280 GLACIAL GEAVBLS OF MAINE. 



forward (Niles^ and Spencer^), also over sand and gravel without disturb- 

 ing the stratification (Chamberlin^), may be due in part to radiant heat 

 absorbed by the bowlders or communicated directly to the ice. It is uncer- 

 tain to what depth solar heat penetrates the ice, yet during the decay of 

 the ice-sheet a time must have come when the sun could penetrate the thin 

 ice to the moraine stuff scattered through it. 



In attempting to sum up this controversy I find too much hypothesis 

 and too little fact. It is important to study in the field, if possible, the 

 effect of a large amount of englacial matter on the rate of flow. In the 

 present state of the case it must be considered doubtful if any large accu- 

 mulations of till have been made in this way. If there were such masses 

 of ice embayed because of the contained till, the till would be upper rather 

 than lower till, or at least a transition between them. The matter of the 

 lenticular hills has been thoroughly glaciated, and it remains to be proved 

 that the mutual friction of till fragments in a mass of partially stagnant 

 ice could simulate the greater attrition which must inevitably mark those 

 which have been ground against the solid rock or against each other at the 

 bottom of the ice. 



Some other questions demand attention. 



DRtlMLINS. 



1. Were the drumlins accumulated at the ice front during the retreat 

 of the ice at unequal rates, so that they are a form of terminal moraine? 

 It can be confidently answered that their shapes and materials are wholly 

 unlike those of the terminal moraines of Maine. 



2. During the final melting of the ice the surface would melt un- 

 equally, since the larger bowlders and deeper masses of till would par- 

 tially protect the ice beneath them from melting. There would be much 

 lateral sliding of till into the depressions thus formed on the surface, as 

 seen by Prof. G. F. Wright on the Muir glacier of Alaska, and this process 

 would originate trains of bowlders, and ridges and mounds of various 



1 Upon the relative agency of glaciers and subglacial streams in the erosion of valleys, Am. Jour. 

 Sci., 3d series, vol. 16, pp. 366-370, 1878. 



=^ Notes on the erosive power of glaciers as seen in Norway, Geol. Mag., new ser., Dec. Ill, vol. 

 4, pp. 167-173, 1887. 



' Observations ou the recent glacial drift of the Alps, Trans. Wisconsin Acad. Sci., Arts, and 

 Letters, vol. 5, pp. 258-270, 1877-81. 



