THE IOWAN ICE INVASION 363 



conspicuous numbers. The last three species are distinctively southern. 

 Helkina occulta has wholly disappeared and its place has been taken by 

 Helicina orbicnlata. The richly fossiliferous loess of Natchez and Vicksburg, 

 Mississippi, also contains the forms common at Hickman and Helena, and 

 the presence of Polygyra obstricta, P. injiecta and P. stenotrema still further 

 stamps the fauna as distinctively southern. 



"But in this variation in the wide loess region there is nothing which sug- 

 gests a transition or change from cold climate to warm climate faunas or vice 

 versa. The variation, as we find it in the loess is practically exactly duplicated 

 in the modern fauna of the surface. The only conclusion, then, which can 

 be drawn from the fossils of the loess is, that during the deposition of the several 

 loesses climatic conditions were not materially different from those which exist 

 in the various parts of the same general region to-day. Such differences as 

 do exist point rather to a drier climate in the northern part of the loess-covered 

 area than that of to-day. 



"Emphasis has sometimes been placed upon the depauperation in size of 

 certain loess shells, as evidence that the climate in which they existed was colder 

 than that of to-day. These depauperate shells are found only in the northern 

 part of the loess area, in Iowa, Nebraska, etc. Their exact counterparts are 

 found living to-day in the drier portions of the same region. And correspond- 

 ing differences do not occur in more easterly series which represent differences 

 in latitude. It is evident that the depauperation is due to drouth and not 

 to a low temperature, and the abundance of these depauperate shells in the 

 northern loess reinforces the evidence already noted that the climate of this 

 region was then somewhat drier than at present. " 



TABLE OF LOESS FORMATIONS 



Prof. Shirnek 45 presents the following ideal section as representing the rela- 

 tion of the different loesses to the drift sheets. 

 Kansan drift 



Kansan residual sands and gravels (Buchanan) 

 Gumbo (Loveland) 

 Black soil (Yarmouth; 

 Post-Kansan loess 

 Illinoian drift 



Illinoian residual sands and gravels 

 Black soil (Sangamon) 

 Post-Illinoian loess 

 Iowan drift 

 Iowan residual sands and gravels 



* Bull. Lab. Nat. Hist., State Univ., Iowa, V, p. 363. 



