LAND WATERS STREAMS 



165 



(p. 116) developed on the latter may find itself out of structural 

 adjustment when it has cut its channel down to the level of the 

 heterogeneous beds below. Such a stream is said to be super- 

 imposed (Fig. 130) on the underlying structure. Structural adjust- 

 ment is likely to follow in time. 



Influence of joints. It has been pointed out that joint planes 

 have somewhat the same influence upon erosion, that bedding 

 planes have when beds are tilted at a high angle. Most rocks are 

 affected by joints, and joints are often nearly vertical. Two sets are 

 generally present, and sometimes more. When there are but two, 

 they usually meet at a large angle (Fig. 2). These joints allow 

 the ingress of water, roots, etc., which help to weather and disrupt 

 rocks. Their effect on erosion may often be seen along a stream 

 which flows in a rock gorge. In such situations, the outlines of 



Fig. 130. 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. 



the banks are sometimes angular, and sometimes crenate (Fig. 131), 

 the re-entrants being located at the joints. By working into and 

 widening joints, running water sometimes isolates masses of rock 

 as islands (Fig. 132). 



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

 meagre, joints often determine the courses of valleys by directing 

 the course of surface drainage. This is shown in many parts of the 

 arid West. In regions where the rocks are notably faulted, the 

 courses of the streams are in many cases controlled by the courses 

 of the fault planes. 



