4 
Pleistocene Deposits of Illinois. — Hersliey. 295 
First, there is no question that this formation was deposited 
in water. The structure alone, without considering the other 
evidence, would make this a certainty. No seolian or glacial 
hypothesis can account for the stratification and ripple-marks. 
But the question remains as to whether it was deposited 
under fluvial or lacustrine conditions. When the Tertiary 
era came to a close, the valleys of northwestern Illinois were 
shaped somew T hat as now, although the slopes were steeper 
and outcropping rocks more numerous. The cause of the 
termination of the peculiar conditions of the Pliocene period, 
and of the institution of the Pleistocene period, is generally 
recognized to have been a great epeirogenic uplift of nearly 
the whole of North America. Northwestern Illinois, of course, 
participated in this elevatory movement, and the streams, 
which had long before reached baselevel and widened their 
valleys to a great extent, now- again trenched their river beds 
down into the solid rock, so that during the early part of the 
Quaternary era they excavated not inconsiderable valleys in 
the bottom of the old Tertiary valleys. These new valleys 
are, in the upper Mississippi basin, mostly buried under an 
accumulation of glacial and later deposits, and for this 
reason they cannot be examined. In the Pecatonica basin all 
the larger valleys have their rock bottoms 100 feet or more 
below r the present stream level. If there are any considerable 
remnants of preglacial river deposits still remaining in nor¬ 
thern Illinois, they must lie on the top and slopes of the rock 
terraces which careful studies of the valleys and of w T ell sec¬ 
tions show” to exist below the present bottoms of the valleys. 
Large quantities of a peculiar river gravel have been plow r ed 
up by the ice from some low-lying position in the Pecatonica 
valley and have been incorporated with the drift over the up¬ 
land country to the west. No such gravel is known to be 
exposed in situ anywhere in the region, and I am strongly 
inclined to believe that it really represents some preglacial 
deposit buried in the valley. 
It is assumed that the glacial periods of past times have 
been largely due to elevation of the land. It seems to be a 
fact, however, that the climax of the Quaternary stages of 
glaciation w T as reached long after the elevatory movement had 
culminated and the land had returned nearly to its former 
