the Cretaceous period, and hence is sometimes known as the Cretaceous base-level. The next lower plain, imperfectly developed, has been called the Shenandoah Plain,[62a] from the Shenandoah Valley where it is well seen (Fig. 132 and Fig. 2, Pl. XII). It is to be noted that the terms base-level and peneplain have both been used in connection with these old plains. Graded plain is equally applicable. The truth is that the topographic types represented by these three terms grade into one another. It may be questioned whether definitions should be insisted on which differentiate these types more sharply than Nature has.
Many of the peculiarities of the drainage of the Appalachian Mountain system are intimately connected with the history just outlined. Thus three great rivers, the Delaware, the Susquehanna, and the Potomac, have their sources west of the Appalachians proper, cross the system in apparent disregard of the structure, and flow into the Atlantic. The James and Roanoke head far to the west, although not beyond the mountain system, and flow eastward, while the New River (leading to the Kanawha) farther south, heads east of the mountain-folds, and flows northwestward across the alternating hard and soft beds of the whole Appalachian system, to the Ohio (Fig. 160). The French Broad, a tributary to the Tennessee, has a similar course. Such streams are clearly not in structural adjustment, and afford good opportunities for piracy. Their courses were apparently assumed during the time of the Kittatinny base-level, when the streams had so low a gradient as not to be affected by the structure (p. 150). Elevation rejuvenated them, and they have held their courses in succeeding cycles across beds of unequal resistance, though smaller streams have become somewhat thoroughly adjusted. Crustal deformations have also helped them to hold their courses, for the Cretaceous peneplain seems to have been tilted to the southeast at its northern end, and to the southwest at its southern, when the succeeding cycle began.
Streams which hold their early courses in spite of changes which have taken place since their courses were assumed are said to be antecedent. They antedate the crustal movements which, but for pre-existent streams, would have given origin to a different arrangement of river courses. As a result of crustal movements, therefore, a consequent stream may become antecedent. Master streams are more likely to hold their courses, and therefore to become antecedent, than subordinate ones.
The uplift of base-leveled beds, especially if the beds are tilted so as to bring layers of unequal resistance to the surface at frequent intervals, affords conditions favorable for extensive adjustment. The numerous wind-gaps in the mountain ridges, representing the abandoned courses of minor streams, and the less numerous water-gaps, which indicate the resistance of large streams to structural adjustment, are instructive witnesses of the extent to which adjustment has gone. So extensive has been the adjustment among the streams of the Appalachian Mountains that there is probably no considerable stream in the whole system which has not gained or lost through its own or its neighbors’ piracy. The history of the rivers of the Appalachian Mountains has been further complicated by a considerable amount of warping during the periods of uplift.[63]