306 



GEOLOGY. 



The most common form of such deposit is a valley train (Fig. .281) 

 of glacial wash stretching indefinitely down the valley. The silt/ sand, 

 and gravel of such trains can usually be distinguished from valley 



Fig. 282. — Alluviation bv glacial stream below Hidden glacier, Alaska. (Gilbert, 



U. S. Geol. Surv.) 



deposits of non-glacial origin by the character of the material, as much 

 of it is the product of grinding, crushing, and fracture, rather than of 

 ordinary surface decay. Its materials are, therefore, fresh and often 

 include rock material which, if long exposed at the surface, would be 

 decomposed or dissolved. 



Where an ice sheet ends in a broad face, as did the ancient conti- 

 nental glaciers, numerous streams flow from it and spread their debris 

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

 ^' apron" {outwash plain) along it. Where streams of considerable 

 size form tunnels under or in the ice, these may become more or less 

 filled with wash, and when the ice melts the aggraded channels appear 

 as long ridges of gravel and sand known as eskers (osars and serpentine 

 kames and kames of authors. See chapter on glacial period). It has 

 been thought that similar ridges are sometimes formed in valleys cut 

 in the ice from top to bottom, and even that they arise from gravel and 

 sand lodged in superglacial channels. The latter at least is probably 

 rare, as the surface streams usually have high gradients, swift currents, 

 and smooth bottoms, and hence give little opportunity for lodgment. 



