112 THE ILLINOIS GLACIAL LOBE. 



depend upon the bearing they may have upon glacial history and their 

 relation to each other. Should it be found that any of them bear evidence 

 of a distinct stage of glaciation to which no name has yet been applied, or 

 to any part of geological time not assigned a name, it would seem proper 

 to introduce a name. But if they simply mark the deposits of small lakes 

 of temporary character formed during an ice advance to which a name has 

 already been applied, it would seem better to extend the name of the ice 

 invasion to the silts. Thus we might speak of buried silts of Kansan age 

 or of Illinoian age. The relation to the glacial deposits would then be 

 more clearly seen than if a separate name were applied to the silts. In the 

 case of the silts which Hershey has named Silveria there appears to be 

 evidence that they do not connect definitely with the advancing Illinois ice 

 lobe, for, as noted above, they seem to have suffered some erosion prior 

 to the Illinoian ice invasion. It therefore may be necessary to retain the 



Eilra-glacial silt. 



FIG. 1. — Section showing the "Silveria formation " near Freeport, Illinois: prepared by Oscar H. Hershey. </ and h com- 

 bined are the " Silveria." 



name Silveria, but to restrict it to deposits which are clearly of similar age 

 to those examined by Hershey. If, on examining the silts of valleys in 

 other parts of the region covered by the Illinois lobe, it is found that a 

 similar interval of erosion separates their deposition from that of the Illi- 

 noian drift, the name may be extended perhaps to such deposits. But if it 

 is found that valleys contain silt deposits which immediately precede the 

 Illinoian till sheet in date of deposition, it seems advisable to Avithhold the 

 name Silveria and refer to them as the silts of the Illinoian stage. It is 

 probable that many if not all the valleys in which the Illinoian till sheet 

 blocked the lower course and formed the temporary lakes contain deposits 

 of this latter class. It is likely, therefore, to be more extensive than the 

 Silveria formation. Hershey recognized this restricted application of the 

 name, and gives the extraglacial silts a separate place (see fig. 1). 



In western Illinois, as was indicated above, there are silts which appear 



