THE LOESS. 421 



water species of shells in the loess is what we should expect 

 in an accumulation which took place in periodical or annual 

 overflows. Land snails still love the flood-plains of variable 

 streams. In the vicissitudes of the last stages of the glacial 

 period we nave the temporary presence of water provided for 

 to suit the habits of such shells as are found in the loess. 



Fig. 120. — Deep gulley in the loess at Helena, Arkansas. Courtesy of U. S. Geo. Sur- 

 vey. Photo, by A. F, Crider. 



