LAND WATERS STREAMS 147 



flows over the surface of a cultivated field gathers earthy matter, 

 and the process is continued all the way to the channel of the 

 istream. Thus sediment is gathered at the very sources of the 

 .streams. It is also contributed by all slope wash from whatever 

 part of the valley, and the stream gathers more load from its bed 

 wherever it flows with sufficient velocity over loose 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 of most streams is made 

 jup of material loosened in advance by the processes of weathering. 

 !The stream, and the waters which get together to make the stream, 

 jfind them ready-made; but rivers frequently wear rock which 

 is not weathered, for the principal valleys of the earth are in solid 

 rock, and many of them in rock of great hardness. How does the 

 stream wear the solid rock? 



When a stream flows over a rock bed, the wear which it accom- 

 iplishes depends chiefly on the character of the rock, the velocity 

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

 by bedding planes and joint planes, the water of a clear stream of 

 | even moderate strength may dislodge bits of the rock. This con- 

 Idition of things is often seen where streams run on beds of shale 

 'or slate. If the rock is hard and without bedding-planes and 

 ! joints, or if its layers are thick and its joints few, clear water will be 

 much less effective. If massive hard rock presents a smooth sur- 

 face 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 is sometimes 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 illustration furnishes a clue to the method by which the erosion 

 of solid rock in a stream's bed is effected. 



