AFTONIAN INTERVAL 139 



many instances of the same kind and it tells the story of an interglacial 

 interval as clearly and forcefully as the gravels and their remarkable 

 faunas. Interglacial peat beds confirm the testimony of the old soils and 

 buried gravels. One of the best known examples of peat at the Aftonian 

 horizon is that in the Oelwein cut. The organic deposit is three feet in 

 thickness ; it contains the remains of a tamarack forest, together with great 

 quantities of pressed moss, almost as fresh as when it grew ; the whole is 

 underlain by dark pre-Kansan and covered with 20 feet of Kansan and 

 Iowan till. The Aftonian peat bed described by Savage in the eleventh 

 volume of the Proceedings of the Iowa Academy of Science is, in some 

 respects, even more important than that at Oelwein; the fauna and the 

 flora of the deposit have been worked out in greater detail. Additional 

 testimony to the same effect is found in the buried forests encountered in 

 digging farm wells over practically the whole of Iowa. Facts relating 

 to this phase of the subject, so far as concerns some of the northeastern 

 counties of the state, were collected by McGee, and are set forth in 

 great fullness and with masterly clearness in his memorable paper, "The 

 Pleistocene History of N ortlieastern Iowa." The Aftonian, more than 

 any other of the interglacial intervals, was a time of luxuriant forests, 

 and forest beds are at present unknown at any horizon in the region 

 studied by McGee except that between the Kansan and pre-Kansan drifts. 

 In view of the evidence, clear and positive, and multiplied over and over 

 again, we can but repeat that the Aftonian was a real interglacial interval, 

 an interval of long duration, an interval of moist climate and swollen 

 streams, an interval when the winters' cold was not so severe or the snow- 

 fall so excessive as to preclude the continued occupation of the region by 

 the great stag, the horse, the mastodon, and the elephant — an interval 

 when the modern types of river mollusks flourished in the streams of 

 Iowa. 



PHYSICAL RELATION OF THE KANSAN TO THE OLDER DEPOSITS 



The Kansan till, which overlies the Aftonian beds, records what now 

 appears to have been the maximum phase of Pleistocene glaciation. Kan- 

 san drift is superficial, except for a partial covering of loess over southern 

 and western Iowa. Its physical properties, color, texture, and toughness, 

 which have often been described, distinguish it readily from till of any 

 other age. In some cases the Kansan ice plowed into the Aftonian 

 gravels, broke them up while frozen, and transported masses varying in 

 some of their dimensions from a few inches to more than one hundred 

 feet. It follows, therefore, that sand and gravel boulders, derived from 

 Aftonian beds and incorporated with other morainic detritus, are among 



