IO2 



WORK OF RUNNING WATER 



are already in existence. The rejuvenated streams erode their 

 valleys after the manner of youthful streams. They excavate new 



valleys in the bottoms of older 

 ones (Figs. 101 and 102), deep- 

 ening them until they reach the 

 new grade plane. Young val- 

 leys in the bottoms of old ones 

 are one of the evidences of re- 

 juvenation. The new valley in 

 the old one may be developing 

 all along its course at the same 

 time, or it may begin at the de- 

 bouchure of a stream and work 

 headward. In either case, the 

 tributaries are rejuvenated when 

 their main is lowered at the 

 point of union. 



Another evidence of rejuve- 

 nation is found in entrenched 

 meanders. When an old wind- 

 ing stream is rejuvenated, the 

 deepened channel follows the 



course of the stream before re- 

 Fig. 100. The Natural Bridge of , rrvr i, ,1 

 Virginia. (U. S. Geol. Surv.) juvenatiorh 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 (Fig. i,Pl. IX), and 

 are known in other parts of the 

 world. 1 With rejuvenation of 

 the drainage, a new cycle of ero- 

 sion is begun, whether the pre- 

 ceding one was complete or not. 

 The principles involved in 

 the recognition of cycles of ero- 

 sion, separated by uplifts, are 



Fig. 101. Cross-section of a wide 

 valley, ab, in the bottom of which a 

 younger valley, cd, has been excavated, 

 as the result of uplift. 



illustrated by Fig. 103, which represents an ideal profile of consider- 

 able 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 



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

 pp. 181-202, and 228-238. 



