530 THE JOURNAL OF GEOLOGY. 



be mentioned below. Whatever the terminology, it seems 

 important to distinguish the long branching gravel ridges that 

 represent the longitudinal drainage of the ancient glaciers from 

 those gravel accumulations that are associated with terminal 

 moraines, and that appear to be the debouchure deposits of glacial 

 waters. A variety of osars or eskers may have been produced 

 by superglacial streams, as will be recognized below. 



( 2 ) Simple tracts or patches of drift formed by subglacial drain- 

 age. — Thin sheets and lenses of sand and gravel in the midst of 

 subglacial till are common phenomena and, while they may in 

 many cases be produced by streams running in tunnels which 

 afterward shifted their position and left no other mark than these 

 patchy deposits, it seems probable that many of these detached 

 sheets and lenses were produced by a diffuse and local drainage 

 developed by any one of several combinations of conditions while 

 the ice was still present and continuing its deposition of till. 

 Similar patches on the surface were perhaps produced in a sim- 

 ilar way. 



2. Deposits of superglacial streams. 



For the most part superglacial streams, after short courses, 

 descend through crevasses or moulins and become englacial or 

 subglacial. This was doubtless true of the Pleistocene glacial 

 streams, and the material which they bore along found its final 

 deposition either in connection with subglacial streams or with 

 the glacial waters after they had emerged from the ice. Never- 

 theless, there are two forms of deposition by superglacial streams 

 that probably find some representatives among Pleistocene forma- 

 tions. The first embrace those which were carried along by 

 superglacial streams that succeed in reaching the edge of the ice 

 or a channel which they cut back into the margin of the ice. In 

 the one case it probably took the form of delta cones, in the other 

 of narrow ridges formed through the restraining aid of the channel 

 walls. The other variety is presumed to have been formed from 

 deposits which gathered along the course of superglacial streams 

 and were let down by the melting of the ice after the glacier 

 became stagnant, retaining essentially the form of their original 



