326 PLEISTOCENE OF INDIANA AND MICHIGAN. 



GILBERT GULF. 



Finally the sea, which then stood relatively higher than now (523 feet higher near Covey 

 Hill on the northern base of the Adirondacks) / entered and turned what had been a glacial lake 

 into a marine gulf, known as Gilbert Gulf. 



GLACIAL LAKES IN THE LAKE MICHIGAN BASIN. 



LAKE CHICAGO. 



While this complicated history was being enacted in' the Huron-Erie-Ontario area, the 

 glacial waters in the basin of Lake Michigan were also undergoing expansion. Here, however, 

 the changes were extremely simple, for until the very last no critical ground affording a new 

 outlet was encountered. The changes that occurred in Lake Chicago were due to erosion of its 

 outlet or to changes in the volume of its discharge. From its first beginning as a narrow crescent- 

 shaped lake at the extreme southern end of the Lake Michigan basin Lake Chicago had expanded 

 northward as the ice receded until two-thirds or three-fourths of the basin was uncovered. In 

 all probability the retreating ice front performed here the same series of oscillations, with strongly 

 marked steps of retreat and readvance, that took place in the Huron-Erie basin. The evidence 

 of these oscillations, however, are not generally so well marked, because critical changes were 

 not produced by them; but some of the stronger moraines mark readvances that override 

 beach ridges which had been made just previously. 



The Port Huron morainic system skirts the north side of the high ground of the southern 

 peninsula of Michigan and appears to be correlated in part with the morainic ridges which pass 

 beneath Lake Michigan just south of Manistee. The Whitehall moraine of the Lake Michigan 

 basin is considered to be a part of the complex Port Huron morainic system. 



Studies by Alden, 3 under the direction of Chamberlin, on the west side of the Michigan 

 basin, have developed evidence of a distinct readvance of the ice to Milwaukee characterized by 

 a deposit of red till, and this probably correlates with the Whitehall moraine (p. 302). Later 

 ridges of red till, which come down to the shore of Lake Michigan near Manitowoc and Two 

 Rivers, Wis., also probably correlate with the Manistee moraine. This correlation is inferred 

 not only because of similar position on opposite sides of the lobe and in reference to earlier 

 moraines, but also because these later ridges do not appear to carry the Calumet and Glenwood 

 beaches of Lake Chicago (see pp. 354, 355), which are present on the Whitehall moraine. Two 

 lower beaches in the same area seem to have wider connections. The upper one is the Toleston 

 beach, 24 or 25 feet above Lake Michigan, and the Nipissing beach, about 15 feet above the 

 lake. The summit in the Chicago outlet in the south part of the city is only 8 feet above Lake 

 Michigan, and is a broad, flat region much obstructed by low, sandy ridges. The Toleston 

 beach passes over this broad divide at a level high enough to have permitted Lake Chicago to 

 discharge over it, even when it received the discharge of Lake Whittlesey or Lake Warren. 

 Nevertheless, the Toleston beach seems to be continuous with the Algonquin beach, which 

 traverses all of the upper three lake basins, and part of the overflow of Lake Algonquin may have 

 been by way of Chicago for a time. The relation of the Chicago outlet to Lake Algonquin is 

 still somewhat problematic. 



When the ice lobe in the Lake Michigan basin retreated into the northern part of that basin 

 it uncovered ground of critical interest on both sides. On the west side the glacial waters of 

 the Lake Superior basin had been held up to a higher level than those of Lake Chicago, and 

 when an opening occm - red around the hills southeast of Marquette these waters drained south- 

 ward along the western edge of the Green Bay lobe of the ice sheet and ultimately into Lake 

 Chicago. 



Bordering the west shore of Lake Michigan and extending into the Green Bay-Lake Win- 

 nebago trough and the Fox and Wolf river valleys is an extensive deposit of red clay, partly 



1 Fairchild and Goldtnwait, personal communications. 2 Alden, W. C. F personal communication. 



