Oldal 82 [82]
first time Lake Erie and Lake Ontario were separated, and then for the first time the Niagara
River carried the surplus water of Lake Erie to Lake Ontario. Various changes contributed to
modify the history of the Niagara River. In the beginning, when the cataract was at Lewiston,
the margin of Lake Ontario instead of being twelve miles away as now, was only one or two
miles distant, and the level of its water was about 75 feet higher than at present.
The characters of the gorge are in general remarkably uniform from endtoend. Its width
does not vary greatly ; its course is flexed but slightly ; its walls exhibit the same alteration
of soft and hard rocks. But there is one exceptional point. Midway its course is abruptly
turned at right angles. On the outside of the angle there is an enlargement of the gorge, and
this enlargement contains a deep pool, called the Whirlpool.
At this point, and on this side only, the material of the wall has an exceptional character.
At this point limestone, sandstone and shales disappear, and the whole wall is made of drift.
Here is a place where the strata that forms the plateau are discontinuous, and must have been
so before the last occupation of the region by the glazier, for the gap 1s filled by glacial drifts.
If we consider as a geological period the entire time that has elapsed since the beginning of the
age of ice, then the history of the Niagara River covers only a portion of the period. In the
judgement of most students of glacial geology, and, I may add in my own judgement, it covers
only a small portion of that period. The great life work of the river has been the digging of
the gorge through which it runs from the cataract to Lewiston. The beginning of its life was
the beginning of that task. The length of the gorge is 1n some sense a measure of the river’s
age, The problem of the time consumed in this great work has been attacked by numerous
writers, and the resulting estimates have ranged from three to four thousand years to three or
four million years.
A critical story of data lends to the belief that the rate of recession in the central part of
Horseshoe Fall is approximately determined, and that it is somewhere between four feet and
six feet per annum. There can be no question that the cataract is the efficient engine, but
what kind of an engine is it? It is a matter of direct observation that from time to time large
blocks of the upper limestone fall away into the river, and there seems no escape from the
inference that this occurs because the erosion of the shale beneath deprives the limestone of
its support. |
At the margin of the Horseshoe Fall and at thé American Fall in which places the body of
failing water is much less, the process is different. ‘There the fallen blocks of limestone form a
low talus at the foot of the cliff, and upon them the force of the desending water is broken and
spent. The differences between the two processes is of great importance in the present connec¬
tion, because the two rates of erosion are very different.