THE DATE OF THE GLACIAL PERIOD. 335 



Mohawk and of the St. Lawrence, and held the water in 

 front of the ice up to the level of the passes leading into 

 the Mississippi Valley. Niagara Elver, of course, was not 

 born until these ice-barriers on the east and northeast 

 melted away sufficiently to allow the drainage to take its 

 natural course. 



Of these barriers, that across the Mohawk Valley doubt- 

 less gave way first. This would allow the confluent waters 

 of this great glacial lake to fall down to the level of the old 

 outlet from the basin of Lake Ontario into the Mohawk 

 Valley, in the vicinity of Home, N. Y. The moment, how- 

 ever, that the water had fallen to this level, the plunging 

 torrents of Niagara would begin their work ; and the 

 gorge extending from Queenston up to the present falls is 

 the work done by this great river since that point of time 

 in the Glacial period when the ice-barrier across the Mo- 

 hawk Valley broke away. 



The problem is therefore a simple one. Considering 

 the length of this gorge as the dividend, the object is to 

 find the rate of annual recession ; this will be the divisor. 

 The quotient will be the number of years which have 

 elapsed since the ice first melted away from the Mohawk 

 Valley. We are favoured in our calculation by the sim- 

 plicity of the geologic arrangement. 



The strata at Niagara dip slightly to the south, but 

 not enough to make any serious disturbance in the prob- 

 lem. That at the surface, over which the water now 

 plunges, consists of hard limestone, seventy or eighty feet 

 in thickness, and this is continuous from the falls to 

 the face of the escarpment at Queenston, where the river 

 emerges from the gorge. Immediately underneath this 

 hard superficial stratum there is a stratum of soft rock, 

 of about the same thickness, which disintegrates readily. 

 As a consequence, the plunging water continually under- 

 mines the hard stratum at the surface, and prepares the 

 way for it to fall down, from time to time, in huge blocks, 



