176 



EARTH FEATURES AND THEIR MEANING 



been invaded is said to have been beheaded. By this act of piracy 

 the stronger stream now develops exceptional activity because of 

 the local steep grades near the point of capture, and with this 

 newly acquired cutting power the invader is competent to ad- 

 vance still further and enter the territory of the stream that lies 

 next beyond. The type of drainage network which results from 

 repeated captures of this kind is known as " trellis drainage " 

 (Fig. 184), a type well illustrated by the rivers of the southern 

 Appalachians. 



In general it may be said that, other conditions being the 

 same, of two neighboring streams which have a common base 

 level, that one which takes the longest route will lose territory 

 to the other, since it must have the flatter average slope. Stream 

 capture may thus come about without the discovery of hard 

 rock layers which are more unfavorable to one stream than an- 

 other. 



Water and wind gaps. In the Allegheny plateau rivers cross 

 the range of harder rocks in deep mountain narrows which upon 

 the horizon appear as gateways through the barrier of the moun- 

 tain wall. Such gate- 

 ways are sometimes 

 referred to as "water 

 gaps," of which the 

 Delaware Water Gap 

 is perhaps the best 

 known example, 

 though the Potomac 

 crosses the Blue 

 Ridge at the historic 

 Harper's Ferry through 

 a similar portal. The 

 valley of the tributary 



Shenandoah has been the scene of an interesting episode in the 

 struggle of rival streams which is typical of others in the same 

 upland region. The records which may be made out from the 

 landscapes show clearly that in an earlier but recent period, 

 when the general surface stood at a higher level which has been 

 called the Kittatinny Plain, the younger Potomac of that "time 

 and a younger but larger ancestor of Beaverdam Creek each 



FIG. 185. Sketch maps to show the earlier and the 

 present drainage condition about the Blue Ridge 

 near Harper's Ferry. 



