THE WORK OF RUXXIXG WATER. 



153 



an open joint in the bed of the stream (as at h, Fig. l-i2), some portion 

 of the water would descend through it. After reaching a lower level it 

 might find or make a passage through the rock to the river below the 



falls. If even a httle water took such a course, the flow would enlarge 



Fig. 140. — Effect of colinnnar structure on weathering. Material unconsolidated 

 Spur of south end of Sheep Mountain. (Lippincott, U. S. Geol. Surv.) 



its chamiel, making a passageway between the joint through which 

 the water descended and the valley below the falls (hcde, Fig. 142). 

 This passageway might become large enough to accommodate all the 

 water of the river. In this case, the entire fall would be transferred 

 from the position wliich it pre^'iously occupied \f) to the position ol 

 the enlarged jomt (6). The fall would then recede. The imdergroimd 

 channel between the old falls and the new would be bridged by 

 rock (;6/" and /'",, Fig. U3).. making a natural bridge. The natiu"a] 

 bridge near Lexington, Va. (Fig. 144), ahnost 200 feet above the 

 stream which flows beneath it., is beheved to have been developed 



