Popular Science Monthly/Volume 1/September 1872/The Past and Future of Niagara
|THE PAST AND FUTURE OF NIAGARA.|
IN October, 1842, the Falls of Niagara were made the subject of careful study by the New York State Geologists. Under their direction a trigonometrical survey was made, and the river-banks—ancient and recent—the contours of Goat and Luna and Bath Islands, and the periphery of the Falls, were mapped with the utmost precision. The map is preserved in the archives of the State, at Albany, and the copper bolts and little stone monuments, which were placed to mark the trigonometrical points, remain—all, except those which fell with Table Rock. The American Association for the Advancement of Science, at its session last summer, petitioned the New York Legislature to provide for another survey. The expense would have been a mere trifle, but gentlemen of "the Reform Legislature" would not even consider the proposition. It is to be lamented, for another survey would give data by which we could translate into time nearly half a mile of the channel. Until the Falls shall be examined again by instrumentation, in estimating the rate of recession we must depend on the eye alone.
In 1840 old citizens told Lyell that the Falls recede about a yard in a year. I hear the same estimate from citizens now. They see a notch in the Horseshoe which was not there thirty years ago, and they see it growing deeper year by year; they see the American Fall more indented than it was when they used to observe it, and from such changes they construct a scale and apply it to the entire periphery. They deceive themselves. A careful study of the Falls from the trigonometrical points, even without instruments, and a comparison of what you see, with the map of 1842, would convince you that the recession during the past 30 years would fall inside of 15 feet. Let us take six inches a year as an approximation to the rate at which the Falls are eating back through the ledges of shale and limestone. The scale which answers to the last 30 years will apply to the channel from the Horseshoe to Ferry Landing, nearly half a mile. Through this part of the channel the Falls have cut through the same rocks they are cutting now. When they were at the site of Ferry Landing, a hard limestone, a member of the Clinton group, No. 4 of our section (Fig. 1), lay at their base, and the recession must have been arrested. Again, when they were at the site of the Whirlpool, a very hard, quartzose sandstone, marked 2 in the section, a member of the Medina system, lay at their base and checked their recession. Here the great cataract must have stood for ages almost stationary. With these two exceptions, the Falls, in every stage of their retreat, have cut through shale below, and the Niagara limestone above.
Another element in the problem of Niagara's age is the flow of water. To construct a scale from the present and apply it to the past, we should know that the amount of water in past ages has been essentially the same as now.
About 9,800 cubic miles of water—nearly half the fresh water on the globe—are in the upper lakes, and 18,000,000 cubic feet of this plunge over Niagara Falls every minute, all the water of the lakes making the circuit of the Falls, the St. Lawrence, the ocean, vapor, rain, and lakes again, in 152 years. Through the Illinois Canal about 8,000 cubic feet of water are taken every minute from Lake Michigan to the Illinois River; through the Welland Canal 14,000 cubic feet flow every minute from Lake Erie into Lake Ontario, and through the Erie Canal 30,000 cubic feet pass every minute from the same lake into the Hudson. Thus, 52,000 cubic feet of water, which Nature would give to Niagara, are diverted every minute by artificial channels, some into the Mexican Gulf and some into the Bay of New York. Add this to 18,000,000, it is as a drop in the bucket, and would make no appreciable difference in the character of the Falls or their rate of recession. Was there ever a time when the Niagara was appreciably a greater river than now?
Below the Falls, on the Canada side, is a terrace, extending along the river-bank, and attaining a height of 46 feet. It contains river-shells, and is an old river-bank. A corresponding bank is found on the New York side, although much broken and eroded. If a tourist will stand on the New Suspension Bridge and cast his eye along these ancient banks, his first impression will be that the Niagara which flowed against them was vastly greater than the river which flows now nearly 200 feet below him. But, if his eye will follow the Canadian terrace above the Horseshoe, he will see it falling lower and lower, till, at the head of the Rapids, it merges into the present bank. From this point upward the river is contained within low banks, and bounded by a plain whose monotony is not broken by a hill or terrace. A glance at the section (Fig. 1) will make this clear to the eye of the reader. The surface of the river from Buffalo to Lake Ontario is represented by the line R, R; the banks, from Buffalo to the Rapids, by the dotted line t, t and the old banks, from the Rapids to the Whirlpool, by a continuation of the same line. It will be seen that this line rises as the surface of the river falls. The slope from the head of the Rapids to the Falls is nearly 50 feet, and the terrace opposite the Falls attains a height of 46 feet.
