78 



PHYSIOGRAPHY 



natural bridge may be formed. Some of the water of such a stream 

 may descend through a crack (as at b, Fig. 76). After reaching a 

 lower level, it may find or make a passage through the rock to the 

 river at the fall. If even a little water follows such a course, it will 

 make its passageway (b c d e, Fig. 76) larger, and in time it may 

 become large enough to carry all the water of the river. The fall will 



Fig. 76. Diagram to illustrate the initial stage in the development of a 

 natural bridge. Longitudinal section at the left, cross-section at the 

 right. 



Fig. 77. A stage later than that shown in Fig. 76. 



then be shifted from its first position at / (Fig. 76) to 6. The fall 

 will then recede up-stream. The underground channel between the 

 old fall and the new would be bridged by rock (&/" and /", Fig. 77) . 

 A natural bridge of this sort is now in process of development in 

 Two Medicine River in northwestern Montana. The Natural Bridge 

 near Lexington, Va. (Fig. 2, PL XV, p. 60), almost 200 feet 

 above the stream which flows beneath it, was probably developed 

 in this way. It is not to be understood, however, that all natural 

 bridges have had this history. 



Narrows. A valley often becomes narrow where it crosses a 

 layer of hard rock. Such a constriction of the valley is a narrows, 

 or a water gap (Fig. 78). The Delaware Water Gap through the 

 Kittatinny Mountain (Pa.-N. J.) is a well-known example. Unlike 

 falls, narrows are not most conspicuous in the youth of the stream, 

 but later, after the valley has been much widened except where it 

 crosses the hard rock. Falls are common in horizontal or nearly 

 horizontal beds, but narrows are developed only in tilted beds. 



Narrows sometimes serve as gateways through mountains, and 



