Wonderful changes have taken place in the contour of the greater Fall
at Niagara since Docteur Gendron recorded that the Indians traded in
those "Erie Stones." The additional Fall, which Father Hennepin
pictured in 1697, as pouring eastward from the Canadian end of the
Horse Shoe Fall, was formed by the waters flowing around a large rock,
which stood at the very edge of the cliff. Before the middle of the
18th Century that rock had disintegrated and been swept away; and that
separate Fall then merged itself into the greater cascade; as is shown
in a view of Niagara accompanying Peter Kalm's description thereof in
1751. But it must be remembered that in Hennepin's time that Canadian
end of the Horse Shoe Fall extended very much farther down the Gorge
than it does to-day--probably 800 feet farther. That Fall then extended
its shallow end down to where old Table Rock stood. Then the levels of
all the upper lakes were higher than they are to-day, those levels
having been considerably lowered through the white man's denudations of
the forests in the Basin of the Great Lakes. As the downpour of Niagara
thereby diminished in volume, that end of the Canadian Fall receded; so
that, as far as can be deduced, that Fall was some 400 feet shorter in
contour (all taken off its western end) in 1900 than it was when
Hennepin saw it--two and a quarter centuries before. Since 1900, the
policy of the Province of Ontario, to turn its share of Niagara into
cash--in renting out to corporations the right to use the waters of the
Cataract for the development of electrical horsepower ("at so much
per")--has resulted in still further shortening the contour of the
Horse Shoe Fall, by another 400 feet. The contour of that Fall was
given by survey in 1840 as 3,060 feet. Hence, in Hennepin's time, it
must have been about 3,500 feet. To-day, owing to the filling in of the
old river-bed, along the edge of the precipice at the Cataract's
western end, that contour line would not be more than 2,700 feet.
But it must be recalled that the recession at the apex of that Fall has
been very marked since 1840; and as that recession is V shaped it has
added somewhat--fully two hundred feet--to the figures of that old
contour line; making the contour line of the Falls to-day about 2,900
feet.
By reason of that shortening of that Fall, two scientific questions are
brought up in regard to those deposits of gypsum, or "Petrified Spray
of the Falls."
First--to what extent has that concretion formed behind the falling
water? Has it formed there in greater quantities than it has where the
face of the cliff has been open to the air? In greater quantities might
have been expected, on account of the greater amount and absolute
continuity of the moisture on the rocky face. The 400 feet length of
cliff, from which the waters have now been permanently shut off,
furnishes the answer. Practically, none of that concrete has ever
accumulated in the crevices of the rock on the face of the cliff
immediately behind the Falls. The currents of air, and the furious
blasts of water which they create, rush constantly away from the under
surface of the falling sheet, and continuously against the face of the
cliff. These scour and cut away the rock, even as a sand blast would
do, though more slowly. They allow no chance for deposits. The strata
of the Clinton Formation (which commences at about the level of the
water in the Gorge, and of the Niagara shale, which overlie it--the two
combined having a depth of about eighty feet) are eaten away the
faster. The eighty-feet-deep layer of Niagara Limestone, which overlies
the shale, being harder, is eaten away slowly; its lower layers being
attacked by the winds and waters from below (as the underlying shale
disappears) and also on its face, yielding faster than the upper ones.
[Illustration: AMERICAN FALL--NIAGARA. CAVE OF THE WINDS IS
BEHIND SMALL FALL.]
That this concretion has always formed in the limestone, back from the
face of the cliff, behind the falling sheet (where the blasts of wind
and water cannot reach nor effect it) is certain.
That it forms under the river bed, and back from the face of the gorge
on both sides of the river, and wherever the water percolates through
the upper layer of rock, is also certain.
It is so found in the limestone (but not in the shale) wherever deep
excavations have been made near the river in the vicinity of the Falls
and wherever tunnels have been driven through the limestone--in the
crevices and especially where a pocket or hollow space exists in that
formation.
This process of eating away the lower rocks, undermining the upper
limestone, which, as its support is taken away, tumbles into the Gorge,
shows the means by which the Falls gradually recede.
It is shown to the best advantage in the Cave of the Winds, which,
during the past thirty years, by this wind-and-water-blast process, has
been enlarged to four times its former size. Some day the layer of rock
at the top of that cave will fall; the edge of the Luna Island Fall
will be thus moved back a number of feet; the Cave of the Winds will
become merely a narrow space between the outward-curving fall of water
and the perpendicular rock; and the wind-and-water-blast will continue
its erosive work on that rocky face;--and in the course of years will
again produce a distinct cave.
The other scientific question--which the future will answer--is, How
fast does this Niagara concrete form? With that 400 feet length of
cliff on the Canadian shore--which was formerly covered by the end of
the Horse-shoe Fall--exposed to the air and to observation (the outer
end of those crevices in its face being now free from any such
deposit); with the extensive excavations on the debris slope for the
Power House below the bank, exposing new surfaces, where little such
deposit now appears; with other probable excavations in connection with
the power development, exposing similar surfaces at other points along
the Gorge; it will be possible to approximately determine the yearly
amount of accumulation and deposit of this ancient Niagara product. For
that deposit will go on as ceaselessly as it has been going on, ever
since the time--possibly many thousands of years ago--when the waters
of a great lake (which was formed by the melting of the ice sheet)
covered all this region; finally breaking over its northern barrier at
the Lewiston escarpment, where, seven miles from its present location,
Niagara was born.