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 614 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. 
UY 
