EVIDENCE THAT BEDS ARE AFTONIAN 125 



The gravels vai^ in coarseness and composition, but the pebbles are 

 commonly water-worn and frequently stained with iron to a deep rusty 

 red. The sands are sometimes similarly stained, but vary to a pure white. 



Both sands and gravels almost universally show black streaks, bands, 

 and cloudings of MnOg.^^ 



Very soft, small, white nodules of CaCOg are also usually present in 

 the sandy and gravelly parts. 



The silt is usually yellowish, bluish, or nearly white and appears in 

 thick strata, or more frequently in smaller plates and layers. It com- 

 monly shows lamination, and sometimes it is more or less mingled with 

 sand. 



In some parts of Iowa peat beds also occur, but they are practically 

 absent from the western beds. 



DISTRIBUTION OF FOSSILS 



Much of this western Aftonian is fossiliferous. Twenty fossiliferous 

 exposures have been carefully studied. Of these 1 is in eastern Nebraska, 

 18 in western Iowa, and 1 in eastern Iowa. Non-fossiliferous exposures 

 were studied in much greater numbers in both states. 



Of the twenty fossiliferous exposures, eight yielded mammals only, 

 seven yielded mollusks only, and five contained both mammals and mol- 

 lusks. 



STRATIGRAPHIC RELATION, PLEISTOCENE SECTION 



These fossiliferous beds lie unconformably between the Nebraskan 

 (pre-Kansan) drift below and the Kansan above, and are interglacial and 

 Aftonian. 



This was first clearly shown to the writer in the county line exposure 

 in the northern part of Harrison County, Iowa, where both tills are 

 present, though he had in the Cox pit previously determined that they 

 are below the Kansan. 



Both tills are not always present, but the Pleistocene members which 

 are present always maintain a consistent relative position, excepting in 

 those limited local cases where the Kansan caused a little confusion by 

 plowing up the Aftonian and Kebraskan. 



In this connection a section of the Pleistocene of western Iowa may be 

 of interest. The Pleistocene rests on the older rocks, which vary with 

 locality, but in western Iowa are usually Missourian or Cretaceous. 

 ' Passing upward, a complete section will reveal the following: 



^ The presence of so much MnOj also indicates the occurrence at one time of large 

 quantities of organic matter. 



