0. II. Hershey — Silveria Formation, 327 



center of the valleys it is almost invariably reached at about 20 

 feet under the present stream level. Being a stiff, clay-like silt, 

 but slowly pervious to water, all wells upon encountering it 

 must goto the bottom, wherea supply of water maybe obtained 

 from the basal gravel or from fissures in the solid rock below. In 

 thickness it usually varies at present between 40 and 60 feet, 

 hut is occasionally penetrated to the depth of 100 feet. Over 

 the abandoned valley of Yellow Creek about 3 miles S.W. of 

 Freeport, there is a low ridge of drift. A well beginning on 

 this ridge was sunk 183 feet without reaching the solid rock at 

 the bottom of the valley. The deposits passed through were 

 first, brown loam, followed by drift, beneath which " blue clay" 

 was reported to have been penetrated to the thickness of 150 

 feet. This " blue clay " represents the Silveria formation, 

 which fills up the abandoned valley of Yellow Creek to above 

 the present stream level. 



The frequency with which the blue silt is encountered at the 

 height of 20 feet above the present stream level, along the 

 sides of the valleys, shows that, were the later formations 

 stripped from the region, they would be found to have rested 

 upon narrow terraces, bounding on either hand trough-shaped 

 valleys about 40 feet in depth. The horizontal banding, the 

 horizontality of the stratification at the exposure near Freeport 

 and other phenomena connected with the formation, have been 

 considered to indicate that the buried terraces represent the 

 nearly level surface of the deposit when completed. In those 

 portions of northwestern Illinois which are outside of the 

 Pecatonica basin, the deposition of the buried blue silt was 

 probably due solely to subsidence of the land, but in Stephen- 

 son county we can explain its unusual development only under 

 the supposition that the Pecatonica valley was closed on the 

 east by a glacier, thus raising the water level considerably 

 above that which the mere subsidence of the region would 

 have done. The probable existence of this extra-glacial lake 

 somewhat complicates the subject, but still it is believed that 

 the valleys in the surface of the silt are valleys of erosion. 

 Not only do these valleys show erosion forms, but they are in 

 proportionate size to the present streams, and are never acci- 

 dentally partially closed by a strong development of the blue 

 silt. Moreover, erosion is clearly indicated in the exposure 

 near Freeport. In short, the evidence is sufliciently strong to 

 warrant my assumption of their origin as valleys of erosion. 

 Therefore, I shall consider the formation to have originally 

 extended over the central portions of the valleys to practically 

 the same height which it now attains on their sides. 



If the importance of the formation be judged by its bulk it 

 will be seen to compare favorably with all the others of the 



