66 



WORK OF RUNNING WATER 



establish for themselves in youth. Thus the Potomac River 

 deepened its valley across the Blue Ridge (Fig. 52) faster than 

 Beaverdam Creek deepened its valley. The head of the young 

 Shenandoah River worked back and tapped Beaverdam Creek, 



THE 



KITTATINNY 

 PLAIN 



THE 



SHENANDOAH 

 PLAIN 



Fig- 52 Fig. 53 



Figs. 52 and 53. The capture of the head of Beaverdam Creek by the Shenan- 

 doah River. Virginia-West Virginia. (After Willis.) 



diverting its head waters to the Potomac (Fig. 53). The Shenan- 

 doah was a pirate, and Beaverdam Creek was beheaded. The stream 

 to which waters are diverted is increased in size, and the beheaded 

 stream is correspondingly diminished. 



J A Cycle of Erosion 



From what has preceded it is clear that the topography of a 

 region undergoing erosion will change greatly from time to time. 

 The first effect of erosion by running water is to roughen the sur- 

 face by cutting out valleys, leaving ridges and hills. The final 

 effect is to make it smooth again by cutting the ridges and hills 

 down to the level of the valley bottoms. When this has been done 

 the plain resulting is called a base-level. The time necessary to 

 produce a base-level is a cycle of erosion. 



Base-level, peneplain, grade. The development of a base-level 

 may be illustrated further in the light of the preceding discussion. 

 Suppose a land surface affected by a series of parallel young valleys 

 without tributaries (a and b, Fig. 54). On either side of them 



