143 



greater at such places than elsewhere, and tributary depres- 

 sions (gullies) will result. Even if an equal amount of water 

 entered the parent valley at all 

 points, the same result would follow, 

 provided the rocks of the valley sides 

 were of unequal strength, for erosion 

 would be more rapid where the rocks 



133. Cross section of 

 a young valley. 



were weaker, giving rise to tributary FlG - 



depressions. 



Stages in topographic development. It is apparent from 



the preceding paragraphs that valleys pass through careers 



just as men do. 

 Each stage in the 

 career of a valley 

 is characterized by 

 certain features, 



FIG. 134. Cross section of a mature valley. so *&** by observ- 

 ing the form of a 



given valley, one may determine readily what point it has 

 reached in its development. Valleys are young when still 

 narrow and steep - 

 sided (Fig. 133). 

 These features in- 

 dicate that as yet 

 down cutting is keeping ahead of other processes. Most young 



valleys have few and im- 

 perfectly developed trib- 

 utaries and relatively 

 steep gradients. (What 



things will determine 



FIG. 136. Diagram showing changes in whether or not young 

 the shape of a valley as it advances from 11 flrp fo^ <?\ Younff 



youth to old age. The material in which Valleys art 



the valley is cut is all of the same char- valleys in various stages 



of development are 



shown on Plate II. Mature valleys are wider, deeper, and 

 have gentler gradients and more and larger tributaries. In 



FIG. 135. Cross section of an old valley. 



