124 B. SHIMBK AFTONIAN FOSSILIFEKOUS GRAVEL AND SAND BEDS 



yield fossils of such interest be set at rest. For that reason the writer 

 presents here a detailed account of the evidence of stratigraphic position 

 furnished by each of the twenty fossiliferous exposures which have been 

 studied in Iowa and Nebraska, and also includes a list of the fossils ob- 

 tained from each, in order that the distribution of these fossils may be 

 more clearly set forth.^^ 



Evidence that Beds are Aftonian 

 general characteristics of the aftonian 



While specific attention is here given to the exposures of fossiliferous 

 Aftonian, it should be borne in mind that there are many more. exposures 

 in the same region which have yielded no fossils, but which possess 

 Aftonian characteristics to a greater or lesser extent. Not every ex- 

 posure presents equally conclusive evidence, and some of the exposures 

 would probably be unintelligible but for the light which comes from the 

 study of the entire series. 



Whatsoever may be the variation in this respect, it seems to be re- 

 stricted within definite limits, for there are certain characteristics which 

 prevail to such an extent that they may be regarded as collectively deter- 

 mining typical Aftonian, and while not all of them are present in every 

 section, yet in the great majority of cases they are sufficiently well devel- 

 oped to make the identification entirely satisfactory. 



COMPOSITION AND STRUCTURE 



The composition and structure of the Aftonian beds, while somewhat 

 variable, are sufficiently constant to assist materially in the identification 

 of this formation. In the western part of Iowa and in eastern Nebraska 

 they consist of gravel, sand, and fine silt, variously cross-bedded and 

 interstratified. Usually each of these types of material appears distinct 

 in a well defined w^edge or stratum, and this segregation evidently repre- 

 sents local variation in the force of the currents. Sometimes only one or 

 two of them make up the deposit, but more frequently all three types will 

 be found in the same exposure, though varying very much in relative 

 amount. Sometimes they are in part blended or mixed. Occasionally, 

 especially where disturbed by the overlying Kansan, the Aftonian con- 

 tains masses of Nebraskan or Kansan till. Very large boulders, usually 

 granite or Sioux quartzite, sometimes occur in the sandy or gravelly parts. 



10 Th'e mammalia mentioned throughout this paper were identified by Professor Calvin, 

 the mollusks by the writer. The collections and the stratigraphic studies in the field 

 were made chiefly by the writer for the Iowa Geological Survey. 



