126 



GEOLOGY. 



where the beds dip up-stream, many more division-planes are crossed 

 in the same distance. Since bedding-planes are planes of weakness, 

 it follows that horizontal and nearly horizontal strata are not, under 

 ordinary conditions of erosion, in a position favorable for most rapid 

 wear. When strata are horizontal, it makes no difference which way 

 the stream runs, for the current sustains the same relation to the cleav- 

 age-planes whatever its course. 



In the case of incoherent material the position of the beds, or even 

 their existence, has little influence on the rate of erosion. Such forma- 

 tions are weak in all directions, not simply along bedding-planes. 



When the strata are vertical, three distinct cases may arise (Fig. 105). 

 The stream may flow (1) with the strike (aa) ; (2) at right angles to the 

 strike (65) ; or (3) obhque to it (cc) at any angle whatsoever. It is 

 perhaps not possible to say which of these positions is most favorable 

 for erosion, for the character of the rock, the thickness of its layers, 

 its abihty to stand with steep slopes, and the strength of the currents 

 concerned, would influence the result. A stream which flows at right 

 angles to the strike (hb, Fig. 105) would cross more cleavage-planes in 



a given distance than a stream 

 flowing in any other direction, and 

 would strike the outcropping edges 

 of layers at the angle of greatest 

 advantage. A stream flowing 

 along the strike (aa), on the other 

 hand, has better opportunity to 

 sink its channel on cleavage- 

 planes, and the current obhque 

 to the strike (cc), has some of the 



Fig. 105. — Diagram to illustrate the various advantages of each of the others. 



relations a stream may sustain to the When the strata are inclined 



outcrop of vertical layers of rock. „ . /-,\ rr^ 



nve cases may arise. (1) Ine 

 stream may be parallel to the strike {aa , Fig. 106), when it makes no differ- 

 ence which way the current flows; it may be at right anglas to the 

 strike (bh'), and (2) flowing with the dip (toward h'), or (3) against 

 it (toward h) ; it may be obhque to the strike, and flowing (4) in the 

 general direction of dip (toward c'); or (5) in the opposite direction 

 (toward c). As before, the stream flowing at right angles to the strike 

 would cross the largest number of layers in a given distance, and so 

 have an opportunity to take advantage of more cleavage-planes than 



