MODE OF DEPOSITION OP THE LOESS. 179 



lodgment and the erosion of the surface. " Erosion is ordinarily more than 

 a match for the dust accumulations. The conditions must have been 

 extraordinary which would give a dust deposition sufficient to supply 

 erosion and still leave such a large residuum as the loess mantle implies. 

 The unleached and unweathered nature of the body of the loess is specially 

 in point here. These considerations warn us of the theoretical danger of 

 too greatly circumscribing the fluvial action." 1 



An undue extension of the fluvial hypothesis is thought to be antago- 

 nistic to the existence of molluscan life and also encounters the topograph- 

 ical and physical difficulties previously urged. In conclusion Chamberlin 

 calls attention to the efforts made by himself and colleagues to find criteria 

 of discrimination between aqueous and seolian loess, and remarks that 

 " while individual types of both deposits are not difficult to find, a crite- 

 rion or series of criteria of general applicability which shall distinguish the 

 two and assign to each its appropriate part are wanting." 



Turning now to the region under discussion in order to test the appli- 

 cability of the hypothesis suggested by Chamberlin, it is found that the 

 variations in level are sufficiently great to put the hypothesis of fluvial 

 deposition to severe tests, though they are not so conspicuous as in border- 

 ing districts on the east or west. The district covered by the Illinois glacial 

 lobe is largely embraced between the levels of contours lying 800 and 500 

 feet above tide, the general elevation in the northern portion of the district 

 being about 800 feet and the southern portion about 500 feet. This descent 

 of 300 feet in passing from the northern to the southern portion of the State 

 is about the same as the fall of the Mississippi River along its western borders. 

 The Mississippi Valley, however, is cut to an average depth of about 200 feet 

 below the bordering uplands. A small portion of northern Illinois slightly 

 exceeds 1,000 feet and portions of southeastern Illinois and southwestern 

 Indiana fall below 400 feet, thus giving a range of fully 600 feet within 

 the limits of the State, aside from the deepening of valleys since the loess 

 deposition. The most conspicuous abrupt variation in elevation of loess- 

 covered districts is found in the southern end of the State, where, as above 

 noted, a ridge rises 300 feet or more above border tracts. A few conspicu- 

 ous reliefs occur along the border of the Mississippi in northwestern Illinois 

 and in the portion south from Quincy. 



1 For a partial dissent from Chamberlin's views see Udden : Bull. Geol. Soc. Amer., Vol. IX, 1897, 

 pp. 7-8 ; also Sniniek : Proc. Iowa Acad. Sci., Vol. VI, pp. 109-110, and Jour. Geol., Vol. VII, 1899, p. 135. 



