506 PLEISTOCENE OF INDIANA AND MICHIGAN. 



of Lake Whittlesey. The second position, AA, is that of the ice barrier of Early Lake Algon- 

 quin. The third position, A'A', which marks the place of the ice front when the lake was at 

 its greatest extent and the upper group of beaches had reached their farthest northward 

 extension, and the fourth position, A "A", which marks the place of the ice front at the last 

 stage of Lake Algonquin (the lower of the Fort Brady beaches) just before the inauguration of 

 the Nipissing Great Lakes, are only rough approximations, the actual position of the ice front 

 having not yet been well determined. 



The curved isobases in the region south of St. Lawrence Eiver and east of Lake Ontario 

 show present knowledge concerning the deformation of the marine shore line, as recently 

 represented by Goldthwait. 1 These lines have no connection with the glacial lakes, but relate 

 solely to the marine waters. They are added merely to show the general relation of the 

 deformation in the marine area to that of the Great Lakes region. 



ICE ATTRACTION. 



The effects of ice attraction in deforming the old beaches are discussed on pages 343-348. 

 RESILIENCE FOLLOWING DEPRESSION BY ICE WEIGHT. 

 RELATIONS OF UPLIFT TO RETREAT OF ICE. 



The causes that deformed the old shore lines of the Great Lakes region acted within fairly 

 well denned limits of space and time; that is to say, although the deforming causes affected a 

 vast area of the northern and northeastern parts of the continent, they had clearly defined 

 southward limits in the region of the Great Lakes, and the time of their action is recorded in 

 the deformed beaches of a long series of lakes and lake stages covering almost the entire period 

 since the maximum of the last or Wisconsin ice sheet. The areal relations of these limits to the 

 area covered by that ice sheet and the time relations of the uplift to the retreat of the ice are 

 so nearly in accord that to many minds they prove causal relation between the ice sheet and 

 the elevation of the land. The apparent great strength of this explanation is not denied, but 

 serious difficulties, that ought not to be overlooked, stand in its way. 



The southern boundary of the uplifted lands, marked by the first or Whittlesey hinge line 

 (see fig. 14), is roughly parallel to the boundary of the Wisconsin ice sheet and is not far north 

 of the mean course of that boundary from northwestern Dakota to the Atlantic coast in 

 Massachusetts. Further, the Algonquin hinge line is substantially parallel with the first, the 

 interval between them, however, being about twice as wide in the west as in the east. 



The attitudes of the old shore lines show three distinct periods of land elevation in the 

 Great Lakes region, separated by two fairly well defined times of stability. The relations of 

 the uplifting movements to the retreating ice sheet may be briefly summarized as follows : 



1. The first elevation began when the ice front had retreated nearly halfway across the 

 Great Lakes region. Just before this uplift the ice front had been resting on the main moraine 

 of the Port Huron system, which marks the ice barrier of Lake Whittlesey. This position of the 

 ice front (see wavy line WW in fig. 14) has been traced continuously through nearly the whole 

 length shown. The uplifting movement continued in spasmodic fashion until the ice front had 

 retreated nearly to the position marked AA. During this interval the hinge line kept shifting 

 north. This movement deformed all the beaches before the Algonquin, the Maumee to the 

 Lundy, inclusive. 



It is entirely possible that uplifts occurred before the formation of the earliest glacial lakes 

 and hence were not recorded in the beaches. The Lum and Rochester channels seem to show 

 a movement occurring just after these channels were abandoned, but the beaches show none 

 that are clearly discernible. The movement, if it took place, must have been relatively slight 

 and could have extended only a few miles, if at all, south of the Whittlesey hinge line. 



2. At the position AA the ice barrier was holding up the water of Early Lake Algonquin. 

 But this short-lived stage soon gave place to the longer second stage with outlet at Kirkfield, 



i Bull. Geol. Soc. America, vol. 22, 1911, pp. 723-724. 



