320 
The American Geologist. 
November, 1893 
of the old deserted valley of a small tributary of thePecaton- 
ica river five miles northwest of the city of Freeport. 
s. 
A. Drift. B. Loess. C. Alluvium. D. Present stream. 
The obstruction in the old valley is a ridge of sand. The 
greatest depth of the gorge is 60 feet, which certainly is less 
than the hight t<> which the sand ridge must at one time have 
attained. Its crest is now only about 30 feet above the level 
of the creek, and the fact that it is completely covered with 
loess shows that the removal of the upper 30 feet or more was 
accomplished before the deposition of the loess. It is evident 
that the bottom of the gorge had been lowered at least to half 
of its depth during the same time. This is only one of a num- 
ber of similar cases. 
4. It has been shown that lakes existed at one time in vari- 
ous parts of northwestern Illinois, but repeated searches have 
failed to reveal any deposits overlying the loess in any of the 
old lake basins. It does not seem probable that every vestige 
of such deposits would have been removed, without the de- 
struction of the loess, throughout large tracts of the lake 
basins ; and this is considered conclusive evidence that tile 
lakes had ceased to exist, and hence that their outlets had 
been cut down to the present bottoms of the gorges previous 
to the deposition of the loess. 
5. The foregoing figure shows a remnant of loess resting on 
the bottom of the gorge along the north side. This from cer- 
tain characteristics which it possesses, is known to be in situ. 
The bluff immediately back of it as well as all along that side 
of the valley, is not perpendicular as it should be if it had 
been eroded within a very recent period, but is a moderately 
steep slope of 20° to 80°. Atmospheric degradation has re- 
duced its slope to that amount since its base was last under- 
mined by a stream. That this degradation occurred previous 
