176 THE ILLINOIS GLACIAL LOBE. 



of Iowa and Nebraska to-day — the evidence, if it suggests any difference, indicating 

 rather less moisture than is found iu eastern Iowa at the present time. This state- 

 ment, which can not be too strongly emphasized, is based upon the study of the modern 

 mollusks of Iowa and Nebraska and their fossil prototypes, extending over a period 

 of nearly twenty years. * * * 



Moreover the molluscau loess-fauna of any region is on the whole like the 

 modern fauna of the same region. For example, Binney reports a number of species 

 from the "post-pliocene" (evidently tlie loess) of the lower Mississippi Valley, of 

 which eleven are southerly species, and all now live iu the same region. Call reports 1 

 fifteen species from the loess of Arkansas, three of them included in the southerly 

 list, and all belong to the modern molluscau fauna of that State. The same is true 

 of the faunas of Iowa and Nebraska, as has been stated. 



This does not indicate transportation from a distance. It is interesting and 

 noticeable that for the most part the species of the loess are common over the same 

 region now. There are some exceptions, for there have been changes no doubt, but 

 these changes, as indicated by the distribution of the shells, are no greater than may 

 now be observed in any limited region in the course of a few years. Species are 

 sometimes disposed to appear, disappear, and reappear in a surprising manner in a 

 given locality, and if we may judge from the vertical distribution of the fossil shells, 

 the same was true during the deposition of the loess. 



The horizontal distribution of the fossils is likewise such that it suggests at once 

 that they are deposited in situ. 



As there are surface areas to-day which have no mollusks, lying in close proximity 

 to those on which mollusks are abundant, so there are deposits of loess without fossi i s 

 adjacent to those which are fossiliferous. As the lands, high or low, lying adja- 

 cent to larger streams have greater numbers of mollusks to day than the outlying- 

 prairies, so the loess bordering these streams is usually much more fossiliferous than 

 that which covers more remote areas, but the distribution of the fossils is not in 

 bands, as if drifted, but is similar to that of the modern specimens at the surface. 2 

 Summing up the evidence of the fossils, we may assert that it points to conditions not 

 unlike those which exist today, and that geologists in seeking for the cause and 

 manner of the deposition of the loess must give up the assumption of widely submerged 

 areas over which fossiliferous loess now occurs, and of a cold climate. 



MODE OF DEPOSITION. 



The mode of deposition of the loess still remains one of the most 

 puzzling problems of Pleistocene geology. Both the seolian and aqueous 

 hypothesis have strong adherents among the students of the- Mississippi 

 Valley portion as well as of other portions of this formation. The students 

 of the Mississippi Valley portion, however, all grant that the influence of 

 wind has been important, and probably all would concede that water has 



i Geol. Survey Ark., Vol. II, pp. 49, 165, and 166. 



- In a recent paper Shimek has discussed more fully the distribution of loess fossils: Jour. Geol. 

 Vol. VII, 1899, pp. 122-140. Also Proc. Iowa Acad. Sci., Vol. VI, 1898, pp. 98-113. 



