494 W. UPHAM STAGES OF THE ICE AGE 



the same limit as the Illinoian and Ohio drift, which was early borne 

 southward from the Patrician central area of outflow, but later from the 

 more eastern Labradorian area. On the southwest edge of the very broad 

 Quebec lobe of that eastern icefield, from the reentrant angle of the 

 glacial boundary near the east end of Lake Erie to the coast of Xew 

 Jersey, the earliest drift extends beyond the latest Wisconsin glaciation, 

 which is bordered by a belt of prominent morainic ridges and hills. 



With so great southward extension of the ice-sheet in the Xebraskan 

 stage on the east half of our continent it doubtless covered nearly all of 

 Minnesota and the upper part of the Mississippi basin, excepting the 

 large driftless area, mostly in Wisconsin, which was exempt from glacia- 

 tion through all the Ice Age. 



Across the Canadian province .: Saskatchewan, Alberta, and British 

 Columbia, and over the adjoining border of the United States, the K 

 watin and Cordilleran icefields probably reached nearly as far south in 

 Xebraskan time as in any of the later and shorter stages of glaciation. 

 Fur that western half of the continent, through the admirable studies of 

 Gilbert and Russell on Lakes Bonneville and Lahontan in Utah and 

 Xevada, and from the Quaternary history of Lake Mono, in California, 

 it is ascertained that the somewhat moister climatic conditions producing 

 the remarkable enlargement of the lakes in the Great Basin were attended 

 with increased snowfall and glaciation upon the contiguous mountain 

 ranges. The two stages of great rise of these lakes in the Pleistocene 

 period, separated by the intervening stage of their desiccation, when they 

 were nearly or wholly evaporated away, are very certainly correlative on 

 more northern and eastern parts of this continent with two divisions of 

 the Ice Age that were marked by abundant snowfall and ice accumula- 

 tion, while the interval of desiccation has its analogue eastward in an 

 mterglacial stage of fluctuating retreat and readvance of the ice bound- 

 ary. Gilbert wrote of Lake Bonneville as follows, noting the parallelism 

 of its historv with climatic changes that grave alternate stasres of growth 

 and decline of glaciers and icefield ; 



"The Bonneville basin originated by distortion of the earth's crust, and came 

 into existence long before the Bonneville epoch. Little is known of its earliest 

 climatic and physical conditions, but it was comparatively dry for a long 

 period immediately preceding the ' formation of the great lake. During this 

 period, alluvial cones were formed about the bases of all its greater mountain 

 ranges, and the smaller ranges were wholly or partly buried by valley deposits. 

 The valley deposits may have been entirely alluvial, but were probably also 

 partly lacustral, the lakes being of small extent. 



3 U. S. Geol. Survey, Monograph I, 1890. pp. 316-318. 



