Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

EFFECT OF CHANGES OF LEVEL.

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.

Fig. 152.—Cross-section of a wide valley, ab, in the bottom of which a younger valley, cd, has been excavated as the result of uplift.

Fig. 153.—Diagram to illustrate in ground plan an ideal case of rejuvenation as the result of uplift.

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.

PLATE XIII.

U. S. Geol. Surv.

Scale, 2+ mile per inch.

Fig. 1. COLORADO.

U. S. Geol. Surv.

Scale, 2+ mile per inch.

Fig. 2. KANSAS.

PLATE XIV.

U. S. Geol. Surv.

Scale, 1+ mile per inch.

Fig. 1. PENNSYLVANIA.

U. S. Geol. Surv.

Scale, 1+ mile per inch.

Fig. 2. CALIFORNIA.

Fig. 154.—Profile of a rejuvenated stream. The Lockatong River (N. J.) to the head of Mud Run.

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.

Fig. 155.—Diagram to illustrate cycles of erosion where the beds are tilted.

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.

Fig. 156.—Diagram to illustrate cycles of erosion where the beds are horizontal.

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.

Fig. 157.—Cross-section of a portion of the Appalachian Mountains to illustrate the phenomena of erosion cycles. (After Rogers.)

Fig. 158.—A diagrammatic longitudinal profile of an Appalachian Mountain ridge.

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.

Fig. 159.—The Kittatinny Mountains and Delaware Water-Gap from Manunka Chunk. (N. J. Geol. Surv.)

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.

Fig. 160.—Showing certain peculiarities of Appalachian drainage. 1 = the Susquehanna; 2 = the Potomac; 3 = the James; 4 = the Roanoke; 5 = the Coosa; 6 = the Tennessee; 7 = the Kanawha; 8 = head of New River; 9 = head of the French Broad.

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]

Fig. 161, 162.—Diagrams to illustrate the effect of crustal warping on stream erosion. The dotted lines represent the profiles of the streams before deformation; the full lines, after. Erosion will be stimulated between a and b in each case, and between c and d in Fig. 162. Below b, Fig. 161, the stream will be drowned, and erosion therefore stopped. Erosion will also be stopped or retarded above a, between b and c, and below d in Fig. 162.

Sinking.—The land on which a river system is developed may be depressed relative to sea-level. In this case the sea would occupy the lower ends of valleys, converting them into bays and estuaries. A stream in this condition is said to be drowned. Of drowned rivers there are many examples along the Atlantic coast. Thus the St. Lawrence River is drowned up to Montreal, and the Hudson up to Albany. If the drowned portion of the latter valley were not so narrow, it would be a bay. Delaware and Chesapeake Bays, as well as many smaller ones, both north and south, are likewise the drowned ends of river valleys (see figures, [Chapter VI]). If all parts of a drainage basin sank equally, the velocities of the streams above the limit of drowning would not be changed, for the gradients would remain the same as before. The fact that a river’s channel is below sea-level is not to be taken as proof that the valley is drowned. Thus the bottom of the channel of the Mississippi is as much as 100 feet below the level of the Gulf, some 20 miles above New Orleans.[64]

Differential movement. Warping.—Where a land surface on which a river system is established suffers warping, some parts going up and others down, the opposite movements being either absolute or relative, various phenomena would result. This may be illustrated by the accompanying diagrams (Figs. [161] and [162]), where the profiles of the streams are represented as warped from the positions represented by the dotted lines, to the positions shown by the full lines. The velocity will be accelerated below the points of differential elevation (between a and b, [Fig. 161], and between a and b, and c and d, [Fig. 162]), but checked above (above a, and between b and c, [Fig. 162]). Above an elevation which notably checks its flow, a stream is ponded. If the ponding is slight, a marsh may develop above the obstruction; if more considerable, a lake is formed. Lakes of this class are likely to be short-lived, since the ponded waters are likely to soon overflow and lower their outlet so as to drain the lake. The elevation which ponds the stream may be great enough and rapid enough so that the resulting lake finds an outlet by some course other than that originally followed by the stream. Where a stream holds its course across an uplift athwart its valley, either with or without ponding, it becomes an antecedent stream (see [p. 169]), since it has a course assumed before the latest deformation of the crust and in apparent disregard of present surface configuration. Thus the Columbia River holds its antecedent course across areas which have been uplifted (differentially) hundreds and even thousands of feet.[65] Some of the striking scenic features of this noble valley are the result of these changes in the country through which it flows. A lesser stream would have been diverted, as many of its tributaries have been. Even its course across the Cascade ranges is believed to be antecedent.[66]

Fig. 163, 164.—Piracy stimulated by warping. Uplift along axis 1–2.

