It has been estimated that the surface of the United States is now being pared down by erosion at the average rate of an inch in 760 years. The derived material is being deposited in the flood plain and delta regions of its principal rivers. Some 513 million tons of suspended matter is in the United States carried to tidewater each year, and about half as much more goes out to sea as dissolved matter. If this material were removed from the Panama Canal cutting, an 85-foot sea-level canal would be excavated in about 73 days. The Mississippi River alone carries annually to the sea 340 million tons of suspended matter, or two thirds of the entire amount removed from the area of the United States as a whole. It is thus little wonder that great deltas have extended their boundaries so rapidly and that the crust is so generally sinking beneath the load. CHAPTER XIII 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). [Illustration: FIG. 175.—Gorge of the River Rhine near St. Goars, incised within an uplifted plain which forms the hill tops.] =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. [Illustration: FIG. 176.—V-shaped valley with well-rounded shoulders characteristic of the stage of adolescence. Allegheny plateau in West Virginia.]