HENNEPIN COUNTY, J888. N. H. WINCHELL. 



a layer of till that came from the northeast. It therefore interfered with the course 

 of the Mississippi, compelling it to shift to the west. It remained in its westerly 

 channel sufficiently long to form a gorge from the Minnesota valley to the mouth of 

 Bassett's creek. This, which might be designated an interglacial channel, passes 

 through the western limits of Minneapolis. Then the second (or last) glacial epoch 

 supervened, its forces moving from the west and northwest and depositing the gray 

 till. This filled and totally obstructed the interglacial gorge, driving the river east- 

 ward again, when it chose the route which it now holds, and on the retirement of 

 the glacier began the erosion of the gorge now seen extending from Fort Snelling 

 to Minneapolis. 



These steps in the history of the Mississippi at Minneapolis have been used to 

 compute the length of time involved, not only in post-glacial geology, but also in 

 interglacial.* The datum for . measurement is the ascertained rate of recession of 

 the present falls since their discovery in 1680. This rate is about five and a half feet 

 per year. The time for post-glacial recession would amount to about 8,000 years. 

 That required for interglacial recession about 15,000 years. This result agrees sub- 

 stantially with that derived later from an investigation of the gorge of Niagara, and 

 also with several other methods of computing post-glacial time. 



The power generated by St. Anthony falls is from 25,000 to 35,000 horse-power 

 at low water, and it is used to operate the flour mills and other manufacturing 

 industry, as well as to develop electricity which runs the city street cars. 



The Trenton limestone and shales in Hennepin county have furnished many 

 fossils, described in volume iii of this report. N. H. w. 



*The discussion of the data as to interglacial time will be found in the American Geologist, vol. x, pp. 69-81 and 802, 1892, 

 and a discussion of the interglacial climate by Mr. Upham will be found in the same journal, vol. xv, pp. 273-295, 1895. 



