POSTGLACIAL BIOTA OF THE GREAT LAKES REGION 115 



When the ice withdrew from the Adirondacks the lake waters were lowered 

 and an outlet was found lower than the Mohawk Valley. The first lower out- 

 let was into glacial Lake Champlain and thru the Hudson River. Further re- 

 treat of the ice uncovered the Atlantic coast and the basin became filled, more 

 or less completely, with sea water (Plate LII) during a period when the ice had 

 opened a passage to the eastward. Subsequent differential uplift brot Lake 

 Ontario to its present level and gave it an outlet thru the St. Lawrence River. 



I. LAKE ALGONQUIN 



The early history of the post glacial lakes has up to this time required a 

 separate discussion for each lake basin. At this time, however, the waters 

 of the basins of Lakes Superior, Michigan, and Huron united, forming a lake 

 considerably larger than the present Great Lakes, and known as Lake Algon- 

 quin (Plate LII). Lake Algonquin may be divided into four stages which are 

 summarized as follows: 



1. Early Lake Algonquin? At the beginning the lake probably had a brief 

 stage when it was confined entirely to the southern half of the Huron basin 

 (Plate LII). This was of short duration, a slight additional retreat of the ice 

 uncovering outlets to the northwest to Lake Chicago and to the east to Georgian 

 Bay and the Trent Valley. The St. Clair and Detroit rivers were also used by 

 the waters to find an outlet thru the Niagara River to Lake Iroquois. 



2. Kirkfield Stage. As soon as the ice melted back far enough to uncover 

 the Trent Valley, the level of the waters fell, the outlets at Port Huron and 

 Chicago were abandoned, and the full discharge of the waters was thru the 

 Trent Valley at Kirkfield. It is believed that Lake Iroquois had already been 

 established when the Kirkfield outlet was opened. 



3. Port Huron-Chicago Stage. When the ice sheet had almost entirely 

 disappeared from the lake basins, a movement of differential elevation began 

 which raised the land at Kirkfield and the outlet was again shifted to Port 

 Huron and Chicago. At first the Chicago outlet carried the greater part of the 

 water, but as this outlet rested on a sill of rock while the Port Huron sill was of 

 till, the latter was soon cut down and carried almost all of the overflow. 



During this third stage the remarkable uplift of the Great Lakes region 

 occurred. This uplift caused a northward splitting and divergence of the 

 beaches below the highest Algonquin beach, the difference in elevation between 

 the beach at Port Huron and the high beach at Lake Gondreau, Canada being 

 900 feet. The beaches fall into three groups, (1), the upper main Algonquin 

 group, (2) the Battlefield group, and, (3) the Fort Brady group. It is be- 

 lieved that these deformations were not slow and evenly distributed but were 

 spasmodic, and relatively sudden and rapid (Taylor). 



7 Taylor, op. cit., pp. 316-320. 



