A CLOCK OF RECENT GEOLOGICAL TIME 



3C3 



waters then bathed all the lower portion of the escarpment, so 

 that the foot of the Fall was upon the borders of the Lake. 



In order to interpret the history of the Niagara gorge, we must 

 remember that the effective drilling of this gorge was in each stage 

 dependent mainly upon 

 the volume of water dis- 

 charged from Lake Erie, 

 a large discharge being 

 recorded by a channel 

 drilled both wide and 

 deep, while that pro- 

 duced by the discharge 

 of a smaller volume was 

 correspondingly narrow 

 and shallow. To-day 

 the gorges of large cross 

 section have, moreover, 

 a relatively placid sur- 

 face, whereas through the 

 constricted sections the 

 water of the river is un- 

 able to pass without first 

 raising its level at the 

 upper end and under the 

 head thus produced rush- 

 ing through under an in- 

 creased velocity. The 

 best illustration of such a constricted section is the Gorge of the 

 Whirlpool Rapids. 



Our reading of the history should begin at the site of the present 

 cataract, since the records of later events are so much the more 

 complete and legible, and it should ever be our plan to proceed 

 from the clearly written pages to those half effaced and illegible. 



As we have learned, the most abrupt change in the cross section 

 of the gorge is found a little above the railroad bridges, where the 

 Upper Great Gorge is joined to the Gorge of the Whirlpool 

 Rapids (Fig. 389). In view of the remarkably uniform cross 

 section of the Upper Great Gorge, there is no reason to doubt that 

 it has been drilled throughout under essentially the same volume 



FIG. 389. Sketch map of the greater portion of 

 the Niagara Gorge to show the changes in cross 

 section in their relations to Niagara history 

 (based upon a map by Taylor). 



