EARTH FEATURES SHAPED BY RUNNING WATER

The newly incised upland and its sharp salients.—The successive stages of incising, sculpturing, and finally of reducing an uplifted land area, are each of them possessed of distinctive characters which are all to be read either from the map or in the lines of the landscape. Upon the newly uplifted plain the incising by the young rivers is to be found chiefly in the neighborhood of the margins. In this stage the valleys are described as V-shaped caÑons, for the valley wall meets the upland surface in sharp salients (plate 12 A), and the lines of the landscape are throughout made up from straight elements. Though the landscapes of this stage present the grandest scenery that is known and may be cut out in massive proportions, often with rushing river or placid lake to enhance the effect of crag and gorge, they lack the softness and grace of outline which belong only to the maturer erosion stages. The grand caÑon of the Colorado presents the features characteristic of this stage in the grandest and most sublime of all examples, and the castled Rhine is a gorge of rugged beauty, carved out from the newly elevated plateau of western Prussia, through which the water swirls in eddying rapids (Fig. 175).

The stage of adolescence.—As the upland becomes more largely invaded as a consequence of the headward advance of the caÑons and their sending out of tributary side caÑons, the sharp angles in which the caÑon walls intersect the plain become gradually replaced by well-rounded shoulders. Thus the lines in the landscape of this stage are a combination of the straight line with a simple curve convex toward the sky (Fig. 176). In this stage large sections of the original plateau remain, though cut into small areas by the extensions of the tributary valleys.

The maturely dissected upland.—Continued ramifications by the rivers eventually divide the entire upland area into separated parts, and the rounding of the shoulders of valleys proceeds simultaneously until of the original upland no easily recognizable compartments are to be found. Where before were flat hilltops are now ridges or watersheds, the well-known divides. The upland is now said to be completely dissected or to have arrived at maturity. The streams are still vigorous, for they make the full descent from the upland level to base level, and yet a critical turning point of their history has been reached, and from now on they are to show a steady falling off in efficiency as sculpturing agents.

Viewed from one of the hilltops, the landscape of this stage bears a marked resemblance to a sea in which the numberless divides are the crests of billows, and these, as distance reduces their importance in the landscape, fade away into the even line of the horizon (Fig. 177).

The Hogarthian line of beauty.—Since the youthful stage of the upland, when the lines of its landscape were straight, its character rugged, and its rivers wild and turbulent, there has been effected a complete transformation. The only straight line to be seen is the distant horizon, for the landscape is now molded in softened outlines, among which there is a repeated recurrence of the line of beauty made famous by Hogarth in his “Analysis of Beauty.” As well known to all art students, this is a sinuous line of reversed or double curvature—a curve which passes insensibly at a point of inflection from convex to concave (Fig. 178). The curve of beauty is now found in every section of the hills, and it imparts to the landscape a gracefulness and a measure of restfulness as well, which are not to be found in the landscapes of earlier stages in the erosion cycle. In the bottoms of the valleys also the initial windings of the rivers within their narrow flood plains add silver beauty lines which stand out prominently from the more somber background of the hills.

Considered from the commercial viewpoint, the mature upland is one of the least adaptable as a habitation for highly civilized man. Direct lines of communication run up hill and down dale in monotonous alternation, and almost the only way of carrying a railroad through the region, without an expenditure for trestles which would be prohibitive, is to follow the tortuous crest of a main divide or the equally winding bed of one of the larger valleys.

The final product of river sculpture—the peneplain.—When maturity has been reached in the history of a river, its energies are devoted to a paring down of the valley slopes and crests so as to reduce the general level. From this time on hill summits no longer fall into a common level—that of the original upland—for some mount notably higher than others, and with increasing age such differences become accentuated. There is now also a larger aggradation of the valleys to form the level floors of flood plains, out of which at length the now slight elevations rise upon such gentle slopes that the process of land sculpture approaches its end. Gradually the vigor of the stream has faded away, and can now only be renewed through a fresh uplift of the land, or, what would amount to the same thing, a depression of the base level. Upland and river have reached old age together, and the approximation to a new plain but little elevated above base level is so marked that the name peneplain is applied to it. Scattered elevations, which because of some favoring circumstance rise to greater heights above the general level of the peneplain, are known as monadnocks after the type example of Mount Monadnock in New Hampshire (Fig. 179).

