27/2 LLLP OW RINATN OF ANGTAOLOGN 
to be such as to operate in like ways on both continents at the 
same time, it is nevertheless interesting to make such correlation 
as the formations permit, and it may be profitable if held in due 
restraint. The parallelism between the Kansan and the Saxonian 
formations is fairly close and certainly suggestive. 
The Aftonian ( = Helvetian ?).—Subsequent to the formation of 
the Kansan sheet of till, and accompanying assorted deposits, 
there was a very notable retreat of the ice. Mr. Upham, whose 
inclination to unify and limit the extent of the glacial period 
makes his judgment especially valuable on this point, expresses 
the opinion that the retreat reached an extent of 500 miles.* 
This judgment is based upon the occurrence of interglacial peaty 
deposits in the basin of Lake Agassiz. If we recognize a subse- 
quent retreat of considerable moment between the Iowan and 
Wisconsin stages, it will not perhaps be clear that these deposits 
do not belong to the later interval; but even under this interpre- 
tation, the retreat must still probably have exceeded 300 miles. 
During. this stage of retreat there were accumulations of muck 
and peat reaching a reported depth of twenty-five feet. One of 
the best exposures of this horizon is found between Afton and 
Thayer, Iowa, and from the former a euphonious name may be 
taken. Owing to the scarcity of gravel in the drift territory of 
southern Iowa, the Chicago, Burlington & Quincy Railway has 
made very extensive excavations upon three gravel deposits lying 
between an upper sheet of till reaching a thickness of forty to 
sixty feet and a lower till of less depth. The gravels appear to be 
kame-like accumulations, at least they are great lenses lying upon 
the surface of the lower till. This lower till is believed to belong 
to the Kansan stage and the upper to the Iowan. On the surface 
of the gravels there accumulated, at points, a deep mucky soil, 
in which occur considerable quantities of vegetal débris. This 
is believed to occupy the same horizon as the numerous peaty 
deposits described by McGee in eastern Iowa.” To this horizon 
also is referred the peat beds lying between the two outer till 
WARREN UPHAM: Am. Naturalist, Vol. XXIX., 1895, March, p. 237. 
?Eleventh Annual Report U.S. Geologicai Survey, p. 487. 
