Having established the existence of the Niagara barrier, it remains to analyze its structure, and then to search out the agencies by which it was broken down. First, in regard to its organization. An examination of the locality reveals the fact that the portion of the ridge lying between old Fort Grey and Brock's Monument was of a peculiar character. At the former point the hard, compact clay had in it but a slight mixture of gray loam and sand. At the latter point, fine gravel was plentifully mingled with this loam. This latter mass, being quite porous, would rapidly become saturated with water, and its component parts be easily separated. The declivity of the high, hard, clay bank, down to the rock at the edge of the precipice, is abrupt on the American side, while on the opposite side the ascent toward Brock's Monument and above is gradual. This formation extends upward about one mile and a half, when the gravel and loam disappear, and the hard clay succeeds and continues upward with a gradual downward slope nearly to the Falls. This upper drift was about twenty feet thick, and rested on a laminated stratum of the Niagara limestone. This stratum, though quite compact, and having its seams closely jointed, was not so thoroughly indurated as the lower strata of the Niagara group, and its thin plates were more easily displaced and broken up. The depression marked in the sixth mile of the profile referred to was evidently cut out by the waters of Fish Creek, after the barrier had been removed, since the land near the head-waters of this stream is higher than at the point where the line runs through the ridge. It is also noticeable that the ridge, at this point, approaches the brink of the escarpment more nearly than at any other, and the sharp declivity of its northern face is clearly shown on the profile in the accompanying map. Within the last century there have been two, and perhaps more, large tidal waves on the Great Lakes. There have also been many severe gales, which have inundated the low lands around their shores, and attacked, with destructive effect, their higher banks. One of these gales is mentioned in another place. It came from about two points north of west, and, as noted, raised the water six feet on the rapids above the Falls. In the narrow portions of the river above, it must have elevated the water still more. Of course a much higher rise would have been produced by the force of such a gale acting upon the vastly increased surface of the larger lake. The first serious impression upon the Niagara barrier must have been made by these two mighty forces. By them, undoubtedly, was made the first breach over its top, thus commencing that slow but sure denudation which finally reached the rock below. And by their aid even the rock itself was removed.