GEOLOGY OF THE TORONTO REGION 



outlet or to the Mattawa-Ottawa outlet, leaving only 

 the supply from the Erie basin to continue the work 

 of cutting back, which then progressed much more 

 slowly. 



It has been determined by Dr. Spencer that, 

 since accurate surveys have been made, the rate of 

 recession of the Horseshoe Fall is 4.2 feet per annum, 

 and that the shape of the rim has been greatly 

 changed. The American Fall has a much smaller 

 flow of water, about 7 per cent, of the whole, and 

 can scarcely handle the great blocks of rock which 

 have tumbled from the cliff. Dr. Spencer estimates 

 its recession at only 0.6 of a foot per annum. 



If the falls have receded at the uniform rate of 

 4.2 feet per annum for a distance of 6^/2 miles, the 

 time required would be about 8,000 years; but the 

 times of much slower recession suggested above may 

 greatly increase this estimate, and Dr. Spencer's 

 figure, 39,000 years, is perhaps not excessive. 



One of the most interesting points along the 

 * Niagara gorge is the Whirlpool, a mile below the 

 ! railway bridges, where the river makes a complete 

 loop, the entering current plunging under the escap- 

 ing current. The cause of the sudden bend in the 

 river channel and of its westward bulge is to be 

 found in an old drift-filled channel which the falls 

 discovered in the course of its recession and par- 

 tially cleared out. The ancient stream was much 

 smaller than the Niagara. 

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