GROu:^rD moraine. 279 



into the ice before it had been much overridden and glaciated; that the 

 region of chief deposition of the subglacial till was near the front of the 

 ice-sheet, where the ice was thinner; that back of this region there was 

 another zone, perhaps found not very far below the distal margin of the 

 n^v(l, where pressure and velocity of motion united to produce great ero- 

 sion of the rock and transportation of subglacial debris, while under the 

 nevd transportation was less active, whether because of the depth, or the 

 structural condition of the snow and ice, or other causes, is unknown. But 

 while the details are thus uncertain, the fact of the existence of the ground 

 moraine is satisfactorily established. 



In the foregoing discussion it has been assumed that the ground moraine 

 was wholl}^ formed between the ice and the rock, and that the flowing out- 

 lines of accumulations were carved by the ice flowing over them. Thei'e 

 is a possible alternative theory that perhaps ought to be noticed. No mat- 

 ter whether we consider the flow of the ice as plastic or viscous, we can 

 conceive of a mass whose internal friction is so great — forming, as it were, a 

 mass of till infiltrated with films and threads of ice — that it could remain 

 embayed while the purer ice flowed over or around it. Such an embayed 

 mass (half-till, half-ice) would be carved into the lenticular foi'm as the 

 glacier flowed over it, just as if it were a mass of true subglacial moraine. 



In reply to this it may be said that if ddbris scattered through the 

 lower part of the ice were reached by heat rays from the sun or other 

 source of heat external to itself it would absorb the heat and, by melting 

 the ice in contact with it, might increase the fluency (or plasticity) even 

 more than the friction of the stones diminished it. Certain bowlders in the 

 Alps have been supposed to rise upward in the ice in consequence of the 

 absorption of solar heat by their upper surfaces.^ The instances where thin 

 glaciers have been observed to flow over bowlders without pushing them 



'It has been oontenaed that rocks •warmed from above naturally rise in the ice. This is doubt- 

 ful as a general proposition. The ice melts with contraction of volume, leaving a small cavity above 

 the water. Molecular heat could then no longer produce melting except where the ice was in contact 

 with the water. The upward melting could proceed from radiant heat alone, while the radiant molec- 

 ular heat communicated to the water would be largely transferred to the bottom of the cavity, the 

 water of 39'' seeking the bottom. Whether as a net result the melting would be most rapid upward 

 or downward would depend on the size of the fragments, their shape, etc. If it be contended that 

 the ice from beneath would push the atone upward fast enough to fill the cavity of melting, we must 

 demand the proof of such action against the force of gravity. This note applies only to the supposed 

 rising of ddbris in the ice owing to heat from above. Whether vertical ice movements could raise the 

 debris is a very different question. 



