The Columbia Formation in iV^. W. Illinois. — Hershey. 21 
movement has set in, and the waters begin to subside. Tliis 
movement must have been comparatively rapid, for no shore 
lines were formed and no considerable body of loess appears 
to have been laid down after the elevatorj"^ movement was well 
advanced. 
These conclusions, although formed chiefly from a stud}^ of 
the loess deposits of the Pecatonica basin and surrounding re- 
gion, are, I believe, generally applicable to this portion of the 
upper Mississippi valley. It has been held by a number of 
glacialists that the loess of the driftless area in Wisconsin, 
Iowa, and Minnesota, was laid down in a large lake produced 
by the ponding up of the waters through the meeting of two 
lobes of the ice-sheet south of that area. But it does not yet 
appear that ice on the Illinois side came into contact with the 
Iowa ice at the time required. The Upland loess, once com- 
pletely mantling northwestern Illinois, west of the newer drift 
sheet mentioned in this paper, extends north into Wisconsin, 
where the waters depositing it seem to have been limited, at 
least for some distance in Green county, bj' a shore line of 
land instead of the border of a glacier. The same sheet of 
Upland loess reappears on the Iowa side of the Mississippi, 
where Mr. W J McGee has shown it to be connected with a 
drift sheet similar to that with which it is connected in Illi- 
nois.* But on our own side of the river this drift sheet does 
not reach the Mississippi at the place assigned to it, if it does, 
indeed, at all. The earlier drift sheet of northwestern Illi- 
nois underlies all the country(except where removed by ero- 
sion) that the writer has been over between Freeport and 
Quincy. Furthermore, the Upland loess, which overlies the 
ancient drift sheet in the Pecatonica basin, is continuous 
(save as severed by erosion) over all the country west to the 
Mississippi river and south to the Rock river. It emerges 
from under deposits of a later age on the south side of the 
Green river basin, and thence continues over the country^ to 
the south as far as the writer's studies have been carried. f 
*"Tlie Plcistoceni' Histoi-y of Xorthj-aslcrri Iowa." Elevt'iilh Annual 
Report of the U. S. (icol. Survey, for IHSit- ''.)(). 
fThe relation bt'lwoen the loess of the Pecatonica basin and that of 
the central Mississippi valley was first i)ointe(l out to the writer by Mr. 
Frank Leverett. It has since been verified by personal observation. 
