950 STUDIES FOR STUDENTS 



seas. When the source of the water was the melting ice, the 

 water may have been running, when it was actively concerned in 

 the deposition of stratified drift, or it may have been standing 

 (glacial lakes and ponds) when it was passively concerned. 

 When ice cooperated with water in the development of stratified 

 drift the ice was generally a passive partner. 



GLACIAL DRAINAGE. 



The body of an ice-sheet during any glacial period is prob- 

 ably melting more or less at some horizons all the time and at 

 all horizons some of the time. Most of the water which is pro- 

 duced at the surface during the summer sinks beneath it. Some 

 of it may congeal before it sinks far, but much of it reaches 

 the bottom of the ice without refreezing. It is probable that 

 melting is much more nearly continuous in the body of a mov- 

 ing ice-sheet than at its surface, and that some of the water thus 

 produced sinks to the bottom of the ice without refreezing. At 

 the base of the ice, so long as it is in movement, there is doubt- 

 less more or less melting, due both to friction and to the heat 

 received by conduction from the earth below. Thus in the ice 

 and under the ice there must have been more or less water in 

 motion throughout essentially all the history of an ice-sheet. 



If it be safe to base conclusions on the phenomena of exist- 

 ing glaciers, it may be assumed that the waters beneath the ice, 

 and to a less extent the waters in the ice, organized themselves 

 to a greater or less degree into streams. For longer or shorter 

 distances these streams flowed in the ice or beneath it. Ulti- 

 mately they escaped from its edge. The subglacial streams 

 doubtless flowed, in part, in the valleys which affected the land 

 surface beneath the ice, but they were probably not all in such 

 positions. 



The courses of well-defined subglacial streams were tunnels. 

 The bases of the tunnels were of rock or drift, while the sides 

 and tops were of ice. It will be seen, therefore, that their 

 courses need not have corresponded with the courses of the val- 

 leys beneath the ice. They may sometimes have followed lines 



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