68 IOWA ACADEMY OF SCIENCES. 



its, which recognized the Kansan, lowan and Wisconsin as the 

 only glacial stages that had been worked out with any satis- 

 factory degree of definiteness. It was in these publications 

 that the Aftonian beds were referred to the interval between 

 the Kansan and the lowan. The adjustment following Bain's 

 demonstration of the true position of the Aftonian left the 

 Buchanan gravels as the only recognized deposit, so far pub- 

 lished, representing this interval, and the term Buchanan offered 

 itself as a convenient designation for the second interglacial 

 stage. In the meantime Leverett* was pushing investigations 

 on a sheet of till younger than the Kansan, but much older 

 than the lowan, and furnishing proof that the enormously long 

 interval between the Kansan and lowan ice invasions is rep- 

 resented not by one, but by three distinct stages of the glacial 

 series. One of these stages, the Illmoian, is glacial, the other 

 two interglacial. When, therefore, in 1896, Chamberlinf revised 

 his classification of glacial deposits there were five drift sheets 

 to be recognized in Iowa in place of three. The Aftonian beds 

 were assigned their true place beneath the Kansan drift and 

 the term Buchanan was used for the second interglacial interval. 



The Buchanan gravels are connected genetically with the 

 events immediately following, or intimately attending the with- 

 drawal of the Kansan ice. The materials were evidently 

 derived directly from the Kansan drift. So far as their deposi- 

 tion is concerned, they belong to the very beginning or initia- 

 tion of the interglacial stage following the Kansan. They 

 remained exposed and undisturbed, subject to all the changes 

 incident to weathering, during the interval following the 

 Kansan, during all the Illinoisan glacial stage, and during the 

 interval that followed, until the invasion of the lowan ice. At 

 no point so far as yet known are they seen beneath the Illinoian 

 drift. They are usually covered with lowan drift within the 

 area which this drift sheet occupies. For some distance out- 

 side the margin of the lowan drift they are known to occur 

 covered with loess. 



The Buchanan gravels are much more generally and widely 

 distributed than was at first supposed. They are known to 

 underlie lowan drift over thousands of acres in Buchanan 

 county alone. A bed is extensively worked for road material 

 near Winthrop. A still heavier bed is exposed a mile or two 



♦Leveretc had recogaizeS the lUinoiaQ drift as the representatlveoof a distinct 

 glacial stage as early as 1891, but the fact was not published until 1896 

 tjour.of Giol , Vol. IV, p. 874, Oct jber-November, 1896. 



