256 '['lie A lucricail Gcoloijl^i. October, 1897 
eral spurs alinost to the very verge of the canon valley of the 
Mississippi river. They form heavily wooded, even-orested 
ridges which rise nearly 500 feet above the stream. South- 
ward from the northern portion of Jackson county the Niag- 
ara plateau disappears, and the hard limestones of this form- 
ation constitute the general upland areas whose surface rep- 
resents the Tertiary peneplain. This extends with an average 
altitude of 900 feet above the sea but a slight southward slope, 
to the northern edge of the broad basin occupied b}^ the lower 
course of the Wapsipinicon river. 
Peneplains of northwestei'n IlUnois. — Upon crossing the 
Mississippi river into northwestern Illinois, we find the Ter- 
tiary peneplain well represented by the general upland surface. 
In Jo Daviess county it slopes perceptibly toward the south- 
west from an axis of deformation which crosses the Wisconsin- 
Illinois line near Warren, and trends southeastward. Through- 
out this region the peneplain corresponds approximately with 
the surface of the Galena limestone, but in Stephenson county 
it bevels the edges of the slightly tilted strata of that forma- 
tion, cutting away 100 feet of it, ten miles east of Freeport. 
Yet the skyline of the upland areas remains remarkably even, 
indicating that, before the uplift, deformation, and dissection 
of the peneplain, it was about as flat as a plain of denudation 
can well be. In northern Carroll county, its southwestward 
slope is quite perceptible, but it maintains an altitude of 
nearly 900 feet above the sea into the northern part o-f White- 
side county. This is another area where the original altitude 
of this peneplain must have been remarkably even, 3-et it is 
underlain by the Galena and Niagara limestones, and the Hud- 
son River shales, never indicating their dilference by any ine- 
quality of surface. 
The Niagara plateau with its outliers or "mounds," which 
we left in Iowa, ending abruptly near the Mississippi river 
in a steep, crenulate escarpment, reappears in Jo Daviess 
county, Illinois, where it is first represented by Pilot knob, a 
prominent cone-shaped elevation, near the mouth of the Ga- 
lena river. This rises to about the same altitude as the Niag- 
ara plateau in the adjoining portion of Iowa. The central 
portion of the county is a dissected plateau, or rather a series 
of small plateaus which do not dilfer from the Niagara plateau 
