These different variations must of necessity have had a great effect on the work of Niagara River. When the Niagara began to flow, instead of its terminal being nearly seven miles from the escarpment, it was only between one and two miles away, and the surface of the lake was about seventy-five feet higher than now. While the outlet remained at Rome, the eastern end of the lake was continually rising, which caused the waters at the western end to rise over one hundred feet. This placed the shore of Ontario almost at the foot of the beautiful cliff at Queenston and Lewiston. After having occupied this position for a long period, the surface of the waters again fell over two hundred feet, carving an old shore line which is now submerged. After this, various changes of level in the land and shiftings of the ice barrier caused numerous old shore lines to be faintly carved. These changes continued until the present outlet was established and the waters began to flow along the present course of the St. Lawrence.
One might think that with these changes all the variable factors of our problem have been discussed; but these same factors also had their effect upon the upper lakes. In a study of the old markings of all the lakes of this region, it seems that the northern shores were continually rising; this, of course, points to an occupation of a more northerly position by the lakes than at present, and also a laying bare of northern parts, and shifting of waters south, or possibly both of these changes at once.
In the most ancient system of which we can obtain an approximately definite knowledge, Lake Huron was not more than half its present size, while Georgian Bay formed the main body, connecting with Huron by a narrow strait. Michigan and Superior occupied about their present limits, but were connected with Huron by rivers rather than short straits; Erie occupied only a fraction of its present position, having no connection with Huron. The waters of the upper lakes were doubtless discharged from the eastern end of Georgian Bay, which then included Lake Nipissing, by way of the Ottawa River, into the St. Lawrence. Thus the Niagara was deprived of about seven-eighths of its present drainage area, and consequently was totally unlike its present self. There is some indication that there may have been an outlet from Georgian Bay by a more southerly route, namely, the Trent River. If this were so, the northern route must have been blocked by the ice, since the Trent Pass is much higher than the one leading from Lake Nipissing, by way of the Ottawa. These are some of the possibilities which must be taken into consideration before any sure calculation can be made as to the age of the Falls, for there must have been an epoch in the history of the river, were it short or long, during which it carried only a very small fraction of the waters which it bears at present.
Let us turn again to the gorge of the river itself. We have noted the similarity of structure of its two sides. This similarity is continuous throughout except at about half-way from Queenston to the Falls, where the river makes a turn in its course of almost ninety degrees. On the outside of this angle is the only place in the whole course where the material of the cliff changes. Here there is a break in the solid rock of the bank, which is filled with loose rock and gravel. This rift, to whatever it may be due, is of pre-glacial origin, for it is filled with the same material, the glacial drift, which covers the whole region. The cliff along Lake Ontario also presents very few breaks; but a few miles to the west of Queenston at St. Davids a broad gap is found in the otherwise unbroken wall. This gap is also filled with glacial drift. On its first discovery it was supposed to be a buried valley, and no connection with the Whirlpool was attributed to it. Later it was supposed that the break in the side of the Gorge, and the one at St. Davids, were parts of one and the same course of some pre-glacial stream. This supposition has been proven by the course having been traced through most of its distance by the wells sunk in the region. Later this interpretation of the facts found was destined to furnish further explanations. The question at once arose: How far and where did the upper course of this ancient valley extend? If it had cut across the course of the modern river, there would have been a break in the continuity of the cliff somewhere on the opposite side of the Gorge; but this can nowhere be found to be the case. The upper course of this ancient channel, therefore, must have coincided with that of the present channel. When, then, the Falls had receded to the side of the present Whirlpool, it reached a point where the greater part of its work had been performed. From here to whatever distance the upper course of the ancient river extended, the only work to do was to remove the loose gravel and boulders with which the glacier had filled its channel. This, of course, was effected much more rapidly than the wearing away of the hard limestone bed. Just what was the depth, and how far this old deserted valley extended, it is almost impossible to estimate. These changes are some of the most potent with which one must reckon in any calculation of the time since the beginning of Niagara's history. However, some work has been done in this line; and a broad field is still open for future investigation.
Ice Mountain on Prospect Point.
At a very early date (1790), and when it was supposed by many to be almost sacrilegious to discuss the antiquity of the earth, Andrew Ellicott made an estimate of the age of the Falls by dividing the length of the Gorge by the supposed rate of recession. This gave as a result 55,000 years as the age of Niagara River. The next estimates which commanded attention were those of Bakewell and Sir Charles Lyell. Each of these men made separate estimates, but were compelled to take as the basis of their calculation the recession as given by residents of the district. Bakewell's calculations preceded Lyell's by several years, and resulted in ascribing to the Falls an age of 12,000 years. Lyell found the age to be about 36,000 years. The popularity of the latter caused his estimate to be accepted for a long period; many persons undoubtedly placing more faith in his results than he himself did. This method of dividing the distance by the rate of recession would be correct if there were no variables entering into the problem, and if the rate of recession were known; but these first calculations involved errors in the rate of movement of the Falls besides making no allowance for the variations which have been mentioned above.
In order to obtain a sure means for measuring the recession of the Falls, Professor James Hall made a survey of the Horseshoe Falls in 1842, under the authority of the New York Geological Survey. This survey plotted the position of the crest of the Falls, and established monuments at the points at which the angles were taken; thus leaving lasting marks of reference to which any future survey might be referred. In 1886, Professor Woodward of the United States Geological Survey, by reference to the markings left by Hall, found the rate of recession for the period to be about five feet per annum. It would, however, be necessary to extend these observations over a long period of time, since certain periods are marked by large falls of rock. Sometimes the centre of the Falls recedes very rapidly, while at other times the centre is almost stationary and the sides show the greater action. One of the most recent calculations of the age of the Falls was made by J. W. Spencer. Having made a thorough study of the history of the river revealed in its markings, and also of the Lakes, making allowance for all the variable factors, he calculated the duration of each epoch separately; and found the age of the river to be about 32,000 years. This result is about the same as that obtained from those based upon the relative elevations of different parts of the old deserted shore lines; and another based upon the rate of the rising of the land in the Niagara district.