ILLINOIAN DRIFT AND SECOND INTERVAL 141 



which estimates of relative age may be based are fuller and more satis- 

 factory than in the case of the pre-Kansan intervals. 



Illinoian Drift and the Yarmouth or Second interglacial 



Interval 



The drift which followed the Kansan, the third in the known order, is 

 the Illinoian, and the interval between the Kansan and the Illinoian 

 is the Yarmouth. For the last two names, Illinoian and Yarmouth, and 

 for the first discussion of the records to which they are applied we are 

 indebted to Leverett. The Yarmouth seems to have been the longest of 

 the interglacial intervals. The Illinoian glaciation, which followed the 

 Yarmouth, affected directly only a small part of Iowa. Ice from the 

 Labradorean center came from the northeast, crossed the channel of the 

 Mississippi, and pushed a broad lobe into a relatively small area in south- 

 eastern Iowa. If glaciers corresponding to the known Illinoian were 

 developed in the Kewatin region, they terminated in that part of the 

 glaciated area which was subsequently covered with Iowan or Wisconsin 

 drift, or with both. Illinoian of Kewatin origin has not so far been 

 anywhere recognized. The strip of territory occupied by the Illinoian 

 drift west of the Mississippi river is rarely more than 25 miles in width, 

 and is limited to portions of Scott, Muscatine, Louisa, Des Moines, and 

 Lee counties. In this area it overlaps the Kansan, and at various points 

 near the margin of the lobe wells have gone through Illinoian into Kan- 

 san, revealing a zone of weathering, a definite soil band, extensive alluvial 

 deposits, and well developed peat beds between the two bodies of drift. 

 The facts have been worked out by Leverett, and are recorded in volume V 

 of the Proceedings of the Iowa Academy of Sciences and in his mono- 

 graph on the Illinois glacial lobe. Owing to the greater recency of the 

 Illinoian as compared with the Kansan, the amount of valley cutting 

 since it was deposited has been much less than in the Kansan, and the 

 opportunities for observing the Yarmouth interglacial deposits in natural 

 exposures are fewer than in the case of the Aftonian. The Yarmouth, 

 however, is a true interglacial interval. It had its forests and its terres- 

 trial faunas, as shown by the sections near the village of Yarmouth, 

 which Leverett has recorded. The faunas embraced some modern mam- 

 mals, for the peat encountered in digging one of* the wells furnished 

 bones of the modern wood rabbit and the common skunk. The mammals 

 of the Aftonian beds so far collected are all extinct. 



The relative length of the Yarmouth interval may be determined ap- 

 proximately by comparing the changes which time has wrought in the 

 Illinoian drift sheet with the changes recorded in the adjacent and sub- 



