374 The Ainerican Geologist. June, i899 
the vicinity of Rush City, Minn., however, and on most of 
the area of the lowan till in eastern Iowa, the somewhat uni- 
form but remarkably thin development of this upper deposit 
of till seems to me consistent only with its having remained 
chiefly englacial until it was laid bare on tlie ice by ablation. 
Being thus exposed, as it could not be in a subglacial position, 
to the waters of the glacial melting and of rains, it yielded 
the very large volume of the loess washed from it and de- 
posited just outside the boundaries of the lowan glaciation. 
Indeed, in the case of the paha plateaus and ridges, which 
McGee has very fully described, loess occurs within the gla- 
ciated area on tracts that projected in depressions of the thin 
ice-sheet, and along the courses of streams that flowed in 
channels walled on each side with ice, being also at their be- 
ginning in some instances underlain by ice. The loess seems 
to me to prove for the lowan glaciation, like the coarser 
modified drift of New England and Minnesota, that the mar- 
gin of the departing ice-sheet at last became covered by its 
previously englacial drift, whence so vast deposits of gravel, 
sand, clay and loess were readily washed away to be laid down 
mostly near the receding ice boundary. 
Similarly I believe, also, that the older lUinoian and Kan- 
san till sheets include as their upper portion a large amount of 
drift that was finally superglacial. This part of the early drift 
comprises much preglacially decayed rock material eroded 
from the Tertiary land surface; and hence decaying and dis- 
integrating boulders and pebbles are usually frequent or abun- 
dant in it, while its finer drift is prevailingly leached like the 
preglacial residuary clays from which it was largely derived. 
Occasionally, however, as Bain has described the old till, prob- 
ably Kansan, of western Iowa,*it has in some localities an un- 
leached condition even at its surface, which may have been 
due to peculiar marginal glacial currents, during the departure 
of the ice, carrying lower parts of the englacial drift upward 
in these places and removing the normal upper till, so that 
undecayed boulders and calcareous drift are there anomalously 
predominant. 
*Am. Geologist, vol.xxiii, pp. 168-176, March, 1899. 
