THE IOW AN ICE INVASION 361 



Iowa State University, ably discusses this subject and some extracts from his 

 paper may fittingly close this chapter. 



"In order that the value of these molluscan faunas may be properly meas- 

 sured it is necessary that they be taken collectively. A single terrestrial shell 

 does not make a land deposit, neither does a single aquatic shell make a water 

 deposit. In water deposits aquatic shells always form a conspicuous part of 

 the fauna, even tho locally they may not predominate. In subaerial deposits 

 aquatic shells may occur, but they are rare and local, and the dominant types 

 are terrestrial. Strictly terrestrial Pleistocene deposits are of two types: 

 buried sand dunes and the loesses. Buried sand dunes are not uncommon in the 

 upper Mississippi Valley, excellent illustrations being found near Gladstone, 

 Illinois; north of Iowa City, Iowa; at Hooper and West Point, Nebraska, and 

 at other points. Neither buried nor surface dunes contain shells so far as 

 observed. 



"The loesses are much more satisfactory for our purposes, because they fre- 

 quently contain fossils and offer by far the best opportunity for the study of 

 Pleistocene terrestrial mollusks. In these deposits terrestrial forms vastly 

 predominate, and fluviatile forms are wholly wanting. So much has been 

 written on this feature of the subject that only reiteration is here possible. 



"Fresh- water shells in the loess are very few. They belong to species 

 which inhabit small ponds and boggy places. They are not of the types found 

 in streams and lakes. They are local in distribution, and in a number of cases 

 clearly associated with buried ponds. Ponds are not rare in high places in 

 loess regions. They frequently contain the smaller Lymnoea, etc., which are 

 sometimes found in the loess, aquatic birds and insects probably being respon- 

 sible for their introduction. Such ponds, if buried by subsequent depositions 

 of loess, would present exactly the conditions under which aquatic shells are 

 usually found in the loess. The vastly predominating forms are terrestrial — 

 upland terrestrial at that. Some have become extinct in the loess region, but 

 occur westward and southwestward in the drier part of the continent. Such 

 are Pupa muscorum, P. blaiidi, Sphyradium edentulum var. alticola, Pyramidula 

 shimekii and Oreohelix iowensis. Others like Succinea grosvenori and Vallonia 

 gracillicosta are still found in the loess region, but they prefer dry, often open 

 grounds. The land species which prefer wet grounds are conspicuously absent 

 from the loess. 



"The fossil mollusks do not enable us to determine the age of any of the 

 Pleistocene formations. The fossils of the Aftonian are not sufficiently dis- 

 tinct from those of modem alluvium to permit the drawing of any conclusion 

 other than that the conditions of deposition were much the same. They do 

 not enable us to distinguish between the loesses, for the fossils of the gray and 

 the yellow loesses are, in larger series, essentially the same. But they give us 



