GLACIAL SEDIMENTS. 305 



BASAL WATERS OF ICE-SHEETS. 



Ice-sheets covering- all the land obviously receive beneath them no 

 water from adjoining land bare of ice. 



The waters found beneath ice-sheets are due to various causes, as 

 follows: 



1. Water of surface melting that has gotten beneath the ice through 

 crevasses. This is by far the largest source of subglacial waters. 



2. Basal melting due to the internal heat of the earth. This normally 

 occurs under all the parts of the glacier and neve having a basal tempera- 

 ture of 32°. Assuming the correctness of Taine's estimate of the internal 

 heat, the annual basal melting- equals a stratum having a thickness of 0.36 

 inch covering an area equal to that of the ice. This might be modified by 

 the circulation of subterranean waters. 



The fact that the subglacial rivers continue to flow during the winter 

 has sometimes been in-ged as a proof of basal melting. But it is known 

 that in winter the larger crevasses become filled with a large amount of 

 snow, even down to the distal extremity of the glacier. This snow partly 

 melts, partly sinks into the depths, where it is only slowly consohdated to 

 ice. So also in the zone of surface slush there is a large quantity of snow 

 capable of holding water like a sponge. It is certain that there is a large 

 amount of unconsolidated snow on all large glaciers or within their wounds 

 that is saturated with water at the end of summer. These granular masses 

 act, like the soils and other porous strata, as reservoirs to moderate the 

 flow, and thus they hold back the water till long after surface melting has 

 ceased for the season. Waters of springs issuing from the earth would 

 continue to flow during the winter. We can thus account for large streams 

 continuing to flow from glaciers during the winter irrespective of basal 

 melting from the internal heat of the earth. Such melting in winter must 

 be proved by other evidence than the mere presence of water beneath the 

 glacier at that season. 



3. Basal melting caused by friction of the ice against its bed. 



• In connection with the friction of the ice against the underlying rocks 

 and till, we may also consider the friction of debris held in the ice against 

 the bed or of one piece against another. When we consider the great amount 

 of rock that was planed off beneath the ice-sheet and reduced to rock flour 

 or broken into fragments, we must conclude that the doing of so great an 



MON XXXIV 20 



