120 



GEOLOGY 



a gully grown big by head erosion, and the valley would not pre- 

 cede the stream. 



If a narrow coastal plain is limited landward by a steeper slope, 

 valleys might develop as shown in Figs. 79 and 80. Again, in 



Fig. 79. Fig. 80. 



Figs. 79 and 80. Diagram to illustrate one mode of valley lengthening. 

 In Fig. 79 there are two small valleys, a and b, and the former ends at 

 the base of the steep slope. In Fig. 80, valley 6 is represented as having 

 been lengthened so as to join a, and the two have become one. 



mountain regions valleys are sometimes formed by the uplift of 

 parallel mountain folds, leaving a depression between (Fig. 81). 

 Drainage will appropriate such a valley so that it 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 



Fig. 81. Structural valley with a river valley developing in its bottom. 



a structural valley. A river valley may be developed in its bottom 

 (a, Fig. 81) and it may be in process of development throughout 

 the structural valley at the same time. 



These illustrations do not exhaust the list of conditions under 

 which valleys develop, but they suffice to show that valleys origi- 

 nate and develop in different ways. 



