



CORRASION 79 



of the stream, and the load it carries. If the rock is much divided 

 l>y bedding planes and joint planes, the water of a clear stream of 

 even modi-rate strength may dislodge bits of the rock. This con- 

 dition of things is seen where streams run on beds of shale'or slate. 

 If the rock is hard and without bedding planes or joints, or if its 

 layers are thick and its joints few, clear water is much less effective. 

 If massive hard rock presents a smooth surface to a clear stream, the 

 mechanical effect of even a swift current is slight. 



This general principle is illustrated by the Niagara River. Just 

 above the falls the current is swift. When the river is essen- 

 tially free from sediment, the surface of the limestone near the bank 

 beneath it sometimes is distinctly green from the presence of the 

 one-celled plants (fresh-water algae) which grow upon it. The whole 

 force of the mighty torrent is not able to sweep them away. Were 

 the stream supplied with a tithe of the sand which it is capable of 

 carrying, it would not take many hours, and perhaps not many 

 minutes, to remove the last trace of the vegetation. This illus- 

 tration furnishes a clue to the method by which the erosion of solid 

 rock in a stream's bed is effected. 



The gravel rolled along the channel wears even solid rock, and 

 as the moving stones wear the stream's bed, they are themselves 

 worn by impact both with the bed and with one another, and are 

 reduced to rounded, water-worn forms. The particles broken off 

 may make grains of sand, or, if very fine, particles of silt or mud. 

 In the course of time the pebbles and cobbles rolled along may be 

 literally worn out. 



The sediment carried in suspension, as well as that rolled along 

 the bottom, wears the rock bed of a stream. The coarser the sedi- 

 ment and the stronger the current, the greater the wear.- The 

 gravel, sand, and mud carried by a stream are therefore the tools 

 with which it works. Without them it is relatively impotent, so 

 far as the abrasion of solid rock is concerned; with them, it may wear 

 any rock over which it passes. 



Swift and slow streams corrade their valleys differently. The 

 erosion of a swift stream is chiefly at the bottom of its channel. 

 The sluggish stream lowers its channel less rapidly, or not at all, and 

 lateral erosion is relatively more important. The result is that slow 

 streams increase the width of their valleys more than the depth, 

 while swift streams increase the depth more than the width. It 

 follows that slow streams develop flats, while swift ones do not. 



