Pleistocene Deposits of Illinois. — Hershey. 
297 
hills, of a luxuriant vegetation like that of a warm temperate 
climate. On the contrary, cold temperate grasses and small 
scrubby trees, mostly red cedar, appear to have been the flora 
of the region in that period. Of the contemporaneous fauna 
little is known. There are, sparsely scattered through the de¬ 
posit, small gastropod shells of two species. Unfortunately, 
owing to a want of knowledge of the importance of these fos¬ 
sils, none were preserved by the writer, and at present the 
falling of the banks buries them under a talus. It was par¬ 
ticularly observed, however, that these shells seemed to be 
depauperate forms of species that existed in larger forms in 
the streams of the later Aftonian interglacial epoch. Where 
most numerous, their number may be roughly estimated at 
1.000 per cubic foot, and throughout the deposit they may av¬ 
erage several hundred in the same bulk of silt. Portions of 
the river alluvium of the subsequent Florence subepoch of the 
Columbia deposits of the same region contain as much as fifty 
shells per cubic inch. The exceedingly limited number of 
species, the scarcity of their remains, and the apparently de¬ 
pauperate forms, strongly indicate a cold and almost frigid 
climate in northwestern Illinois at the time of the deposition 
of our -‘buried loess” or blue silt. 
Digging into the blue silt in the bottom of the ravine, I en¬ 
countered a stratum, about eight iflches in thickness, which 
contained a great number of pieces of angular white chert, 
such as is common in the Galena limestone and its residuary 
material. This chert was imbedded in the blue silt in a 
definite stratum, but not in stratified form. As already stated, 
such white angular chert made up a large part of the pregla¬ 
cial soil of the region, and it undoubtedly was strewn quite 
plentifully on the surface of the surrounding hills and shores 
of the lake. I was at first inclined to believe that a landslide 
had brought it into its present position ; but the entire ab¬ 
sence of the red clay which must have accompanied such a 
landslide, and the want of any suitably situated ridge near 
by, induced me to reject this hypothesis and to adopt the fol¬ 
lowing. In the winters the lake was covered with ice, which, 
along the stony shore, incorporated much of the chert gravel 
into its peripheral portion. Upon the breaking up of the ice 
in the spring, this gravel was carried out into the lake and 