The river cross sections of successive stages.—To the successive stages of a river’s life it has been common to carry over the names from the well-marked periods of a human life. If neglecting for the moment the general aspect of the upland, we fix our attention upon the characteristic cross sections of the river valley, we find that here also there are clearly marked characters to distinguish each stage of the river’s life (Fig. 180). In infancy the steep, narrow, and sharp-angled caÑon is a characteristic; with youth the wider V-form has already developed; in adolescence the angles of the caÑon are transformed into well-rounded shoulders, and the valley broadens so as in the lower reaches to lay down a flood plain; in maturity the divides and the double curves of the line of beauty appear; while in the decline of old age the valleys are extremely broad and flat and are floored by an extended flood plain.

The entrenchment of meanders with renewed uplift.—Upon the reduced grades which are characteristic of the declining stage of a river’s life, the current has little power to modify the surface configuration. On the old land of this stage a renewed uplift starts the streams again into action. This infusion of driving power into moving water, regarded as a machine capable of accomplishing certain work, is like winding up a clock that has run down. Once more the streams acquire a velocity sufficient to enable them to cut their valleys into the land surface, and so a new erosional cycle may be inaugurated upon the old land surface—the peneplain. After such an uplift has been accomplished and the rivers have sunk their early valleys within the new upland, we may look out from this now elevated surface and the eye take in but a single horizontal line, since we view the plain along its edge.

By the uplift the meanders of the earlier rivers may become entrenched in the new upland, the wide lobes of the individual meanders being now separated by mountains where before had been plains of silt only. The New River of the Cumberland plateau and the Yakima River of central Washington (Fig. 181) furnish excellent American examples of intrenched meanders, as the Moselle River does in Europe. Upon the course of the latter river near the town of Zell a tunnel of the railroad a quarter of a mile in length pierces a mountain in the neck of a meander lobe in which the river itself travels a distance of more than six miles in order to make the same advance. The Kaiser Wilhelm tunnel in the same district penetrates a larger mountain included in a double meander of the river. Although intrenched, river meanders are still competent to scour and so undermine the outer bank, and with favoring conditions they may by this process erode extended “bottoms” out of the plateau. (See Lockport quadrangle, U. S. G. S.)

The valley of the rejuvenated river.—Whenever a new uplift occurs before an erosional cycle has been completed, the rivers become intrenched, not in a peneplain, but in the bottoms of broad valleys. The sweeping curves which characterize mature landscapes may thus be brought into striking contrast with the straight lines of youthful caÑons which with V-sections descend from their lowest levels (Fig. 182). The full cross section of such a valley shows a central V whose sharp shoulders are extended outward and upward in the softened curves of later erosion stages.

The arrest of stream erosion by the more resistant rocks.—The capacity of a river to erode and carry away the rock material that lies along its course is dependent not only upon the velocity of the current, but also upon the hardness, the firmness of texture, and the solubility of the material. Particularly in arid and semiarid regions, where no mantle of vegetation is at hand to mask the surfaces of the firmer rock masses, differences of this kind are stamped deeply upon the landscape. The rock terraces in the Grand CaÑon of the Colorado together represent the stronger rock formations of the region, while sloping talus accumulations bury the weaker beds from sight.

