THE WORK OF RUNNING WATER. 169 



pleted early in the Cretaceous period, and hence is sometimes known 

 as the Cretaceous hase-level. The next lower plain, imperfectly de- 

 veloped, has been called the Shenandoah Plain,* from the Shenandoah 

 Valley where it is well seen (Fig. 132 and Fig. 2, PL XII). It is to be 

 noted that the terms hase-level and peneplain have both been used in 

 connection with these old plains. Graded plain is equally applicable. 

 The truth is that the topographic types represented by these three 

 terms grade into one another. It may be questioned whether defini- 

 tions should be insisted on which differentiate these types more sharply 

 than Nature has. 



Many of the peculiarities of the drainage of the Appalachian Moun- 

 tain system are intimately connected with the history just outhned. 

 Thus three great rivers, the Delaware, the Susquehanna, and the Po- 

 tomac, 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 eastward, while the New River (lead- 

 ing 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. 160). 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 (p. 150). 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 ante- 

 cedent. They antedate the crustal movements which, but for pre- 

 existent streams, would have given origin to a different arrangement 

 of river courses. As a result of crustal movements, therefore, a con- 

 sequent stream may become antecedent. Master streams are more 



1 Willis. Physiography of the United States. The Northern Appalachians. 



