Science, Geology and Physics 



Ice age and has ever since been in progress, and from computa- 1903 

 tions or estimates of the duration of that period, obtained by p " 

 comparing the length and other dimensions and features of the 

 gorge with the present rates of its erosion by the falls and its 

 widening by decay, and the wear of rain and frost upon its 

 inclosing cliffs. And the chief reasons for wishing to determine, 

 approximately, the length of the Postglacial period consists in 

 its application as a key to unlock the difficult problem of the 

 age of the earth. Geology knows ratios of the relative lengths 

 of its periods and eras, which, if the length of this latest period 

 can be learned, will supply the approximate duration of all the 

 geologic ages. 



In the careful studies of the history of the Niagara river and 

 gorge by Pohlman and Gilbert, as in the earlier observations of 

 Lyell and Hall, the coincidence of the postglacial Niagara gorge 

 with the preglacial St. David's channel at the whirlpool is clearly 

 recognized. The present river here has washed out the drift 

 that filled the ancient channel and apparently reached to the 

 bottom of the whirlpool, about 1 30 feet above the sea. Thence 

 the preglacial St. David's stream bed, beneath the drift, has 

 probably this depth of 1 1 7 feet below the level of Lake Ontario, 

 or more, along its course past St. David's and onward to the 

 deep central part of the Lake Ontario basin. 



The preglacial stream, as Pohlman has shown, drained the 

 shallow Tonawanda valley, but not the area of Lake Erie. At 

 the whirlpool this St. David's stream, according to Pohlman, 

 plunged down in a cataract from the hard Medina sandstone bed, 

 which is underlain and overlain by soft shales. Having at this 

 place eroded a valley or ravine 400 feet deep beyond the Medina 

 falls and a quarter of a mile wide, this stream doubtless also 

 had cut an important ravine, though of smaller size, along its 

 higher course for a considerable distance before reaching the 

 site of the whirlpool. Dr. Pohlman supposes, with sufficient 

 reasons, that the St. David's ravine reached along the part of the 



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