BLUE EARTH COUNTY. 439 



Glacial drift. J 



Soda, Na> 3.17 traces traces 0.24 



Ferric oxide, Fe.. Oj traces traces 0.25 5.45 1.53 



Sulphuric oxide, 8 d 0.23 



Phosphoric oxide, Pi O; 0.09 



Organic matter traces traces 



Water, H 2 1.98 traces 2.25 4.71 1.40 



99.99 99.34 98.02 98.58 97.08 



In the absence of palaeontological evidence, it is impossible to deter- 

 mine to which part of the Cretaceous series these beds in Blue Earth county 

 should be referred ; but there can be little doubt that they belong some- 

 where in this age. Scanty exposures of Cretaceous strata are found in many 

 parts of the western two thirds of Minnesota, enclosing sometimes marine 

 fossils, sometimes impressions of leaves, and at a few places thin layers of 

 lignite. 



Before the Cretaceous age, during which western Minnesota and the 

 region of the upper Missouri were depressed and covered by the sea, deep 

 channels had been cut by rivers in the Lower Magnesian strata of this 

 county; and the slopes and course of drainage seem then to have been partly 

 like those of the present day. At least we find where the Minnesota river 

 now flows a remarkably water-worn and deeply excavated valley, in which 

 these Cretaceous beds of clay and sand were deposited. 



Glacial drift. The drift in Blue Earth county has the same characters 

 in its composition and sources of material, manner of formation, diverse 

 deposits, and topography, as are found generally, except in its belts of ter- 

 minal and medial moraines, throughout a very large area of southern and 

 western Minnesota and upon much of Iowa and Dakota. In describing the 

 surface features of the county, the topography of the drift-sheet, in its 

 gently rolling or undulating and partly quite flat expanse, and the deep, 

 trough-like valleys which intersect it, have been already sufficiently noticed. 

 The thickness of this sheet of glacial drift is principally from 100 to 200 feet, 

 but in the Mankato well it was found to be 290 feet. Its average upon the 

 whole county is probably 150 feet. Before its erosion by rivers, this was a 

 mantle entirely concealing the bed-rocks, which had no exposure in this 

 region. 



The formation of the drift, including removal, intermixture and depo- 

 sition, took place in the last completed period of geological history, and is 

 found to have been accomplished by the agency of a vast ice-sheet that 



