THE PEORLAN SOIL AND WEATHERED ZONE 247 



ville till was found to be very calcareous immediately above the 

 loess. The evidence of a weathered zone at the top of the 

 loess is as clearly shown as at the top of the underlying 1111— 

 noian till and several exposures occur in which the two weath- 

 ered zones may seem in vertical sections. It is probable, how- 

 ever, that such a zone would be developed more rapidly in the 

 loess than in the till, because of the greater porosity of the 

 former. 



Evidence of an interval between the Iowan and early Wis- 

 consin glaciations is found in the great dissimilarity in the out- 

 line of the two ice sheets. The outline is more out of harmony, 

 both with the early Wisconsin and the Illinois, than the outline 

 of those sheets with each other. The great extension 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 Wis- 

 consin 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 

 change in glacial conditions that can scarcely be assumed to 

 have taken place suddenly. 



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 interglacial stage 

 than during the Sangamon. The breadth of excavation, how- 

 ever, was reduced to but a fraction of that in the Sangamon 

 stage. The amount of change in altitude can as yet scarcely be 

 even conjectured, 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. Compar- 

 ing the work with substages of the Wisconsin it appears that 

 the interval may not greatly exceed that between the Shelby- 



