THE IOWAN ICE INVASION 347 



ing the Iowan stage of glaciation. It is therefore a near-correlative of the 

 Iowan drift, though it really represents the early part of the Peorian stage 

 of deglaciation (p. 158). 



If studies of the loess deposits covering other drift sheets outside the Wis- 

 consin area should be in complete accord with the results obtained by these 

 authors, the heavy loess deposits of the Mississippi Valley would have a new 

 significance and would indicate, as affirmed by many of the older geologists, 

 a definite period of loess deposition during the Pleistocene. A reexamination 

 of the loess fossil deposits now referred to Yarmouth and Sangamon time would 

 place at least some of these in Peorian time. Until such examinations are 

 made and more extensive study is given the loess overlying the Kansan and 

 Ilhnoian drift sheets, it has been thot best to leave the biota of these earlier 

 deposits as they were listed before the studies of Alden and Leighton appeared. 

 Many of these lists of loess fossils contain fresh water mollusks (Planorbis, 

 Galba, Physa) and the suggestion is at once presented that possibly these shells 

 came from the lower part of the deposit and represent the ground surface of 

 the Yarmouth or Sangamon interval before the heavy deposit of loess was laid 

 down. The qustion as to whether loess was deposited in any degree during 

 these earlier intervals must also be settled before any satisfactory readjustment 

 of the faunas of the loesses can be made. 



IV. The Peorian Biota 



1. IOWA 



In Harrison and Monona counties this yellow loess is quite fossiliferous 

 and Prof. Shimek has identified a large number of species. 11 A section of the 

 strata in this county, as well as a list of the mollusks noted therein, is shown be- 

 low: 12 



Section oj loess in Harrison County 



Yellow loess (post-Iowan) 6-7 feet 



Ferruginous band 4-6 inches 



Bluish loess with iron tubules and small calcareous nodules (post-Kansan) 3 feet 



List of species in post Iowan loess, Harrison County 



Vallonia gracilicosta Zonitoides arborea 

 Polygyra monodon " minuscula 



" multUineala Pyramidula altcmata 

 Strobilops labyrinlhica " cronkhitei anthonyi 



" virgo ' ' shimekii 



Leacochila fallax? Helicodiscus parallelus 



Papilla muscorum Saccinea avara 

 Bifidaria armijera " grosvenori? 



11 Op. cit., pp. 395-396. 



u Op. cit., p. 382. The correlations are the author's. 



