

CHANGES OF LEVEL 105 



uplift (relative or absolute), and a third cycle was inaugurated. 

 The third cycle began so recently that it has not yet advanced far. 



Some of the features just described are illustrated by Fig. 81. 

 The even mountain crest in the background is the Kittatinny 

 Mountain of New Jersey and its continuation in Pennsylvania. 

 In common with other corresponding crests, it is a remnant of the 

 oldest recorded base-level (or peneplain) of the region. Below the 

 mountain crest there is another plain, developed in a subsequent 

 cycle of erosion, while the valley plain in the foreground represents 

 the work of a still later cycle. 



Many of the peculiarities of the drainage of the Appalachian 

 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. The French Broad, a tributary to the Tennessee, has 

 a similar course. Such streams are clearly not in structural adjust- 

 ment, and afford good opportunities for piracy. Their courses were 

 apparently assumed during the time of the Kittatinny peneplain, 

 when the streams had so low a gradient as not to be affected by 

 structure. Elevation rejuvenated them, and they have held their 

 courses in succeeding cycles across beds of unequal resistance, 

 though smaller streams have become somewhat thoroughly adjusted. 

 Crustal deformations have also helped them to hold their courses, 

 for the peneplain seems to have been tilted to the southeast at its 

 northern end, and to the southwest at its southern, when the suc- 

 ceeding 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. 



