THE WORK OF RUNNING WATER 67 



Figs. 58 and 59, and if the head of the valley a is lower than the 

 valley it reaches, b, the valley a will steal the water which would 

 otherwise flow down 6. The valley a (Fig. 59) is thus lengthened. 

 Streams are sometimes lengthened at their lower ends. This is the 

 case where the sediment which they deposit at their lower ends 



Fig. 58. Fig. 59. 



Figs. 58 and 59. Diagrams to illustrate a phase of piracy. By the headward 

 growth of a, Fig. 58, it reaches b, and finally carries off its upper waters; 

 a, Fig. 59, is a pirate; 6, Fig. 59, has been diverted, and c has been beheaded. 



builds the land out into the sea. The stream then finds its way 

 across the new-made land. 



Summary. All valleys are being made deeper in some places 

 all the time; all valleys are being made wider all the time; and 

 some valleys are growing longer. All streams sooner or later 

 develop flats in their valleys, and these flats may increase in width 

 till the divides between them become low, or even until they are 

 worn away altogether. 



The History of Rivers and Valleys 



Since valleys grow year by year, they must formerly have been 

 smaller than now. If we could trace their history backward, we 

 should find that there was a time when the large valleys of the 

 present day were small, when many of the small valleys were only 

 ravines, and when the present ravines and gullies did not exist. 

 Going still farther back, we may imagine a time when even the large 

 valleys had a beginning. 



One method of valley birth and growth is illustrated by the 

 development of a gully, already outlined. A gully started during 



