Niagara Falls 



1907 the draftsman sees the landscape as though faintly pictured on a 

 sheet of paper, and at the same time sees the pencil with which 

 he traces its outlines. Before photography this method was the 

 most accurate known for recording the outlines of a landscape, 

 and in skillful hands it gives results of notable precision. There 

 is much internal evidence that Captain Hall's sketches at Niagara 

 were made with care and fidelity, and in view of these facts I 

 have thought it worth while to endeavor to combine his record 

 with the records by mapping. 



The American Fall. 



The recession of the American Fall is much slower than that 

 of the Horseshoe. The sheet of water on its brink is compara- 

 tively thin, and the force the water acquires in falling is not suffi- 

 cient to remove the larger of the limestone blocks broken from 

 the ledge above. The blocks are therefore heaped at the base 

 of the cliff and serve as a natural riprap to protect the shale 

 against wear. (See PI. XI, p. 62, and fig. 8, p. 62.) Since the 

 Horseshoe Fall parted from the American, leaving it stranded 

 at the side of the gorge, there has evidently been some falling 

 away of the crest of the American Fall, else there would be no 

 limestone blocks at its base. But as the talus increases in 

 height it becomes more and more protective, and the rate of 

 recession should theoretically diminish. 



It has already been observed that the geologist's interest in 

 the rate of recession applies primarily to the Horseshoe Fall, 

 because the work of that fall makes the gorge longer. If the 

 conditions of erosion had been uniform during the whole period 

 of the excavation of the gorge the work of the American Fall 

 would have little bearing on its time estimates, but the volume of 

 the river has not always been so great as at present, and there 

 were two epochs in the history of the gorge when the volume 

 was very small. During those epochs the discharge of the whole 

 river was probably not much greater than the present discharge 



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