The Columbia Formation in X. If. Illinois. — Ilershey. 15 
upper 3 to 5 feet of the Upland loess has been weathered to a 
loose porous clay of a bulf color, which often breaks up into 
small cubical blocks. 
The distribution of this deposit is nearly coextensive with 
all that part of northwestern Illinois west of an irregular line 
entering the state at the northeast corner of Stephenson 
oounty. thence running east-southeast to the mouth of Sugar 
river, thence west-southwest along the Pecatonica river to the 
Stephenson county line near the village of Pecatonica, and 
thence eastward to near Rockford, from which point onward 
it is not yet traced but is supposed to extend in a very irreg- 
ular course southwestward along the valley of the Rock river 
to some point below Oregon, whence it probably trends off to 
the south, passing out of the district covered by this paper. 
Throughout the country between this line and the Mississippi 
river, the Upland loess must have originally existed as a man- 
tle of remarkably uniform thickness resting alike on the Val- 
ley loess in the deeper vallej's and the old interglacial soil on 
the ridges. Its altitudinal limit has not yet been determined 
with certainty, as it has suffered great erosion and so has been 
removed from many of the steeper ridges. Beds of clay which 
are apparently Upland loess have been observed on some of 
the "mounds" of Jo Daviess and Stephenson counties, and it 
is probable that it was originally deposited on them all. It 
certainly attains a greater altitude than 1,000 feet aVjove the 
sea, or more than 250 feet above the Pecatonica river at 
Freeport. 
Northwestern Illinois is a very hilly region, as compared 
with the central portion of the state. The range in altitude 
between the "mounds" and the Mississippi river is as much 
as 600 feet, reached sometimes within a distance of a few 
miles. On these steep slopes the loess remains only in 
patches; but in Stephenson county, where the hills attain an 
elevation above the valleys of only a few hundred feet and 
the slopes are gentler, it occupies nearly the entire surface. 
Perhaps more than half of the formation has already been re- 
moved by erosion, but in favorable situations nearly the en- 
tire thickness remains and is found to be about as follows : 
In the northeast portion of Stephenson county, and in the 
northwest portion of Winnebago county, 7 or 8 feet, main- 
