THE DATE OF THE GLACIAL PERIOD. 



551 



considerable portion of the erosion of the Niagara gorge, 

 both above and below the Whirlpool, had been accomplished 

 before the glacial period, so that all the present stream had to 

 do was to clear of its unconsolidated till the portions of the 

 preglacial gorge which it occupied, and somewhat widen the 

 channel. 



For future reference, if for nothing else, it is worth while 

 to introduce at this point a portion of our detailed study upon 

 the rate at which lateral erosion is proceeding through 

 atmospheric influences alone along the face of this gorge. 



J(C a. s. a. r<x LS. 



Sec I 



-?e £ m 



Fig. 141 — Sections showing the actuol rate of erosion along the sides of the Niagara 

 Gorge. 



Section I was made 860 feet south of the tunnel at the north 

 end of the gorge, and the measurements were taken from the 

 outer rail of the track at points where a perpendicular excava- 



