22 The American Geologist. January, 1895 
Correlation with the Columbia Formation in the Lower 
Mississippi Valley. 
It is a well known fact that the loess of central Illinois is 
continuous with that of the extreme southern portion of the 
state, which is also known to be a continuation of the loess 
and loam, or upper member, of the Columbia formation in the 
Mississippi embayment. The absence of any known barrier 
across the Mississippi river below Savanna, 111,, the extension 
of loess from the drif tless area to the lower Mississippi valley, 
the fact that the surface of the loess-depositing waters grad- 
ually rose (in relation to the land surface), at least in north- 
western Illinois, and that a subsidence of the land was al- 
ready in progress before the loess began to be deposited, seem 
to necessitate the rejection of the theory of an ice-dam to ac- 
count for the Upland loess of northwestern Illinois. 
We have been referring to the Upland loess as deposited in 
a great fresh-water lake ; but it may not have been a true lake, 
for it seems quite probable that it had connection with the 
ocean waters in the Mississippi embayment. 
It may be assumed that a great submergence of the land in 
the upper Mississippi region and a similar submergence in 
the lower Mississippi valley, the deposits of each of which ap- 
pear to bear the same relation to the earliest drift sheet of 
Illinois both north and south, being apparently continuous 
with each other, atford sutficient evidence of the Columbia age 
of the loess and underlying alluvium of northwestern Illinois. 
Accordingly we will endeavor to show a parallelism between 
the various members of the Columbia formation in the Missis- 
sippi embayment and in the Pecatonica basin. 
McGee divides the Columbia formation in the lower Missis- 
sippi area into four members.* The lowest member, or the 
Port Hudson clays, is described as '"a vast bed of blue, black, 
gray, or brown laminated clay, commonly clean, though some- 
times parted with sand, silt, or fine gravel, and often charged 
with calcareous or ferruginous nodules. * * * Jt, is pre- 
eminently a low-level deposit, seldom rising far above the 
modern base-level. * * * This phase of the formation 
lines the broad ancient valley of the Mississippi from Cairo to 
*Twell'th Annual Report of the U. S. Geol. Survey, for 1890-'91, pp. 
392-407. 
