140 S. CALYIX PRESENT PHASE OF PLEISTOCENE IN IOWA 



the notable features of the Kansan drift, whether seen in the Oelwein 

 cut in northeastern Iowa (plate 2, figure 2) or in the large cuts near 

 Thayer, in the southwestern part of the state (plate 3, figure 1). Near 

 Missouri valley, at one of the newly discovered exposures, there is a great 

 mass of transported and badly distorted Aftonian, the full size as yet 

 unknown, but it has been worked quite extensively as an independent 

 gravel pit. The strata, originally horizontal and cross-bedded, are 

 faulted and folded, and in some cases they are tilted through an angle 

 of more than ninety degrees. On the side showing the greatest deforma- 

 tion the pit has been worked out up to a vertical wall of Kansan drift; 

 and in this body of till, against which the gravels, standing on edge, sud- 

 denly end, we recognize the instrument used by the Kansan glacier in 

 plowing into the Aftonian beds and breaking them into blocks. Here is 

 a mass of deformed and displaced Aftonian, an enormous sand and gravel 

 boulder, and here, in the exact position assumed while the work was 

 being done, is the agent through which was transmitted the shove and 

 the thrust recorded in the distorted gravels (plate 3, figure 2). Masses 

 of the dark pre-Kansan, rolled and kneaded and showing the effect of the 

 tremendous squeeze and push of a continental glacier, also occur as 

 boulders embedded in the Kansan. These sharply defined masses of 

 Aftonian gravels and pre-Kansan till, large or small, incorporated in the 

 Kansan, add confirmation to the evidence, if confirmation is needed, that 

 this till and these gravels existed as distinct geological formations before 

 the Kansan ice invaded Iowa. 



RELATIVE AGE OF KAXSAX AXD PRE-KAXSAX 



"While the Kansan is younger than the pre-Kansan, younger than any 

 of the Aftonian deposits, it is not possible to say, even approximately, 

 how much younger it is. The gravels were greatly altered by weathering 

 before the Kansan drift was deposited, the oxidation and kaolinization 

 requiring time. It required time to clothe the cold, bare surface of the 

 pre-Kansan drift with forests, and forests did grow luxuriantly, genera- 

 tions of them probably, during the Aftonian interval. The accumulation 

 of the Aftonian peat required time. Time was needed for the develop- 

 ment and distribution of the aquatic and terrestrial Aftonian faunas. 

 But, granting all the time required for these things, it is still probable 

 that the length of the Aftonian interval was equal to but a small part of 

 post-Kansan time. If the time since the Kansan be represented by unity, 

 the time since the pre-Kansan would be represented by one and a small 

 fraction. On the other hand, the Kansan till is certainly very much 

 older than anv of the later drift sheets, and fortunately the data on 



