86 IOWA ACADEMY OF SCIENCES. 



that the Illinoian is a markedly younger sheet than the Kan- 

 san. This difference in age was suspected to occur from a 

 comjDarison of maturity of valleys in the two districts, but the 

 testimony of the weathered zone preserved below the Illinoian 

 was of value to confirm it. 



THE AFTONIAN AND PRE-KANSAN DEPOSITS IN 

 SOUTHWESTERN IOWA. 



BY H. FOSTER BAIN. 



INTRODUCTION. 



The Aftonian deposits of southwestern Iowa have peculiar 

 interest in that within the area is the type locality for the 

 Aftonian. So far neither the drift of the region nor the Afton- 

 ian as a unit has received a general discussion. The refer- 

 ences to the beds extant are merely incidental to broader stud- 

 ies. The type locality and several other critical exposures 

 have been visited by many geologists but no one has presented 

 a complete account of the beds in question. The time has not 

 even yet arrived for an adequate discussion of the Aftonian, 

 but in order to 'prevent possible misapprehensions it seems 

 advisable to present a brief summary of present knowledge. 

 It should be remembered that the exposures of the Aftonian 

 and the sub- Aftonian are scattered; that their importance was 

 unsuspected until quite recently; that in the nature of things 

 the phenomena may be expected to be somewhat illusive, and 

 that but little of the area has received detailed study. In view 

 of these facts the present must be taken as a preliminary state- 

 ment only and subject to considerable future revision. 



Scattered evidence of a forest bed was found by White in his 

 survey* of the region. The most noteworthy occurrence 

 recorded by him was that of a peat bed two to three feet in 

 thickness in Adair county, f There is some uncertainty, how- 

 ever, whether 'this peat occurs below the loess -merely, or is 

 beneath true bowlder clay, and hence, presumably of Aftonian 

 age. A recent visit to the locality by Mr. -Cowles, of the United 

 States Geological Survey, failed to clear up the doubt on this 

 point. I 



* Geol. Iowa, Vol. I, p. 97, 1870. 

 t Op Clt., p. 339. 



* Private communication. 



