496 W. UPHAM STAGES OF THE ICE AGE 



stages of Lakes Bonneville and Lahontan to the time of the Sangamon 

 recession of the ice-border. Chains of lakes in Martin Comity, Minne- 

 sota, mark courses of interglacial rivers that flowed southward into the 

 basin of the East Fork of the Des Moines, which were likewise regarded 

 in that paper as of Sangamon age, being thought to afford proof of a 

 great recession of the ice-sheet between the Illinoian and Iowan stages of 

 glaciation, so far, probably, as to uncover the southern half of Minnesota. 



More full consideration, however, of the relationship of the fluctuating 

 western Quaternary lakes with the recognized diverse stages of growth 

 and wane of the icefields, as here stated, has since led me to refer both 

 the drying up of the western lakes and the erosion of interglacial river 

 courses in southern Minnesota, with all the many occurrences of fossil- 

 iferous interglacial formations showing a very great interruption of the 

 Ice Age on this continent, to the Aftonian stage of glacial recession. In 

 comparison with that time of far withdrawal of the ice boundary, its 

 Sangamon oscillation was of brief duration and relatively narrow areas. 



Under this new review of the changeful and complex records of secular 

 climatic conditions during the Ice Age, the Nebraskan and Aftonian 

 stages appear very long; but the later stages, represented by drift sheets 

 and interglacial beds that are far more extensively accessible to observa- 

 tion, forming the surface upon nearly all the glaciated area of this conti- 

 nent, are seen to have occupied less time than was before supposed. The 

 most noteworthy revision of the sequence of changes in climate and oscil- 

 lations of the ice-margin, as shown by the drift and associated interglacial 

 beds, consists in the reference of the four glacial stages after the Aftonian 

 interval to a practically continuous prevalence of widespread snowfall, 

 bringing the ice-sheet to its maximum area and thickness, from which it 

 waveringly receded, but was at last, in a geologic sense, rapidly melted 

 away. 



Short distances and duration of ice recession and return, or of fluctua- 

 tion in adjacent or confluent icefields, one superseding or pushing back 

 another, may sufficiently account for the observed Yarmouth, Sangamon, 

 and Peorian land surfaces and fossiliferous beds that were covered by 

 ensuing glaciation. Even the moraine-forming Wisconsin stage had 

 changes of glacial currents on some areas during the final melting of the 

 ice-sheet, which brought new outlines of its border and an overlapping 

 of consecutive series of the marginal moraines. 



In Iowa the fauna of the Aftonian deposits included the extinct mam- 

 moth and mastodon, an extinct horse, and species of the deer and rabbit 

 yet living in this region. The flora comprised the pine and other conif- 



