66 H. A. GLEASON VEGETATIONAL HISTORY OF MIDDLE WEST 



There is also some geological evidence of a climate during Wis- 

 consin time well suited to a xerophytic vegetation. Various rivers of 

 the region, notably the Wisconsin, the Green, and the Illinois, which 

 rise within the Wisconsin drift and flow out to the west or southwest, 

 occupy, beyond the Wisconsin terminal moraine valleys entirely out of 

 proportion to the size of the present rivers and choked with immense 

 quantities of glacial outwash. This may indicate unusually rapid 

 melting, due to a mild (and presumably also a dry) climate. 



During this period, forest belts of the more mesophytic species may 

 have existed along the Mississippi, Wabash, Illinois, Missouri, Cedar, 

 and Des Moines rivers in the eastern portion of this semi-arid region, 

 just as they do today in a similar climate in Nebraska and Kansas. 



Climate in the Early Post-Wisconsin. — Whether or not the 

 conclusion is accepted that a mild and semi-arid climate existed in the 

 Peorian and persisted through the Wisconsin stages, it seems almost 

 certain that such a climate characterized at least a portion of the 

 time involved in the post-Wisconsin glacial retreat. Palaeontological 

 and geological evidence for such a condition is scanty, and is concisely 

 summarized by various authors in a recent volume. 35 Alden 35a believes 

 " that during the deposition of the post-Wisconsin loess the climate iu 

 the northern interior may have been somewhat drier than at present, 

 but was not greatly different," Knowlton says 35e " There is some little 

 palaeobotanical support for the contention that there was a slightly 

 warmer period following the close of the glacial epoch." Tyrrell con- 

 cludes 3 ^ that the glacial climate of the Canadian northwest was " suc- 

 ceeded by a dry continental climate, under neither of which conditions 

 was a forest growth possible." Basing his opinion on the occurrence 

 of certain animal remains, Hay believes 35d " that after the retreat of 

 the ice-sheet a warmer period ensued," at the culmination of which 

 " the region along the southern shores of Lakes Ontario, Erie, and 

 Michigan enjoyed a climate similar to that now prevailing in Tennessee 



as Die Veriinderungen des Klimas seit dem Maximum der letzen Eiszeit. Eine 

 Sammlung von Berichten . . . herausgegeben von dem Exekutivkomitee des 11. 

 Internationalen Geologenkongresses. Stockholm, 1910. Including: 



35 a Alden, Wm. C. Certain geological phenomena indicative of climatic conditions 

 in North America since the maximum of the latest glaciation. 353-363. 



ssd Hay, O. P. On the changes of climate following the disappearance of the 

 Wisconsin ice sheet. 371-374. 



35e Knowlton, F. H. The climate of North America in late glacial and subsequent 

 post-glacial time. 367-369. 



35f Tyrrell, J. B. Changes of climate in northwestern Canada since the glacial 

 period. 389-391. 



