INFLUENCE OF JOINTS 



99 



commonly than elsewhere, and where they cross the hard layers it 

 is in most cases at right angles to the strike. This is shown in Fig. 



t)4, where the arrows indicate 61 10' S s_'oo' 



the direction of strike. 



As base-level is approached, 

 the outcrops of hard rock are 

 brought low. When the resist- 

 ant beds have been reduced to 

 base-level, streams may flow 

 without regard to the resistance 

 of the rock beneath, for down- 

 ward cutting has ceased. 



It happens in some cases 

 that rocks of unequal resistance 

 are covered by beds of uniform 

 hardness. A consequent stream 

 (p. 61) developed on the latter 



ar'oo 



may find itself out of structural 



adjustment when its channel 



is sunk to the level of the 



heterogeneous beds below. Such 



a stream is said to be superimposed (Fig. 95) on the underlying 



structure. Structural adjustment is likely to follow in time. 



Fig. 94. Adjusted drainage in a region 

 of folded rocks. The many nearly paral- 

 lel streams are flowing with the strike. 



INFLUENCE OF JOINTS ON EROSION 



It has been pointed 

 out that joint planes have 

 somewhat the same influ- 

 ence upon erosion that 

 bedding planes have when 

 the beds are tilted at a 

 high angle. Most rocks 

 are affected by joints, 

 and many of them are 

 nearly vertical. Two sets 

 are generally present, and 

 in some places more. When 

 there are but two, they 

 usually meet at a large 

 angle (Fig. 2). These 



Fig. 95. Diagram to illustrate superimposition. 

 The consequent stream on the upper formation 

 was superimposed on the underlying structures 

 when the upper bed had been cut through. 



