308 GLACIAL GEAVELS OF MAINE. 



6. There is another possible, though hardly probable, source of subgla- 

 cial waters, which we admit into our list simply as a subject for investiga- 

 tion. Possibly it depends for its basis wholly on our ignorance of the 

 structure of the nivd. It has often been observed that at the margin of 

 the snow fields the solid ice extends under the snow. In the Mount St. 

 Elias region Russell has seen it to a depth of 100 to 200 feet beneath the 

 snow. But the snow there does not melt at elevations above 13,000 feet, 

 but comes down as avalanches upon the ndvd. These conditions can not 

 be typical of ice-sheets, for though the latter may perhaps sometimes rise 

 above surface melting, there are no avalanches to compact the ice, nor any 

 crevasses to admit water from rocks nearly bare of snow. Both Russell 

 and Chamberlin regard it as probable that even in such a supposed ice- 

 sheet the dry ndve grows more compact as we go downward, and finally 

 becomes solid ice. A hole bored to the bottom of the Greenland n^ve 

 would answer all these questions of fact, but in the absence of observations 

 it must be considered as possible that there are conditions under which the 

 coarse granular snow or partially consolidated ice extends beneath the zone 

 of surface melting, so as to become charged with seeping water, and near 

 enough to the ground to permit its contained water to escape to the bottom 

 of the ice without the aid of crevasses as the grains are slowly pressed 

 together to form consolidated ice. This could happen only under snow 

 fields unbroken by crevasses. If this ever happens, the granular zone would 

 form the fountain head of subglacial streams. 



BASAL FURROWS AS STREAM TUNNELS. 



As the glacier flows over an obstruction a furrow is formed in the base 

 of the ice. Though viscous to a certain extent under ordinary pressures, 

 the ice can not at once fit itself to the lee side of the obstruction. This is 

 proved not only by the general laws of the flow of fluids but also by field 

 phenomena, such as the subglacial till that has been seen to gather beneath 

 the ice of a tongue that crossed a low part of a hill in Greenland, the 

 phenomenon of crag and tail, the existence of hollows in the rock that 

 were glaciated not at all or only imperfectly, etc. The ice does not always 

 change its direction and bend downward when the rock surface does so, 

 and thus small caves may exist beneath the ice. This is proved by the 

 facts elsewhere recorded as observed at Rockland. It is to be noted that 



