Corrcspo/idcncc. 253 
tocene deposits has moved with tremendous strides during the past two 
years. A few points only can be noted. First, Bain showed that the 
till overlying the Aftonian beds was Kansan; the lower till of McGee, 
and not the lowan as had been assumed. This observation necessitated 
adjustment of views previously held. It added a new drift sheet to the 
known glacial series of Iowa. It demonstrated that the Aftonian and 
Buchanan interglacial beds belonged to different horizons. Before that 
adjustment Chamberlin* had published his classification of American 
glacial deposits which recognized only the Kansan, lowan and Wiscon- 
sin glacial stages, with two interglacial stages, the Aftonian being re- 
ferred to the interval between the Kansan and the lowan. Bain's -demon- 
stration of the true position of the Aftonian left the Buchanan gravels 
as the only recognized deposit, so far published, representing this inter- 
val, and the term Buchanan offered itself as a convenient designatioii 
for the second interglacial period. In the meantime Leverettf was 
pushing investigations on a sheet of till younger than the Kansan, but 
much older than the lowan, and furnishing proof that the enormousl}- 
long interval between the Kansan and lowan ice invasions was not a 
imit, but comprised three distinct stages of the glacial series. One of 
these stages, the Illinoian, was 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 place of three. The 
Aftonian beds were assigned to their true place beneath the Kansan, 
and the term Buchanan was used for the second interglacial stage. 
The Buchanan gr;!vels are connected genetically with events imme- 
diately following, or intimately attending the withdrawal of the Kansar. 
ice. The materials were evidently derived directly from the Kansan 
drift. So far as their deposition is concerned they belong to the very 
beginning of the interglacial stage following the Kansan. They are 
much more widely distributed than was at first supposed. They are ex- 
posed, in cases with a thickness of thirty feet, at scores of points in each 
of a number of comities examined, and sometimes hundreds of acres arc 
embraced in a single continuous area. Within the region invaded by 
lowan ice they are usually overlain by lowan till with characteristic 
lowan bowlders strewn over the surface. In the northeast corner of 
Delaware county, and at other points within the Kansan area but out- 
side the margin of the lowan drift, they are overlain by loess. 
The use of the term Buchanan as a name for an interglacial stage 
is open to criticism. It came into use tentatively before the recognition 
of the Illinoian drift, as a stage distinct from either Kansan or lowan. 
liad been published; when the whole period of time between the retreat 
<^)f the Kansan. and the invasion of the lowan ice was supposed to be 
a single, uninterrupted interglacial interval. It was first used in the 
l)recise sense in which the term Aftonian was originally used, and as a 
*Jour. of Geol. Vol. Ill, p. 270, April-May. 1S7.".. 
The Great Ice Akp, James Geikie, :i(l cd., pp. 724-774, l.S!)"'. 
tLeverett had rc^coynized the Illinoian drift as tlic rcpro.sentativc' of a distinrt 
1,'lacial stage as i^arly as IS94, but tin- fact was not ptibli.slied until WM. 
iJonr. of Geol., vol. IV, p. ,S74, Oct. -Nov., IMH). 