Each area of harder rock which rises athwart the course of a stream causes a temporary arrest in the process of valley erosion and is responsible for a noteworthy local contraction of the river valley. The valley is carved less widely as well as less deeply, and since a river can never corrade below its base, a “temporary base level” is for a time established above the area of harder rock. Owing to the contraction of the valley under these conditions, the locality is described as a river narrows (Fig. 183). The narrows upon the Hudson River occur in the Highlands where the river leaves a broad expanse occupied by softer sediments to traverse an island-like area of hard crystalline rocks. Within the narrows of a river the steep walls, characteristic of youth and the turbulent current as well, are often retained long after other portions of the river have acquired the more restful lines of river maturity. The picturesque crag and the generally rugged character of river narrows render them points of special interest upon every navigable river.

The capture of one river’s territory by another.—The effect of a hard layer of rock interposed in the course of a stream is thus always to delay the advance of the erosional process at all levels above the obstruction. When a stream in incising its valley degrades its channel through a veneer of softer rocks into harder materials below, it is technically described as having discovered the harder layer. Where several neighboring streams flow by similar routes to their common base level, those which discover a harder rock will advance their headwaters less rapidly into the upland and so will be at a disadvantage in extending their drainage territory. A stream which is not thus hindered will in the course of time rob the others of a portion of their territory, for it is able to erode its lower reaches nearer to base level and thus acquire for its upper reaches, where erosion is chiefly accomplished, an advantage in declivity. The divide which separates its headwaters from those of its less favored neighbor will in consequence migrate steadily into the neighbor’s territory. The divide is thus a sort of boundary wall separating the drainage basins of neighboring streams, and any migration must extend the territory of the one at the expense of the other. As more and more territory is brought under the dominion of the more favored stream, there will come a time when the divide in its migration will arrive at the channel of the stream that is being robbed, and so by a sudden act of annexation draw off all the upper waters into its own basin. By this capture the stream whose territory has been invaded is said to have been beheaded. By this act of piracy the stronger stream now develops exceptional activity because of the local steep grades near the point of capture, and with this newly acquired cutting power the invader is competent to advance still further and enter the territory of the stream that lies next beyond. The type of drainage network which results from repeated captures of this kind is known as “trellis drainage” (Fig. 184), a type well illustrated by the rivers of the southern Appalachians.

In general it may be said that, other conditions being the same, of two neighboring streams which have a common base level, that one which takes the longest route will lose territory to the other, since it must have the flatter average slope. Stream capture may thus come about without the discovery of hard rock layers which are more unfavorable to one stream than another.

Water and wind gaps.—In the Allegheny plateau rivers cross, the range of harder rocks in deep mountain narrows which upon the horizon appear as gateways through the barrier of the mountain wall. Such gateways are sometimes referred to as “water gaps”, of which the Delaware Water Gap is perhaps the best known example, though the Potomac crosses the Blue Ridge at the historic Harper’s Ferry through a similar portal. The valley of the tributary Shenandoah has been the scene of an interesting episode in the struggle of rival streams which is typical of others in the same upland region. The records which may be made out from the landscapes show clearly that in an earlier but recent period, when the general surface stood at a higher level which has been called the Kittatinny Plain, the younger Potomac of that time and a younger but larger ancestor of Beaverdam Creek each crossed the Blue Ridge of the time through similar water gaps (Fig. 185, map, and Fig. 186). The Potomac of that time was, however, the more deeply intrenched, and possessing an advantage in slope it was able to advance the divide at the head of its tributary, the Shenandoah, into the territory of Beaverdam Creek. Thus the beheading of the Beaverdam by the Shenandoah was accomplished (Fig. 185, second map) and its upper waters annexed to the Potomac system. With the subsequent lowering of the general level of the country which yielded the present Shenandoah Plain, the former water gap of Beaverdam Creek was abandoned of its stream at a high level in the range. Known as Snickers Gap, it may serve as a type of the “wind gaps” of similar origin which are not altogether uncommon in the Appalachian Mountain system (Fig. 186).

Character profiles.—For humid regions the landscapes possess characters which, speaking broadly, depend upon the stage of the erosion cycle. For the earliest stages the straight line enters as almost the only element in the design; as the cycle advances to adolescence the rounded forms begin to replace the angles of the immature stages, and with full maturity the lines of beauty alone are characteristic. As this critical stage is passed irregularity of feature and ever more flattened curves are found to correspond to the decline of the river’s vital energies. There are thus marks of senility in the work of rivers (Fig. 187).

