134 S. CALVIN PRESENT PHASE OF PLEISTOCENE IN IOWA 



relative age and differential characters of the several sheets of drift. It 

 is not to be understood from this, however, that in the favored area 

 selected for discussion there are no unsettled Pleistocene problems. Im- 

 portant questions, many of them, are still waiting for solution ; but while 

 knowledge is admittedly incomplete in many particulars, it may be 

 Avorth the while at this stage in the interpretation of Pleistocene records 

 to set out the points that seem to be indicated with a fair degree of clear- 

 ness. In the discussion which is to follow no attempt will be made to 

 give an historical outline of the growth of knowledge relative to the 

 problems under consideration, such as would be appropriate in an address 

 dealing finally with the subject; nor will any effort be made to assign the 

 degree of credit which belongs severally to the masters who have worked 

 so effectively and so illuminatingly in the Pleistocene field. It will be 

 enough to say that Pleistocene geolog} 7 , as represented in Iowa, is in- 

 debted to a host of men, among whom may be mentioned Chamberlin, 

 McGee, Bain, IJpham, Leverett, Udden, Beyer, Macbride, Shimek, and 

 Savage ; while of men who have contributed to our knowledge of Pleisto- 

 cene conditions outside of Iowa the number is still larger. Noteworthy, 

 epoch-making, have been the scholarly contributions of such students of 

 Canadian geology as Hinde, Coleman, and Dawson. 



The fact that within the limits of Iowa at least five distinct drift 

 sheets are clearly differentiated, a number greater than may be readily 

 distinguished in any corresponding area elsewhere, affords some justifica- 

 tion for limiting the discussion to so small a portion of the glaciated 

 area. The problems of this limited field, however, are the problems of 

 the continent, so far as the age of ice is concerned. 



Pleistocene Problem of Twenty Years ago and of Today 



Less than twenty years ago there was at least one eminent geologist on 

 this side of the Atlantic who denied the evidence of any glacial invasion 

 of any of the present habitable portions of North America. There were 

 many who admitted the evidence of one, but only of one, episode of 

 glaciation, and some of this class still survive. A few men whom we all 

 honor were laboriously collecting facts which proved that glaciers had 

 overridden portions of the continent at least twice; and they discussed 

 phases of glaciation which, in accordance with the best knowledge of the 

 time, were known as the "First" and "Second" glacial epochs. Upper and 

 Lower till became familiar terms in the literature of Pleistocene geology. 

 Now we point to evidence showing five ice invasions, possibly six; and 

 only a few, if any, are left to question the adequacy of the evidence. As 



