Читать книгу The Niagara River - Archer Butler Hulbert - Страница 35

In following the course of this old shore line a gradual slope is noticed, and if this was a shore line, we must account for this variation in elevation, since the surface of the water is always level. The explanation is to be found in the fact that portions of the earth's surface are gradually rising while others are as gradually sinking. On comparing the old coast line with the level of the present one, we find that the lake has gradually inclined to the south and the west. This change in elevation had its share in determining the configuration of the lake as well as the relief features of the surrounding region. The point of discharge was at Rome, New York, as long as the barrier blocked the regions north of the Adirondack Mountains. As soon as the encroaching warmth of the south had removed this barrier to the level of the Rome outlet, the water began flowing by the St. Lawrence course. True the first outlet was not the same as the present one; but it must have been many times shifted in the course of the retreat of the ice. As a result of this alternate shifting, together with the changing of the level of the lake, there are to be found the markings of numerous shore lines, some of which pass under the present level of the waters.

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.