The following extracts from an article written by Prof. Tyndall will be
of interest in this connection:--
“The fact that in historic times, even within the memory of man, the
Fall has sensibly receded, prompts the question, How far has this
recession gone? At what point did the ledge which thus continually
creeps backward begin its retrograde course? To minds disciplined in
such researches the answer has been and will be, At the precipitous
declivity which crosses the Niagara from Lewiston on the American to
Queenston on the Canadian side. Over this traverse barrier the
affluents of all upper lakes once poured their waters, and here the
work of erosion began. The dam, moreover, was demonstrably of
sufficient height to cause the river above it to submerge Goat Island,
and this would perfectly account for the finding by Mr. Hall, Sir
Charles Lyell, and others, in the sand and gravel of the island, the
same fluviatile shells as are now found in the Niagara River higher
up. It would also account for those deposits along the sides of the
river, the discovery of which enabled Lyell, Hall, and Ramsay to
reduce to demonstration the popular belief that the Niagara once
flowed through a shallow valley.
“The physics of the problem of excavation, which I made clear to my
mind before quitting Niagara, are revealed by a close inspection of
the present Horseshoe Fall. Here we see evidently that the greatest
weight of water bends over the very apex of the Horseshoe. In a
passage in his excellent chapter on Niagara Falls, Mr. Hall alludes to
this fact. Here we have the most copious and the most violent whirling
of the shattered liquid; here the most powerful eddies recoil against
the shale. From this portion of the Fall, indeed, the spray sometimes
rises without solution of continuity to the region of clouds, becoming
gradually more attenuated, and passing finally through the condition
of true cloud into invisible vapor, which is sometimes re-precipitated
higher up. All the phenomena point distinctly to the center of the
river as the place of the greatest mechanical energy, and from the
center the vigor of the Fall gradually dies away toward the sides. The
horseshoe form, with the concavity facing downward, is an obvious and
necessary consequence of this action. Right along the middle of the
river the apex of the curve pushes its way backward, cutting along the
center a deep and comparatively narrow groove, and draining the sides
as it passes them. Hence the remarkable discrepancy between the widths
of the Niagara above and below the Horseshoe. All along its course,
from Lewiston Heights to the present position, the form of the Fall
was probably that of a horseshoe; for this is merely the expression of
the greater depth, and consequently greater excavating power, of the
center of the river. The gorge, moreover, varies in width as the
depth, of the center of the ancient river varied, being narrowest
where that depth was greatest.
“The vast comparative erosive energy of the Horseshoe Fall comes
strikingly into view when it and the American Fall are compared
together. The American branch of the upper river is cut at a right
angle by the gorge of the Niagara. Here the Horseshoe Fall was the
real excavator. It cut the rock, and formed the precipice over which
the American Fall tumbles. But since its formation the erosive action
of the American Fall has been almost _nil_, while the Horseshoe has
cut its way for five hundred yards across the end of Goat Island, and
is now doubling back to excavate a channel parallel to the length of
the island. This point, I have just learned, has not escaped the acute
observation of Prof. Ramsay. The river bends; the Horseshoe
immediately accommodates itself to the bending, and will follow
implicitly the direction of the deepest water in the upper stream. The
flexibility of the gorge, if I may use the term, is determined by the
flexibility of the river channel above it. Were the Niagara above the
Fall sinuous, the gorge would immediately follow its sinuosities. Once
suggested, no doubt geographers will be able to point out many
examples of this action. The Zambesi is thought to present a great
difficulty to the erosion theory, because of the sinuosity of the
chasm below the Victoria Falls. But assuming the basalt to be of
tolerably uniform texture, had the river been examined before the
formation of this sinuous channel, the present zigzag course of the
gorge below the Fall could, I am persuaded, have been predicted, while
the sounding of the present river would enable us to predict the
course to be pursued by the erosion in the future.
“But not only has the Niagara River cut the gorge--it has carried away
the chips of its own workshop. The shale being probably crumbled, is
easily carried away. But at the base of the Fall we find the huge
boulders already described, and by some means or other these are
removed down the river. The ice which tills the gorge in winter, and
which grapples with the boulders, has been regarded as the
transporting agent. Probably it is so to some extent. But erosion acts
without ceasing on the abutting points of the boulder, thus
withdrawing their support, and urging them down the river. Solution
also does its portion of the work. That solid matter is carried down
is proved by the difference of depth between the Niagara River and
Lake Ontario, where the river enters it. The depth falls from
seventy-two feet to twenty feet, in consequence of the deposition of
solid matter caused by the diminished motion of the river. Near the
mouth of the gorge at Queenston, the depth, according to the Admiralty
Chart, is 180 feet; well within the gorge, it is 132 feet.”