SNAILS AND THE SOIL — VAN CLEAVE 279 



have been washed down from higher grounds where colonies of living 

 snails may be located. 



Wind. — Winds are likewise effective agents in transporting dead 

 shells. The evidences in support of this statement come largely from 

 past geological times. Fine sands and shells from some prehistoric 

 dustbowl were carried great distances by the winds and were then 

 laid down in the characteristic deposits called loess. These forma- 

 tions, rich in fossil snail shells, are highly desirable agricultural lands. 



Ice. — The fauna of the loess probably lived under conditions prac- 

 tically the same as those prevailing today. Many of the larger species 

 where inhabitants of forested areas along river bluffs. Some of the 

 smaller species doubtless lived in open woods or thickets, or even on 

 the open prairies as they do today. In southern Illinois, the loess 

 deposits are entirely different from those in the northern part of the 

 State with respect to the molluscan fauna. In the southern loesses, 

 there are large land shells abundantly represented although these are 

 almost wholly lacking from the loess of northern Illinois and their 

 place is taken by minute species. A part of this variation is probably 

 due to climatic differences, similar to and even exaggerated above 

 those existing today. Loess deposits in northern Illinois probably 

 began soon after the retreat of the ice and under such conditions that 

 the climate would have been considerably colder than at present. 

 Geologists seem convinced that isotherms were displaced southward 

 during considerable part of the period while loess was being deposited. 

 Under these conditions it would be highly probable that temperature 

 conditions in central and southern Illinois at that time were more 

 nearly what we find now in central or southern Michigan and 

 Wisconsin. 



Human agency. — Shell heaps of enormous size have often been 

 formed by human agency. Pre-Columbian Indians, especially along 

 the coast, used shellfish for food and some tribes produced very ex- 

 tensive beds of shells at their camp sites. Even in the Central States, 

 primitive Indians left extensive accumulations of snail shells in some 

 places (Baker, 1936), probably having utilized the snails as food. In 

 like manner, shells and shell artifacts are scattered through the 

 mounds produced by the race of mound builders, though not in the 

 quantities characteristic of the kitchen middens. 



On top of natural deposits of shells, there are frequently found 

 shells the presence of which would entail extreme changes in tempera- 

 ture and in the extent of the oceans if they were natural deposits. In 

 dredge cuts in northern Illinois, there have been found fairly good 

 samples of shells that have never been known to live anywhere except 

 in Florida and many of these are strictly marine species. The only 

 logical explanation of this condition is that Indians left them at camp 



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