178 THE ILLINOIS GLACIAL LOBE. 



The leading- difficulties cited by Chamberlin as attending the hypoth- 

 esis that the loess is simply an outwash of glacial grindings distributed by 

 glacio-fluvial waters, are its vertical distribution and the presence of shells 

 of land raollusks. "The extreme vertical range is not far from 1,000 feet. 

 The range within a score of miles is frequently from 500 to 700 feet." In 

 its interfluvial phase it mantles an undulatory surface and apparently 

 reaches a greater elevation on the east than on the west side of the 

 main valleys. It is difficult to bring its border into strict accord with a 

 horizontal plain as required by the lacustrine and marine phases of the 

 hypothesis, or even into a consistent gradient as required toy the fluvial 

 phase, without an arbitrary warping of the surface. It seems also extremely 

 difficult to conceive how a great flood which had the ice sheet for its northern 

 border could have been peopled so widely with land mollusks. In view of 

 these difficulties, Chamberlin proposes to divide the influence of wind and 

 water as follows. He adopts the glacio-fluvial hypothesis as the funda- 

 mental explanation, assuming, (a) the presence of the Iowan ice at the chief 

 stage of deposition; (b) a very low slope of the land and consequent wide 

 wandering* of the glacial waters; (c) the development of extensive flats over 

 which the glacial silts were spread; (<i) great periodic extension of glacial 

 waters caused by (1) periods of warm weather in the melting season, and 

 (2) by warm rains. He considers it probable that the periodic extensions 

 of the floods were not always destructive to vegetation over the flat region, 

 and that land mollusks and other animals dependent upon the vegetation 

 mav have found temporary retreat from the flood on the taller vegetation. 

 Upon the retreat of the Avaters, extensive silt-covered flats would become 

 exposed to the sweeping influence of the wind, and when dried the silt 

 would be borne in great quantities over the adjoining uplands. 



This hypothesis demands an accommodation between the breadth of 

 the fluvial deposits and the extent and massiveness of the a?olian deposits, 

 for a restriction of the glacial floods to narrow channels would render the 

 sweeping ground for the winds too limited in area to supply material for 

 the great mantle of silt found on the uplands. In proportion as the river 

 work is narrowed the wind work is expanded. It follows that the seolian 

 factor will cut away its own ground if pushed too far. It is further urged 

 that the seolian deposits are measured not by the quantity of silt borne by 

 the winds and lodged on the surface, but by the difference between such 



