THE WORK OF RUNNING WATER. 



121 



rock of the stream's bed they are themselves worn by impact 

 with it and with one another. In all cases the softer material suffers 

 the more rapid wear. The first effect of wear on materials in 

 transportation is the reduction of their rugosities of surface. The 

 projecting points and sharp angles are worn off, and the stones are 

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

 grains of sand, or, if very fine, particles of silt or mud. Even after a 

 stone has been rounded it is subject to further wear and reduction, 

 and in the course of time may be literally worn out. 



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

 the bottom^ may wear the rock bed of a stream. When a grain of 

 sand in suspension escapes from an upward moving current it may 

 not sink quietly. If it be caught by a downward current it may be 



Fig. 102. — Some of the tools with which a stream works. The cobbles and bowlders 

 have been shifted by the stream in its flow. Other stones and bowlders now in 

 transit cause the ripples in the stream. The Chelan River, Wash., just above its 

 junction with the Columbia. (WilHs, U. S. Geol. Surv.) 



made to strike a blow on the bed of the stream, and the effect of the 

 blow is to wear the surface which receives it. The larger the grain 

 and the stronger the current the greater the wear. 



