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 61/2 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 escaping 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 partially cleared out. The ancient stream was much smaller than the Niagara.