We turn now to Goat Island. A walk around the island, by the margin of the river, will show us what immense denudation its limestones have suffered. The extent of this denudation can be seen in our section of the island (Fig. 2). To wear away such beds of limestone, the river, for many ages, must have flowed over the island. And as the upper beds of fluviatile drift, marked, d in our section, are a little below the level of the highest terrace, we must infer that the river, when contained in these ancient banks, covered the island, and was eroding its beds of limestone.
By all this we see that the Niagara itself has made the Rapids, and that, as it cut its way downward, its forsaken banks have assumed the character of terraces. And we see, by the low banks and absence of old banks above the Rapids, that even the highest of these ancient banks did not contain a greater river than this which flows through the narrow gorge to-day.
We assume, then, from all the monuments the river has left of its own history, that the present rate of recession would be a fair measure of the past, except at the Whirlpool and Ferry Landing. Six inches a year, measured on the channel, would place the Falls at Lewiston 74,000 years ago. We have no means of knowing how long the quartzose sandstone, which forms the lowest part of the bank at the Whirlpool, would have arrested the cataract. This stratum is 25 feet thick, and, as its southward dip is 20 feet a mile, and the slope of the river-channel 15 feet a mile, the Falls would have to cut back through this rock more than half a mile. The halt may have been many thousand years. Add another period for the halt at the landing,
No. 6, the Niagara limestone (6 also of Fig. 1); No. 7, the shaly limestone (marked 7 in Fig. 1). The Rapids have been formed by the erosion of this limestone. D, alluvial drift covering the eroded limestone. The section will show how great had been the denudation of the limestone by the river before the drift D accumulated, and before the river had found its present level.
and the age of the channel, from Lewiston to the Horseshoe, may not fall below 200,000 years. Unquestionably the channel has been excavated since the close of the glacial epoch, which science has well-nigh demonstrated occurred about 200,000 years ago. But this channel is only the last chapter in the history of Niagara.
Standing by the Whirlpool on the east, and looking over the river, we see a break in the ledges of rock which everywhere else form the bank. On the western side, around the bend of the Whirlpool, for a distance of 500 feet, bowlders and gravel take the place of ledges of rock. Many of these bowlders are granite and greenstone and gneiss, which have travelled hundreds of miles from the northeast. This mass of northern drift fills an old river-channel, which we can trace from the Whirlpool to the foot of the escarpment at St. David's—about two miles and a half. The reader will see by the map (Fig. 4) that this old channel marked 13 lies in a line with the present channel above the Whirlpool. The opening at St. David's is two miles wide. Here the Falls stood "in the beginning," wide, but not deep. They had cut back two miles and a half when the glacial period came, and lakes and rivers, and the great cataract, were buried under a colossal sheet of ice. If we can trust astronomical data (Stone's Tables of the Eccentricity of the Earth's Orbit), the glacial epoch lasted about 50,000 years. Add this to the age of the present channel, and 25,000 years for the preglacial channel, and we have 275,000 years as an approximation to the age of Niagara River.
Of course these figures are given merely as an approximation to the truth. To the general reader the time seems immense. But to the geologist it seems short, and his concern is to account for the æons in which the lakes and their water-shed must have stood above the ocean, but which the Niagara has not registered. Let us attend for a little while to the earlier history of this Niagara region.
From the Old Suspension Bridge three geologic systems can be seen on the river-banks. The lowest is a red, mottled, shaly sandstone, the Medina sandstone. It is marked 3 on the section (Fig. 1). Above this, and having the same dip, is a thin group of green shale and gray limestone, the Clinton group, No. 4 of the section. Overlying the Clinton is dark shale, and over the shale a thick band of gray limestone, the two forming the Niagara group, designated on the section by Nos. 5 and 6.
Below the escarpment at Lewiston, as the diagram will show, the lowest member of the Medina sandstone (No. 1, Fig. 1) appears as the surface rock. We find it ripple-marked and carrying the Lingida cuneata and Fucoides Harlani, its characteristic shell and seaweed. It underlies a good part of Western New York and Canada, and extends southward into Pennsylvania and Virginia, with everywhere the same characters, indicating a quiet, shallow sea, fed by rivers which for ages brought down the same sediments. It is eighth in the series of palaeozoic rocks which form the first volume of the world's history after the beginnings of life, and is the oldest rock which shows itself about the Falls.
