GLACIERS 



227 



tion with their work which need be discussed. Like other 

 stream-laid beds, such deposits are in layers and consequently 

 unlike the till deposited directly by the ice. 



FIG. 245. Diagram to illustrate the building of a valley train. 

 Describe and account for what you see along the front of the ice sheet. 



Valley trains. Streams flowing away from glaciers in 

 valleys of moderate slope are generally overloaded with 

 debris derived from the ice and washed from tributary slopes 

 beyond the ice. They therefore make deposits along their 

 braided channels, building river plains of sand and gravel. 

 Such aggradational plains are valley trains (Fig. 245). The 

 stream deposits more and coarser material near the ice, and 

 less and finer sediment farther from it. The downstream 

 slope of valley trains is accordingly steepest near the ice 

 and increasingly gentle away from it (Fig. 246). Much of 

 the material of valley trains -is cross-bedded. 



Many remnants of valley trains, in the form of terraces, 

 occur along the rivers of northern and northeastern United 

 States. The longer the edge of the ancient ice sheet from 



FIG. 246. Diagram of a valley train, showing the slope of its surface, its 

 structure, and its relation to the terminal moraine in which it heads. 



which the aggrading streams issued remained stationary, 

 the greater the valley fillings. Heavy valley train deposits, 

 like massive terminal moraines, therefore indicate protracted 

 stands of the edge of the ice. 



