1 1 6 PROCEEDINGS OF THE AMHERST MEETING 



Presented in abstract extemporaneously, with lantern-slide illustra- 

 tions. 



Discussed by M. M. Leighton and W. M. Davis. 



FURTHER DATA ON THE DIFFERENTIATION OF THE GLACIAL DRIFT SHEETS 



OF NORTHERN ILLINOIS 



BY MORRIS M. LEIGH TON 



{Abstract) 



The work of the past season has yielded additional evidence which verifies 

 the conclusions of the previous season, as presented at the Chicago meeting of 

 the Geological Society of America, that the glacial drift west of the outer Wis- 

 consin moraine must be differentiated into two distinct sheets — one of Illi- 

 noisan age, which extends to the border of the Driftless Area, and the other, 

 apparently of Early Wisconsin age, which overlies the Illinoisan and extends 

 as far west as Stillman Valley, in Ogle County. The limits of the younger 

 drift have been determined, and because of its lobate form and its typical 

 development immediately south of Belvidere it is referred to as the Belvidere 

 lobe. 



South of Dixon, in territory far beyond the Bloomington moraine, and which 

 at one time was mapped as Iowan, and later as Illinoisan, relatively fresh drift 

 was found bearing the same definite aspects of youth and the same relations to 

 the Illinoisan as the Belvidere lobe. Some evidence was also found suggesting 

 that the strip of drift on the south side of the Green River basin, between 

 Sheffield and Geneseo, is also of the same age, which, if true, indicates that a 

 lobe of Early Wisconsin ice invaded the Green River basin as far west as Rock 

 River. It is hoped that a short additional period of work will definitely clear 

 up this remaining question as to the drift of northwestern Illinois. 



Presented in full extemporaneously. 



Discussed by Frank Leverett, J. W. Goldtliwait, and W. M. Davis, with 

 response by the author. 



Discussion 



Mr. Leverett : The boundary drawn by Professor Leighton does not seem to 

 be in accord with the observations made some years ago by Leverett of the 

 extent of a drift that is separated from the underlying Illinoisan drift by fos- 

 siliferous silts and by a fresher aspect. The extent of this overlying drift is 

 set forth in Monograph XXXVIII, U. S. Geological Survey, under the name 

 "Iowan drift/' The leaching tests presented by Leighton seem less conclusive 

 than supposed by him, for texture and other features need to be considered. 



Professor Leighton (in reply) : I appreciate the criticisms of Mr. Leverett. 

 In regard to the exposure at Polo, I may say that the cut is now obscured by 

 slump and sod, but an attempt to reexpose the materials resulted in finding 

 only one till, and another worker had previously failed to identify the two 

 tills and an intervening loess with an auger, although they may be there. At 

 Polo a gumbotil lies at the top of the till, the like of which is not known for 

 the Belvidere area. The calcareous portion of the loess of this area also rests 



