170 GEOLOGY 



with a canoe-shaped valley within. If two hard layers were in- 

 volved, instead of one, there would be two encircling ridges, with 

 a curved valley between them, and a canoe-shaped valley within 

 the innermost (Fig. 137). A succession of plunging anticlines 



Fig. 138. Cross-section of a portion of the Appalachian Mountains to il- 

 lustrate the phenomena of erosion cycles. (Rogers.) 



and synclines might give rise to a very complex series of ridges 

 and valleys. Illustrations of the above phenomena are found at 

 various points in the Appalachian Mountains. 1 



In the structural adjustment which goes with the erosion of 

 folds, it often happens that the valleys come to be located on the 

 anticlines, after they have been worn down, while the outcrops 

 of the hard layers on the flanks of the anticlines, or even in the 

 original synclines, become the mountains (Fig. 138). 



EFFECT OF CHANGES OF LEVEL 



Rise. If, after being base-leveled, or notably reduced by 

 erosion, a region is uplifted so as to increase the gradients and veloc- 

 ities of its streams, they are said to be rejuvenated, and a new cycle 

 of erosion is begun. The renewed youth differs from the first 

 youth, in that the streams are already in existence in well defined 

 channels. The rejuvenated streams erode their valleys after the 

 manner of youthful streams. They excavate new valleys in the 

 bottoms of older ones (Figs. 139 and 140), deepening them, or 

 tending to, until they reach the new grade plane. 



The new valley in the old one may be developing all along its 

 course at the same time, or it may begin at the debouchure of a 

 stream and work headward. In either case, the tributaries are re- 

 juvenated when their main is lowered at the point of union. 



Another evidence of rejuvenation is found in entrenched mean- 



1 See Willis, The Northern Appalachians, in Physiography of the United 

 States. 



