ABSTRACTS OF PAPERS 115 



The papers composing the symposium will be printed in full in this 

 volume of the Bulletin. 



The Society adjourned about 12.30 p. m. for lunch. 



Session of Thursday Afternoon 



On reassembling in the afternoon the Society met in two sections. 

 The first, under the chairmanship of Vice-President Woodworth and the 

 secretaryship of Douglas W. Johnson, listened to the following papers : 



TITLES AND ABSTRACTS OF PAPERS PRESENTED AT THE AFTERNOON 

 SESSION AND DISCUSSIONS THEREON 



COMPARATIVE STUDY OF THE NEBRASKAN AND KANSAN TILLS IN IOWA 



BY GEORGE F. KAY 



(Abstract) 



The prevalent opinion among Pleistocene geologists has been that Nebraskan 

 till can be distinguished without difficulty from Kansan till by differences in 

 physical characters of these two tills. Recent detailed investigations of Ne- 

 braskan and Kansan tills, where the evidence admits of no doubt as to their 

 correct identification, has shown conclusively that this view must be modified. 

 Many excellent exposures of these two tills, separated by materials which 

 could have been formed only during an interglacial epoch of long duration, 

 have been made available for study in recent years in connection with railway 

 construction and the improvement of the roads of the State. The evidence 

 gained from a study of these exposures in widely distributed areas in Iowa 

 justifies the conclusion that, whereas it is true that in some parts of Iowa the 

 Nebraskan and Kansan tills can be distinguished by their differences in color, 

 texture, structure, lithologic composition, and related features, as was pointed 

 out by Carman in his report on the Pleistocene history of northwestern Iowa, 

 nevertheless there are many areas in Iowa within which these two tills resem- 

 ble each other so closely that it is impossible to determine by physical char- 

 acters whether a particular outcrop of till is Nebraskan till or Kansan till. 

 In such places the only satisfactory basis thus far found in Iowa by which it 

 is possible to decide whether the till is Nebraskan till or Kansan till is the 

 relationship of the till to interglacial material the age of which can be deter- 

 mined. Among the most widespread and most significant of these interglacial 

 materials is gumbotil. If a till is overlain by Nebraskan gumbotil or can be 

 shown to be related to Nebraskan gumbotil, which in Iowa is found as rem- 

 nants of a former extensive Nebraskan gumbotil plain, the till is Nebraskan 

 till. If, however, the till is overlain by Kansan gumbotil or can be shown to 

 be related to Kansan gumbotil, which also is found as remnants of a former 

 extensive Kansan gumbotil plain, the till is Kansan till. 



