248 T'lie American Geolofjlst. October, 1897 
issippl rivers, there are, under the thick drift of this region, 
isolated areas of Trenton shales and underlying strata which 
rise to nearly a common level forming an almost destroyed 
plain which has an average altitude of slightly less than 900 
feet above the sea. Over northern Dakota county and south- 
ern Ramsey, the soft Trenton shales which form these buried 
upland areas, are not capped by any specially resistant stratum, 
removing their summit plane from the category of structural 
plains. As, also, higher strata of Silurian age undoubtedly 
existed over them at one time, we can only explain the exist- 
ence of tlie plain at the present time, under the hypothesis 
that it represents a baselevel of erosion — in fact, constitutes 
the remains of a nearly destroyed peneplain. Now, marine 
Cretaceous strata outcrop along the Crow river in Hassan 
township, in the northwestern part of Hennepin county, at 
an altitude of about 900 feet above the sea. This indicates 
that the 900-foot plain in the vicinity of St. Paul and Minne- 
apolis corresponds to the land surface of late Cretaceous time, 
so that it deserves to be known as the Cretaceous peneplain. 
In going southward across Dakota county, we find that the 
rock surface of the general upland rises slightly so that while 
it is less than 900 feet near St. Paul, it is from 950 to 1000 
feet above the sea in the southeastern corner of the county. 
We shall next pass into Goodhue county, lying south and 
southeast of Dakota county, which is for our purpose one of 
the most interesting in the entire state. The general upland 
surface near the canon valley of the Mississippi river, is a 
slightly undulating plain rising gently and evenly from an al- 
titude of 950 to 1000 feet in the northern portion where it 
connects with the Dakota county plain, to 1100 feet above the 
sea in the southeastern portion of the county. In northern Da- 
kota and Ramsey counties, because the peneplain was under- 
lain by soft Trenton shales 75 to 125 feet in thickness, it has 
been nearly destroyed, but in Goodhue county and southeast- 
ward, its underlying strata (the Lower Magnesian series), be- 
ing more resistant, it is represented by a great number of fllat- 
topped ridges separated by comparatively narrow valleys or 
caiions often 400 or more feet in depth. In the southern por- 
tion of the county, the Trenton limestone underlies the surface 
and forms a plain which is only slightl}^ higher than the Low- 
