The estimate is that nine hundred millions of cubic feet of water pour
over Niagara every hour, and great as this mass is, there is a belief
that half the water passing into Lake Erie from the upper lakes does
not go over the falls, but finds its way into Ontario through a
subterranean channel. Nothing demonstrates this theory, but it is
advanced to account for the difference between the amount of water
accumulated in the upper lakes and that going over the falls. The
actual current is sufficiently enormous, however, and steadily wearing
away the rocks over which it descends, it has during the past ages
excavated the gorge of the lower rapids. The land surface, which is
low at Lake Erie, scarcely rising above the level of its waters,
gradually becomes more elevated towards the north, till near Lewiston
it is about forty feet above Erie. The Niagara River thus flows in the
direction of the ascent of this moderately inclined plane. Beyond this
the surface makes a sudden descent towards Lake Ontario of about two
hundred and fifty feet down to a plateau, upon which stands Lewiston
on the American side and Queenston on the Canadian side of the river.
There thus is formed a bold terrace looking out upon Ontario, from
which that lake is seven miles away, and from the foot of the terrace
the surface descends gently one hundred and twenty feet farther to the
lake shore. The gorge through which the river flows is three hundred
and sixty-six feet deep at this terrace. There is no doubt the first
location of the great cataract was on the face of the terrace near
Lewiston, and it has gradually retired by the eating away, year after
year, of the rocky ledges over which the waters pour. This, however,
has not been done in a hurry, for the geologists studying the subject
estimate that it has required nearly thirty-seven thousand years to
bring the falls from Lewiston back to their present location. In fact,
from the stratification, Professor Agassiz expressed the opinion that
at one time there were three distinct cataracts in Niagara River.
During the brief time observations have been made, great fragments of
rocks have been repeatedly carried down by the current pouring over
Niagara, the frosts assisting disintegration. This caused not only a
recession but decided changes in appearance. Since 1842 the New York
State geologists, who then made a careful and accurate topographical
map, have been closely watching these changes, and the average rate of
recession is estimated at slightly over two feet annually. In Father
Hennepin's sketch of 1678 there was a striking feature, since entirely
disappeared, a third fall on the Canadian side facing the line of the
main cataract, and caused by a large rock turning the diverted fall in
this direction, this rock falling, however, in the eighteenth century.
The rate at which recessions occur is not uniform. No change may be
apparent for several years, and the soft underlying strata being
gradually worn away, great masses of the upper and harder formations
then tumble down, causing in a brief period marked changes. At the
present location of the cataract, sheets of hard limestone cover the
surface of the country, and from the top of the falls to eighty or
ninety feet depth. Shaly layers are under these. All the strata slope
gently downward against the river current at the rate of about
twenty-five feet to the mile. Above the falls, in the rapids, the
limestone strata are piled upon each other, until about fifty feet
more are added to the formation, when they all disappear under the
outcropping edges of the next series above, composed of marls and
shales. Through these piles of strata the cataract has worked its way
back, receding probably most rapidly in cases where, as at present,
the lower portion of the cutting was composed of soft beds of rock,
which being hollowed out and removed by frost and water, let down the
harder strata above. The effect of continual recession must be to
diminish the height of the falls, both by raising the river level at
their base and by the sloping of the surmounting limestone strata to a
lower level. A recession of two miles farther, the geologists say,
will cut away both the hard and the soft layers, and then the cataract
will become almost stationary on the lower sandstone formation, with
its height reduced to about eighty feet. This diminution in the
Niagara attractions might be startling were it not estimated that it
can hardly be accomplished for some twelve thousand years.