O. P. Hay — Deposit of the Glacial Drift. 57 



so great, that it powerfully denuded those regions and, bearing 

 the debris along with it, constantly exposed a new surface to 

 erosion. As it was urged on over the region of the present 

 Great Lakes and was confined to comparatively narrow chan- 

 nels, it continued to gather up the wreck of the abraded strata 

 below : but when it had debouched from these channels and 

 had become spread out into a much broader sheet, its motion 

 was much slower, and its pressure on the underlying strata 

 much less than formerly. As it was in motion, however, it at 

 first planed off the surface of all parts of the country that were 

 not specially protected, and polished and scored the underlying 

 rocks, as we find them to-day. But the earth's heat was con- 

 stantly invading the lower layers of the glacier and melting it 

 away. Thus it would happen that a larger and larger propor- 

 tion of heavy materials would accumulate at the bottom of the 

 ice-sheet. Stones would also doubtless often reach the bottom 

 through crevasses, and streams of water from superficial melt- 

 ing would carry thither sand and clay. 



It can scarcely be doubted that this accumulation of coarse, 

 earthy, materials in the lower portions of the glacial mass 

 would greatly retard the movement there; and with the in- 

 crease of these materials, the retardation would go on until a 

 time would come when all movement in those lower layers 

 would cease, the small proportion of ice be melted out, and a 

 permanent deposit formed. Other horizons higher up would 

 then in their turn be similarly affected; and thus the bottom 

 moraine might attain almost any thickness. 



While contending that the great bulk of the Drift deposits 

 were formed as above described, we may believe with Prof. 

 Newberry and others that much of the glacial debris was car- 

 ried forward and deposited in terminal moraines ; and recently 

 a number of these moraines have been described by Profs. 

 Chamberlain and Wright, and others. We may again believe 

 with Prof. J. D. Dana that large quantities of detritus were held 

 in the body of the glacier and deposited on its final dissolution 

 during the Champlain. Such a deposit, not having been sub- 

 jected so long to the grinding action of the glacial mill, would 

 naturally contain more and larger stones than the deeper parts 

 of the Drift. Nor are we precluded from believing with Prof. N. 

 H. Winchell that there may have been, here and there, tracts of 

 soil visible on the face of the wide extended whiteness. 



The conclusions reached above may be thus summed up : 



1. A glacial ice-sheet moving over a nearly level surface 

 would possess far less power of abrading its bed than the same 

 glacier would have while descending a slope of high angle. 



2. Through subsidence of the glacial mass, caused by the 

 earth'5 heat, and through other influences, a constantly increas- 



