256 The American Geologist. April, i89s 
unit has received a general discussion. 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 statement only and 
bubject to considerable future revision. 
The Afton-Thayer exposures were visited by McGee and Chamber- 
lin in company, and the evidence of an interglacial interval here, in con- 
nection with the facts derived from a study of other portions of the Mis- 
sissippi valley, was considered .sufficient to warrant the reference of the 
beds to two distinct periods of glaciation. With a wise conservatism, 
the two periods were assumed to be the same as had been demonstrated 
in northeastern Iowa, and accordingly in the nomenclature eventualh. 
proposed by Chamberlin* the upper drift at Afton was considered to 
be the lowan, and the lower the Kansan. The Aftonian beds proper 
were considered to represent the interval between the Kansan and the 
Towan. It is important to note that in the original paper by Chamber- 
lin the term Aftonian was not applied to the gravels which form so 
conspicuous a feature of the Afton-Thayer sections. These were con- 
sidered to represent rather kame-like accumulations upon the surface 
of the older drift sheet. This distinction has not been always clearly 
observed. 
The Afton-Thayer outcrops are for many reasons the most important 
of those bearing on the question of an interglacial interval in south- 
western Iowa and will be described in some detail. Preliminary to 
this it is desired to examine briefly what sort of evidence may properly 
be required to establish the presence of two drift sheets. In southern 
Iowa the most important criteria have been found to be forest beds 
.lud buried soils, leached horizons, ferruginated zones ("ferretto hori- 
zons"), water-laid beds, topographic changes, and the physical charac- 
ter of the till. The cumulative value of this sort of evidence is believed 
to be important. 
TJir Affo/i'T/myrr Exposifjrs. 
The Aftonian beds are not positively known to occur in or immedi- 
ately adjacent to the city of Afton; the latter is. however, the best 
known place near the original exposures. The beds are seen well ex- 
posed at three abandoned gravel pits located three to six miles east of 
Afton proper. These are (i) between Afton Junction and Talmage: 
(2) about one mile southeast of the Junction on the south side of Grand 
river; (3) about three-quarters of a mile west of Thayer on the south 
side of the C, B. and Q. railway. For convenience these will be called 
the Afton Junction, Grand river and Thayer pits respectively. The 
Afton Junction pits show the overlying loess, the Kansan drifts and the 
gravels, with certain buried silts or loess below the latter. The Grand 
liver exposure shows the upper and lower drifts with the gravels bc- 
*GreatIce Age (Geikie), pp. 773-774, 1893; Jour, (icol., vol. Ill, 270-277, 1895. 
