ALLUVIAL DEPOSITS 



in 



is degraded later, and its materials spread more widely, as suggested 

 1>\ Kig. no. 



1 H-posits of this sort have probably been far more important in 

 the past than has been generally recognized. Much of the material 

 of the Coastal Plain of the Atlantic and Gulf slopes of the United 

 States appears to have been deposited in this way. A large part 

 of the Great Plains is covered with wash from the Rocky Moun- 

 tains, and similar deposits are of great extent and depth east of the 

 Andes and south of the Himalayas. They are, indeed, of signifi- 

 cant extent and depth on the plains about almost every mountain 

 range which has been carefully studied. It seems clear that similar 

 deposits must have been made at all stages in the past history of 

 the earth, whenever and wherever mountainous lands bordered 

 plains. 



Formations of this general sort, made at the bases of high lands, 

 have now been recognized among the ancient formations of the 



V\K. in. A branching stream. Junction of the Cooper and Yukon rivers, 

 Alaska. Shows also bars, etc. (U. S. Geol. Surv.) 



earth, as well as among the recent ones, and some of the ancient 

 beds of sediment deposited in this way attained thicknesses of 

 hundreds and even thousands of feet. They probably attained their 

 greatest thickness, as now, in basins. 



2. In valley bottoms. A stream which makes deposits in 

 its channel, makes the channel smaller. In time it may become 

 too small to hold all the water. A part then breaks out, and follows 



