THE WORK OF RUNNING WATER 57 



that their bottoms and banks are of such fine, loose material that 

 even their slow currents can get it and carry it forward. 



Some river valleys are in solid rock, even in rock which is very 

 hard (Fig. 1, PI. XIII, p. 52). Do rivers gather load from such 

 valleys? In answering this question, we must remember, in the first 

 place, that rock exposed to water, as in the bed of a stream, 

 decays. As it decays, it crumbles, and the crumbled part is readily 

 swept away by a swift current. Again, the sand and gravel rolled 

 along by a stream wear its bed, even if it is of hard rock. The sedi- 

 ment which a stream carries, therefore, becomes a collection of tools 

 with which the running water works, and with these tools even hard 

 rock is worn away. The rock in which valleys are cut is sometimes 

 broken by cracks or joints (Fig. 2, PL XIII, p. 52), and in such cases 

 the stream may carry away the pieces if they are not too large. 



Clear water, flowing over a bed of firm, hard rock, effects little 

 mechanical wear. This is well shown in the case of clear streams 

 like the Niagara. Tiny plants, similar to those which make moist 

 stone walls green, may sometimes be seen growing on the limestone 

 of its bed, where the water is shallow enough to allow the bed to be 

 seen. This is the case even at the brink of the falls, where the cur- 

 rent is very swift, but where all the force of the mighty torrent is 

 unable to sweep the tiny plants away. If the stream had a partial 

 load of sand or mud, these little plants would be torn away in a 

 hurry. The sediment carried by a stream, therefore, helps it to 

 erode, especially where the bed is of solid rock. 



Carrying sediment. As already stated, coarse materials, such 

 as pebbles, are generally rolled along the bottom, while fine materials, 

 such as particles of mud, are often carried in suspension, that is, in 

 the water above its bottom. The movement of the coarse materials 

 rolled along the channel is easily understood, but the behavior of the 

 fine sediment in suspension needs explanation. 



Mud is composed chiefly of tiny particles of rock, nearly three 

 times as heavy as water; yet these particles, heavy as they are, 

 often remain in suspension for long periods of time. They are held 

 up in the water much as dust is held up in the air. Since they are 

 heavier than the water, they tend to sink all the time. They do in 

 fact sink; but as gravity brings them down, many of them are 