Up the river, about two miles from Lewiston, the railroad, which descends the river-bank, takes us to the junction of the Medina sandstone with the Clinton group. The green shale is barren here, but at Lockport we have found it full of Agnostics lotus, a little ill-defined crustacean. The overlying limestone is exceedingly rich in fossils, Atrypa neglecta being the characteristic shell. The sea had changed both its life and the rock material on its bottom.
Another change, and to the Clinton succeeded the Niagara period. The change was not abrupt, for many species, common in the Clinton sea, lived in the Niagara as well.
In the Niagara shale we have found Conularia Niagarensis, a shell which must be referred to a Pteropod mollusk. Pteropods of the living world are seen only on mid-ocean. They flap themselves over the water by wing-like appendages from the side to the head. Their shells do not drift ashore, but the dredge has brought them up from the ooze of the deep-sea bottom. Now, this Niagara shale is only the hardened ooze of an ancient sea-bottom, and the Conularia tells us that here the sea was open and deep.
A time came when mud-sediments were no longer brought down, and the bottom of an ocean, clear, warm, placid, over an area which extended from the Hudson far beyond the Mississippi, was a vast grove of coral. In sheltered nooks of the coral-grove were gardens of waving crinoids, and three-lobed, many-jointed, many-eyed trilobites were crawling over the coral sand, and mollusks in richly-sculptured shells were everywhere on sand and coral. The Niagara limestone is a monument of that ancient life. With the formation of this rock and its uplift from the sea, the geologic record here about Niagara closed, until the coming of the Ice.
We turn now to the geology of the lake-region. The area of the lakes is estimated at 90,000 square miles; and the area whose streams flow into the lakes, at 400,000 square miles. This immense area is one of the oldest on the globe. On the north shore of Lake Huron and Lake Superior we find the azoic rocks, and on the borders of Lake Erie and Lake Michigan we find no rock newer than the lowest members of the Devonian. The whole water-shed of the St. Lawrence was reclaimed from the ocean before the close of the Devonian epoch. If the drainage has always been through the gulf of St. Lawrence, the Niagara should be one of the oldest rivers on the globe. And yet, as we have seen, in the geological calendar it is very young. How shall we account for this gap between ocean-history and river-history? A little more of geology and something of topography will help us to understand why the Niagara has recorded such a small segment of the time which lies between us and the Devonian seas.
Ideal Section of the St. Lawrence and its Lakes.
Borings made a few years ago at La Salle, on the Illinois, revealed the fact that the valley had been eroded forty feet below the present river-bed. Pot-holes and water-worn ledges at Athens mark the course of an ancient river. Other evidences of the ancient river are found in the valley of the Des Plaines and along the Calumet feeder of the Illinois Canal.
The topography of the lake-basins and the Niagara plateau will explain that old river-bed.
Lake Erie, as everybody knows, and as we have indicated in the ideal section of the St. Lawrence and its lakes (Fig. 3), fills a shallow basin eroded in a plateau 333 feet above the level of Lake Ontario, and 565 feet above the ocean. The surface of Lake Michigan is 600 feet above tide-level, and, as the lake is 1,000 feet deep, its bottom is 400 feet below the level of tide-water. Lake Superior is 900 feet deep, and its surface about 20 feet above that of Lake Michigan. The Niagara, from Buffalo to the head of the Rapids, has a fall of 15 feet. The fall from Lake Michigan to Goat Island is 50 feet—just equal to the slope of the Rapids. A barrier 15 feet high, stretching across the plateau at the head of the Rapids, would throw the river back on Lake Erie, and such a barrier, 50 feet high, would hold back the waters of Lake Michigan.
We can see the significance, now, of a few features of topography about the Falls.
