138 S. CALVIN PRESENT PHASE OF PLEISTOCENE IN IOWA 



collection of equine bones which ma}' belong to either of the two species. 

 There are vertebrae, scapulas, and calcanea; nearly all the limb bones are 

 represented, including metapodials and phalanges. The large amount of 

 the material and its unexpectedly perfect state of preservation, these are 

 the impressive facts, and these indicate very clearly that the fossils are 

 not mere chance intrusions, but are the remains of a fauna which was 

 living in the- region at the time the gravels were accumulating. Further- 

 more, it will be evident that the presence of this fauna, whether we take 

 into consideration either its molluscan or its mammalian phase, is incon- 

 sistent with the view that the floods indicated by the deposits were fed 

 by melting glaciers. When this fauna lived and the gravels were depos- 

 ited there had been time enough since the disappearance of the pre- 

 Kansan ice to allow the region to become clothed with an abundant vege- 

 tation suitable for the support of the elephant and the horse; and the 

 temperature of the streams did not preclude the presence of fypes of 

 mollusks which find a congenial habitat in the rivers of modern Iowa. 

 The Afton Junction-Thayer beds have yielded fossils, quite a number; 

 but among these are such forms as Favosites from the Silurian and Pla- 

 centiceras from the Upper Cretaceous. Among the other things there 

 occurred some foot bones of a small, slender-limbed horse less than half 

 the height of our domestic species. The stage of equine development 

 indicated by the bones showed that the animal could not be older than 

 the Pliocene; but it was assumed that, like the other fossils from the 

 same beds, it must be pre-Glacial. In the light of the new finds in Har- 

 rison and Monona counties we may conclude that this beautiful little 

 Equus was probably contemporary with the deposition of the gravels, 

 and that there may have been at least three species of Aftonian horses. 



Concerning the precise age of these widely distributed gravels between 

 the Kansan and the pre-Kansan, it may be said that the Aftonian faunas 

 show that they were not laid down at the beginning of the interval, when 

 the earlier ice-cap was melting. Extensive weathering and alteration of 

 the materials, especially in the upper zone, show that they were not depos- 

 ited at the close of the interval, when the Kansan ice was advancing. 

 Forest material and remnants of an old soil, observed by Chamberlin and 

 McGee between the gravels and the overlying drift, support the conclu- 

 sion based on weathering. All lines of evidence now indicate that the 

 beds in question record conditions which existed at some time during the 

 progress of the interval, neither at its beginning nor at its close, but in 

 the light of present knowledge the precise age of the deposits can not be 

 more definitely stated. 



The soil band in the railway cut near Tama, already noted, is one of 



