108 S. CALVIN — lOWAN DRIFT 



region under consideration, no one liiid suH})ected that nioi-e tlian one 

 sheet of till was re])resented in the Pleistocene deposits of Iowa ; but the 

 studies of McCJee in Iowa and of Chamhcrlin in ^\'isc<Jnsin led to the 

 very general recognition of the niultiplii character of our drift dej)0sits. 

 Other investigators, using the methods of ol)servation employed h}^ the 

 ])ioneer workers named, and guided by tlie criteria for discriminating 

 different drift-sheets Avhich they pointed out, have been able to map, 

 with a fair degree of accurac}^ the limits of the successive ice invasions. 



Previous Work on the Iowan 



The earlier work on the Pleistocene of northeastern Iowa was remarkalJe 

 in many respects. The energy and intellectual acumen with which the 

 investigations were ])rosecuted, the enormous mass of details collected 

 and classified, and the masterly way in which the facts were handled, 

 and conclusions of epoch-making significance deduced from them, com- 

 manded the admiration of all geologists. Nevertheless, as was practi- 

 cally unavoidable in doing pioneer work in an unexplored field, some 

 important facts w'ere overlooked. Three sheets of till are now known to 

 be present in the ai'ea studied, but the hypothesis employed recognized 

 only two ; and the attempts to harmonize the drift sections in Bremer, 

 Buchanan, and other counties where the Iowan is typically developed, 

 in accordance with the hypothesis, led to some confusion of statement. 



Origin of the Name 



McGee applied the names " Upper till " and " Lower till " to the re- 

 spective drift-sheets which he recognized in northeastern Iowa. It is 

 obvious from numerous statements that what we now call the Iowan drift 

 must be regarded as the typical phase of his upper till. For example, 

 he tells us that the prevailing color of the upper till is yellow or buff, 

 running into gray,* and that boulders,t so large that they rise above the 

 drift as high as houses or haystacks and give character to every landscape, 

 are conspicuous elements of the upper till. Furthermore, the boulders 

 characterizing the upper till are said to culminate in size and abundance 

 in Butler, Bremer, Black Hawk, and Buchanan counties. And so the 

 details of structure and distribution are described so minutely as to leave 

 no possible doubt concerning the particular drift-sheet which stood in his 

 mind as the embodiment of all the typical characteristics of the upper 

 till. There is only one till in all the area described that expresses itself 



♦Pleistocene Hist. Northeastern Iowa, llth Ann. Kept. U. S. Geol. Sur., p. 47fi. 

 + Iliiil.. p. 481. See also the Resume, p. .")40. 



