Oldal 82 [82]
OCR
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 connection, because the two rates of erosion are very different.
