The Development of 3fisslssippi Vallei/. — Hershey. 249 
er Magnesian plain which adjoins the canon valle^^ of tlie 
great river. 
In Goodhue township, about 12 miles southwest of lake 
Pepin, quite an extensive outlier of Cretaceous strata lias 
been mapped by the Minnesota survey. It is surrounded b}^ 
outcropping St. Peter sandstone and Shakopee limestone. 
Contours of 1060 to 1150 feet above the sea cross the area 
mapped as Cretaceous. The general upland plain, here about 
1100 feet above the sea, seems to pass across these strata, 
but in the text we learn that the hight and topograj^hy of 
this area are similar to those of the "Trenton plain" on the 
west, so that it seems to be an outlier of an upland surface 
wiiich has been destroj'-ed eastward from it, but is .represented 
by the Trenton plain. Now, whether we recognize the Tren- 
ton plain as due to structural features or not, the presence 
of the Cretaceous strata proves that it corresponds approx- 
imately with the uplifted, deformed and widel}'" dissected 
Cretaceous peneplain. It is also quite clear that there being 
no lower peneplain than the Lower Magnesian plain near the 
Mississippi river, if an}' Tertiar^y peneplain is here represent- 
ed, it must be very nearl}' identical in altitude with the Cre- 
taceous peneplain. 
We will now go eastward into Wabasha county. The gen- 
eral upland or Lower Magnesian plain near the river, has an 
altitude of 1100 feet in the country west of lake Pepin, but 
rises gently to about 1200 feet above the sea in the southeast- 
ern corner of the county. In Olmsted county, which is direct- 
ly south of it, the Trenton plain which, in Goodhue county, 
was scarcely distinguishable from the Lower Magnesian plain 
so far as altitude was concerned, now rises into a plain which 
is distinctly higher than the other. It attains an altitude in 
the south portion of the county of about 1300 feet above the 
sea. Beyond its eastern outcrop or escarpment, isolated areas 
of the Trenton rise like "mounds" from a lower plain, notabl}^ 
Lone Mound near Potsdam. 
In Winona county, which lies between Olmsted county and 
the Mississippi river, the upland forms one or more dissected 
plains, which have an average altitude of 1300 feet above the 
sea in the southwestern corner of the county, where the Tren- 
ton plain appears, and 1200 feet eastward from the Trenton 
