90 GEOLOGY. 



their flat-bottomed valleys (Fig. 1, PI. VII, Central Kansas) and their 

 erosion, except in time of flood, is largely lateral. 



If the processes of degradation were to continue until the land 

 surface was brought to sea-level, and this might be done by solution 

 though not by mechanical erosion of running water, the rivers would no 

 longer flow, and the drainage system would have reached the end of 

 its history — death. 



Not only do valleys normally pass from birth to youth, from youth 

 to maturity, and from maturity to old age, but a single river system 

 may show these various stages of development in its various parts. 

 Thus in the area shown in Fig. 2, Plate VII (north central Kansas), 

 there is a tract (extreme southwest) where the erosion history is scarcely 

 begun. The zone of land a httle farther northeast, and just reached 

 by the heads of the valleys (same figure), is in its youth. The well- 







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Fig. 74. — A peneplained surface where the elevations are small but steep-sided. 

 Near Camp Douglas, Wis. (Atwood.) 



drained and uneven tract southwest of the flat of the Solomon River 

 is in maturity, while the flat of the main valley has the general charac- 

 teristics of old age. 



The age of valleys in terms of erosion is also expressed more or 

 less perfectly by their cross-sections. The line 1-1 (and I'-l') of Fig. 64 

 represents in cross-section a narrow V-shaped valley. Such a section 

 is always indicative of youth. The stream which developed it cut 

 chiefly at its bottom, not at its sides. It was therefore rapid, and 

 rapid streams are young. 'The line 2-2, (2'-20 (Fig. 64) shows the 

 same valley at a later and maturer stage when downward cutting 

 has nearly ceased. The widening of the valley by slope wash has 



