THE DATE OF THE GLACIAL PERIOD. 344 



and was given to recording his observations for the in- 

 struction of the outside world and of future generations. 

 From his description, printed in Amsterdam in 1704, 

 Professor N. H. Winchell is able to determine the pre- 

 cise locality of the cataract when discovered in 1680. 



Again, in 1766 the Catholic missionary Carver visited 

 the falls, and not only wrote a description, but made a 

 sketch (found in an account of his travels, published in 

 London in 1788) which confirms the inferences drawn 

 from Hennepin's narrative. The actual period of reces- 

 sion, however (which Professor Winchell duly takes into 

 account), extends only to the year 1856, at which time 

 such artificial changes were introduced as to modify the 

 rate of recession and disturb further calculations. But 

 between 1680 and 1766 the falls had evidently receded 

 about 412 feet. Between 1766 and 1856 the recession 

 had been 600 feet. The average rate is estimated by 

 Professor Winchell to be about five feet per year, and the 

 total length of time required for the formation of the 

 gorge above Fort Snelling is a little less than eight thou- 

 sand years, or about the same as that calculated by Messrs. 

 Woodward and Gilbert for the Niagara gorge. 



To these calculations of Professor Winchell it does 

 not seem possible to urge any valid objection. It does 

 not seem credible that the amount of water in the Missis- 

 sippi should ever have been less than now, while during 

 the continuance of the ice in the upper portion of the 

 Mississippi basin the flow of water was certainly far greater 

 than now. 



If any one is inclined to challenge Professor Win- 

 chell's interpretation of the facts, even a hasty visit to 

 the locality will suffice to produce conviction. The com- 

 parative youth of the gorge from Fort Snelling up to 

 Minneapolis is evident : 1. From its relative narrowness, 

 when compared with the main valley below. This is rep- 

 resented by the shading upon the map. The gorge from 



