GLACIO-FLUVIAL DEPOSITS 165 



by the fact that they are largely of undecayed rock material, espe- 

 cially if deposited recently. 



Numerous streams flow from an ice-sheet, spreading their debris 

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

 gravel and sand (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 

 volleys. 



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 

 gnvel and sand, known as eskers (Fig. 171). It has been thought 

 that eskers represent deposits formed in superglacial channels; but 

 this is probably rarely if ever the case, for most surface streams have 

 high gradients, swift currents, and smooth bottoms, and hence give 

 little opportunity for lodgment. Furthermore, ice-sheets, in con- 

 nection with which eskers are developed, have no surface material 

 except at their immediate edges. 



At the mouths of ice-tunnels or ice-channels, and in the re-entrant 

 angles of the edge of the ice, sands and gravels are liable to be 

 bunched in quantity, giving rise, after the adjacent ice has melted, 

 to peculiar hills and hollows of knob-and-basin type. The hills 

 and short ridges of stratified drift formed in this way are known as 

 kamcs. Much stratified drift (gravel, sand, and silt) deposited by 

 glacial streams has no distinctive topographic form, and therefore 

 no special name. 



All fluvio-glacial deposits are stratified. Kames and eskers 

 made in immediate association with the ice, and more or less affected 



Fig. 172. The end of a glacier in Spitsbergen. (Rabot.) 



