REVIEWS 473 



that most American glacialists will agree, each in his own way, that the 

 interpretations of the middle drift of America, if we may use the term rather 

 broadly, are those that most invite question and perhaps further readjust- 

 ment. And so a few of the salient features of the shifting history of Amer- 

 ican opinion on the middle drift may be pertinent to this review. By 

 American middle drift let us understand the formations that lie between the 

 Aftonian interglacial beds that cap the lowest till and the base of the declared 

 glacial sheets of Wisconsin age. 



In an early comparison of American and European glacial formations 

 put in print sixteen years ago in the revised edition of Geikie's Great Ice 

 Age (1894, p. 774), the American deposits were given a threefold grouping, 

 with a basal member, the Kansan, a middle member, the lowan, and an 

 upper member, the Wisconsin, using here the simplified forms the terms 

 subsequently took. The first, the Kansan, was made to embrace the till 

 below the "Forest Beds" and the "Noah's Barn- Yards" that were then 

 supposed to form a common interglacial horizon; the second, the lowan, 

 was made to include the till above the "Forest Beds" and below the young 

 moraine-ridged sheet which was made to constitute the third or upper 

 division, the Wisconsin. This last division has held its place and name 

 with firmness throughout and shows no signs of instability, but the two 

 earlier divisions have suffered a serious shifting of names and of interpre- 

 tations, and the end is perhaps not yet at hand. The name Kansan has 

 been shifted from the sub-Aftonian till to the super- Aftonian till originally 

 called lowan. The term lowan thus displaced has been transferred from 

 the middle drift to the uppermost and least member of the grouped beds 

 originally covered by the term. The reasons for these shifts seemed cogent 

 to the workers in the lowan field at the time they were made and perhaps 

 seem so still. They were accepted with slight reluctance by the glacialist 

 who had given the terms their original applications. The cogency of the 

 reasons for the changes has, however, from his point of view, largely dis- 

 appeared with the progress of study, and if it were practicable to return 

 essentially to the original usage, making the sub-Aftonian till Kansan and 

 the super-Aftonian till lowan, and to take the exceptional exposures of 

 both formations near Afton Junction, Iowa, as the types, as was originally 

 done, it would seem to him to accord best with the inherent fitness of the 

 case. Particularly does this seem so in the application of the term lowan, 

 for the super-Aftonian till not only has a broader and more distinctive 

 expression in Iowa that anywhere else, but it is the greatest of Iowa's 

 drifts; it is inherently the lowan drift. When the shift of terms was made, 

 it was supposed that the uppermost till sheet in eastern Iowa with 



