THE WORK OF STREAMS 



141 



be to narrow the valley there, while widening it above. Us- 

 ually, however, it is carried away by the stream. By the 

 widening of two adjacent valleys, the intervening ridge may 

 be worn out, the two becoming one (Fig. 130). The contin- 

 uation of this process among neighboring valleys would ulti- 

 mately reduce the entire surface of the area affected to the 

 level of the valley bottoms. 



Valley lengthening. The heads of valleys are usually 

 without permanent streams, for they are commonly above 

 the lowest level of the 

 ground-water surface. 

 The stream, therefore, 

 does not assist in the 

 lengthening of its valley 

 headward, but all the 

 other agencies which 

 widen valleys help also 

 to lengthen them. A valley ceases to grow by headward 

 erosion when a permanent divide (Fig. 131) is established. 

 This is when the wear accomplished by the run-off which 



a 



FIG. 131. Diagram of a divide. The 

 crest of the divide (at a) is permanent if 

 the conditions of erosion are the same on 

 the two sides. Rainfall may lower it, 

 but it cannot shift its position horizon- 

 tally. 



FIG. 132. Bad-land topography near Grand Junction, Colo. Shows many 

 gullies. (Baker.) 



enters the head of the valley is balanced by the erosion of the 

 water which runs from the divide in the opposite direction. 

 Thus limits are set to the growth of a valley in all three 



