LAND WATERS STREAMS 175 



Mountain system are intimately connected with the history just 

 outlined. Thus three great rivers, the Delaware, the Susquehanna, 

 and the Potomac, have their sources west of the Appalachians 

 proper, cross the system in apparent disregard of the structure, 

 and flow into the Atlantic. The James and Roanoke head far to 

 the west, although not beyond the mountain system, and flow east- 

 ward, while the New River (leading to the Kanawha) farther south, 

 heads east of the mountain-folds, and flows northwestward across 

 the alternating hard and soft beds of the whole Appalachian system, 

 to the Ohio (Fig. 143). The French Broad, a tributary to the 

 Tennessee, has a similar course. Such streams are clearly not in 

 structural adjustment, and afford good opportunities for piracy. 

 Their courses were apparently assumed during the time of the 

 Kittatinny base-level, when the streams had so low a gradient as 

 not to be affected by the structure. Elevation rejuvenated them, 

 and they have held their courses in succeeding cycles across beds 

 of unequal resistance, though smaller streams have become some- 

 what thoroughly adjusted. Crustal deformations have also helped 

 them to hold their courses, for the Cretaceous peneplain seems to 

 have been tilted to the southeast at its northern end, and to the 

 southwest at its southern, when the succeeding cycle began. 



Streams which hold their early courses in spite of changes which 

 have taken place since their courses were assumed are said to be 

 antecedent. They antedate the crustal movements which, but for 

 pre-existent streams, would have given origin to a different arrange- 

 ment of river courses. As a result of crustal movements, therefore, 

 a consequent stream may become antecedent. Master streams 

 are more likely to hold their courses, and therefore to become ante- 

 cedent, than subordinate ones. 



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 been the adjustment among 



