248 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 2 



In southern Yukon and northern British Columbia, where coastal 

 mountains are lower, enough moisture was carried across the coastal 

 barrier to build up a thick ice sheet in the rough but lower country 

 between the Coast Ranges on the west and the Rocky Mountains on 

 the east. 



The Laurentide Ice Sheet. — Most of North America east of the 

 Rocky Mountains was overspread during the glacial ages by a vast, 

 coalescent mass of ice to which the name Laurentide Ice Sheet has 

 been given (G. M. Dawson, 1890, p. 162) . 5 The area of this glacial 

 carapace at its maximum probably exceeded 5 million square miles. 

 Its exact eastern limits are not known because the ice extended to sea- 

 ward of the present coast and the evidence is therefore submerged. 

 To the north the vast glacier covered the southern and eastern islands 

 of the Canadian Arctic Archipelago and overspread the network of 

 narrow straits that separated them. Whether the most northerly and 

 westerly islands were ever wholly buried beneath ice is not known, 

 simply because the geology of much of the region has never been 

 investigated. 



The ice grew thick enough to overtop most or all of the highlands in 

 northeastern North America, and may possibly have reached an ex- 

 treme thickness of 10,000 feet, although this figure is a matter of 

 conjecture. 



Greenland Ice Sheet* — Eastward across Baffin Bay and Davis Strait 

 the Greenland Ice Sheet, during the glacial ages, was thicker and more 

 extensive than it is today, as is clearly shown by the ubiquitous signs 

 of glaciation both vertically above and outward beyond the existing 

 ice sheet. It can hardly be doubted that the Greenland and Lauren- 

 tide ice bodies were firmly coalescent across the narrow straits that 

 separate northwest Greenland from Ellesmere Island. It is even pos- 

 sible that the two ice sheets, partly aground and partly afloat, were 

 coalescent across Baffin Bay and Davis Strait. 



Despite its great area, the Greenland Ice Sheet failed to cover the 

 extreme northeastern tip of Greenland, just as it fails to do today. 

 The explanation lies in lack of nourishment. In that region the pre- 

 cipitation is now very small, and during the glacial ages it must have 

 been as small or smaller, literally starving the northeastern tip of 

 the Greenland Ice Sheet. 



GROWTH AND DISAPPEARANCE OF THE GLACIERS 



The hypothesis that best fits the facts now known is that each of the 

 glacial ages began with a world-wide gradual reduction of tempera- 

 ture. In consequence the proportion of snowfall to rainfall increased, 



8 For good descriptions of some of the glacial features see Bell, 1890 ; J. W. Dawson, 

 1893 ; Tanner, 1944, pp. 173-253. 



6 Systematically discussed in Kayser, 1928. 



