The Elements of Geology. William Harmon Norton
the lake basins of its area, are gradually effaced. By the furrowing action of the rain wash and the head ward lengthening, of tributaries a branchwork of drainage channels grows until it covers the entire area, and not an acre is left on which the fallen raindrop does not find already cut for it an uninterrupted downward path which leads it on by way of gully, brook, and river to the sea. The initial surface of the land, by whatever agency it was modeled, is now wholly destroyed; the region is all reduced to valley slopes.
Fig. 50. Drainage Maps
A, an area in its infancy, Buena Vista County, Iowa; B, an area in its maturity, Ringgold County, Iowa
Fig. 51. Successive Longitudinal Profiles of a Stream
am, initial profile, with waterfall at w, and basins at l and l´, which at first are occupied by lakes and later are filled or drained; b, c, d, and e, profiles established in succession as the stream advances from infancy toward old age. Note that these profiles are concave toward the sky. This is the erosion curve. What contrasting form has the weather weather curve (p. 34)?
The longitudinal profile of a stream. This at first corresponds with the initial surface of the region on which the stream begins to flow, although its way may lead through basins and down steep descents. The successive profiles to which it reduces its bed are illustrated in Figure 51. As the gradient, or rate of descent of its bed, is lowered, the velocity of the river is decreased until its lessening energy is wholly consumed in carrying its load and it can no longer erode its bed. The river is now at grade, and its capacity is just equal to its load. If now its load is increased the stream deposits, and thus builds up, or aggrades, its bed. On the other hand, if its load is diminished it has energy to spare, and resuming its work of erosion, degrades its bed. In either case the stream continues aggrading or degrading until a new gradient is found where the velocity is just sufficient to move the load, and here again it reaches grade.
Fig. 52. A V-Valley—the Canyon of the Yellowstone
Note the steep sides. What processes are at work upon them? How wide is the valley at the base compared with the width of the stream? Do you see any river deposits along the banks? Is the stream flowing swiftly over a rock bed, or quietly over a bed which it has built up? Is it graded or ungraded? Note that the canyon walls project in interlocking spurs
V-Valleys. Vigorous rivers well armed with waste make short work of cutting their beds to grade, and thus erode narrow, steep-sided gorges only wide enough at the base to accommodate the stream. The steepness of the valley slopes depends on the relative rates at which the bed is cut down by the stream and the sides are worn back by the weather. In resistant rock a swift, well-laden stream may saw out a gorge whose sides are nearly or even quite vertical, but as a rule young valleys whose streams have not yet reached grade are V-shaped; their sides flare at the top because here the rocks have longest been opened up to the action of the weather. Some of the deepest canyons may be found where a rising land mass, either mountain range or plateau, has long maintained by its continued uplift the rivers of the region above grade.
Fig. 53. Section of the Yellowstone Canyon
This canyon is 100 feet deep, 2500 feet wide at the top, and about 250 feet wide at the bottom. Neglecting any cutting of the river against the banks, estimate what part of the excavation of the canyon is due to the vertical erosion of its bed by the river and what to weathering and rain wash on the canyon sides
In the northern hemisphere the north sides of river valleys are sometimes of more gentle slope than the south sides. Can you suggest a reason?
The Grand Canyon of the Colorado River in Arizona. The Colorado River trenches the high plateau of northern Arizona with a colossal canyon two hundred and eighteen miles long and more than a mile in greatest depth (Fig. 15). The rocks in which the canyon is cut are for the most part flat-lying, massive beds of limestones and sandstones, with some shales, beneath which in places harder crystalline rocks are disclosed. Where the canyon is deepest its walls have been profoundly dissected. Lateral ravines have widened into immense amphitheaters, leaving between them long ridges of mountain height, buttressed and rebuttressed with flanking spurs and carved into majestic architectural forms. From the extremity of one of these promontories it is two miles or more across the gulf to the point of the one opposite, and the heads of the amphitheaters are thirteen miles apart.
The lower portion of the canyon is much narrower (Fig. 54) and its walls of dark crystalline rock sink steeply to the edge of the river, a swift, powerful stream a few hundred feet wide, turbid with reddish silt, by means of which it continually rasps its rocky bed as it hurries on. The Colorado is still deepening its gorge. In the Grand Canyon its gradient is seven and one half feet to the mile, but, as in all ungraded rivers, the descent is far from uniform. Graded reaches in soft rock alternate with steeper declivities in hard rock, forming rapids such as, for example, a stretch of ten miles where the fall averages twenty-one feet to the mile. Because of these dangerous rapids the few exploring parties who have traversed the Colorado canyon have done so at the hazard of their lives.
Fig. 54. Grand Canyon of the Colorado River, Arizona
The canyon has been shaped by several agencies. Its depth is due to the river which has sawed its way far toward the base of a lofty rising plateau. Acting alone this would have produced a slitlike gorge little wider than the breadth of the stream. The impressive width of the canyon and the magnificent architectural masses which fill it are owing to two causes. Running water has gulched the walls and weathering has everywhere attacked and driven them back. The horizontal harder beds stand out in long lines of vertical cliffs, often hundreds of feet in height, at whose feet talus slopes conceal the outcrop of the weaker strata (Fig. 15). As the upper cliffs have been sapped and driven back by the weather, broad platforms are left at their bases and the sides of the canyon descend to the river by gigantic steps. Far up and down the canyon the eye traces these horizontal layers, like the flutings of an elaborate molding, distinguishing each by its contour as well as by its color and thickness.
Fig. 55. Diagrams illustrating Conditions which produce Falls or Rapids
A, vertical succession of harder and softer rocks; B, horizontal succession of the same. In A the stream ab in sinking its bed through a mass of strata of different degrees of hardness has discovered the weak layer s beneath the hard layer h. It rapidly cuts its way in s, while in h its work is delayed. Thus the profile afb´ is soon reached, with falls at f. In B the initial profile is shown by dotted line.
The Grand Canyon of the Colorado is often and rightly cited as an example of the stupendous erosion which may be accomplished by a river. And yet the Colorado is a young stream and its work is no more than well begun. It has not yet wholly reached grade, and the great task of the river and its tributaries—the task of leveling the lofty plateau to a low plain and of transporting it grain by grain to the sea—still lies almost entirely in the future.
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