TESTS OF SUBGLACIAL OR SUPERFICIAL DEPOSITION. 421 



tated by a body of water rising above the moutli of the stream tunnel, 

 such as the sea, or a glacial lake, or even the dam found on the proximal 

 side of hills over whicli subglacial streams flow. Thus it might often 

 happen that the same osar river was in different portions of its course 

 subglacial, englacial, and superglacial. The important matter, from the 

 geological standpoint, is to be able to recognize the deposits of these 

 different kinds of streams in the field. We therefore make a preliminary 

 inquiry as to the tests by which to distinguish them. 



LENGTH OP RIDGE. 



I have been able to devise no crucial test between the two kinds of 

 streams depending on the length of the ridge, yet there is much to prove 

 that the deposits in a subglacial tunnel are more likely to be longer and 

 those in superficial channels shorter. We omit from this discussion the case 

 of subglacial streams becoming supei-ficial by the disappearance of their 

 roofs, since that is a late phenomenon which happened at some time to all 

 subglacial tunnels, and is of significance only when the deposit of gravel 

 continued after the collapse of the roofs. 



Obviously the normal place for the subglacial river is beneath the ice, 

 and the cases where it rises for a time into englacial or superglacial chan- 

 nels are exceptions. Such portions of its course must be shorter than the 

 subglacial. We may therefore eliminate from this comparison all except 

 two cases: The rising of a subglacial river onto the surface near the ice 

 front, like the kame river of the Malaspina glacier, and the case of the 

 channel supposed to be wholly due to superglacial waters. 



Regarding such terminal or marginal superglacial channels as those of 

 the Malaspina glacier, we must admit that the conditions under which they 

 occur are unusually favorable as compared with other glaciers or known 

 ice-sheets. This glacier is situated near sea level; it is so nearlj^ stagnant 

 that large areas have become covered with forest; it is in slow retreat, 

 though almost fossil, and has rather steep terminal slopes. For some 

 reason the glacial streams have either formed no subglacial tunnels under a 

 marginal zone of uncertain breadth, or the original tunnels have become 

 blocked by ice or sediment or moraines so that the streams have been 

 forced to form englacial tunnels, which become superglacial by the melt- 

 ing away of the overlying ice, and the streams continue such as they flow 



