0. H. Uershey — Silveria Formation. 329 



The second factor which creates a difficulty in placing this 

 formation in the geologic time scale, is the exact significance 

 of the eroded valleys in the surface of the blue silt. The drift 

 sheet in the deeper valleys has been mostly removed by subse- 

 quent erosion (Aftonian mainly). But locally v^e find the 

 drift in situations which indicate that the erosion of the valleys 

 in the blue silt was accomplished mainly before the completion 

 of the Kansan epoch. The question remains, however, as to 

 whether this erosion was sub-aerial or glacial in nature. In 

 the exposure near Freeport it is settled in favor of the former 

 hypothesis, for here we find an erosion unconformity with a 

 slightly developed soil and remains of vegetation at its surface, 

 overlain by another extra-glacial lake deposit which itself pre- 

 ceded the arrival of the ice in that vicinity. Strong presump- 

 tive evidence that the erosion of all the valleys in the surface 

 of the blue silt was mainly sub-aerial and antedated the arrival 

 at Freeport of the ice-sheet, is furnished by the regularity of 

 the valleys, their nearly uniform depth, and the absence from 

 the till west of Freeport of any large amount of material which 

 could have been derived from this formation. Therefore, we 

 can say that if these valleys are truly valleys of erosion as they 

 appear to be, their excavation was accomplished mainly by 

 sub-aerial agencies, and antedated the glaciation of the region. 



It has been assumed, as before stated, that the unusual devel- 

 opment and relatively great height which the Silveria forma- 

 tion attains in Stephenson County, was due to obstruction of 

 the valley by the advancing ice-sheet. The sub-aerial erosion 

 within the limits of the former lake area, above partially 

 demonstrated, requires the disappearance of the ice from the 

 lower portion of the valley to give free drainage through the 

 ancient Rock-Illinois valley, then occupied by the ancestor of 

 the present Hock river. The relative importance of this 

 fiuctuation in the border portions of the ice-sheet depends 

 largely on the length of the erosion interval above indicated. 

 While the amount of material removed from the immediate 

 vicinity of the streams during this post-Silveria erosion interval 

 was apparently greater than the erosion which has been accom- 

 plished in the same position since the lowan loess series was 

 deposited, I do not think that any very long period was 

 required. Although the completion of the work within 500 

 years would have converted the ancient Pecatonica river into a 

 stream of "liquid mud," the somewhat increased precipitation 

 of the glacial period, and otherwise more favorable conditions 

 of erosion, may have enabled its accomplishment."^ 



* The length of time required to deposit the Silveria formation in Stephenson 

 County can only of course be conjectured. If we assume the annual amount of 

 sediment deposited to have had a thickness of one inch, and the average original 



