ICE AGE IN NORTH AMERICAN ARCTIC — FLINT 253 



all the arctic glaciers underwent at least some reduction in size during 

 the warm period. Future research is likely to bring forth evidence 

 that this reduction occurred. 



GLACIAL LAKES 



The great deglaciation that has been in very irregular progress 

 throughout the last few tens of thousands of years was accompanied 

 by the appearance of many temporary lakes, held in between glacier 

 ice on one side and sloping ground on the other. Most of the lakes 

 were localized in preglacial stream valleys, depressions that could be 

 converted into basins by glacial erosion, or damming by ice, or both. 

 Some of the most conspicuous preglacial valleys had developed along 

 the contact between the igneous and metamorphic pre-Cambrian rocks 

 of the Canadian Shield and the surrounding Paleozoic sedimentary 

 rocks. Major lakes of today, such as Great Bear and Great Slave 

 Lakes and Lake Winnipeg, consist of segments of these valleys con- 

 verted into lake basins by glacial action. These, together with other 

 lakes in the region west of Hudson Bay (notably Lake Athabaska and 

 Wollaston, Eeindeer, Cree, and Lesser Slave Lakes), show by the 

 abundance of lake deposits and abandoned strandlines in the terrain 

 surrounding them that they were considerably larger during the 

 shrinkage of the ice sheet than they are at present. 



The two larger lakes formed partly within the arctic region during 

 the deglaciation have almost entirely disappeared. Lake Agassis ex- 

 tended from latitude 46°N., in Minnesota, nearly to latitude 58°N., 

 in northern Manitoba, and had an area equal to that of the existing 

 Great Lakes combined. It was held in on the north and east by the 

 edge of the ice sheet, and when this melted away the water drained off 

 to Hudson Bay, leaving a few basins, Lakes Winnipeg and Winnipe- 

 gosis, to contain residual pools. Lake O'fibw ay -Barlow, south of 

 James Bay, was held in in the same manner. It stretched from the 

 76th meridian to the 88th, and when glacial melting destroyed its 

 northern shore it drained away, leaving upon the bedrock surface a 

 veneer of silt and clay to mark its former extent. This is the well- 

 known "clay belt" which has made a wide region possible for agricul- 

 ture, in contrast with the rocky country surrounding it. 



THE POSTGLACIAL SEA AND RISE OF THE LAND 



The greater part of the coastline of Arctic North America is fringed 

 with superficial deposits of sand and silt, in places containing the fos- 

 sil shells and bones of marine animals. Throughout great distances 

 these deposits are fashioned into beaches, bars, and other shore 

 features sweeping along the contour of gently sloping terrain. Shore 

 • and sea-floor deposits of this kind form a discontinuous belt that varies 



