THE NEBRASKAN ICE INVASION 227 



manganese dioxide. Indications are that this should be referred to the Afton- 

 ian. Shimek remarks that ''while the fossil shells from this stratum are some- 

 what unlike those which were collected in the Aftonian beds, the difference 

 is no greater than that which we might expect in different parts of the same 

 region, especially since no doubt special conditions existed. It is probable 

 that the deposit was formed in a swamp or shallow lake, and both its shores and 

 bottom would produce environment unlike that of the Aftonian streams. The 

 preponderence of terrestrial species is, however, very unusual. " The life of 

 this deposit is indicated below: 



Polygyra monodon Hclicodiscus parallelus 



" midtilineata Succinea w&ara 

 " profunda " ovalis 



Strobilops labyrinthica " retusa 



" virgo Carychium exiguum 

 Bifidaria armifera " exile 



contractu Helicina occulta 



Vitrea hammonis Galba caperata 

 Euconidus fidvus " humilis modkella 



Zonitoides arborea Aplexa hy prior um 



minusada Physa gyrina 



Pyramidula alternata Planorbls parvus 



cronkhitei anthonyi Pisidiwn abdilum 



C. DISTRIBUTION OF THE AFTONIAN BIOTA 



There are a number of deposits beyond the limits of Harrison and Monona 

 counties which are referable to this interglacial stage. 



1. Iowa 



Near Oelwein, Fayette Count}', a fine white sand deposit, overlying a peat 

 bed, occurs in a railway cut, which is referred to the Aftonian stage. 9 The 

 section may be summarized as follows: 



5. Iowan drift to 10 feet 



4. Buchanan gravel (Yarmouth stage) to 2 " 



3. Kansan drift, with remains of wood 3 to 20 " 



2. (a) Sand, fine-white, well water worn, often with little silt and clay (Aftonian) to 6 inches 

 (b) Vegetal layer and soil, 2-4 inches almost pure carbonaceous matter, the 

 rest largely charged with humus. Moss (Hypnum) common in peat layer 



(Aftonian) to 4 feet 



1. Sub-Aftonian (Nebraskan) drift, greenish blue when wet, greenish cast 



when dry 10 feet exposed 



•Beyer, Proc. Iowa Acad. Sci., IV, p. 59, 1897. 



