But, to prepare the way for a more definite settlement of the question, the New York Geological Survey, under Professor Hall's direction, had a careful trigonometric survey of the Horseshoe Fall made in 1842, erecting monuments at the points at which their angles were taken, so that, after a sufficient lapse of time, the actual rate of recession could be more accurately determined. In 1886 Mr. Woodward, of the United States Geological Survey, made a new survey, and found that the actual amount of recession in the center of the Horseshoe Fall had proceeded at an average rate of about five feet per annum. The subject was thoroughly discussed by Drs. Pohlman and Gilbert, at the Buffalo meeting of the American Association in 1886, when it was proved, to the satisfaction of every one, that, if the supply of water had been constant throughout its history, the whole work of eroding the gorge from Lewiston to the Falls would have been accomplished, at the present rate of recession, in about seven thousand years.
But the question was immediately raised, Has the supply of water in Niagara River been constant? It was my privilege, in the autumn of 1892 (see Bulletin of the Geological Society of America, vol. iv, pp. 421-427), to bring forth the first positive evidence that the water pouring over Niagara had for a time been diverted, having been turned through Lake Nipissing down the valley of the Mattawa into the Ottawa River, following nearly the line of Champlain's old trail and of the present Canadian Pacific Railroad. The correctness of this inference has been abundantly confirmed by subsequent investigations of Mr. F. B. Taylor and Dr. Robert Bell.[A] The occasion of this diversion of the drainage of the Great Lakes from the Niagara through the Ottawa Valley was the well-known northerly subsidence of the land in Canada at the close of the Glacial period. When the ice melted off from the lower part of the Ottawa Valley the land stood five hundred feet lower than it does now, but the extent of this subsidence diminished both to the south and the west, making it difficult to estimate just how great it was at the Nipissing outlet. A subsidence of one hundred feet at that point, however, would now divert the waters into the Ottawa River. That it actually was so diverted is shown both by converging high-level shore lines at the head of the Mattawa Valley and by the immense delta deposits at its junction with the Ottawa, to which attention was first called in my paper referred to above.
The indeterminate question which remained was, At what rate did this postglacial elevation of land which has brought it up to its present level proceed? Dr. Gilbert, Professor Spencer, and Mr. Taylor have brought forth a variety of facts which, according to their interpretation, show that this rate of elevation was so slow that from twenty thousand to thirty thousand years was required to restore to the Niagara River its present volume of water. Their arguments are based upon the varying width and depth of the Niagara gorge, proving, as they think, the presence of a smaller amount of water during the erosion of some portions. Dr. Gilbert has also brought forward some facts concerning the extent of supposed erosion produced by the diverted waters of Niagara when passing over an intermediate outlet between Lake Simcoe and Lake Nipissing. But the difficulty of obtaining any safe basis for calculation upon these speculative considerations has increased the desire to find a means of calculation which should be independent of the indeterminate problems involved. That I think I have found, and so have made a beginning in obtaining desired results. The new evidence lies in the extent of the enlargement of the mouth of the Niagara gorge at Lewiston since the recession of the falls began.
It is evident that the oldest part of the Niagara gorge is at its mouth, at Lewiston, where the escarpment suddenly breaks down to the level of Lake Ontario. The walls of the gorge rise here to a height of three hundred and forty feet above the level of the river. It is clear that from the moment the recession of the falls began at Lewiston the walls of the gorge on either side have been subject to the action of constant disintegrating agencies, tending to enlarge the mouth and make it V-shaped. What I did last summer was to measure the exact amount of this enlargement, and to obtain an approximate estimate of the rate at which it is going on.[B] As this enlargement proceeds wholly through the action of atmospheric agencies, the conditions are constant, and it is hoped that sufficiently definite results have been obtained to set some limits to the speculations which have been made upon more indefinite grounds.
Fig. 2.—View looking east across the gorge near the mouth, showing the railroads and the outcrops of Clinton and Niagara limestones above the steam road.
The face on the east side of the gorge presents a series of alternate layers of hard and soft rocks, of which certain portions are very susceptible to the disintegrating agencies of the atmosphere. The summit consists of from twenty to thirty feet of compact Niagara limestone, which is underlaid by about seventy feet of Niagara shale; which in turn rests upon a compact stratum of Clinton limestone about twenty feet thick, which again is underlaid by a shaly deposit of seventy feet, resting upon a compact stratum of Medina sandstone twenty feet thick, below which a softer sandstone, that crumbles somewhat readily, extends to the level of the river.
Fig. 3.—Looking up the gorge from near Lewiston, showing on the left the exposed situation of the eastern face of the gorge at the extreme angle, where the measurements were made.