0. H. Ilershey — Florencia Formation. 91 



much as 90 per cent of the mass. This was derived by stream 

 erosion from the Pleistocene rock gorges. There is also a cer- 

 tain percentage of drift pebbles which were secured in the 

 erosion of the till and gravel ridges of the district. The rock 

 fragments are sometimes angular, but usually are as well 

 rounded as the Galena limestone will admit by stream action. 

 They are much stained by the hydrated sesquioxide of iron. 

 Some of these stains are a dull earthy black in color, but the 

 greater part are a reddish brown. Locally the iron oxide is 

 present in such amount as to cement the gravel into a soft 

 conglomerate rock. There are rarely any shells or woody mat- 

 ter in this lower or gravel division, as it formed an unfavorable 

 environment for the animal life of the streams, and was depos- 

 ited at so low a level as not to catch any of the driftwood. 



The structure of the Florencia gravel is not very distinct, 

 but in places it is seen to be irregularly stratified. The sur- 

 face is everywhere uneven or rapidly undulating. As it is a 

 river deposit, we may presume that this irregularity of the sur- 

 face is due to its having constituted the stream-bed and not the 

 flood-plain deposit of the Florencia subepoch. The depressions 

 between the ridge-like elevations of the gravel represent the 

 deeper portions of the streams, while the higher portions of 

 the deposit are the sites of the ancient gravel bars. It is a 

 curious circumstance that the subsequent erosion of this gravel 

 has been so slight that the ancient gravel bars now form rapids 

 in some of the present streams, as in the lower course of Yel- 

 low creek. Indeed, the beds of all the larger streams of 

 Stephenson county, Illinois, are nearly everywhere composed 

 of this gravel, overlain in the deeper portions by a little brown 

 silt and mud. That the gravel which forms bars in their beds 

 is not the Modern river gravel except in a few instances, is 

 known from its outcrop in the banks at one or both ends of 

 the bars. The Florencia gravel is distinguished from the 

 Modern stream gravel by its much greater rounding, a larger 

 percentage of drift pebbles, and its peculiar ferruginous stain- 

 ing. Its stratigraphic relations, also, serve to distinguish it. 



The ancient stream gravel now under discussion outcrops in 

 the banks of Crane, Yellow, and tributary creeks and just 

 under the low- water level of the Pecatonica river. It is never 

 exposed to a greater height than two feet, and its total thick- 

 ness is unknown. Near Bolton a well was reported to have 

 penetrated " blue gravel and driftwood " to a thickness of ten 

 feet. In the Pecatonica river valley it seems to be largely 

 replaced by ferruginous sand, and to extend twenty or more 

 feet below the present river level. It constitutes the main 

 body of the Florencia formation ; but to the geologic student 

 the upper division is of vastly greater interest. 