The reader will turn to the map of Niagara River, which we have drawn, with some modifications, from the official maps of the Boundary Commission. He will see that, from the foot of Grand Island to the Falls, the course of the river is almost due west. At the Falls it makes an elbow, and extends thence, with no abrupt winding except at the Whirlpool, northward to Lake Ontario. At Schlosser Landing, about a mile above the Rapids, a stream called Gill Creek empties into the river. It is not more than six miles long, and its course is parallel to that of Niagara below the Falls. Its source is a swamp about two miles east of the river, and nearly the same distance north of Old Fort Gray. We have the anomaly of two streams flowing side by side, within two miles of each other, in opposite directions, and through an apparently level country. Gill Creek, flowing southward, has a fall in six miles, of 60 feet. Its source is 60 feet higher than the surface of Niagara at Schlosser Landing. This high land is not a hill, but a ridge—an anticlinal axis extending from northeast to southwest across the Niagara channels. Before it was broken through and eroded, it formed a barrier a few feet higher than the surface of Lake Michigan. Then Niagara was not, and the upper lakes sought the ocean through a great river, sections of whose channel, as we have seen, can still be traced from Chicago to the Illinois.
We have lingered long in the past. What of the future? The intelligent tourist who stands by the great cataract cannot allow the beauty, the grandeur, the vast magnificence of the scene, to bear down his imagination and bind up all his powers in the present. He looks and listens, and, while he stands overpowered by the falling torrent and rising spray, and thunderous pounding of torrent on fallen torrent, his imagination breaks the spell, and his thoughts wander away into the past and the yet to be. Are future ages to see this wonder, and find it as great as our eyes see it?Mr. Hall, in his report on the Fourth District, and Sir Charles
The reader will remember that the dip of the strata here is 20 feet a mile southward. He will remember, too, that the current below the Falls is 15 feet a mile northward. If he will turn to the section it may help him to see that a stratum which, a mile below the Falls, crops out along the bank 35 feet above the river, would be brought down, at the Falls, to the level of the river; and he will see that, for every mile the Falls have cut their way southward, they have lost 35 feet in height—the dip of the strata and slope of the channel. Let them cut back two miles farther (this is the reasoning of Hall and Lyell), and they will have passed the head of the Rapids. The shale which now lies at their base and forms the lower part of the precipice will have disappeared beneath the river-bed, and the limestone which has always been at the top of the precipice will have reached the bottom. As the Falls have receded by the action of the spray on the shale below, and the breaking and falling down of the undermined limestone above, now that the entire precipice is limestone, the features of the cataract will begin to change. The rock will wear away faster at the top than at the bottom, and the great Niagara—only a hundred feet high now—will dwindle away into a succession of cascades and rapids. This is the future as shaped in the minds of Hall and Lyell. They have overlooked an important fact the change in the course of the river.
A reference to the map will show that the American Fall (8) is cutting eastward, and the Horseshoe (9) southward. But, after a few hundred feet have been cut away, the direction of the Horseshoe will change, and both Falls will move eastward. Above Goat Island they will unite and move on, one Fall, of immense width, till Navy Island cuts it in two. The greater Fall will then be on the American side, and its recession will still be eastward. A little. Fall on the Canada Bide will retreat southward around Navy Island and then Grand Island. About a mile above the northern point of Grand Island this Fall will have moved southward far enough to leave the shale and have the precipice all of limestone. The water will then wear away the rim faster than the base, and the Fall will become a series of cascades and rapids.
But the main Fall will have to cut back to within a mile of Tonawanda Island—by the course of the river, nearly eight miles from the Horseshoe before it makes the same southing. The Fall will have cut back, not with the dip, but nearly at right angles across it. And by the present rate of recession it must continue its work of excavation for 80,000 years before the shale will disappear under the bed of the river and the limestone form the entire precipice. Then the same fate will overtake this greater Fall which, ages before, awaited the other. All this on the assumption that Nature is to go on selecting her own channels and seeking her own ends.
But man is, here, greater as a mere dynamic than any other force acting on the globe. Already Niagara has felt his power. Fifty-two thousand cubic feet of water which belong to her, every summer minute be diverts to his own uses. Another century will see him on every acre along the borders of the upper lakes. Every forest he fells, every acre he ploughs, will affect, though inappreciably, the flow of water over the Falls. Time may come when his hand, laid on the earth in gigantic enterprise, will cause the Falls to shrink into insignificance. He will make these lakes furnish him highways to the ocean, east and south. A canal from Lake Michigan to the Illinois, great enough to float ships laden for the marts of Europe, and another from Lake Erie to Lake Ontario, are achievements in the near future.