168 



PHYSICAL GEOLOGY 



outcrops of the weaker layers, while their tributaries join them 

 at right angles, producing a regular drainage pattern (Fig. 173). 



FIG. 171. Pawnee Buttes, Weld Co., Colorado. The dark beds are sand- 

 stone ; the light ones, shale. (Darton, U.S. Geol. Sun.) 



In such a region streams are in some cases diverted from 

 courses across hard layers to courses over soft layers. The 



method by which the 

 change is accomplished 

 may be illustrated from 

 Figures 174, 175, and 17G. 

 In Figure 174 the farther 

 stream crosses the resistant 

 ridge-making layers in 

 water gaps, and is unable 

 to cut its valley in the 

 weak rock just above the 

 gaps any faster than it 

 does in the hard rock at 

 the gaps. The nearer 

 stream does not cross the 

 hard beds and, therefore, 

 has cut its valley consider- 

 ably lower, and is lengthen- 

 ing it rapidly by headward 

 erosion. Presently it will 

 reach and enter the farther 

 valley, and the waters of 



FIG. 172. A valley formed along a 

 joint plane. Enfield Gorge, near 

 Ithaca, N.Y. (Tarr.) 



the latter above the point 



