108 LIFE OF THE PLEISTOCENE 



The sequence of these postglacial episodes is known to be very complicated, 

 the biota varying with the changes of conditions. 



2. During the Glenwood Stage there was no life excepting, perhaps, wander- 

 ing mammoths, mastodons or other mammals which may have strayed north- 

 ward. 



3. Following the Glenwood Stage there was a period of low water during 

 which the level of the lake dropped to about ten feet above the present level, 

 or 590 feet above sea level. At this time an abundant and varied fauna, 

 consisting principally of mollusks, took possession of the shallow bays. 

 This fauna consisted of shallow water types, such as are now found in small 

 bays connected with the Great Lakes. A rich forest of spruce and tamarack, 

 fir, arbor vitae, poplar and oak took possession of the dry land, and shallow 

 water vegetation, such as Char a and Potamogeton, must have filled the water; 

 Typha and probably other aquatic plants, lined the shores of the embayments. 

 This biota extended as far north as Manitowoc County, Wisconsin. The 

 reasons which may be assigned for the fall of the water are the rapid melting 

 of the ice in the Michigan basin, the possible opening of a temporary outlet 

 north of the Green Bay Valley or the shifting of the outlet in the Huron-Erie- 

 Ontario basin to the Mohawk Valley past Syracuse. All of these factors may 

 have contributed. 



4. Following the low water stage the lake again raised its level, due to a 

 readvance of the ice, or to the return of the use of the Grand River outlet, 

 and formed the Calumet Stage, the water standing from 35 to 40 feet above 

 the present lake level, or 615 to 620 feet above sea level. It is possible that 

 the lake may have returned to the lower Glenwood level and then suddenly 

 dropped to the Calumet level. At this time an off-shore barrier was formed, 

 extending from Wilmette south to Bowmanville. West of this bar Wilmette 

 Bay developed. No life has been found in the beaches referable to this period 

 of the lake's history, but it is quite probable that between the Calumet and 

 Toleston stages the forerunners of the rich Toleston biota took possession of 

 the waters. The naiad fauna observed in the Sag-Calumet channel may belong 

 to this transition period. 



5. The water gradually fell (due to cutting down of the outlet) to about 20- 

 25 feet above the present lake level (600-605 feet above sea level) forming the 

 Toleston Stage. The Rose Hill bar became wide and peninsular-like and an 

 off-shore barrier developed (the Graceland bar) and extended from Bowman- 

 ville south to Lincoln Park. Wilmette Bay was now ten miles long, two to 

 three miles wide, and five to twenty feet deep. A rich fauna of river mussels 

 (Naiades, Unionidse) took possession of Wilmette Bay and Lake Chicago, 

 forming huge beds comparable to those now exsiting in the larger rivers of the 

 Mississippi Valley. Two species of naiads and one gastropod do not now live 

 in the Chicago area; one, Elliptic crassidens, extended its range as far north as 



