At the southern terminal of the glacier, where it ended in the ocean, the ice broke away in large bergs, as in the northern seas to-day; but where the advancing ice met the warmer climate on land, it was melted and thus deposited at its terminal all the material it carried. The eroding power of this ice sheet, together with the deposit of its materials on melting, brought about a great change in the configuration of the country. Many old valleys were obliterated, while a number of new ones were carved. As the ice retreated northward with the change of climate, new lakes and rivers were formed. Many times the streams escaping from the lower level of lakes were forced to find an entirely new course, and so to carve a new channel of their own. The region of the Great Lakes and the Niagara River is no exception to this rule; and it is with the ending of the Ice Age that the history of the river begins.
A glance at a map shows a low range of hills or rather a gentle swell in the land surface forming the watershed between the lakes and the streams flowing to the south. At the time of the farthest southerly extension of the glacier it reached beyond this elevation; and its waters were discharged into the rivers flowing to the south. When the southern terminal had retreated to the north of this divide, but still blocked all outlet to the north or east, there was doubtless a number of lakes here discharging their waters across the present low watershed to the south. Some of these ancient valleys can still be traced for long distances of their course. These lakes passed through their varying history as those of to-day, their surface troubled by wind and storm and their waves leaving indelible carvings upon their shores.
One of these lakes occupied what is now the western end of Lake Erie, shortly after the ice front had passed to the north of the watershed mentioned. There are still very definite markings which show that its waters were discharged across the divide by a channel into the present Wabash River and thence into the Ohio. This channel can be traced throughout most of its course very easily. There are at least four distinct shore lines preserved to us, which show four successive levels of the lake as it reached lower outlets before the Niagara River was born. All of these old shore lines can be traced throughout most of their courses.
As the ice continued to retreat, next we notice the greatest change in elevation of the surface of the water. The ice front finally passed to the north of the present Mohawk River, thus allowing the waters to escape by that outlet, and, as a consequence, lowering the surface of the lakes by over five hundred feet. This drained a great extent of land and dropped the surface of Ontario far below the present level of the Niagara escarpment. Then for the first time the Niagara began to flow, and its Falls began their work. Immediately upon the formation of this new, lower lake it began the work of leaving its history carved upon the rocks, sands, and gravels which formed its shores. Its first ancient beach is more easily traced for almost its entire course than any of the other old levels. It does not even take the trained eye of the scientist to see its unmistakable history written in the sands. The earliest western travellers describe the Ridge Road running along this old, deserted beach as showing unmistakable signs of having been an ancient shore line of the lake.
The Horseshoe Fall, July, 1765.
From an unsigned original drawing in the British Museum.
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.