Ross: The Dunesland Heritage of Illinois 



17 



over portions of the continent, each one eventually melting away. The 

 third of the four major ice sheets extended over Illinois as far south 

 as Carbondale. During the fourth major ice advance, called the Wis- 

 consinan, the tremendous Lake Michigan glacial lobe spread to central 

 Illinois, after which its southern margin retreated and readvanced, 

 retreated and readvanced, time after time until finally it melted away. 

 The last ice thrust of this Lake Michigan lobe to enter Illinois oc- 

 curred about 13 thousand years ago. A subsequent thrust 11 thousand 

 years ago covered much of southeastern Wisconsin but did not quite 

 reach Illinois. 



Evidence from many sources indicates that the four major glacial 

 periods were associated with four climatic periods. There is an indi- 



^^^^^M^W^ 



Fig. 11. — General outline of glacial events during the Pleistocene in North America. White, 

 extent of the last major glacial periocJ, the Wisconsinan; northern rhombic area, shelf ice; hori- 

 zontal lines, southern extent of previous glaciations in the Midwest; stipple, areas thought never 

 to have been glaciated. 



