134 B. SHIMEK 



Moreover in many years' experience in dredging in ponds and 

 streams, the writer has seldom seen a land shell which had been 

 carried with the finest sediment into ponds or lakes, though such 

 shells are sometimes found in sand and other coarse material. 

 Currents of water which could carry most of the shells now 

 found fossil, would also carry coarser material than that which 

 makes up the loess. Another fact which bears out this conclu- 

 sion is the presence of opercula in fossil shells of Helicina occulta 

 in the northern loess and Helici?ia orbiculta in the southern loess. 

 As the operculum so readily falls from the decaying animal, it 

 would scarcely remain in place if the shell had been transported 

 any distance. 



2. The occurrence of fossiliferous loess chiefly in the vicinity 

 of streams is consistent with the theory of loess formation pre- 

 sented by the writer before the Iowa Academy of Science. 1 

 Plants, and especially forests, develop chiefly and' primarily along 

 streams. This creates conditions favorable to land molluscs, 

 and at the same time forms a trap for the dust carried from 

 adjacent more barren regions. The occurrence of loess in the 

 eastern part of Iowa chiefly along the border of the Iowan drift 

 sheet may also be explained on the same ground. After the 

 melting of the ice the terminal moraines offered the first lodging 

 place for plants. Here forests early developed, and the condi- 

 tions for entrapping the dust from adjacent less favored territory 

 which was probably dry during a part of the year were here first 

 created. We are in the habit of describing the lobed ridges 

 of loess regions as characteristic of loesc topography, yet they 

 are quite as much characteristic of some drift areas, as for 

 example, along the Big Sioux River in Iowa and South Dakota. 

 In eastern Iowa the surface of the loess is largely shaped by the 

 underlying moraines which first presented conditions suitable 

 to the deposition of the loess, and where consequently the deposit 

 is best developed. The loess at Natchez does not show this 

 loess topography in the same degree. 



3. The depauperation of some forms of shells, and the pres- 



1 Proc. Iowa Acad. Sci., Vol. Ill, p. 82 el seq. 



