CONCLUSION 51 



wanting, as already described. The rate at this stage is the one which 

 Winchell very nicely calculated. He believed that the same rate applied 

 to the making of the entire gorge, which it evidently does not. 



Very possibly the falls were only entering the Nicollet Island rapids at 

 the time when Hennepin and Carver saw them, and the upper limestone 

 was not all gone from the fallscarp. "Winchell calculated the recession 

 of the falls from the time when Hennepin saw them, A. D. 1680, to that 

 of Carver, 1766, to be 413 feet, or 4.79 feet per year, and from 1766 to 

 1856 to be 606 feet, or 6.73 feet per year. The increase of 40 per cent in 

 the rate of the second period over the first indicates, I think, that the falls 

 , were in transition stage. In that case also the first must be an increased 

 rate over that of stages preceding the entrance of the falls into the Nicol- 

 let Island rapids. Since 5 feet, or one-third, is lacking from the upper 

 limestone at Tenth Avenue bridge, it seems probable that the same was 

 only half eroded away at the time of Father Hennepin's visit. We may 

 therefore assume four periods of accelerated recession, of which the two 

 just cited as calculated by Winchell are the last. If we accordingly dis- 

 count the rate of the third, 4.79 feet per year, by 40 per cent twice, we 

 may find the probable rate of recession (2.44 feet per year) previous to 

 the entrance of the falls into the Nicollet Island rapids. 



While the rate of recession of the falls as taken by Winchell is twice too 

 great, the length of recession which he takes for the time since the falling 

 of the river Warren and ending of the ice-barrier in the north is twice too 

 long. At the time of that event he supposed the falls to have begun at 

 Fort SnelHng, while most probably they were half way along in retreat, or 

 at the Lake Street stage. From there to Tenth avenue the recession was 

 fairly uniform. Taking, then, one-half the rate and one-half the length, 

 the number of years, as calculated by Winchell for the event of the reduc- 

 tion of the river Warren, is not necessarily changed. Eight or ten thou- 

 sand years may have elapsed while the last 4 miles of the gorge were 

 making. 



Probably no less an interval elapsed while the preceding 4 miles were 

 making from above Fort Snelling to Lake street, though in this case the 

 rate was not evidently uniform, and calculation of years from distance 

 divided by a supposed rate would be very uncertain. The recession was 

 at first comparatively rapid, but the falls during that interval were in- 

 creasing in height, tending toward slower recession. The volume of the 

 stream may have been reduced much below that which it was later, and 

 this would tend toward slow recession. Here, also, for a part of the dis- 

 tance, perhaps a mile, as said, the limestone was augmented by crystalline 

 limestone and shales. Of the effect of all this we have no good record 



