328 0. H. Ilershey — Silveria Formation. 



district. It underlaid one-seventh of the county with an 

 average original thickness of 100 feet. This spread out over 

 the entire county would make a uniform layer of 14+ feet. 

 For the purpose of comparison I will introduce a table in 

 which all the other Quaternary formations of the county have 

 been treated in the same manner. 



Thickness if spread 

 over entire county. 



1. Silveria formation 14 + feet. 



2. Lake Pecatonica formation ... 1 foot. 



3. Till 4feet. 



4. Stratified drift gravel 2 feet. 



5. Angular limestone grav-el (drift) 1 foot. 



6. Florencia formation 6 inches. 



7. Valley loess _.. 2 feet. 



8. Upland loess __ V feet. 



9. Modern alluvium 10 + inches. 



Superficial deposits of Stephenson County, 111. Total 32 feet, 



4 inches. 



Its mass is thus seen to be twice as great as the drift series 

 proper (Nos. 3, 4 and 5) and about one and one-half times as 

 great as the combined mass of the lowan loess series of the 

 region (Nos. 6, 7 and 8.) 



The determination of the exact age of the Silveria forma- 

 tion depends on several factors, the precise value of which 

 cannot at present be positively demonstrated. The first is the 

 positive identification of the drift sheet which overlies this 

 formation with that which is exposed in Kansas, and therefore 

 known as the Kansan drift sheet. For reasons which it is 

 hardly necessary to mention here, the oldest drift sheet of 

 northwestern Illinois, southwestern Wisconsin and northeastern 

 Iowa, has been considered as Kansan in age. Moreover, it is 

 supposed to represent the culminating stage or time of maxi- 

 mum advance of the great Kansan ice-sheet. As this correla- 

 tion is probably correct, I shall assume that the drift sheet of 

 Stephenson County, Illinois, is Kansan in age. Now this drift- 

 sheet rests upon the Silveria formation. Therefore, the age of 

 the latter is not later than the Kansan epoch. Following the 

 usual custom of correlating all formations which are inter- 

 glacial in position, with the inter-glacial epochs, I should pro- 

 nounce this deposit as a pre-glacial formation. But its 

 evident derivation from material formed during glacial action 

 and (presumably at least) the known position of the glacial 

 front during its deposition, connect it with some stage of 

 glaciation. Therefore, it is neither properly pre-glacial nor 

 inter-glacial in age. 



