Erosion Cycles In Xortli ireslvm fllliiois. — Jlersliei/. 75 
direction, upon ascending to its summit we should look into 
a similar but broader basin, and bej'ond it to another higli 
ridge in the distance. 
During the first two views we stood on " mounds," and 
looked down upon a broad gentl}' undulating and deepl_y dis- 
sected plain. During the last two views we stood on some of 
the ridges which compose this plain, and looked across lower, 
dissected, basin-like plains. It is evident that the topographic 
forms of the district fall naturally into four classes. There 
is apparentl}^ a well defined but deeply eroded peneplain, be- 
neath the surface of which there Jiave been excavated shallow 
vallej^s, so broad that they may be considered as another but 
imperfectly developed peneplain. The " mounds " rise above 
the first as monadnocks, and the blufF-bound valleys are 
trenched below the second, constituting the present canon- 
like valleys of the streams. 
Ancient Peneplains or Baselevels. 
The writer recognizes five distinct preglacial baselevels in 
northwestern Illinois. As their age is somewhat uncertain at 
the present stage of the studj^ they will be designated bj^ 
numbers. 
Peneplain Xo. 1. The ''mounds" of the lead region of 
Iowa, Wisconsin, and Illinois, consist of about 100 feet of 
Cincinnati shales, capped by from 25 to 150 feet of Niagara 
limestone. They do not all rise to the same altitude. l)ut. in 
general, adjacent "mounds" have their summits nearly on a 
common level. Many of them are prolonged into ridges and 
broaden into narrow plateaus. The ridges have even crest 
lines, and the plateaus are flat. In short they impress one as 
being remnants of an eroded and slightl}' deformed peneplain. 
There are, however, several objections to the ready acceptance 
of this proposition. Especially it may be urged, from the 
presence of the capping Niagara limestone, that the apparent 
even plain of the summits is a structural plane due to a resis- 
tant stratum. 
There is some evidence that a baselevel plain of erosion ex- 
isted at or above the summit plane of the "mounds." They 
almost invariably stand over the synclinal axes of the district. 
This fact has been so generally recognized tiiat some geolo- 
gists liave ventured to locate such axes by means of the 
