'J'2'1 The American Geologist. November, 1893 
liH'ssial. The supposition is that the old valley had become 
obstructed by a scries of gravel ridges, one spur of which re- 
mains, by till, heavy deposits of which are exposed on the 
south side of the valley, and by other deposits of Glacial age, 
remnants of which can still be found. Subsequently, during 
the deglaciation interval which followed, the gorge just men- 
tioned was excavated, together with other gorges farther up 
the creek, and at the same time the obstruction of the old 
valley was being removed by erosion. Then came the deposi- 
tion of the loess, after which, because an unusually thick de- 
posit of loess had been laid down in the gorge, whereas the 
obstruction in the old valley had previously been almost 
removed, the creek again regained the old course. 
It is natural for us to believe that several successive gorges 
cut by the same creek, in the same time, in the same kind of 
rock, and with other conditions alike, would differ but little 
from each other in dimensions and shape. Now by comparing 
these two gorges, which are evidently pre-loessal, in age, with 
the Others along Yellow creek, we may be able to determine, 
approximately, the amount of erosion that has been accom- 
plished in them, both before and since our great time-mark, 
the deposition of the loess. It will thus be found that a large 
part, perhaps four-fifths or more, of the erosion of all the 
gorges was accomplished in the deglaciation interval which 
preceded the loess. 
A comparison of the relative size and shape of the gorges 
which are undoubtedly pre-loessial, with the figure on page 
320, shows the probability that the situation of the dotted 
line, representing an ideal restoration of the gorge just pre- 
vious to the deposition of the loess, is approximately correct. 
Similar comparison maybe extended to all the gorges ot the 
region, and in the majority of cases we find that the amount 
of rock removed in post-loessial times is apparently repre- 
sented by a triangular section, the perpendicular of which 
now forms a precipitous bluff. 
The history of the Pleistocene rock gorges of northwestern 
Illinois appears to have been as follows: a. Obstruction of 
old valleys by the early drift, b. Formation of lakes, c. Ex- 
cavation of deep, narrow gorges, similar to those of postglacial 
age on the later drift, and consequent destruction of lakes. 
