The Development of 3Ilssissippi Valley. — Hershey. 253 
face was a low-lying plain through nearly the whole of the 
Tertiary era. This idea is, I understand, in accordance with 
the opinion of Messrs. Haj'^es and Campbell, and of the geolo- 
gists of the Minnesota survey. This long period of baselevel- 
ing was suddenly terminated by the most profound movement 
of elevation which has affected the • territory since the Paleo- 
zoic era, enabling the streams to dissect the upland peneplain 
by canon valleys from 400 to 700 feet in depth, (excluding the, 
at present, buried portions). 
Peneplains of northeasfern Iowa* Crossing the state line 
into Iowa, we find the main topographical features of the ter- 
ritory whose investigation has just been completed, continued 
and even better defined than in the more northern district. 
In the.northern one-third of Allamakee county, the Tertiary 
peneplain is underlain by the St. Peter sandstone, but over 
the remainder of the county, the Trenton group forms its 
nearly level surface. The altitude averages 1200 feet above 
the sea, and hence the canon valleys which, in this county, 
have tortuous courses suggesting an origin from the mean- 
ders of the streams on the peneplain before uplift, are nearly 
600 feet in depth. A few mound-like elevations reaching 1300 
feet above the sea occur on a ridge trending northeast from 
near Ludlow. Going westward into Winneshiek and Howard 
counties, we rise rather abruptly to the surface of the Tren- 
ton plateau, having an average altitude slightly exceeding 
1300 feet above the sea. This plain is indistinguishable so far 
as altitude is concerned frOm the surface of the Niagara stra- 
ta west of it, which latter also merges into the plain of the 
Cedar Valley limestone. The surface of these three limestone 
formations together constitute the 1300-foot plain or plateau 
in distinction from the 1200-foot plain or Tertiary peneplain 
east of the Trenton escarpment. Now, in the vicinity of For- 
eston and Lime Springs, in northern Howard countj^ Creta- 
ceous strata occur with characters which demonstrate their 
affinity to the strata of practically the same age outcropping 
in Minnesota on this same 1300-foot plain. This Cretaceous 
*The information concerning northeastern Iowa was derived largely 
through a study of the line hypsographical and geological maps, (sup- 
plemented by the admirable descriptions contained in the text) accom- 
panying Dr. W J McGee's excellent memoir on "The Pleistocene Hist- 
ory of Northeastern Iowa," in the Xlth Annual Report of the U. S. 
Gcol. Survey. 
