Reviews 



The Aftonian Gravels and Their Relations to the Drift Sheets in the 

 Region about Ajton Junction and Thayer. By Samuel Calvin. 

 {Proceedings oj the Davenport Academy oj Sciences, Vol. X, 31 

 pages, 1905.) 



In this small brochure Dr. Calvin presents the results of an excellent 

 and much-needed re-study of the type locahty for the Aftonian. It will be 

 remembered that, when Chamberhn in 1895 proposed a classification of 

 American glacial deposits, he named one of the important interglacial 

 epochs from certain beds occurring at Afton in southwestern Iowa. These 

 beds were correlated with others in eastern Iowa and in Minnesota, and 

 assigned to the interval between the lowan and Kansan glacial periods. 

 When the Iowa Geological Survey took up the study of the drift deposits", 

 it was very shortly determined that the lower drift of eastern Iowa was the 

 upper drift of southwestern Iowa, and presumably the one to which the 

 term "Kansan" should be applied. This being so, the beds at Afton must 

 represent a pre-Kansan rather than a post-Kansan period of deposition; 

 and if they were truly interglacial, any drift below them represented an 

 earher glacial interval than any at that time recognized in the region. Ten 

 years ago, beUef in the complexity of the glacial period was not so unanimous 

 as now, and there was some hesitancy in taking so radical a step as was 

 involved in the recognition of an additional glacial advance. In the years 

 which have since passed, the pre-Kansan has been widely recognized, 

 and its existence is now fully established. 



In 1898, when the reviewer studied the Aftonian locahties, certain of 

 the exposures were obscure, and the evidence was confusing, so that it 

 was thought wiser to make no certain deductions as to the exact relations 

 of the Aftonian to the Kansan. Professor Calvin has been so fortunate 

 as to be able to study new exposures which completely explain the puzzling 

 irregularities noted before, and which leave no doubt of the Aftonian 

 marking a true interglacial interval separating the pre-Kansan from the 

 Kansan by a notable period of time. Mr. Savage had already shown 

 from paleo-botanic evidence' that the correlation of the beds at Afton with 



1 Proceedings of the Iowa Academy of Sciences, Vol. XI, pp. 103-9. 



68 



