132 PROCEEDINGS OF THE BOSTON MEETING 



CERTAIN ASPECTS OF GLACIAL EROSION IN ALASKA 

 BY W. O. CROSBY 



(Alistract) 



Describes and discusses examples, believed to be typical, which tend to 

 localize glacial overdeepening and to show that valley broadening is by far 

 the more important phase. 



Presented by title by request of the author. 



FURTHER DISCUSSION OF THE AFTONIAN GRAVELS AND THEIR RELATION 



TO THE DRIFT-SHEETS IN THE REGION ABOUT AFTON 



JUNCTION AND THAYER, IOWA 



BY GEORGE FREDERICK KAY 



{Abstract) 



The gravels near Afton Junction and Thayer, in Union County, southwesterji 

 Iowa, are so well known to students of Pleistocene geology, and their Aftoniau 

 age has been so generally accepted, that one may well hesitate to state that 

 a restudy of these famous exposures and other exposures in the same region 

 has revealed evidence which seems to justify further discussion of the origin i 

 and relationships of these gravels, and to warrant question being raised with 

 regard to former interpretatioiis. 



Ever since the year 1895, when Dr. T. C. Chamberlin made reference in 

 Geikie's Great Ice Age to the interesting characteristics of these gravels, and 

 interpreted them to be kamelike deposits closely associated with the drift on 

 which the gravels lie and overlain by a later drift-sheet, many glacial geolo- 

 gists of America' and of Europe have visited the locality. Some have come 

 merely to see the type sections of the two oldest drifts, now known as the 

 Nebraskan drift and the Kansan drift, separated by the gravels which years 

 ago were named the Af tonian interglacial gravels ; others have come to study 

 carefully the characteristics of the drifts and gravels and their interrelation- 

 ships. The most important contributions dealing with these gravels and re- 

 lated deposits have been made by Chamberlin, Bain, and Calvin. 



The purpose of this paper is to show that the gravels of Union County, 

 which for so many years have been called the Aftonian interglacial gravels, 

 do not constitute a distinctive stratigraphic horizon separating the Nebraskan 

 drift from the Kansan drift, which is the generally accepted interpretation, 

 but that these gravels are lenses and irregularly shaped masses of gravels 

 within a single drift, the Kansan, or, if in two drifts, the Nebraskan and the 

 Kansan ; it is not possible by means of the gravels to differentiate the two 

 drifts. 



It is shown also in this paper that, although the gravels in the vicinity 

 of Afton Junction and Thayer can not be used to establish the presence of 

 two drifts, there is other evidence in the region which makes it clear that the 

 two oldest drift-sheets are present, and that they are separated in age by a 



