ICE AGE IN NORTH AMERICAN ARCTIC — FLINT 255 



reason to believe that in preglacial time this master stream included 

 the drainage of the Missouri River in North Dakota and Montana, but 

 that that drainage was diverted toward the south by the expanding 

 ice sheet. 



The absence of highly elevated marine features from the western 

 and northern coasts of Alaska results probably from the fact that 

 western and northern Alaska were but scantily covered with ice, so 

 that disturbance of crustal equilibrium there was small. Postglacial 

 emergence of the Pacific coast of Alaska and British Columbia has 

 been conspicuous; this is in keeping with the known thick cover of 

 glacier ice in that coastal region. 



CHRONOLOGY AND CAUSES OF GLACIATION 



The North American Arctic yields very little direct information 

 either on the actual dates of events in the glacial past or on the prob- 

 able causes of the glacial climates. Our knowledge of these matters, 

 still very scanty at best, comes chiefly from the glacial drift sheets 

 in the temperate zones of North America and Europe. Until the 

 use of the radiocarbon method of dating, developed by Libby (1952), 

 it was assumed that the time elapsed since the shrinking ice sheet 

 began to uncover the very young Mankato drift was 25,000 years. 

 Radiocarbon measurements of wood from the Two Creeks peat, im- 

 mediately underlying the Mankato drift, has shown that the age of 

 the wood is only about 11,000 years (Flint and Deevey, 1951). As 

 yet the radiocarbon method is directly applicable only to organic 

 matter less than 30,000 years old. Therefore the degree of chemical 

 alteration of each of the several drift sheets still furnishes the best 

 chronology, inaccurate though it is. At present the lengths of the 

 glacial ages can only be guessed at, but they are widely regarded as 

 having been much shorter than the interglacial ages. The whole 

 group of four glacial and three interglacial ages together is believed 

 to have lasted roughly one million years. 



Because the interglacial times are believed to have been longer than 

 the glacial times, the Arctic has been largely free of a glacier-ice 

 covering during the greater part of the Pleistocene epoch. However, 

 whatever thin soils may have been developed over the surface of the 

 bedrock during the ice-free interglacial ages were almost wholly 

 swept away by the intervening glaciations. 



The fluctuations in the mean annual temperatures of temperate 

 latitudes during the Pleistocene epoch seem to have been no more than 

 10°C. — roughly 8° colder than now during the glacial ages and 2° 

 warmer than now during the interglacial ages. The causes of these re- 

 peated fluctuations constitute a much-debated question to which var- 

 ious answers have been given. To me the cause appears to have been 



