MODE OF FORMA TION OF TILL 



6 1 



topography would not suffice even to remove the residuary clay. 

 When I make the statement that the four classes of glacial 

 deposits discriminated in this district, occupy hills equally as 

 high, as narrow, and as steep, also occupy valleys equally as 



YiQ. I.— The Kansan ice at work in northwestern Illinois. 



deep and broad, and trending in the same direction, we are pre- 

 pared to accept the following conclusions : 



I That certain glaciers, notably that which glaciated north- 

 western Illinois, in passing over a slightly hilly region, exerted 

 very unequal pressure on the land surface, this inequality not 

 always being directly due to the topography of the immediate vicinity. 



2. In crossing hills of moderate height, they sometmies 

 strongly abraded the crests, while on closely adjoining areas 

 they deposited ground moraine on hills of similar height and 



shape. 



3. The areas of maximum and of minimum glacial action 

 were generally permanent, or approximately so, throughout the 

 time of glaciation. 



4. In studying areas in the Kansan drift region or at least 

 that portion of it which is in northwestern Illinois, the relative 

 length of the glaciation in different localities cannot be even 

 inferred from the apparent severity of the glacial action. 



In conclusion, I will state in more definite language what I 

 conceive to have been the position with relation to the mass of 

 the ice, of the unstratified drift of Stephenson county, Illinois. 

 The derivation, transportation, and deposition of the material 

 occurred mainly under the peripheral portion of the glacier. 

 Mere, through the thinning of the edge, the weight was less, and 

 the force of the ice movement having decreased, there was a 



