578 SAMUEL CALVIN 



interbedded shales, are referable to the Dakota stage; while the 

 main body of shale and the calcareous Inoceramus beds represent 

 the Fort Benton division of the Colorado. It is not certain that 

 there is any true Niobrara in Iowa. During the long interval between 

 the Upper Carboniferous and the Upper Cretaceous the surface of 

 Iowa was deeply eroded, and it was on such a surface that the Cre- 

 taceous sediments of the state were unconformably deposited. Since 

 the Cretaceous, these sediments, which were comparatively thin at 

 the most and imperfectly consolidated, have been extensively removed 

 by erosion, and now occur in more or less isolated patches. On 

 the geological map of Iowa the Cretaceous is indicated over the 

 entire area upon which it was originally spread. 



The Pleistocene deposits. — Iowa was exceptionally fortunate in 

 its location with reference to the movements and marginal limits 

 of the successive ice-invasions of the Glacial epoch. The state, 

 therefore, offers unusual facilities for the study of the relative age 

 and differential characters of the several sheets of drift which make 

 up the great body of mantle rock within the limits of the glaciated 

 area. The succession of the glacial and interglacial stages which 

 have been recognized by members of the national and state surveys 

 is indicated in the columnar section, and the subject will be found 

 discussed in the national and state reports. 



