200 W. Uril AM PLEISTOCENE AND PRESENT ICE-SHEETS. 



however, in Great Britain, and Chamberlin and Salisbury, with McGee 



and others, in this country, think that there was only very scanty 

 englacial drift. My reasons for believing that it was of large amount I 

 have stated in several recent papers, three of which have been presented 

 before this Society * 



Rapidity of final Ablation of the Ice-sheets. — The rates of observed abla- 

 tion of the Muir glacier and the Mer de Glace suggest that during the 

 closing stage of the glacial period the ice-sheets may have been melted 

 away very fast. If such ablation prevailed every summer for one or two 

 centuries, it must melt 2,000 ttf 4,000 feet of ice, which was approxi- 

 mately the thickness of the Pleistocene ice-sheet from central New Eng- 

 land westward across the Laurentian lakes to Minnesota and southern 

 Manitoba. This accords with the apparent duration of the glacial lake 

 Agassiz for only about 1,000 years, and with the evidently very rapid 

 accumulation of the eskers, of their associated sand plains and plateaus, 

 of the valley drift, and also, as I confidently think, of the drumlins. 

 There were indeed many times of halt or readvance of the ice-front, in- 

 terrupting its general retreat, as shown by the terminal moraines, of 

 which Professor Chamberlin, Mr Frank Leverettand others have mapped 

 no less than fifteen or twenty in their order from south to north in Ohio, 

 Indiana, Illinois, and southern Michigan; but such halts forming large 

 moraines on each side of Lake Agassiz were demonstrably of short con- 

 tinuance, only for a few decades of years, and the whole departure of the 

 ice-sheet from the southern end of this glacial lake to Hudson bay was 

 geologically very rapid. 



The causes for the sudden departure of the ice were therefore probably 

 as exceptional and unique in their character as for its accumulation, and 

 I think that they were indeed exactly the reverse of those before assigned 

 for the inauguration of the glaciation. Beneath the load of ice thousands 

 of feet thick the land sank, and when the ice-sheets were at their maxi- 

 mum area and during their retreat the glaciated lands stood mostly some- 

 what lower than now. The depression from the preglacial altitude was 

 virtually equivalent to a transfer of the Greenland ice-sheet to the present 

 temperate latitudes and comparatively low lands of the provinces of 

 Quebec and Ontario and the northern United States, where that vast 

 sheet of ice would be rapidly melted during all the summer months and 

 more slowly in spring and autumn, until within probably a few hundred 

 years it would entirely disappear. 



♦"Inequality of Distribution of the Englacial Drift," Bull. Geol.Soc. Am., vol. iii, 1891, pp. 134-148. 

 ''Criteria of Englacial and Subglacial Drift,"' Am. Geo!., vol. viii, pp. 37G-3S5, December, 1891. 

 " Conditions of Accumulation of Drumlins" (presented in preliminary outlines at the Rochester 

 meeting of the Geological Society, August, 1892), Am. Geol., vol. x, pp. 339-362, December, 1892. 

 "Eskers near Rochester, X. V." read before the Society at this Ottawa meeting. 



