North American Tnfer(jlac'ial Deposits. — Upham. 283 
of the ice-front at Chaska, Minn., in the lower part of the 
Minnesota valley, across beds of stratified clay containing 
large Unionidcf shells, in such topographic and stratigraphic 
position that these mollusks are shown to have become al- 
ready well established there in a congenial habitat, with fa- 
voring climatic conditions, while the ice was receding between 
these late marginal moraines.* The forest bed and associated 
molluscan shells before noted record a great interval of glacial 
retreat and reiidvauce, measured by hundreds of miles; but 
the Chaska fossils doubtless represent only a small oscillation, 
probably no more than a few miles and possibly even less 
than one mile. 
Northwestern Iltinois. — Recent studies of the Pleistocene 
formations in the basin of the Pecatonica river, in northwest- 
ern Illinois, by Mr. Oscar H. Hershey,f show that the mol- 
luscan fauna living there in creeks and rivers during the lat- 
ter part of the interval following the Kansan formation and 
preceding the lowan formation, which was accompanied by 
extensive loess deposits, comprised, so far as it is represented 
by a collection of six species, only the same shells which are 
still abundant in the fresh waters of northern Illinois to-day. 
The many air-breathing and fewer fresh-water molluscan spe- 
cies in the loess have also all continued to live in the same 
region to the present time. Mr. Hershey's paper is of special 
interest in proving that after the growth of forests and the 
existence of mollusks betokening a climate nearly the same as 
now, while the conditions of gravel and sand deposition in the 
valleys implied a slightly higher general altitude of the coun- 
try, there ensued a great depression toward the end of the 
lowan time of readvancing glaciation and during the closely 
following glacial retreat with its abundant deposition of loess. 
This depression is thought to have been near the end of the 
Glacial period. In another paper,;); 3Ir. Hersliey estimates 
that the cutting of certain rock gorges in the same district, 
referred to the time between the Kansan and lowan forma- 
tions as these are subsequently named by Chamberlin, re- 
quired this interval to be much longer than the Postglacial 
*Geologyof Minn.. \n\. n. pp. i:!I-i:!l. Ml-Ul. 
f Am. Geologist, vol. xv, y)\). 7-24, .luii., IS!).'!. 
JAm. Geologist, vol. xii, pp. '514-:{2:{, Nov., iS!)."}. 
