94 0. H. Hershey — Florencia Formation. 



Florencia formation dates from about the time of the passage 

 from the mild conditions of the Aftonian to the somewhat 

 severer climatic conditions of the Iowan epoch. 



The Florencia formation is overlain with perfect conformity 

 by the basal member of the Iowan loess series. The latter 

 varies much in constitution from place to place, but in the 

 Crane and Yellow Greek valleys is a highly ferruginous bright 

 red clay, usually only a few inches in thickness. This ocherous 

 layer derived its large constituent of iron oxide from the red 

 Aftonian soil of the surrounding hills. The upper portion of 

 the Florencia formation, as already intimated, presents evidence 

 that the subsidence of the region had already advanced so far 

 as to produce a general flooding of the streams. A little later, 

 either through a sudden increase in the movement of land 

 depression, or the obstruction of the mouth of the Pecatonica 

 valley by the advancing Iowan ice-sheet, the flooded Florencia 

 streams were converted into long narrow lakes which gradually 

 deepened and eroded the red soil on the valley slopes, redepos- 

 iting it at lower levels as a red and brown sand and the " fer- 

 ruginous layer" of our Crane and Yellow Creek sections. 

 Almost immediately the extensive fauna and flora of the Flo- 

 rencia streams and valleys were totally destroyed, so that the 

 Iowan loess deposits are practically unfossiliferous. 



Several miles northeast of the village of Pecatonica in Win- 

 nebago county, there is a section which displays finely the 

 entire series of the Iowan loess resting with perfect conformity 

 upon the Florencia formation. The latter is exposed from the 

 river level to a height of three feet, and is a bed of laminated 

 and variegated clay which, by the erosion of the river, is made 

 to simulate indurated shales. There are a great many black 

 semi-decayed logs projecting from the bank into the river. 

 They reach a thickness of one foot and lie in the position of 

 driftwood. Over this Florencia clay comes, first, a stratified 

 bed of brown sand, then a less distinctly laminated stratum of 

 fine silt or typical loess, followed by the ordinary and easily 

 recognized " main body " of the loess series, or Upland loess, 

 as I have previously denominated it. Each member of the 

 series is distinct and they have a combined thickness at this 

 locality of 30 feet 



As I shall show presently, the climatic conditions at the time 

 of the deposition of the Florencia formation in northwestern 

 Illinois, as indicated by the faunal remains, were apparently 

 nearly identical with those usually considered typical of the 

 Aftonian epoch, while its physical relations to the rock gorges 

 and to the overlying loess series, together with a presumed 

 presence of the ice, supported by some evidence, on the east 



