JErosiori Ci/rles in Xorfliiiu^sfern Illinois. — Heraheii. 11 
cone-shaped elevations and narrow ridges has doubtless been 
lowered somewhat during subsequent cycles of erosion. In 
some cases the limestone cap has been entirely' removed and 
the soft shales underneath have been worn down to a mere 
dome-shaped protuberance on the lower plain. But the flat- 
topped plateaus must attain a hight as near to that of the 
uplifted basal plane of erosion as do an}^ of the ancient pene- 
plains of America. 
This peneplaih No. 1, in nortli western Illinois, attains a 
maximum altitude at least 1,275 feet above the sea, as indi- 
cated by Charles Mound, in Jo Daviess county, the highest 
elevation in the state of Illinois. From here it slopes gently 
towards the south and southeast. The most eastern "mound" 
is situated near the village of Eleroy in Stephenson county. 
It is thirty miles east-southeast fi-om Charles Mound and at- 
tains an altitude of about 1,075 feet above the sea. On this 
"mound" there remains only a thickness of about a dozen feet 
of the Niagara limestone, and we might suppose that its sum- 
mit has been worn down far below the peneplain level. But 
all the "mounds" in Stephenson county rise to a plane which 
gradually ascends as it approaches Charles Mound and passes 
through its summit. Consequently, we may assume that the 
peneplain level is represented by the summits of the Stephen- 
son county "mounds." We thus learn that not only is it de- 
pressed toward the southeast but it also approaches the plane 
which passes through the ordinary upland surface. In west- 
ern Jo Daviess county the upper peneplain stands about 250 
feet above the lower plain, while in Stephenson county the 
difference is only 125 feet. The preservation of many rem- 
nants of the peneplain in central Jo Daviess county is due to 
the considerable thickness of Niagara limestone remaining in 
the structural troughs at the close of the cycle. But the 
peneplain in passing eastward on the strike of the synclinal 
axes beveled the edges of the Niagara strata gently upturned 
by the Grand de Tour-La Salle axis of uplift. This accounts 
for the snuill amount of Niagara limestone remaining on the 
"mounds" of Stephenson county and also for its total absence 
in the country east of Eleroy. 
At the close of cycle No. 1 northwestern Illiiutis was a low- 
lying plain of slight relief, with river systems adjusted to the 
