LAND WATERS STREAMS 



167 



If above a waterfall, for example, there were an open joint in the 

 bed of the stream (as at b, Fig. 134), some portion of the water 

 would descend through it. After reaching a lower level it might 



Fig. 133. Effect of columnar structure on weathering. Material but partial- 

 nsolidated. 

 Geol. Surv.) 



. . . u para- 



ly consolidated. Spur of south end of Sheep Mountain. (Lippincott, 

 U. S. G 



find or make a passage through the~rock to the river at the falls. 

 If even a little water took such a course, the flow would enlarge 

 the passageway through the joint to the valley at the falls (bcde, 

 Fig. 134). This passageway might in time become large enough 

 to accommodate all the water of the river. In this case, the entire 

 fall would be transferred from the position which it previously 



!> ' ( ' 1 _ 



v ! K * S 



< * .*.'** i ; L 



s^^A-y.l 



Fig. 134. Diagram to illustrate the initial stage in the development of a 

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

 right. 



Fig. 135. A stage later than that shown in Fig. 134. 



occupied (/) to the position of the enlarged joint (6). The falls 

 would then recede. The underground channel between the old 



