558 TEE ICE AGE IN NORTH AMERICA. 



Carver, was four hundred and twelve feet ; and between 1766 

 and 1856, six hundred feet, making a total between 1680 and 

 1856, of one thousand and eighteen feet. " These give re- 

 spectively the rates 4*79, 6*73, and 5*08 feet per year, and for 

 the corresponding periods necessary for the recession of the 

 falls from Fort Snelling (a distance of a trifle over eight 

 miles) 8,819 years, 6,276 years, and 8,315 years. The aver- 

 age of these three results is 7,803 years." 



Professor Winchell then proceeds to discuss the possible 

 elements of error in this calculation : 



1. That arising from difference in the volume of the 

 river. The terraces already described in the chapter on 

 " Preglacial Drainage," as characterizing both the Minnesota 

 River and the upper Mississippi, reveal the existence of 

 enormous floods during the closing stages of the Glacial pe- 

 riod. Indeed, these floods in the Minnesota River were so 

 high as to fill it up to the level of the lime-rock at Fort Snel- 

 ling, about one hundred feet. During the existence of this 

 high water, therefore, there could have been no cataract at 

 Fort Snelling or farther up the Mississippi. The Falls of 

 St. Anthony could have begun only after the floods of the 

 Minnesota began to shrink so as to uncover the lime-rock at 

 Fort Snelling. 



2. Difference in the height of the falls at various points 

 from Fort Snelling up to its present position. This is shown 

 to be comparatively insignificant, so that it can be left out of 

 the account. 



3. The stage of the Glacial period when the recession 

 began. Upon this we quote again at length : 



This point has already been considered in the possible va- 

 riations in the volume of the river. It is probable that the 

 Mississippi, in diminutive form, began to flow in its new 

 channel at the acme of the cold,* since the moraine of the sec- 

 ond Glacial epoch runs across the country, approximately 

 through this region, and since it would have remained in its 



* See map of Minnesota in next chapter (Fig 181). 



