164 



GEOLOGY 



exposing the more and the less resistant beds (H and S, Fig. 128, 

 respectively) in alternate belts along the flanks of the truncated 

 folds (truncated at ab and cd). The streams, especially the lesser 

 ones, now flow along the strike of the weaker beds much more 

 commonly than elsewhere, and where they cross the hard layers 

 it is usually at right angles to the strike. This is shown in Fig. 

 129, where the arrows indicate the direction of strike. 



As base-level is approached, the outcrops of hard rock are 

 brought low. When they have been reduced to the level of their 

 surroundings, the streams may flow without regard to the resistance 

 of the rock beneath, for the downward cutting has ceased. 



It sometimes happens that rocks of unequal resistance are 

 covered by beds of uniform hardness. A consequent stream 



79* 



Fig. 129 A. Streams of the upper Potomac basin, illustrating adjustment. 

 The northeast course of many of the streams is with the strike. 



