170 GEOLOGY. 



likely to hold their courses, and therefore to become antecedent, than 

 subordinate ones. 



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

 as to bring layers of unequal resistance to the surface at frequent in- 

 tervals, affords conditions favorable for extensive adjustment. The 

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

 doned 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 adjustment has gone. 

 So extensive has been the adjustment among the streams of the Ap- 

 palachian Mountains that there is probably no considerable stream 

 \jx the whole system which has not gained or lost through its own or 

 its neighbors' piracy. The history of the rivers of the Appalachian 

 Mountains has been further comphcated by a considerable amount 

 of warping during the periods of uphft.^ 



'--^^ 



— — ^---^ 





C 



r^ot 



t^^/7 / /?;/*=»/ 



■ 





Figs. 161, 162. — Diagrams to illustrate the effect of crustal warping on stream erosion. 

 ' The dotted lines represent the profiles of the streams before deformation; the full 

 lines, after. Erosion will be stimulated between a and h in each case, and between 

 c and d in Fig. 162. Below 6, Fig. 161, the stream will be drowned, and erosion 

 therefore stopped. Erosion will also be stopped or retarded above a, between 

 h and c, and below d in Fig. 162. 



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

 depressed relative to sea-level. In this case the sea would occupy 

 the lower ends of valleys, converting them into bays and estuaries. 

 A stream in this condition is said to be drowned. Of drowned rivers 

 there are many examples along the Atlantic coast. Thus the St. 



1 For excellent accounts of the rivers of the Appalachian Mountains see Davis, 

 Rivers of Northern New Jersey, Nat'l Geog. Mag., Vol. II, pp. 81-110; and Rivers of 

 Pennsylvania, op. cit., pp. 183-253; Willis, The Northern Appalachians, Physiography 



