142 PHYSICAL GEOLOGY 



dimensions. In depth, the limit is base level ; in width and 

 length it is fixed by neighboring valleys. 



Struggle among valleys. It is not to be inferred from 

 what has preceded that all gullies become valleys, or even 

 ravines. Quite the opposite is true. Few of the gullies shown 

 in Figure 132, for example, can grow to ravinehood. As 

 they widen, the intervening divides will be worn out, combin- 

 ing adjacent gullies and reducing the number. Many gullies 

 are commonly destroyed in the formation of a single ravine, 

 which in turn is likely to presently find its growth contested by 

 other ravines. Such a conflict is shown in Plate II, among the 

 ravines near Wesley. Little opportunity for growth remains 

 to most of the ravines in the vicinity, and many are doomed 

 to early destruction by their more powerful neighbors. 



Valleys without gullies. Not all valleys have grown from 

 gullies as described above. In the northern part of the United 

 States and in Canada, for example, thousands of lakes were 

 formed during the Glacial period (p. 214). In the moist cli- 

 mate of this region, the lakes received more water as rain on 

 their surfaces and as run-off from tributary slopes, than they 

 lost by evaporation. Consequently, many overflowed their 

 rims, forming streams. Such streams followed the lowest 

 available lines of descent to other streams or lakes, and by 

 erosion developed valleys. Thus the streams existed before 

 the valleys. In this and other ways, streams and valleys of 

 this class are in contrast with those considered first. 



Tributary valleys. Most valleys have tributaries, and 

 these in turn branch repeatedly, like the limbs of a tree. A 

 main valley and all its tributary valleys constitute a valley 

 system, whose streams, the main river and all its branches, 

 form a river system. The entire area drained by a river system 

 is a drainage basin. Tributary valleys commonly start as 

 gullies on the sides of their parent valleys. If the slope of 

 the ground back from the sides of a valley is such that more 

 water enters it at some points than at others, the velocity, 

 and hence the erosive power, of the entering water will be 



