THE WORK OF RUNNING WATER. 



163 



accelerated at its debouchure, and it begins to excavate a new valley 

 in the bottom of its old one. The new valley commences at the lower 

 end of the old one, and develops headward {a and h, Fig. 153). Good 

 illustrations are furnished by the streams in the west central part of 

 New Jersey. The Delaware has here a sharply defined valley, and its 

 tributaries are essentially as deep as their main at the point of junction. 

 Above this point they have high gradients for a short distance (three 

 to six miles), beyond which they wind sluggishly in wide valleys with 

 low gradients across a relatively high plateau. Their profiles are 

 illustrated by Fig. 154. The flat, though high, surface in which their 

 upper courses lie, appears to have been nearly base-leveled in an earlier 

 cycle, and then to have been elevated. The date of the elevation is 

 fixed, in terms of erosion, by the time necessary for the excavation 

 of the Delaware gorge, and the narrow gorges along the lower courses 

 of its tributaries. It v/as so recent that the effects of rejuvenation, 

 proceeding from the debouchures of the tributaries toward their heads, 



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Fig. 154. — Profile of a rejuvenated stream. The Lockatong River (N. J.) to the 



head of Mud Run. 



have not yet advanced far from the Delaware. Similar relations are 

 found elsewhere (Fig. 1, PI. XIII, s. e. Col.). Another pecuharity of re- 

 juvenated drainage is shown in Fig. 2, Plate XIII (s. Kan.). Here Elm 

 Creek flows at a level 200 feet below that of Sand Creek, 4 miles distant. 

 The valley of the former appears to have entered upon a new cycle 

 as the result of uplift, while that of the latter, in the area shown on the 

 map, is still unrejuvenated. Farther down-stream, the valley of Sand 

 Creek shows signs of rejuvenation. It may be noted that a tributary 

 of Amber Creek has good opportunity to capture Sand Creek, for the 

 latter flows about 25 miles before reaching the level of Amber Creek 

 at its junction with Elm Creek. 



Should the lower end of a tributary valley fail to be degraded as 

 fast as the valley of the main at the point of junction, the tribu- 

 tary is out of topographic adjustment with its main. Falls or rapids 

 may result. When the lower end of a tributary valley is distinctly 



