The. Development of Mississippi Valley. — Hersheij. 251 
that direction and soutliward into Iowa, where it becomes the 
surface of the well-marked topographic feature, the Niagara 
plateau. We shall now see whether there is any further evi- 
dence of its having originated under baseleveling conditions. 
Cretaceous strata are not well exposed, but are probably 
developed with considerable areal extent in the southwestern 
corner of Fillmore county, where they overlie Devonian lime- 
stone at an average altitude of 1300 feet above the sea. The 
lower portion, as the Minnesota survey report says, "consists 
of sandstones and lignitiferous clays or shales, the sandstones 
lying at the base of the formation, and being the same that 
Dr. White has denominated in Iowa, the Nishnabotany sand- 
stone, and belonging to the Dakota group of Messrs. Meek and 
Hayden.'" This sandstone so far as observed has a thickness 
of about 100 feet. The shaly member above the sandstone has 
been identified by Mr. F. B. Meek as the Fort Benton group 
of Messrs. Meek and Hayden. Cretaceous strata are also map- 
ped in the eastern part of Mower county, near Hamilton as 
overlying Devonian limestone at about 1300 feet above the 
sea. Near Austin in the western part of the county they are 
represented as occurring over Devonian shale and sand-rock 
at an altitude of about 1150 to 1200 feet above the sea. It 
thus seems evident that the 1300-foot plain in Fillmore and 
eastern Mower counties, because of its possessing extensive 
outliers of marine Cretaceous strata of the age of the earlier 
portion of upper Cretaceous time, represents approximately 
the land surface on which, after its submergence, the ancient 
sea deposited its sediment, and tliat therefore it deserves to be 
known as the Cretaceous peneplain. This ancient plain slopes 
northwestwardly, being represented in western Mower county 
at about 1200 feet and in Brown and Cottonwood counties it 
must pass beneath the Cretaceous strata, which are mapped 
in the Minnesota valley by Upham at levels between 800 and 
900 feet above the sea. This is, also, approximately the alti- 
tude of the top of outcropping areas of Archean granite and 
gneiss throughout the upper Minnesota valley. The highlands 
south of this depression are heavily drift covered, but the 
coteau des prairies is known to consist largely of Cretaceous 
strata. 
In recapitulation of the results of this investigation into 
