io6 



WORK OF RUNNING WATER 



The uplift of base-leveled beds, especially if the beds are tilted 

 so as to bring layers of unequal resistance to the surface at frequent 

 intervals, affords conditions favorable for extensive adjustment. 

 The numerous wind-gaps in the mountain ridges, representing the 

 abandoned courses of minor streams, and the less numerous water- 

 gaps, which indicate the resistance of large streams to structural 

 adjustment, are instructive witnesses of the extent to which ad- 

 justment has gone. So extensive has it been among the streams of 

 the Appalachian Mountains that there is probably no considerable 

 stream in the whole system which has not gained or lost through its 

 own or its neighbors' piracy. 



Sinking. The land on which a river system is developed may 

 be depressed relative to sea-level. In this case the sea occupies 

 the lower ends of valleys, converting them into bays and estu- 

 aries. A valley in this condition is said to be drowned. Of drowned 

 valleys there are many examples along the Atlantic coast. Thus 



Fig. 105 



Fig. 1 06 



Fig. 105. Chesapeake Bay and its surroundings. The bay is a drowned river 

 valley, and the lower ends of its tributary valleys are also drowned. 



Fig. 106. The drainage of the region about Chesapeake Bay as it would have 

 been but for drowning. 



