324 O. II. Hershey — Silveria Formation. 



Art. XLYII. — The Silveria Formation ; by Oscar H. 

 Hershey, Freeport, 111. 



In a recent number of the American Geologist,* the writer 

 discussed, under the name of ^' Kansan Buried Loess," a small 

 series of blue and brown laminated silts, exposed in a ravine 

 one and one-half miles south of the city of Freeport, Stephen- 

 son county, Illinois. At the time, this one very limited expo- 

 sure furnished the only definite information of this formation 

 which had been secured. Subsequent studies, however — car- 

 ried on chiefly through the examination of well-sections — have 

 shown this formation to be an unusually important one, not 

 only from its bulk, but also from the light which it throws on 

 the relative altitude of northwestern Illinois during the earlier 

 portion of the Kansan epoch. It has, therefore, been thought 

 advisable to again place the subject before the geologic public, 

 and this paper may be considered as a preliminary description 

 and definition of the formation. 



The fact is, of course, known to the writer that buried silts, 

 nearly identical in lithologic constitution and stratigraphical 

 position, have been discovered in several counties in northern 

 Illinois, and also in that portion of Iowa which closely adjoins 

 this area. But as there is always more or less uncertainty con- 

 nected with the correlation of buried silts in different areas 

 when their direct continuity has not been traced, and as this 

 formation appears to be best developed in Stephenson county, 

 Illinois, I will confine my remarks to that section of country 

 exclusively. 



The Silveria formation is a thick bed of stratified silt of a 

 nearly uniformly dark bluish-gray color, and which occupies 

 and nearly fills the deep-buried portion of the Pecatonica val- 

 ley and its tributary valleys in northwestern Illinois. It i^ 

 penetrated by nearly all the wells in the vicinity of the present 

 streams, and is popularly known as " blue clay." It is, how- 

 ever, not a true clay as it does not contain the necessary per- 

 centage of exceedingly fine particles. This statement leads 

 us directly to the manner of discriminating it from blue till or 

 bowlder clay, which is sometimes penetrated by wells in this 

 same district. The blue till is lighter in color, more calcareous 

 and finer-grained, presents no evidences of stratification, and 

 contains irregularly disseminated and comparatively abundant 

 pebbles and small bowlders largely of foreign rock species. 

 The blue silt differs from it chiefly in the following particu- 



* Early Pleistocene Deposits of Northern Illinois. Am. GeoL, vol. xvii, No. 5, 

 May, 1896. 



