. Elementary physical geography . Fig. 139. The Yellowstone, a young valley broadening by weathering and being deepened along a narrow line by the river erosion. be certain that, for one reason or another, erosion is now, orhas recently been in progress. That weathering is also pro-ducing an effect, is evident from the fact that the valley iswider at the top than at the bottom, because the former hasfor a longer time been exposed to its action (Fig. 139). Insuch cases the river is often a series of cascades or falls,because (see Chapter XVI.) in its rapid down-cutting, the RIVER VALLEYS. 269.
. Elementary physical geography . Fig. 139. The Yellowstone, a young valley broadening by weathering and being deepened along a narrow line by the river erosion. be certain that, for one reason or another, erosion is now, orhas recently been in progress. That weathering is also pro-ducing an effect, is evident from the fact that the valley iswider at the top than at the bottom, because the former hasfor a longer time been exposed to its action (Fig. 139). Insuch cases the river is often a series of cascades or falls,because (see Chapter XVI.) in its rapid down-cutting, the RIVER VALLEYS. 269. Fig. 140. A bit of drainage in Illinois, showing slightdevelopment of tributaries. stream finds rocks of different powers of resistance, andtherefore cuts its bed irregularly. Therefore, in addition togorges, waterfalls characterize youthfulness in river many cases lakes are also present; and since the processof lake destruction or re-moval is a simple and brieftask, they do not long re-main in the river valley. The development of thestream proceeds most rap-idly near its mouth, andlater in the headwaters ;and consequently, tribu-taries are not numerousat first (Fig. 140) ; butone by one they begin todevelop, until all of the area is brought under the influenceof some stream or rill (Fig. 141). At first the divides are not very definite, and they maybe flat-topped and swampy; butin maturity these become quitesharply defined, and usually everypart of the area is drained. When vertical erosion hasceased, the work of the river be-comes merely that of a transporterof sediment,
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