Reading References for Chapters XII and XIII

General:—

Sir John Playfair. Illustrations of the Huttonian Theory of the Earth. Edinburgh, 1802, pp. 350-371.

J. W. Powell. Exploration of the Colorado River of the West and its Tributaries. Washington, 1875, pp. 149-214.

G. K. Gilbert. Report on the Geology of the Henry Mountains. Washington, 1877, pp. 99-150. (A classic upon the work of rivers.)

C. E. Dutton. Tertiary History of the Grand CaÑon District (with atlas), Mon. 2, U. S. Geol. Surv., 1882, pp. 264.

W. M. Davis. The Rivers and Valleys of Pennsylvania, Nat. Geogr. Mag. vol. 1, 1889, pp. 203-219; The Triassic Formation of Connecticut, 18th Ann. Rept. U. S. Geol. Surv., Pt. ii, 1898, pp. 144-153.

Sir A. Geikie. The Scenery of Scotland. London, 1901, pp. 1-12.

I. C. Russell. Rivers of North America. Putnam. New York, 1898, pp. 327.

M. R. Campbell. Drainage Modifications and their Interpretation, Jour. Geol., vol. 4, 1896, pp. 567-581, 657-678.

Henry Gannett. Physiographic Types, U. S. Geol. Surv., Topographic Atlas, Folios 1-2, 1896, 1900.

W. M. Davis. The Geographical Cycle, Geogr. Jour., vol. 14, 1899, pp. 481-504.

The flood plain:—

Henry Gannett. The Flood of April, 1897, in the Lower Mississippi, Scot. Geogr. Mag., vol. 13, 1897, pp. 419-421.

W. M. Davis. The Development of River Meanders, Geol. Mag., Decade iv, vol. 10, 1903, pp. 145-148.

W. S. Tower. The Development of Cut-off Meanders, Bull. Am. Geogr. Soc., vol. 36, 1904, pp. 589-599.

River terraces:—

W. M. Davis. The Terraces of the Westfield River, Massachusetts, Am. Jour. Sci., vol. 14, 1902, pp. 77-94, pl. 4; River Terraces in New England, Bull. Mus. Comp. ZoÖl., vol. 38, 1902, pp. 281-346.

River deltas:—

G. K. Gilbert. The Topographic Features of Lake Shores, 5th Ann. Rept. U. S. Geol. Surv., 1885, pp. 104-108; Lake Bonneville, Mon. I, U. S. Geol. Surv., 1890, pp. 153-167.

Charts of Mississippi River Commission.

G. R. Credner. Die Deltas, ihre Morphologie, geographische Verbreitung und Entstehungsbedingungen, Pet. Mitt. Ergh. 56, 1878, pp. 1-74, pls. 1-3.

The peneplain:—

W. M. Davis. Plains of Marine and SubaËrial Denudation, Bull. Geol. Soc. Am., vol. 7, 1896, pp. 377-398; The Peneplain, Am. Geol., vol. 23, 1899, pp. 207-239.

Intrenchment of meanders:—

W. M. Davis. The Seine, the Meuse, and the Moselle, Nat. Geogr. Mag., vol. 7, 1896, pp. 189-202.

Stream capture:—

N. H. Darton. Examples of Stream Robbing in the Catskill Mountains, Bull. Geol. Soc. Am., vol. 7, 1896, pp. 505-507, pl. 23.

Collier Cobb. A Recapture from a River Pirate, Science, vol. 22, 1893, p. 195.

William H. Hobbs. The Still Rivers of Western Connecticut, Bull. Geol. Soc. Am., vol. 13, 1902, pp. 17-22, pl. 1.

Isaiah Bowman. A Typical Case of Stream Capture in Michigan, Jour. Geol., vol. 12, 1904, pp. 326-334.

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