THE WORK OF SNOW AND ICE 



273 



deltas. In its transportation, the river-borne drift is assorted, and 

 after its deposition it is stratified. True glacial deposits in the 

 upper part of a mountain valley are, therefore, often connected 

 with glacio-fluvial deposits farther down the valley. The silt, sand, 

 and gravel of valley trains can often be distinguished from valley 

 deposits of non-glacial origin by the fact that they are more largely 

 of undecayed rock material, especially if deposited recently. 



Fig. 231. Esker of Punkaharju, Finland. 



From the ice-sheet, numerous streams flow, spreading their 

 debris in front of the terminal moraine, forming a broad fringing 

 sheet or " apron" (outwash plain) along it. Outwash plains have 

 much in common with piedmont alluvial plains. They differ from 

 valley trains chiefly in being shorter, wider, and not confined to a 

 valley. Where streams of considerable size form tunnels under 

 the ice, the tunnels may become more or less filled with water-worn 

 debris, and when the ice melts, the aggraded channels appear as 

 ridges of gravel and sand, known as eskers (Fig. 231). It has been 

 thought that eskers represent deposits formed in superglacial 

 channels; but this is probably rarely if ever the case, for the surface 



