RELATIVE AGE OF THE IOWAN 145 



most deeply buried parts up to the grass roots in the humus layer at the 

 surface. The amount of weathering in the Iowan must be expressed by 

 a number that is very near to zero. How many times greater must be the 

 number which would fitly express the amount of Illinoian weathering 

 which has affected a zone 3 or 4 feet in thickness ? And if we had these 

 numbers correctly determined, would their relative values correctly ex- 

 press the relative lengths of post-Illinoian and post-Iowan time ? If so, 

 the Sangamon was very long, and the Illinoian is many times as old as 

 the Iowan. It may not be inappropriate here to say that the question 

 of leaching is not emphasized in this discussion. The amount of lime 

 carbonate present in any drift is a criterion of small value in determining 

 its age. Quantitatively, the calcareous material included in the different 

 drift sheets as an original constituent differed very greatly. For exam- 

 ple, the Iowan drift was never as calcareous as the Kansan. There is 

 very little limestone flour even in its deeper parts, and there are prac- 

 tically no limestone pebbles. Within the limestone areas of the region 

 which it traversed, the Iowan ice seems to have been too thin to cut down 

 to bedrock. All the characteristic materials which it carried, all except 

 what may have been picked up from the Kansan, were probably derived 

 from an area near the origin of the ice-sheet, an area of coarse crystalline 

 rocks. On the other hand, the Wisconsin drift is charged to excess with 

 limestone flour and limestone pebbles, greatly outranking in the matter 

 of these constituents all the other glacial deposits of the Mississippi 

 valley. Again, we find that the same till sheet varies in respect to lime 

 content in different localities, the old Kansan having originally had 

 vastly more calcareous material in the southwestern part of the state 

 than in the northeast. Furthermore, the presence or absence of lime 

 carbonate near the surface of any particular drift sheet may depend 

 locally on movements of ground waters. Without movement there can 

 be no transfer of the soluble contents of the drift from one place or from 

 one zone to another. Again, a drift sheet may be never so young — it 

 may have been finished but yesterday — but if the ice which transported 

 it traversed a region containing nothing but siliceous rocks, the most 

 industrious wielder of the acid bottle could get no reaction. The acid 

 bottle may have its place, but it lacks something yet of being an instru- 

 ment of precision when used to determine the relative age of drift. The 

 bearing of all this is obvious, and it is all told to emphasize the point 

 that, owing to its original composition, the superficial portion of the 

 Iowan drift, or any other portion of it, for that matter, may give little 

 or no reaction with acid; but the drift is nevertheless young. Measured 



