114 S. CALVIN — lOWAN DRIFT 



of vegetable matter buried in and beneath the drift we have all learned 

 to call Kansan. The forest zone separates the Kansan of the later lite- 

 rature relating to the Pleistocene from the oldest drift of the region, so 

 far as known, from the lately added member of our Glacial series — the 

 pre-Kansan or sub-Aftonian till. 



Comparisons with other Formations 

 lowan compared with the kansan 



The lowan drift in northeastern Iowa rests directly on what has come 

 to be called the Kansan, or on the secondary product of the Kansan', 

 which has been described under the name of the Buchanan gravels. 

 The Kansan becomes the superficial till around the southern and eastern 

 borders of the lowan. Whether this till bordering and underlying the 

 lowan is in reality the dejDosit to which the name Kansan was originall}' 

 applied, need not now be argued. It is sufficient to say that it is the 

 formation which all recent writers on Pleistocene geolog}^ have, b}^ com- 

 mon consent, adopted as the Kansan, and it is to be hoped that the 

 present usage will not be changed. 



The contact relations of the lowan and Kansan constantly invite com- 

 parisons between the two drift-sheets, and such comparisons bring out a 

 number of striking differences. In the area under consideration the 

 Kansan contains none of the large coarse textured boulders that are so 

 characteristic a feature of the lowan. Kansas boulders are relatively 

 small. Very rarely do they exceed 4 feet in diameter, and in a majorit}' of 

 cases the diameter is less than 10 inches. Dark, fine grained green-stones 

 are common in the Kansan and rare in the lowan, and the same is true of 

 limestone fragments and pebbles. The till of the Kansan is normally 

 blue, while the lowan is yellow. In the intervals or intervals* between 

 the Kansan and the lowan the upper part of the Kansan was profoundly 

 modified by aqueous and atmospheric agencies. The changes wrought 

 in these intervals are expressed in (1) erosion, (2) oxidation, (3) leach- 

 ing, and (4) in the decay of the contained boulders in the zone of weath- 

 ering. 1. The Kansan surface was ver}^ deeply eroded, and mature ero- 

 sional topography was completely developed before the lowan drift was 

 laid down on it. 2. The surface was weathered and oxidized to a depth 

 of from 6 to 12 feet, as shown in many recent exposures. The oxidation 

 is most complete in the upper 3 or 4 feet, from which depth there is a 

 gradual transition to the normal blue, unweathered phase. The weath- 



* Leverette has shown that three stages of the Glacial series are represented by the interval be- 

 tween the Kansan and the lowan. These are respectively Yarmouth (interglacial), Illinoian 

 (glncial), and Sangamon (interglacial). 



