182 



GEOLOGY 



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

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

 ains, 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 about almost every mountain range which 

 has been carefully studied, especially where plains are adjacent to 

 the mountains. It seems clear that similar deposits must have 



Fig. 150 A branching stream. Junction of the Cooper and Yukon rivers, 

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



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 

 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 reduces the size of the channel. In time it may be- 

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

 follows a new course over the valley flat. This process may be 

 repeated again and again (Fig. 150). Streams sometimes deposit 

 bars in their channels, especially in low water. The bars may be 



