As we shall see, there are facts which point rather clearly to an earlier cataract which ended its life immediately above the present Whirlpool. When the later Niagara cataract had receded to near the upper end of the Cove section, or near the present Whirlpool, the falling water must have been separated from this older channel and its filling of till deposits by only a thin wall of rock, and this must have been constantly weakened as its thickness was further reduced. When this weakened dam at last gave way, it must have produced a debacle grand in the extreme. It is hardly to be conceived that the “washout” of the ancient channel to form the Whirlpool Basin could have occupied more than a small fraction of a day, though it is highly probable that the broken rock partition below the Whirlpool was not immediately removed entire. The mandible-like termination of the Eddy Basin immediately above the Whirlpool has led Taylor to believe that the cataract quickly reëstablished itself at this point upon the last site of the extinct St. Davids cataract. If reduced in power for a short interval, as a result of the obstructions still remaining in the lately broken dam below the Whirlpool, the remarkable narrowing of the gorge at this point would be sufficiently accounted for. Being compelled to turn through more than a right angle after it enters the Whirlpool Basin, the swift current of the Niagara River is forced to double upon itself against the opposite bank and dive below the incoming current before emerging into the Cove section below the Whirlpool (Fig. 386). [Illustration: FIG. 386.—Map of the Whirlpool Basin, showing the rock side walls like those of the Niagara Gorge, and the drift bank which forms the northwest wall (after Gilbert).] In tearing out the loose deposits which had filled this part of the buried St. Davids Gorge, many bowlders of great size were left which slid down the slope and in time produced an armor about the looser deposits beneath, so as to protect them and prevent continued excavation. Thus it is found that the submerged northwestern wall of the basin is sheathed with bowlders large enough to retain their positions and so stop a natural process of placer outwashing upon a gigantic scale (Fig. 386).