CHAPTER IV 

 THE POSTGLACIAL BIOTA OF THE GREAT LAKES REGION 



I. General Statement 



A study of the life of any given region naturally and logically leads to a 

 consideration of the life of other regions more or less related to the particular 

 area under discussion. In the present instance this study and comparison is 

 quite essential, since the aquatic repopulation of the glaciated area was prob- 

 ably largely by way of the Chicago outlet. 



Records, more or less complete, are available from various places adjacent 

 to the basin of the Great Lakes, clearly indicating that the biota followed the 

 retreat of the ice and occupied the territory as fast as it became available for 

 the different types of life. The lake sediments and beaches in Wisconsin, 

 Illinois, Michigan, New York, and various parts of Canada contain, in a few 

 localities, an abundance of biotic remains. No studies, comparable with those 

 carried on in the Chicago basin, the details of which have been indicated in the 

 preceding pages, have been made in any of these deposits. 



II. Outline of the History of the Extinct Lakes 



Before proceeding to discuss the various deposits in which evidences of 

 life have been found, it will be necessary to consider briefly the succession 

 of lakes formed at the margin of the retreating ice sheet. 1 



A. THE WISCONSIN ICE SHEET (Plate XL VI) 



The last glacial invasion extended well into the United States, completely 

 covering the Great Lakes with ice, and sending two large lobes into the territory 

 west of the Mississippi River, one into Iowa and one into South Dakota. 

 The life of the englaciated region was driven south and occupied the territory 

 as near the ice margin as the rigor of the climate would permit. The aquatic 

 life found refuge in the Mississippi, Rock, Illinois, Wabash, Ohio, and Missouri 

 rivers and their tributaries, from which territory it was ready to advance as 

 soon as the ice sheet began to retreat. 



B. FORMATION OF GLACIAL LAKES 



"As the ice border withdrew to the north of the divide separating the St. 

 Lawrence basin from the Mississippi basin, the glacial waters were ponded be- 



1 This discussion is compiled from the papers of Leverett, Taylor, Goldthwait, and 

 Chamberlin. The early and late Wisconsin ice sheets are considered together. 



