188 THE ILLINOIS GLACIAL LOBE. 



the development of the Peorian zone as there displayed. It is possible, 

 however, that other lines of evidence next to be considered may call for a 

 period of greater length between the Iowan and Wisconsin stages of glaci- 

 ation than these exposures of the Peorian weathered zone seem to require. 



IOWAN OUTLINE COMPARED WITH SUCCEEDING AND PRECEDING 



GLACIATIONS. 



Evidence of an interval between the Iowan and early Wisconsin glaci- 

 ations is found in the great dissimilarity in the outline of the two ice sheets. 

 The outline is more out of harmony, both with the early Wisconsin and 

 the Illinoian, than the outline of these sheets with each other. The great 

 ex! cnsion toward the south border of the Driftless Area, both in the Iowa 

 and Illinois lobes of Iowan ice, is singularly out of harmony with both 

 succeeding and preceding glaciations. The shifting of lobes involved in 

 the change from the Iowan to the early Wisconsin can scarcely be assumed 

 to have occurred in a brief interval. The moraine-forming- habit of the 

 Wisconsin and absence of distinct morainic belts in the Iowan also implies 

 a chang-e in glacial conditions that can hardly have taken place suddenly. 



CHANGE IN ATTITUDE OF THE LAND. 



Evidence of an interval between the deposition of the Iowan loess and 

 associated silts, and that of the Shelbyville till, is found in a change in the 

 attitude of the land, which resulted in a marked deepening of the valleys. 

 There appears to have been a greater depth of excavation during- the 

 Peorian stage than during the Sangamon. The breadth of excavation, 

 however, amounted to but a fraction of that in the Sangamon stage. 



LENGTH OF THE PEORIAN STAGE. 



The amount of change in altitude can as yet scarcely be even conjec- 

 tured, much less demonstrated, but its effects on the drainage are such as to 

 support the view that it denotes a time interval of considerable length, a 

 view which is also supported by the work accomplished in deepening the 

 valleys. Comparing- the work with substages of the Wisconsin it appears 

 that the interval may not greatly exceed that between the Shelbyville and 

 Bloomington ice advances. The Shelbyville sheet had become channeled 



