300 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



difficulties attending the application of eithei* hypothesis to the entii'e 

 deposit.^ 



The adherents of the eolian hypothesis generally assume that drainage 

 conditions were such that the dust accumulation exceeded the erosion of 

 the land, but they object to the view that much of the surface was under 

 water. The restriction of the deposit to a certain stage in the Grlacial epoch 

 is thought to affect in no way the applicability of this hypothesis so long 

 as it is granted that there were exceptionally poor drainage conditions, it 

 being thought that erosion is ordinarily more than a match for dust accu- 

 mulations. They cite the presence of shells of land mollusks and the wide 

 vei-tical distribution of the loess as fatal to the aqueous hypothesis. 



The adherents of the aqueous hypothesis, while recognizing the diffi- 

 culties of accounting for the land mollusks and the wide vertical distribu- 

 tion of loess, have emphasized the important fact that the thickest and most 

 porous loess is distributed along the main valleys, and they maintain that 

 its distribution was largely dependent upon the great streams of the region. 

 The}^ also have emphasized the occurrence of coarse material in places in 

 the basal portion of the loess, and the occasional development of distinct 

 beds of silt and also of sand that seem to be water laid. 



The close connection between the lowan drift sheet and the loess 

 deposits of eastern Iowa and western Illinois is recognized by the advocates 

 of each hypothesis, and there seems to be unanimity of opinion that water 

 was an important distributer there, though some difference of opinion exists 

 as to the extent of its influence. 



In the region under discussion the variations in level are such as to 

 put the aqueous hypothesis to severe test, for the silt deposits occur from 

 an altitude 500 feet or less above tide up to more than 1,000 feet. In 

 southeastern Indiana this wide range is found within a space of but a 

 few miles. The uplands ordinarily stand 200 feet or more above the surface 

 of the drift accumulations in the main valleys, and nearly 400 feet above 

 the main streams. To submerge this region it would seem necessary to 

 assume a depression that would bring the uplands about to sea level and 

 carry the valleys to a level far below tide, there being no apparent basis 

 for the hypothesis of land barriers or other obstructions which could have 

 held a wide bod}' of water much above sea level. 



^Supplementary hypothesis respectmg the origin of the loess of the Mississippi Valley: Jour. 

 Geol., Vol. V, 1897, pp. 795-802. 



