THE WORK OF RUNNING WATER. 119 



banks of the streams, to those which are partially submerged, and 

 sometimes to those altogether submerged beneath shght depths of 

 water. When the ice breaks up in the spring such bowlders, buoyed 

 up b}^ the ice, may be floated far down the stream. The influence of 

 ice in this connection is most considerable in high latitudes, but it is 

 of consequence as far south as Virginia, where the river deposits some- 

 times contain bowlders which the unaided streams could not have 

 carried. Ground ice sometimes forms about bowlders in the bottoms 

 of streams, especially in the quiet pools of turbulent rivers, and floats 

 them to the surface before the surface itself is frozen/ In the floods 

 of spring rivers often spread their ice widely over their flood-plains. 

 It is sometimes massed in constricted portions of valleys so as to form 

 great dams, the breaking of which is attended with great destruction. 



Corrasion. 



Abrasion. — The wear effected by running water is corrasion. So 

 long as the materials to be carried away are incoherent it is easy to 

 see how running water picks them up and carries them forward. The 

 water which gathers in the depressions on the slope of a cultivated 

 field gathers earthy matter from the surface over which it passes, 

 even before it is concentrated into rills, and the rills continue the pro- 

 cess. Thus the loose materials of the surface are gathered at the very 

 sources of the streams, and the amount of sediment in the water after 

 a heavy shower, even at the head of the stream, may be great. The 

 run-off from the slopes of any valley in any part of its course likewise 

 brings sediment to the stream, which gathers more from its bed where- 

 ever it flows with sufficient velocity over incoherent material. 

 Streams also undercut their banks, and receive new load from the 

 fall of the overhanging material. 



By far the larger part of the sediment acquired by a normal stream 

 is made up of material loosened in advance by the processes of weather- 

 ing. The stream, or the waters which get together to make the stream, 

 find them ready-made; but rivers frequently wear rock which is not 

 weathered, for the principal valleys of the earth's surface are cut in 

 solid rock, and many of them in rock of exceeding hardness. How 

 does the stream wear the solid rock? 



^ W. G. Thompson. Nature, Vol. I, p. 555, 1870. The Matapediac River, N. B. 

 Cited by Russell in Rivers of North America, p. 25. 



