SILVERIA (?) FORMATION AND OTHER SILT DEPOSITS. 113 



to nave been formed at the Kansan stage of glaciation by the blocking of 

 valleys which had westward discharge to the Mississippi. If this be the 

 correct interpretation, these silts can not properly be termed Silveria, for 

 they may be much older than that formation and their relation to the 

 Glacial series would be more clear if they were termed Kansan silts. 



The Silveria formation discussed by Hershey "is a thick bed of strati- 

 fied silt of a nearly uniformly dark bluish-gray color with bands often 

 several feet in thickness which are a lighter tint." But one surface exposure 

 has been found, and is located in a small ravine 1£ miles south of the city 

 of Freeport, yet wells have shown its occurrence in the valleys of nearly 

 all the streams in the Pecatonica drainage basin. At the surface exposure 

 "the upper portion is a false-bedded, calcareous and ferruginous, light- 

 brown fine sand and silt, and appears to represent the shore deposits of an 

 ancient lake in which this formation was apparently laid down." Its 

 calcareousness strongly supports the view that it is a glacial silt. Several 

 species of small shells and also fragments of partially decayed wood have 

 been collected by Mr. Hershey. Specimens of the shells, submitted to Dr. 

 W. H. Dall, of the United States Geological Survey, are found to represent 

 three different species, with fragments of still other species. These species 

 are present in about the following proportionate numbers: Succinea avara 

 50, Pupa blandi 5, Pyramidula striatella 2. Hershey refers the occurrence 

 of this terrestrial fauna in a deposit of lacustrine character, to the position 

 near the shore of the lake. 



This deposit appears to nave considerable bulk in the valleys of 

 Stephenson County. In a well 3 miles southwest of Freeport, in the old 

 valley of Yellow Creek, it was penetrated to a depth of 150 feet without 

 reaching the bottom. Hershey estimates that if spread out over the entire 

 surface of Stephenson County, this deposit would make a uniform layer at 

 least 14 feet in depth. He estimates the total depth of the superficial 

 deposits of Stephenson County to be 32^ feet. It forms therefore nearly 

 half the bulk of these deposits. 



Above the silt which Hershey has called the Silveria formation there 

 is another silt deposit separated from it by an erosion unconformity and 

 a slightly developed soil. This he considers an extraglacial lake deposit 

 formed during the advance of the ice sheet which formed the overlying till. 

 This deposit he estimates to have an average thickness, if spread over the 

 mon xxxvin 8 



