THE IOWAN DRIFT 579 



at that time credited to the region; but if the texts relating to the 

 subject are carefully read and the maps published in connection with 

 them are examined, it will be seen that the view that the lower till, 

 the Kansan, lies below the Aftonian is untenable. For example, 

 the description of the materials and prevailing color of the upper 

 till on p. 476 of the Eleventh Annual Report is true for only the 

 third of the drift sheets and is at variance with the facts if intended 

 to include the middle till. The same is true of the reference to the 

 large granite bowlders as "the most conspicuous element of the 

 upper till," on p. 481. On the other hand, the characteristics 

 assigned to the lower till in the comparisons made between it and 

 the upper on p. 479, are all features that belong to the middle 

 drift sheet; in no true sense are they descriptive of the sub-Aftonian. 

 It is true that at the end of the paragraph there is a reference to 

 the "forest bed" as a plane of separation between the upper and 

 lower tills, but the characters which the author saw and so cor- 

 rectly and graphically described belong to a super- Aftonian till and 

 to nothing else. 



If now we turn to the chapters on " Glacial Phenomena of North 

 America," contributed by Professor Chamberlin to Geikie's Great 

 Ice Age, we shall see again how the preconception that there were 

 but two drifts where three actually exist, led unavoidably to con- 

 fusion. It is no reflection on anyone that such confusion crept in 

 in the earlier discussions. Some things are unavoidably overlooked 

 by the pioneer who opens up for us new fields of science, and we 

 can but admire the genius and the insight of the masters who 

 taught us how to read the complicated history of the Pleistocene 

 deposits of the Mississippi Valley. As in the Eleventh Annual 

 Report, so in the Great Ice Age, it is two super- Aftonian tills that 

 are most frequently referred to in the text, and most accurately 

 represented on the map opposite p. 727 as, East-Iowan and Kansan. 

 The distinguishing characteristics of the upper and middle drift 

 sheets could not be more clearly or more succinctly stated than is 

 done on p. 760 of the work cited where, speaking of the East-Iowan, 

 it is said: 



In Iowa the granitic types predominate. Immense bowlders are freely 

 scattered over a portion of the surface. As greenstones prevail in the lower 



