IOWA ACADEMY OP SCIENCES. 10 L 



It may be mentioned in conclusion that it has been suggested, 

 notably by Chamberlin,* that a complete series of deposits 

 recording a glacial period should theoretically include a series 

 of early deposits made by minor advances of the ice of increas- 

 ing intensity, covered by those of the maximum advance which 

 in turn should be covered by a second series of deposits made 

 by minor advances of decreasing extent. So far only the max- 

 imum and some of the later drift sheets have|been discriminated . 

 It is believed that the pre-Kansan drift probably represents 

 one of these earlier and minor extensions of the ice sheet. 



It is to be noted that the Aftonian as first used was correlated 

 with the ' 'forest bed' ' of McGee. Recent work has shown that 

 in northeastern Iowa two separate horizons were confused 

 under the latter title. Inasmuch as at some points the "forest 

 bed" is now believed to be between the Kansan and pre-Kansan, 

 rather than what is now known as the lowan and Kansan, the 

 original determination was in so far correct, and these would 

 accordingly be legitimate reasons for applying to the upper 

 drift of southern Iowa the term lowan. The tinal usage will 

 be to some extent determined by the fact as to whether or not 

 the pre-Kansan or the Kansan of present usage, is really the 

 surface drift of eastern Kansan. 



The recent changes are in the matter of dividing the forma- 

 tion which McGee called his "upper till." Since the peculiar 

 topographic forms which he so well described, and which are 

 so generally associated in mind with his "upper till" belong to 

 that portion of it now recognized as lowan, and furthermore, 

 since northeastern Iowa has been considered the type region 

 for the lowan, present usage will probably prevail, but this 

 possible change should not be lost sight of. 



In studies in the Alpine glaciation of Europe three periods 

 of glaciation have been made out. These include, (1) a fresh, 

 young till, (2) an older widespread till, and (3) a very old and 

 imperfectly known till, f In a general way the Iowa section 

 may be correlated with these beds. No. 1 answering to our 

 young drift, lowan or Wisconsin, or both; No. 2, the Kansan; 

 No. 3, the pre-Kansan. 



The Illinoian seems to have no correlative in the Alpine 

 section, unless possibly this middle drift should prove capable 

 of division. Until, however, much more is known of the pre- 

 Kansan such correlations must rest on rather slender data. 



♦Great Ice Age (Geikle), p. 736. 1895. 



+Le Systeme glaciare des Alpes, Penck, Bruckner et du Fanquier. 



