The oldest erosion plain of the Appalachian Mountains, the results of which are seen in the even-crested ridges so characteristic of the system, is sometimes called the _Kittatinny base-level_.[1] It was completed early in the Cretaceous period, and hence is sometimes known as the _Cretaceous base-level_. The next lower plain, imperfectly developed, has been called the Shenandoah Plain,[62] from the Shenandoah Valley where it is well seen (Fig. 132 and Fig. 2, Pl. XII). It is to be noted that the terms _base-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 definitions should be insisted on which differentiate these types more sharply than Nature has. 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 eastward, 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. 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 somewhat 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 arrangement 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 antecedent, than subordinate ones.