198 The American Geologist. March, i897 
belt from thirty to lifty miles wide, extending from north to 
south across portions of the counties of Jo Daviess, Stephen- 
son, Winnebago, Ogle, Carroll, Whiteside, Lee, and Rock Is- 
land, besides being widely distributed over western Illinois 
south from the Rock river. The district to whicli it is pro- 
posed to call special attention in the present paper includes 
the part of the hydrographic basin of the Pecatonica river 
which has been glaciated, and also the Leaf river basin imme- 
diately south of it, together comprising about 1,400 square 
miles. 
The surface is essentially an eroded peneplain, having an 
altitude at the northwestern corner of about 1,025 feet above 
the sea, and sloping thence southeastward to about 900 feet 
at the southeastern corner of the district. In the western 
portion of the area a few elevations known ;is "mounds" rise 
from 75 to 200 feet above the peneplain. Eroded below this 
upland plain are broad shallow basins along the main lines of 
drainage. One of these, through which the Pecatonica river 
tlows, is three to five miles wide and 75 to 100 feet deep. 
Trenched beneath the surface of this second and very imper- 
fectly developed peneplain, are the present immediate valleys 
of the streams. The Pecatonica valley proper, belonging to 
this system, has a width of one to two miles, and an average 
depth of 100 feet. These valleys are, next to the main upland 
peneplain, the most prominent topographic features of north- 
western Illinois and southwestern Wisconsin. Formerly thej' 
were much deeper than now, their rock floors being rarely less 
than 100 feet, and possibly in some places as much as 200 feet, 
beneath the present streams. The buried i)ortion of the val- 
leys was largely filled with a blue silt Just previous to the 
glaciation of the region, and on this the drift sheet rests.* 
Description of the Stratified Gravel and Sand. 
The knolls and short ridges of stratified drift in the Peca- 
tonica and Leaf river basins, although occurring at wide in- 
tervals throughout the district covered by the Kansan drift 
sheet, are principally collected into a number of irregular 
belts (shown in plate XL), which cross the country parallel 
*For a complete discussion of the topography of the district the read- 
er is i-eferred to a recent article by the writer in the American Geolo- 
(iisT, vol. XVIII, pp. 72-100, August, 1896. 
