420 GLACIAL GEAVELS OF MAINE. 



WERE.OSARS DEPOSITED BY SUBGLACIAL OR BY SUPERFICIAL STREAMS? 



Neglecting- basal melting, we divide the ice-sheet into a zone or area 

 of diffused superficial waters, a zone of superficial streams, and a zone of 

 subglacial streams. But these superficial streams are formed only where 

 there is considerable thickness of snow and ice, near the margin of the n^v^, 

 and seldom if ever would englacial matter get up to such a height in the 

 ice. These streams may have helped determine the courses of subglacial 

 streams, but they could not have deposited glacial gravels until the ice was 

 so far melted that the bottoms of their canyons approached so near to the 

 ground that they found englacial inatter to roll and transport. The height 

 to which basal morainal matter can rise in the ice, especially in a hilly 

 country, is quite uncertain, but most of the englacial matter must have 

 been low in the ice. Without assiimiug any definite height of the englacial 

 matter, we can safely affirm that if any osars were deposited by streams 

 that flowed in channels open above to the air, it was wlien the ice at the 

 place of deposition was rather thin. Such streams would not be the cor- 

 relatives of the surface streams found far up toward the ndv^, but rather 

 of those described by Russell near the extremity of the Malespina glacier, 

 or by Wright near the retreatal moraines of the Muir glacier. It has been 

 often assumed that those who maintain that the osars were deposited by 

 superficial streams mean that they were deposited far back from the 

 extremity of the glacier toward the neve, whereas, since most of the osars are 

 stratified, this hypothesis postulates channels cut down through the ice to the 

 ground or nearly to the ground, a condition that can occur only near the 

 distal end of the glacier, where the ice is not very deep. Such supposed 

 channels, open on the top to the air^ might have very different antecedents. 

 They might be formed by surface waters eroding and melting a channel 

 downward in the ice, they might have become open to the air by the 

 melting of the roofs of subglacial tunnels, or a subglacial tunnel might 

 have become stopped, either by sediment or by ice, whereby the stream 

 was forced to rise and overflow on the ice or form an englacial channel. 

 In case of a subglacial tunnel proving insufficient to conduct all the water, 

 a portion might often run off on the surface, as happens at the time of the 

 discharge of the Miirjelen See, and thus a single river might have both a 

 subglacial and a superglacial outlet. Such accidents might often be facili- 



