THE ILLINOIS GLACIAL LOBE. 



By Frank Leverett. 



CHAPTER I. 



INTRODUCTION. 



The Illinois glacial lobe includes a portion of the great ice sheet which 

 extended from the highlands east and south of Hudson Bay, southwestward 

 across Michigan, the Lake Michigan basin, and Illinois, to the axis of the 

 Mississippi Valley in southwestern Illinois and southeastern Iowa (PI. I). 

 It finds its natural limits on the northwest at the border of the Driftless 

 Area of southern Wisconsin and northwestern Illinois. On the west it 

 overlaps a region previously glaciated, but its extent there is readily deter- 

 mined, for it has usually a definite border in a belt of ridged drift. On the 

 south and southeast it extends to the unglaciated tracts of southern Illinois 

 and southern Indiana. From central Indiana northward it seems to have 

 b'een merged with the eastern portion of the great ice sheet, except in 

 shrunken stages. In the last stage here considered it extended but little 

 beyond the borders of Lake Michigan and was then distinct from lobes to 

 the east lying in the Saginaw and Maumee basins. The present report 

 discusses the deposits made by the lobe south from the latitude of the 

 Illinois-Wisconsin line, in Illinois, Indiana, and Michigan. 



The drift deposited by this ice lobe has received considerable attention 

 from the geologists of the several State surveys, and also from several 

 students of geology not connected with official surveys. The early publi- 

 cations furnish numerous sections of the drift, of which mention is made 

 below, which throw light on its structure and thickness. They contain 

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MON XXXVIII 1 



