The Develop}ae)itof 3Iississippi Valley. — Hershey. 257 
of Iowa except in areal extent. These plateau masses form a 
dissected plain as perfect as the Niagara plateau in Iowa. 
Around their western, northern, and eastern borders there are 
outliers in the form of "mounds," some cone-shaped and 
others elongated into short flat-topped ridges. The larger of 
these "mounds" attain practically the same altitude as the 
neighboring plateaus, and their summits aj-e evidently parts 
of the same dissected plain. Imagining for a few moments, 
that all the strata which liave been removed from between and 
around these "mounds" and plateaus be i-estored, we would 
see the land surface as a very slightly undulating plain, slop- 
ing gently from an altitude of about 1275 feet on the Wiscon- 
sin line to 1100 feet near the mouth of the present Galena 
river, and 1050 feet above the sea in Stephenson county near 
the village of Eleroy. This plain would be continuous, with- 
out any deformation whatever, with the Niagara plateau in 
Iowa. It would also sink, with reference to the Tertiary pen- 
eplain, from about 250 feet along the Mississippi river near 
Dubuque and 275 feet in Jo Daviess county near Scales mound, 
to only 125 feet in Stephenson county near Eleroy. Further- 
more, its surface would not reflect the slight folds of the in- 
durated formations, several of which cross the area of the 
plateaus. But more remarkable than anything jei stated, 
the Niagara limestone would thin out under this plain toward 
the east, until in Erin township, Stephenson county, less 
than a dozen feet of Niagara limestone would remain, al- 
though the formation occurs even now in much greater thick- 
ness under the Tertiary peneplain, less than ten miles south. 
Whether we consider the summit level of the Niagara plat- 
eaus and outlj^ing "mounds" in Illinois as a structural plain 
or not, the fact remains that they form parts of one great top- 
ographical feature of the upper Mississippi valley — a plateau 
extending from Goodhue couuty, in Minnesota, in a magnifi- 
cent curved course, into Jo Daviess county, Illinois. It is 
everywhere bounded on its eastern and northeastern face by 
an escarpment, whose crest line gradually rises above the Ter- 
tiary peneplain to a maximum of about 250 feet near Dubuque 
and thence gradually declines again. Its western face has 
been obscured by heavy deposits of drift in Minnesota and 
in Iowa, except near the Mississippi river where it is appar- 