Another peculiarity of valleys and streams resulting from changes of level is illustrated in [Fig. 2, Pl. XIV] (southern California). The main valleys of this part of the coast were developed when the land stood considerably higher than now. Later the subsidence of the coast converted the lower ends of the valleys into bays or fiords. The bays were then transformed into lagoons by deposition. Subsequent rise of the land or depression of the sea allowed the drainage from the old lagoons to cut across the deposits which had converted the bays into lagoons. The result is an old, wide valley above, suggested by a young one below.

If the warpings were considerable, much more decisive changes in drainage would result. Suppose the drainage of a given region to be represented by the streams in [Fig. 163]. If there is uplift along the axis 1–2, that part of ac above the axis of uplift would be ponded, or at least have its velocity checked, while the flow of some of the tributaries of d would be accelerated, and might work back and capture the other stream ([Fig. 164]).

Crustal warping was one of the conditions under which the Tennessee achieved its present anomalous course, and its history[67] is illustrative of the complex changes which drainage suffers when warping affects the area where the rock structures are of unequal resistance. At the close of the Cretaceous cycle of erosion, when the Appalachian Mountains had been reduced to a peneplain, the waters falling in the area now drained by the upper course of the Tennessee flowed south-south-west to the Gulf in a stream (the Appalachian River, a, [Fig. 165]) the lower part of which had the general position of the Coosa and the Alabama.

To the west of the Appalachian River, shorter streams flowed west and southwest into the Mississippi embayment ([Fig. 165]) by courses which are not now definitely known. The succeeding cycle of erosion was inaugurated by uplift and deformation of the peneplain. The axis of greatest elevation (AB, [Fig. 166]) was nearly parallel to the Appalachian River, and the effect of the differential uplift was to impose a greater task on this river (a, [Fig. 166]), which flowed along the axis of uplift, than upon the rivers which flowed westward and southwestward to the Mississippi embayment. The result was that the strongest of the southwesterly flowing streams worked its head back into the drainage basin of the Appalachian River, and captured, one by one, the head-waters of its westerly tributaries, establishing some such drainage relations as are shown in [Fig. 166]. Still later, after the land area of the region had been considerably extended by the withdrawal of the sea, the Appalachian River itself was reached by the invading stream, and its waters carried away to the Mississippi Bay by a course the lower part of which is thought to have corresponded approximately with the course of the present Black River (b, [Fig. 167]).

Fig. 165.—Shows the general position of the main drainage lines in the southern Appalachians at the close of the Cretaceous cycle of erosion. The lower part of stream b is made to follow the course of a portion of the present Tennessee.

Fig. 166.—Shows the general position of the main drainage lines in the southern Appalachians, after the capture of the westerly tributaries of the Appalachian River by stream b. Compare [Fig. 165].

Fig. 167.—A stage later than that shown in [Fig. 166]. The sea is represented as having withdrawn from a considerable area which was submerged at earlier stages (Figs. [165], [166]).

Fig. 168.—Shows the final change which resulted in the present course of the Tennessee. The land is represented as somewhat higher than now.[68]

Still later there was further deformation which caused additional changes in the drainage. The whole region was uplifted, relatively if not absolutely, but the uplift was differential, being greatest along the axis represented by AB, [Fig. 167]. The effect of the deformation was to stimulate the tributaries of the Ohio flowing north from this axis. Their growth was further accelerated by the weakness of the strata over which they ran. At the same time, the uplift to the south led the southwesterly flowing stream (b, [Fig. 167]) to discover relatively hard beds of rock in its lower course, and these beds retarded its down-cutting. The result was that a tributary of the Ohio (a, [Fig. 167]) finally tapped the main stream flowing to the southwest (b, [Fig. 167]) and carried its upper part over to the Ohio ([Fig. 168]). This was the beginning of the present Tennessee.