208 S. CALVIN AFTONIAN MAMMALIAN FAUNA II 



Besides the Aftonian, there are two other horizons, conspicuously gravel 

 bearing, in the Pleistocene deposits of Iowa. One is represented by the 

 extensive beds of Buchanan gravels, which were evidently laid down at 

 the time of melting of the Kansan ice and are super-Kansan in strati- 

 graphic position; the other — much younger — is related to the disappear- 

 ance of the Des Moines ice-lobe of the Wisconsin. There is a third post- 

 Aftonian horizon — one which furnishes beds of clean quartz sand with 

 practically no gravel — showing deposits made by floods from melting ice, 

 related to the waning of the glaciers of the lowan stage. Gravels of the 

 later stages are not always readily discriminated unless the stratigraphic 

 positions are unmistakably indicated, and in the field work of the past 

 two years nothing that did not show clearly its relation to the Kansan, or 

 the Nebraskan, or to both, has been set down as Aftonian. In accordance 

 with this rule the gravel pits at Denison are to be transferred, temporarily 

 at least, to the doubtful list. Though they are located in the valley of 

 the Boyer Eiver — the stream on which the noted pits at Logan and Mis- 

 souri Valley occur — and though, according to Shimek's determinations, 

 so far as the scant contained mollusks are concerned, they are faunally 

 the same as the beds farther down the stream, and though structurally 

 and otherwise they appear to be identical with deposits that are certainly 

 Aftonian, the relation to definitely determined drift sheets is not clear. 

 The lower contact of the gravels is not seen ; their upper surface is over- 

 lain by loess, which shows an older and a newer phase, but there is no 

 till-sheet in sight either above or below to fix the exact horizon. Judg- 

 ment relative to the age of the Denison pits should, therefore, be sus- 

 pended until their status has been established. The few mammalian 

 fossils collected from the Denison pits should be checked out of the 

 Aftonian fauna, except so far as they have been or may be confirmed by 

 collections from beds of demonstrated age. The standing of Elephas 

 primigenius and the great stag, Cervalces, will be afi^ected, at least tem- 

 porarily, for these have not yet been found in other localities in beds of 

 undoubted Aftonian. 



FOSSILIZATION AND MODE OF PRESERVATION 



As the collections increase in number of specimens, and as they repre- 

 sent a wider geographic range, it is seen that, with the exception of the 

 few collected from silt beds, all the true Aftonian fossils agree in a re- 

 markable way in the amount of change or fossilization that the bones 

 and teeth have undergone. To a very noticeable and large extent those 

 from the arenaceous beds have all been changed by the infiltration of 

 silica. They are decidedly stony and hard and heavy, and they differ in 



