96 The American Geologist. Xwkma, \>m 
111 the Pecatoniea l)asin the buried i)()r(i()n oi" the valleys is 
tilled mainly with a blue t^ilt of glacial derivation. The var- 
ious members of the Kansan drift sheet rest upon this " buried 
loess," demonstrating the ])re-Kansan age of the lower gorges. 
Hence they may be considered to re])resent a se]>arat<' epoch 
of the Pleistocene period. 
Northeastern Illinois is a heavily drift-covered region, and 
has an average altitude considerably less than the northwest- 
ern portion of the state. Many of the streams are out of their 
preglacial courses; and the writer would not venture to trace 
the various baselevels of the Freeport section into that dis- 
trict. Peneplain No. 2 may be represented by the general rock 
surface in Boone, McHenry, DeKalb, and Kane counties, at an 
average altitude of 700 feet. This portion of the state is de- 
pressed below its normal altitude as the lingering etf'ect of 
the weight of the last great ice-sheet which has melted from 
it in comparatively recent times. 
This discussion has developed several important inferences. 
It is only quite recently that the hypothesis was seriously en- 
tertained that the Kansan drift and the Lafayette formation 
Avere contemporaneous in deposition. Because of its bearing 
on this question it is exceedingly important that the position 
of the Lafayette plane in the upper Mississippi basin be com- 
pletely demonstrated ; for, if the preceding correlations are 
•correct, the Lafayette period and the Kansan epoch were sep- 
arated by at least one and probably two epochs of marked 
erosion. The second inference is that the pre-Kansan portion 
of the Quaternary era was at least several times as long as the 
remainder of it, considering the era to have begun at the time 
of the culmination of the first post-Lafayette uplift in the 
coastal plain. The third inference is that practically the 
whole of the true valleys of the state of Illinois (excluding 
basins in the northwestern corner and new rock-gorges of 
post-Kansan age) are early Quaternary in age. 
All the preceding correlations are provisional and intended 
more as suggestions than as definite facts. It is only quite 
recently that the writer made the mistake of comparing a se- 
ries of supposed Tertiary valleys in northwestern Illinois with 
valleys of an apparently corresponding series in the Ozark 
plateau region of southern Missouri.* It now appears that 
*See the American Geologist, vol. xvi, December, 1895. 
