t 



CHANGES OF LEVEL 101 



In a region free from mantle rock, or where the mantle rock is 

 joints have determined the courses of many valleys by 

 directing the course of surface drainage. This is well shown in 

 many parts of the arid West. In regions where the rocks are faulted 

 the courses of some streams are controlled by the faults. It is 

 probable that joints and fault planes have been more important 

 in locating valleys, especially where the mantle rock is thin, than 

 was formerly recognized. 



Joints in rocks may occasion the development of natural bridges. 

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

 bed of the stream 

 (as at b, Fig. 98), 

 some portion of 



the water will de- 



, , , . rig. 98. Diagram to illustrate the initial stage in the 



tnrougn it. development of a natural bridge. Longitudinal section 



After reaching a at the left, cross-section at the right. 



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

 river at the falls. If even a little water takes such a course, the flow 



will enlarge the ,__ == __ _ j* j* 



passageway 

 through the joint 

 to the valley at 

 the falls (bcde ^8- 99- A stage later than that shown in Fig. 98. 



I-'ig. 98). This passageway may in time become large enough to 

 accommodate all the water of the river. The entire fall will then 

 be transferred from the position which it previously occupied (/) 

 to the position of the enlarged joint (b). The fall will then recede. 

 The underground channel between the old falls and the new will 

 then be bridged by rock (bf" and /'", Fig. 99). The natural 

 bridge near Lexington, Va. (Fig. 100), almost 200 feet above the 

 stream which flows beneath it, is believed to have been developed 

 in this way. It is not to be understood that all natural bridges l 

 have had this history. 



EFFECT OF CHANGES OF LEVEL 



Rise. If, after being base-leveled, or notably advanced in an 

 erosion cycle, a region is uplifted so as to increase the gradients 

 and velocities of its streams, they are said to be rejuvenated. 

 Renewed youth differs from first youth, in that the streams 



1 Cleland, Pop. Sci. Mo., May 'n, and Bull. G. S. A., Vol. XXI, p. 313. 



