116 THE ILLINOIS GLACIAL LOBE. 



extraglacial lake formed in valleys in front of the advancing ice sheet, or, 

 like the surface loess, have a wider distribution less clearly connected with 

 the ice invasion, is not at present known. They may be widely distributed 

 beneath the Illinoian sheet in western Illinois. In texture and general 

 appearance these deposits are very similar to the surface loess. They are 

 apparently not so compact as the laminated silts of northwestern Illinois, 

 described by Hershey under the name Silveria formation. 



The deposits of laminated clay with sand partings, found beneath the 

 till in central Adams County, Illinois, have already been discussed as prob- 

 able products of an obstruction of an eastern tributary of the Mississippi 

 by the Kansan invasion of the Keewatin ice sheet. It is probable that 

 similar deposits fill the valleys of other eastern tributaries of the Mississippi 

 in western Illinois, though as yet none have been observed. The date of 

 these laminated clays, compared with that of the buried silt of Rock Island 

 County, or of the Silveria formation of Stephenson County, is not known. 



The silt beneath the till at Pana, noted above, is so much older than 

 the overlying till that it can scarcely be considered an extraglacial lacustrine 

 deposit formed during the ice advance which deposited the till. Its origin 

 and date are not determined. 



In the reports of the geology of Illinois there appear several instances 

 of the occurrence of a plastic blue clay, or "blue mud," below the till in the 

 vicinity of the glacial boundary in southern Illinois. 1 This blue material 

 appears to be a silt deposit stained by humus. It often contains much wood 

 and other organic matter. The writer has not been successful in finding an 

 exposure and can not pass an opinion upon the character or the origin of 

 the deposit. It is probable that valleys in that region which were obstructed 

 by the advancing ice sheet were filled to some extent by silt, but since the 

 deposit in question contains wood and other vegetal material it apparently 

 antedated the till by a considerable interval, though it is possible that the 

 accumulation of wood and organic matter is due to the introduction of this 

 material by currents of water during the silt deposition, and not by the 

 growth of vegetation on the silt after the completion of the deposition. 



A detailed section of the material penetrated by the Isabella Thompson 



' Geol. Illinois, Vol. I, pp. 299, 300, 316; Vol. Ill, pp. 75, 86, 87, 103. 



