LAND WATERS STREAMS 



171 



ders. If an old winding stream is rejuvenated, the deepened 

 channel follows the course of the stream before rejuvenation. The 

 result is that a new winding gorge 

 is cut; that is, the old meanders 

 are entrenched. Entrenched 

 meanders are rather common in 

 the Appalachian Mountains (PI. 

 XI), and are known in other 



Fig. 139. Cross-section of a wide 

 valley, ab, in the bottom of which a 

 younger valley, cd, has been ex- 

 cavated, as the result of uplift. 



parts of the world. 1 Entrenched 



meanders and new valleys in 



the bottoms of old ones are 



among the commoner marks of rejuvenation. With rejuvena- 

 tion of the drainage, a new cycle 

 of erosion is begun, whether the 

 preceding one was complete or 

 not. 



The recognition of different cycles 

 of erosion, separated by uplifts, 

 is often easy. The principles in- 

 volved are illustrated by Fig. 141, 

 which represents an ideal profile of 

 considerable length (say 20 miles). 

 The points a, a', and a" have about 

 the same elevation. Below them 

 there are areas b, b' ' , and b", which 

 have a nearly common elevation, 

 below which are the sharp valleys 

 d, d' , and d". The points a, a', 



T-,. and a" represent the cross-sections 



* ig 140. Diagram to illustrate ., A , ,, 



an ideal case of rejuvenation of ridges formed by the outcrops of 



as the result of uplift. The layers of hard rock. If the crests 



black area at the bottom ,, . , , , ,, 



represents the sea. f the ridges are level, the points 



a, a', and a" must represent rem- 

 nants of an old base-level, since at no time after a ridge of hard 



. Davis. The Seine, the Meuse and the Moselle. Nat. Geog. Mag., Vol. 

 VII, pp. 181-202, and 228-238; an article which throws much Jight on the 

 behavior of rivers. 



