Erosion Ci/clea in N (jrflnrcsfem Illinois. — J/ersJiei/. 93 
sion, of Cretaceous age, traceable over the southern end of the 
Driftless Area, it is probably not elevated much if any above 
the summit plane of the '"mounds;" (b) this summit plane 
appears to represent an uplifted and eroded peneplain; (c) 
this i^eneplain answers all the conditions of a nearly destroyed 
Cretaceous peneplain. In the absence of evidence to the con- 
trary we will therefore provisionally correlate peneplain No. 
1 of the Freeport section with the widely extended Cretaceous 
peneplain of the. Appalachian and Ozark regions. 
In working out the correlation of peneplain No. 2 we are 
■confronted with one important fact, namely, that baselevels 
Nos. 2 and 8 approach and perhaps merge into each other 
throughout a large portion of the central Mississippi basin. 
Moreover, as the No. 2 peneplain has been largely destroj'-ed 
around the eastern and southern borders of our district, it is 
difticult to say with certainty on which peneplain we stand 
when on an upland surface in the country south of Rock river. 
It is believed, however, that peneplain No. 2 is represented by 
all the higher rock surfaces throughout central and southern 
Illinois at an average altitude of about 700 feet above the sea. 
This upland surface, if continued into northeastern Missouri, 
rises to between SOO and 900 feet above the sea level. It is 
apparently the same plane which passes across the Meramec 
highlands west of vSt. Louis; and, unless I mistake, it pene- 
trates the Ozarks in long, narrow basins, which in the central 
portions of the plateau attain an altitude of 1,000 and 1,100 
feet above the sea. It is there overlain b}^ an ancient river 
deposit whose lithologic resemblance to the Lafayette forma- 
tion, as developed at the inner border of the coastal plain, is 
very striking.* 
Hayes and Campbell recognize the Tertiary peneplain in 
western central Kentucky at an altitude of about 800 feet 
above the sea. At an equal distance north of the Ohio river, 
in southern Indiana, the general upland surface has about the 
same altitude. This Indiana upland plain is apparently con- 
tinuous, through central Illinois, with peneplain No. 2 of the 
Freeport section. 
Our conclusions as to the age of peneplain No. 2 in north- 
western Illinois are as follows: («) the presence on the j)ene- 
*Amkkican Geologist, vol. xvi, December, 1895. 
