ICE AGE IN NORTH AMERICAN ARCTIC — FLINT 247 



glaciated and that glaciers originated in these highlands. Data of 

 the same kind show that the vast lowland region between these high- 

 land groups was also glaciated — evidently by a thick and extensive 

 ice sheet. It was long believed that this ice sheet was built up by the 

 accumulation of fallen snow on the lowlands themselves. However, I 

 have put forward reasons for believing that the great lowland ice sheet 

 grew up through the coalescence and expansion of glaciers from the 

 Atlantic highlands (Flint, 1943, 1947; Demorest, 1943). According 

 to this view the ice sheet was in a sense an immigrant into the lowland 

 country rather than being indigenous. The manner in which the ice 

 sheet developed is not entirely settled, nor can it be until much more 

 has been learned about the glacial geology of the vast region surround- 

 ing Hudson Bay. The discussion that follows is based on the view 

 that the broad ice sheet invaded the lowland from the east, with 

 the reservation that this view is not yet fully proved and is therefore 

 subject to whatever modifications may be made necessary by further 

 study. 



The Cordilleran Glacier Complex. — The Cordilleran or high- 

 mountainous region of western North America, from Alaska to 

 Mexico, is dotted with glaciers today. The colder climates of the 

 glacial ages brought about so great an expansion of glaciers in this 

 region that from southern Washington northwestward to the Bering 

 Sea a complex of coalescent valley glaciers, piedmont glaciers, and ice 

 sheets covered a combined area of more than 950,000 square miles. 4 

 On the west this ice entered the Pacific and probably formed a floating 

 shelf similar to the shelf floating on the Koss Sea off the Antarctic 

 Continent today. On the east the ice descended to the plains where, 

 along a 1,700-mile front stretching from Glacier National Park in 

 Montana to the mouth of the Mackenzie at the Arctic Ocean, it 

 coalesced with the great lowland ice sheet. 



The distribution of the Cordilleran glaciers was controlled then, 

 as today, by two principal factors : high land and atmospheric mois- 

 ture. These controls are clearly evident in Alaska. There the thick- 

 est and most extensive glaciers formed on the coastal mountains, which 

 not only are the highest and most massive mountains in western North 

 America but also stand directly in the path of warm moist air masses 

 coming in from the Gulf of Alaska. The Brooks Eange in Northern 

 Alaska is lower and less massive, and faces the cold Arctic Ocean 

 rather than the warm Gulf of Alaska. In that range the glaciers were 

 correspondingly less extensive, thinner, and less active. Between the 

 coastal mountains and the Brooks Range the lowlands drained by 

 the Yukon River had no glaciers at all ; this intermontane country, 

 although cold, was too low and too dry to support them. 



" * For descriptions of the glaciation of representative districts see Kerr, 1934 ; Capps, 

 1932. 



236639—53 17 



