THE WORK OF STREAMS 



171 



evidence is there of weathering? (7) What are probably the chief 

 agents of weathering here ? (8) Is there evidence of diastrophism ? 

 If so, what ? 



FIG. 177. South Fork of Birch Creek, a tributary of the Yukon River, 

 Alaska. (Prindle, U.S. Geol. Surv.) 



STREAM DEPOSITS 



Causes of deposition. (1) Anything which checks the 

 velocity of a loaded stream occasions the deposition of sedi- 

 ment, (a) A decreasing gradient is an important cause, 



especially in the mid- 



die and lower por- 

 tions of large valleys. 

 (6) Rivers which flow 

 through regions of 

 scant rainfall fre- 

 quently lose water 

 both by rapid evapo- 

 ration and by sinking 

 into the ground (Fig. 



178). (Where in the Fl , G - 178. Tejunga River, southern Cali- 

 TT . . forma, sinking in the sand of its flood- 



United States IS this plain. (Fairbanks.) 



