510 W. TJPHAM STAGES OF THE ICE AGE 



to have advanced like a wave from Minnesota and North Dakota north- 

 ward through Manitoba and northeast to Hudson Bay, permanently 

 elevating the country as now, mostly about 500 feet above the height 

 which it held when first uncovered by the late Wisconsin glacial melting. 



Soon after the ice-front receded, and while it yet continued to be 

 the northern barrier of Lake Agassiz, the uplift along the whole extent 

 of this ancient lake, more than 600 miles from south to north, was prac- 

 tically completed. The southern half of the lake area was first raised 

 nearly to its present height ; later its northern half was elevated, while the 

 southern part received only slight increase of height ; and last the basin 

 of Hudson Bay, near the center of the glaciated area of Xorth America, 

 has been raised from its Champlain marine submergence of 300 to 500 

 feet. A part of this elevation on the shores of Hudson Bay was shown 

 by Dr. Robert Bell to have been very recent, and it is even probably still 

 in progress. 



Nearly the same amount and a similar gradual advance of this Wis- 

 consin uplift, with continuation through Champlain time, are attested 

 likewise by the northward ascent of the beaches of a very complex series 

 of glacial lakes held in the basins of the present Great Lakes tributary to 

 the Saint Lawrence by the barrier of the receding Labradorian icefield. 



The Champlain Stage 



Subsidence of the heavily ice-laden northern half of our continent 

 from its great preglacial elevation brought during the Sangamon inter- 

 glacial stage, and more remarkably at the ensuing Iowan stage of glacia- 

 tion, low slopes and very gentle currents of the Mississippi and its tribu- 

 taries, with restoration of a temperate climate at the ice-margin, permit- 

 ting much loess to be deposited along these valleys. Warm summers, 

 favorable to the accumulation of marginal moraines, continued during 

 the Wisconsin glaciation, although evidence of freer drainage from the 

 ice-front, with deposition of much water-borne sand and gravel, indicates 

 that in the Mississippi basin the maximum subsidence had been followed 

 by a slight reelevation. 



Toward the close of the Ice Age and coincident with the existence of 

 Lake Agassiz and the Laurentian glacial lakes, the moderate uplift of the 

 glaciated region from its Sangamon and Iowan depression extended far- 

 ther north, advancing in general as fast as the ice-borders receded, but 

 apparently it is not yet entirely completed. Indeed, considerable parts 

 of the drift-bearing areas of North America and Scandinavia are still 



