39 



A few paces north of Prospect point is Hennepin's 

 View, supposed to mark the spot from which the first 

 white man beheld the falls. The point is higher and 

 affords a better general view. 



Slow Recession of American Fall. — The crest of 

 the American fall is about 1,000 feet (305 m.) long and 

 forms a nearly straight line continuous with the cliff on 

 either side, so that that part of the cliff which forms the 

 brink of the fall has receded little if any more than the 

 adjacent parts which have not been the crest of a water- 

 fall. Gilbert, Spencer and others have made estimates of 

 the rate of recession of the American fall based on Hall's 

 and later surveys. Spencer found the rate to be about 

 •6 feet (-18 m.) per year, but Gilbert found a very large 

 error in Hall's map. The northern part of Hall's crest 

 line is represented as a large salient projecting about 

 100 feet (30-4 m.) too far. After making this correction 

 from contemporary drawings, Gilbert concluded the rate 

 "is probably as small as -2 (-06 m.) of a foot per annum." 

 But it may be much less than this, as the following consid- 

 erations show. The talus slopes north and south of the 

 American fall are no wider than that directly in front of 

 it, except for an interval of 300 or 400 feet (90 or 122 

 m.) in the central part, where the width is a very little 

 greater. Manifestly, if normal cliff recession due to 

 weathering has produced talus slopes as wide as that in 

 front of the American fall, the production of a slope of the 

 same width along the front of the falls need not be attrib- 

 uted to the work of the falls, but is quite as likely to be 

 due to normal cliff recession. 



The sheet of water passing over this fall is thin and 

 feeble and has not been able to remove or wear the blocks 

 away, even in the 400 or 500 years that have probably 

 elapsed since the Horseshoe fall was finally separated from 

 the American fall. If the slight re-entrant in the central 

 part of the crest line was made by the fall as distinguished 

 from cliff recession by weathering, and was deepened even 

 as much as 50 feet (15-2 m.), the average rate would be 

 about • 1 of a foot (-03 m.), or a little more than one inch 

 (2 • 5 cm.) a year. On the rest of the crest line there is no 

 certain evidence of recession by the action of the falls. 



Goat Island. — This island separates the American 

 fall and rapids from the Horseshoe fall and the rapids 

 above it. The surface is nearly flat, and composed of 



