540 THE ICE AGE IN NORTH AMERICA. 



at Buffalo and great interest was naturally centered upon this 

 question of the rate of the recession of the falls. Mr. G. K. 

 Gilbert, of the United States Geological Survey, whose au- 

 thority is unsurpassed on such subjects, gave it as his conclu- 

 sion that the "maximum length of time since the birth of the 

 falls, by the separation of the lakes, is only seven thousand 

 years, and that even this small measure may need significant 

 reduction." At the same time Mr. R. S. Woodward,* of 

 Washington, made a new survey, and gives the results defi- 

 nitely as follows : The length of the front of the Horseshoe Fall 

 is twenty-three hundred feet. Between 1842 and 1875 four 

 and a quarter acres of rock were worn away by the recession 

 of the falls. Between 1875 and 1886 a little over one acre 

 and a third disappeared in a similar manner, making in all, 

 from 1842 to 1886, about five and a half acres removed. 



Subsequent surveys have amply supported these conclu- 

 sions. From the survey made in 1905, by Mr. Carvel Hall, 

 state engineer of New York, it appears that the recession of 

 the Horseshoe Fall (which is where the principal volume 

 of water descends) during the sixty-three years between 

 1842 and 1905 was 333 feet, or at the rate of 5.3 feet per 

 annum 



The foregoing estimates concerning the recession of the 

 Niagara gorge assume a uniform rate, and that all the work 

 has been done since the glacial period. As to the first of 

 these assumptions, Dr. Julius Pohlman,f of Buffalo, adduces 

 some evidence to show that the present course of the Niagara 

 from the Whirlpool to Queenston follows an old line of drain- 

 age, in which a small stream had eroded a shallow valley 

 previous to the ice period, and thus, by reducing the thick- 

 ness of the upper layer of hard limestone along its course, 

 had greatly facilitated the work of recession, when the whole 

 torrent of Niagara began to pour over the escarpment. Dr. 

 Pohlman has also greatly increased our conception of the 

 work already done before the Glacial period by the stream 

 which had its exit from the Whirlpool to St. Davids. This 



* Report in "Science," September 3, 1836. 



t "Transactions of the Amer. Institute of Mining Engineers," 1888. 



