The present width of the river at the mouth of the gorge is seven hundred and seventy feet. It is scarcely possible that the original width of the gorge was here any less than this, for in the narrowest places above, even where the Niagara limestone is much thicker than at Lewiston, it is nowhere much less than six hundred feet in width. Nor is it probable that the river has to any considerable extent enlarged its channel at the mouth of the gorge at the water level. On the contrary, it is more probable that the mouth has been somewhat contracted, for the large masses of Niagara and Clinton limestone and Medina sandstone which have fallen down as the shales were undermined have accumulated at the base as a talus, which the present current of the river is too feeble to remove. This talus of great blocks of hard stone has effectually riprapped the banks, and really encroached to some extent upon the original channel.
We may therefore assume with confidence that the enlargement, under subaërial agencies, of the mouth of the gorge at the top of the escarpment has been no greater than the distance from the present water's edge to the present line of the escarpment at the summit of the Niagara limestone. This we found to be three hundred and eighty-eight feet—that is, the upper stratum of hard rock on the east side of the gorge had retreated that distance, through the action of atmospheric agencies, since the formation of the gorge first began. The accompanying photogravures and diagram will present the facts at a glance. The total work of enlargement on the east side of the gorge has been the removal of an inverted triangular section of the rock strata three hundred and forty feet high and three hundred and eighty-eight feet base, which would be the same as a rectangular section of one hundred and ninety-four feet base. From this one can readily see that if the average erosion has been at the rate of one quarter of an inch per annum, the whole amount would have fallen down in less than ten thousand years; while if the time is lengthened, as some would have it, to forty thousand years, the rate would be reduced to one sixteenth of an inch per year.
Fortunately, the construction of the railroad along the face of the eastern wall of the gorge affords opportunity to study the rate of erosion during a definite period of time. The accompanying photogravures will illustrate to the eye facts which it is hard to make impressive by words alone. The course of the road is diagonally down the face of the gorge from its summit for a distance of about two miles, descending in that space about two hundred feet to the outcrop of hard quartzose Medina sandstone. The lower mile of this exposure presents the typical situation for making an estimate of the rate at which the face is crumbling away.
Fig. 4.—Nearer view of the upper portion of the face near the mouth, showing the exposure of the situation at that point.
Beginning at what used to be known as the "Hermit's Cave," near the Catholic College grounds, where the Niagara shale is well exposed, and extending to the outer limit of the gorge, the height of the face above the railroad averages one hundred and fifty feet. Now, the crumbling away of the superincumbent cliffs gives continual trouble to the road. Three watchmen are constantly employed along this distance to remove the débris which falls down, and to give warning if more comes down than they can remove before trains are due. The seventy feet of Niagara shale, and the equal thickness of shaly Medina rock which underlies the Clinton limestone, are constantly falling off, even in fair weather, as any one can experience by walking along the bank; while after storms, and especially in the spring, when the frost is coming out, the disintegration proceeds at a much more rapid rate. Sometimes two or three days are required by the whole force of section hands to throw over the bank the result of a single fall of material.
At a rate of one quarter of an inch of waste each year the amount of débris accumulating for removal on the track along this distance would be only six hundred and ten cubic yards per annum—that is, if six hundred and ten cubic yards of material falls down from one mile of the face of the wall where it is a hundred and fifty feet high, the whole amount of enlargement of the mouth of the gorge would be accomplished in less than ten thousand years. Exact accounts have not been kept by the railroad; but even a hasty examination of the face of the wall makes it sure that the actual amount removed has been greatly in excess of six hundred yards annually. This estimate is based partly on the impression of the railroad officials as to the cost of removal, and partly on the impressions of the watchmen who spend their time in keeping guard and in the work of removing it.
Fig. 5.—Showing extent of erosion at base of the Niagara shale since 1854. (See description in the text.)
But that is not all. The accompanying photogravures indicate an actual amount of removal over a part of the area enormously in excess of the rate supposed. Fig. 5 shows a portion of the precipice, a hundred feet high, where the road first comes down to the level of the Clinton limestone, and where, consequently, the whole thickness of the Niagara shale is accessible to examination. Fortunately, Patrick MacNamara, the watchman at this station, was a workman on the road at the time of its construction in 1854, and has been connected with the road ever since, having been at his present post for twelve years. We have therefore his distinct remembrance, as well as the appearance of the bank, to inform us where the face of the original excavation then was. In the picture he is standing at the original face, while the other figure is nearly at the back of the space which has been left empty by the crumbling away of the shale. The horizontal distance is fully twenty feet, and the rocks overhang to that amount for the whole distance exposed in the photograph. All this amount of shale has fallen down in forty-four years, making a rate many times larger than the highest we have taken as the basis of our estimate. Of course, this rate for the crumbling away of the Niagara shale on its fresh exposure is much in excess of the average rate for a long period of time; but it is clear that the rate of erosion at the base of the Niagara limestone at the mouth of the gorge can never have been sufficiently slow to reduce the total average much below the assumed rate of a quarter of an inch a year.