72 The American deologist. Aufnist, 18% 
PREGLACIAL EROSION CYCLES IN NORTHWEST- 
ERN ILLINOIS. 
By Oscar H. Hershey, Freeport, 111. 
Introduction. 
There are three steps to be taken in the study of the geo- 
morphology of the upper Mississippi basin. The first is the 
determination of the various cycles of erosion in the several 
districts as illustrated by the present topographic forms : the 
second is the correlation of the ancient baselevels of the dif- 
ferent districts by comparison and by direct tracing; the 
third is the determination of the relation between the cj'cles 
of erosion in the geologic province and the cycles of deposi- 
tion in the surrounding seas, as illustrated by marine deposits 
of a known age. It is the purpose of the writer in this paper 
to take the first step in one very limited district, namel}^, that 
portian of the state of Illinois which lies north and west of 
the Rock river. This district lies on the southern slope and 
near the base of the Wisconsin Uplift, is a more elevated area 
than the remainder of the state, and has been subject to epeir- 
ogenic movements of a greater amplitude. Furthermore, its 
rock formations are well indurated, and the glaciation of the 
region has had very little effect on its topography. Hence 
the study of the geomorphology of the state may well begin 
in this northwestern corner. Illustrations will be drawn 
chiefly from the Peeatonica basin, because that is the portion 
of the district with which the writer is best acc^uainted. 
Stratigraphy of the District. 
The indurated bed-rock formations of northwestern Illinois 
may be combined into tlie following general table: 
Thickness. 
Carboniferous sandstone and Devonian limestone feet. 
Niagara limestone 200 •' 
Hudson River shales 85 " 
Utica shales? 11 " 
Galena limestone 200 " 
Trenton shale 5 •' 
Trenton limestone 10 ' • 
Peeatonica limestone 10 " 
St. Peter sandstone 200 " 
Lower Magnesian limestone and shale, exposed 10 " 
Total 791 feet. 
