164 



WORK OF SNOW AND ICE 



GLACIO-FLUVIAL WORK 



The streams to which the melting of the ice gives rise are laden 

 with gravel, sand, and silt derived from the ice. Where the mud 

 is light-colored, the streams are sometimes described as "milky." 

 Where the amount of material carried is great, much of it is dropped 

 at a slight distance from the ice, the coarsest being dropped first. 

 Glacial streams are, as a rule, aggrading streams, and therefore 

 develop alluvial plains, called valley trains (Fig. 170), or, where they 



Fig. 170. Diagram to illustrate the profile of a valley train, and its relations 

 to the terminal moraine (m) in which it heads. 



enter lakes, bays, or other streams, deltas. In its transportation, 

 the river-borne drift is assorted, and after its deposition it is strati- 



Fig. 171. Esker of Punkaharju, Finland. 



fied. Glacial deposits in the upper part of a mountain valley are, 

 therefore, generally connected with glacio-fluvial deposits farther 

 down the valley. The silt, sand, and gravel of valley trains can, 

 as a rule, be distinguished from valley deposits of non-glacial origin 



