0. II. Ilershey — Florencia Formation. 93 



Distribution. — Because of the very incomplete condition of 

 the study of the Quaternary geology of northwestern Illinois, 

 the Florencia formation is known to the writer in the Peca- 

 tonica basin only. Here it is found, wherever the proper 

 horizon is exposed, along nearly all the streams of Stephenson 

 county, but has been studied mainly in the valleys of Crane 

 and Yellow creeks and the Pecatonica river. Its outcrops, 

 although fairly numerous, never rise more than a few feet 

 above the stream level, and are usually somewhat obscured by 

 a talus from the bank above. For this reason, the formation 

 has probably failed of investigation in other districts of this 

 portion of the Mississippi basin ; but as it represents condi- 

 tions which were not limited to northwestern Illinois, its exist- 

 ence in all the deeper valleys of this and neighboring states 

 can hardly be doubted. In Stephenson county, the most sig- 

 nificant feature of its distribution is the fact that, unlike the 

 drift and loess deposits, it is confined strictly to a certain level. 

 Its upper surface forms a plane which scarcely varies from 

 eighteen inches to two feet above the present low-water level 

 of the streams. It is, therefore, a fluvial formation and can 

 extend from the present streams only so far as the sides of the 

 valleys which were excavated in the drift and rock after the 

 Kansan epoch. In the Yellow creek and the Pecatonica 

 river valleys near Freeport, its width may vary from an 

 eighth to a mile ; and in western Winnebago county, its borders 

 are probably two miles apart. It has been estimated that if 

 that portion of the formation which is developed in Stephen- 

 son county, were spread as a uniform sheet over the surface of 

 the entire county, it would have a thickness of about six inches. 



Stratigrajphic relations. — Because of the very interesting 

 nature of the fossil contents, it is of the greatest importance 

 that the age of the Florencia formation be definitely fixed, and 

 this necessitates a careful investigation of its stratigraphic 

 relations. It rests upon the Kansan drift sheet everywhere 

 except where post-Kansan erosion has completely removed the 

 till and other glacial deposits. It is, therefore, separated from 

 the latter by an erosion interval of the length of which the 

 interglacial rock gorges of this region* are the gauge. The 

 Florencia formation passes through these rock gorges, com- 

 pletely burying their flat bottoms. Its age is, therefore, not 

 earlier than the practical completion of these gorges. Now 

 the presence of the Iowan loess series at various places within 

 them has demonstrated that in age they correspond mainly to 

 the Aftonian epoch. This would seem to indicate that the 



*The Pleistocene Rock Gorges of Northwestern Illinois, American Geologist 

 vol. xii, No. 5, November, 1893. 



