THE WORK OF RUNNING WATER. 



therefore the normal growth of a valley involves its lengthening at its 

 lower end as well as at its upper. The lengthening of a valley, or at 

 least the lengthening of a stream, also takes place at its lower end if the 

 land in which it hes is being extended seaward by deposition. 



Structural valleys. — In mountain regions valleys are sometimes 

 formed by the uplift of parallel mountain folds, leaving a depression 

 between (Fig. 58). Drainage will appropriate such a valley so that it 



Fig. 58. — Structural valley with a river valley developing its bottom. 



becomes in some sense a river valley. But it is not a river valley in the 

 sense in which the term has been used in the preceding pages. It is 

 rather a structural valley. In its bottom a river valley may be devel- 

 oped (a, Fig. 58). 



The foregoing illustrations by no means exhaust the list of conditions 

 under which valleys develop, but they suffice for the present. 



The courses of valleys. — River valleys are rarely straight. To under- 

 stand why they are crooked it is only necessary to understand the 

 methods by which they grow. In so far as a river valley is a gully grown 

 big, that is, in so far as its length is the result of head erosion, its course 

 was determined by the course of the antecedent gully. If in the case 

 shown in Fig. 59 the slope of the 

 surface above the head of the gully 

 is uniform, its material homoge- 

 neous, and the rainfall everywhere 

 equal, more water will come into 

 the gully from the direction a than 

 from any other. In this case there 

 would be more wear in the direct 

 line of its extension than elsewhere, 

 and the head would advance in a 

 straight hne. But if there be inequalities of slope about the head 

 of a gully at any stage of its development more water may come in 



Fig. 59. Fig. 60. 



Figures to show why the head of a gully 

 (and therefore a valley) departs from 

 a direct course. 



