330 O, H, Hershey — Silver ia Formation. 



The post-Silveria erosion interval has no epochal value, as it 

 was too short, there is no evidence of a marked change of cli- 

 mate, and the ice-sheet probably lay close to the Rock-Illinois 

 valley during the entire time. I should divide the Kansan 

 epoch into a number of sub epochs or stages of which the time 

 of deposition of the Silveria formation would constitute one 

 stage, the post-Silveria erosion interval another, and the time 

 of glaciation of northwestern Illinois still another. Therefore, 

 the Silveria formation in Stephenson County, Illinois, may be 

 chronologically correlated with an early stage of the Kansan 

 epoch. 



The post-Silveria stage of erosion has some bearing on the 

 question of the relative altitude of the region at that time. 

 All the deposits in the valleys of Stephenson County, under 

 the drift sheet and its associated Lake Pecatonica clays, except 

 the Red Gravel before referred to, appear to be genetically 

 connected so that they may be considered as one formation, 

 made up of gravel, sand, and mainly of blue silt — the Silveria 

 formation. Just previous to the beginning of deposition of 

 this formation, the streams flowed from 100 to 200 feet lower 

 than at present. Therefore, the land stood relatively higher. 

 During the post-Silveria stage of sub-aerial erosion, when free 

 drainage was again restored in the Pecatonica basin, the 

 streams flowed about 20 feet under the present water level or 

 over 100 feet above the pre- Silveria stream level. The com- 

 paratively great width of the post-Silveria valleys indicates that 

 the interval of erosion was sufficiently prolonged to enable the 

 streams to cut to at least a local base level. Therefore, the 

 land stood relatively lower at the close than it did at the 

 beginning of the Silveria sub-epoch. In other words, the early 

 stages of the Kansan epoch were characterized by a slow sub- 

 sidence of the land which had returned nearly to its present 

 relative altitude before the maximum extension of the ice- 

 sheet. 



The name which I have proposed for the formation dis- 

 cussed in this paper, is derived from that portion of Stephenson 

 county, where it is, so far as at present known, best developed, 

 and in which its only surface outcrop in the Pecatonica basin, 

 occurs. If the deposit in this county should be correlated 

 with some formation already discriminated and specifically 

 designated in other districts, the name of the latter would, of 

 course, have precedence, but the term Silveria would still be 

 of some value as a local denomination of the conditions pecu- 

 liar to the Pecatonica basin, namely as Lalce Siloeria. 



Freeport, 111., July 13, 1896. 



thickness of the formation to have been 100 feet, 1,200 years would be required. 

 This estimate probably falls rather below than above the actual time. 



