Графични страници
PDF файл
ePub

THE SIGNIFICANCE OF THE LOW WATER LEVELS.

BY PROF. A. P. COLEMAN,
Professor of Geology, Toronto University.

Tictoria Park

Highland Creek

SECTION OF THE SCARBORO' HEIGHTS.

It is hard for us to believe that the splendid fresh-water seas, which we call the Great Lakes, can undergo any important change. They seem so secure and rightful a possession, that we look on the lowering of their waters three or four feet beneath the usual level with a certain irritation, as if nature were treating us unfairly in making us adjust our harbours to a new level. The idea never enters our minds that these lakes could be destroyed, or so swollen as to flood every city on their shores; and yet the geologist has proofs that these broad and beautiful sheets of water represent only a fleeting stage in the series of episodes making up the history of this part of America.

Not only has Lake Ontario more that once brimmed over banks a hundred and fifty feet above its present shores, but it has been at least once, and probably twice or thrice, wiped completely out of existence; and all this within quite recent geological times, that is, since the beginning of the Ice Age; and much the same is true of the other lakes of the St. Lawrence system.

The best record of the history of Lake Ontario is to be found in the picturesque cliffs of the Scarboro' Heights, and the ravines of the Don and the Humber, bits of attractive natural scenery too little valued by Torontonians, but full

of significance to the student of glacial geology.

The history is not so clearly written that "he who runs may read"; on the contrary, one must examine into the matter with something of the detective's patient skill, following up the clues afforded by a broken shell, a beetle's wing, a bit of rotten wood in a clay bank, a scratched pebble here and a crumpled bed of sand there; until at last the web of circumstantial evidence is complete, and takes shape before our eyes.

It is a fascinating study, but the limits of a magazine article make details unadvisable, so that results must be dealt with rather than methods of research. Most of the information used in this article has been obtained by Dr. G. J. Hinde, formerly a resident of Toronto, and the present writer; but the works of Dr. Spencer, Sir William Dawson and others have also been drawn upon.

The most legible of our documents is to be found at the Scarboro' Heights, of which a sketch is given. In our first cut, the dotted boulder which rises from the lake at Victoria Park, reaches a height of nearly a hundred and fifty feet, plunges suddenly down. to the lake only to rise again as suddenly, and finally sinks again,

the lake level at Highland Creek; a sort of Cupid's bow nine miles in length. Another dotted

band caps the summit of the heights three hundred feet above the lake.

In nature these dotted bands consist of boulder clay, the carpet of confused clay and stones spread out irregularly over a country conquered by a glacier, a sort of trail of the icy serpent by which the geologist can track the movements of the monster after he has retreated. There are three of these beds of boulder clay to be seen about Toronto, though the lowest is out of reach beneath the lake at Scarboro', and each of them bears. convincing evidence of a tremendous act in the drama of the world's life in this region, when a chill ice

ing hosts of plants and animals to occupy their old territories.

Each bed of boulder clay marks an invasion of the ice, while the beds of stratified sand and clay between them prove interglacial periods when the waters of the lake were busy spreading out the materials brought down by swollen streams, entombing here and there bits of wood and bark, or insects or shellfish, to give us an idea of the life of the time.

Lake Ontario, then, has been elbowed out of its bed and destroyed more than once by the invasion of glaciers. After each retreat of the ice there was a stage of high water; the first time the

After Sir William Dawson.

TERRACES AT TADOUSAC.

monster gathered its forces in the fastness of Labrador, snows heaped or snows, till they lay to a depth of ten thousand feet in the north and crept slowly southward and westward, overwhelming the continent, driving all living beings to more genial regions, filling the beds of lakes and rivers, and dragging everywhere the spoils of rock and soil it had gathered in its previous

course.

But the scene changes. The warm south wind and the sun at length gain the upper hand and the ice mass melts away faster than it is replenished. It gradually retreats towards its north-eastern home, freeing the earth from an

cubus and allowing the advanc

water rose at least a hundred and forty feet above the present level, for beds of sediment were formed at that height; the second time, two hundred and eighty feet higher than now; and the third time, a hundred and sixty. The last water level left its mark as a well defined beach with sand bars and cliffs, as may be seen along the foot of the Davenport ridge to the north of Toronto, or the grand cliffs near Hamilton. This line of old beaches has been traced by Dr. Spencer from Trenton to Hamilton on the north shore, and on the south as far as Queenston, while Professor Gilbert has followed it along the American shore.

It will be noted that any of the

have

high-water stages would flooded Kingston, Toronto, Hamilton, and a dozen other towns and cities on the north and south shores of the lake.

How are we to account for these tremendous changes in the lake level? For the last episode, which Dr. Spencer has named the Iroquois water, three theories have been formed, and probably the two former stages of high water may be accounted for in the same way. According to one theory the earth's crust was heaved up in the neighbourhood of the present Thousand Islands, thus holding back the water and raising its level. A second theory, which is held by Dr. Spencer, supposes that the whole of eastern Canada was sunk beneath its present level to a depth of some four hundred feet, allowing the sea to flow inland so that the site of Montreal was submerged, and the Gulf of St. Lawrence stretched to the foot of what is now the Mountain," at Hamil

ton.

66

A third theory, which is the one most generally held by geologists, and, to my mind, the most probable one, assumes that the retreat of the glaciers of the Ice Age was arrested for a while in the neighbourhood of Kingston, and that a wall of blue ice lay across the foot of the lake, damming its waters. until they rose high enough to flow off by a new channel toward the Hudson.

There is one very remarkable circumstance to be mentioned regarding the old Iroquois water. Its beach must have been horizontal when it was made, but Dr. Spencer has shown that it is now tilted out of position. It stands a hundred and fourteen feet above Burlington Bay, a hundred and sixty feet above Toronto Bay, about two hundred feet above the lake at Scarboro' Heights, and over four hundred feet above the

How

Bay of Quinte at Trenton. could staid Mother Earth indulge in such a freak as this?

Probably the best explanation is to suppose that the earth's crust rests on a somewhat plastic substratum. Load it down with five thousand feet thickness of ice and it sinks under the burden. Thaw off the ice and it slowly rises again. Since the ice thawed away first from the south-west end, that corner of the raft bobbed up first, while the north-east end was still held down.

Then the Iroquois lake cut its shore line.

When the rest of the ice finally melted, the north-east of Canada rose in its turn, and all the beach lines were tilted out of place. Mr. Warren Upham, who is gifted with imagination, even thinks that this part of the once ice-laden continent popped up too far, and is still oscillating, trying to reach an equilibrium! Dr. Spencer holds, however, that the uplift is not yet ended, and that eastern Ontario is still on the rise.

One curious inference from this "differential uplift," is, that the trough of Lake Ontario was tilted down so far at first as to leave the Hamilton end high and dry.

It

was only as the Thousand Island end rose towards its present position that the lake backed up, filling the basin in which we now find it.

It should be remembered that all these strange events in the life history of Lake Ontario, and the similar events in the history of the other lakes, took place in times that the geologist looks on as very recent, within the last one or two hundred thousand years, at least; the last episode, that of the Iroquois water, probably within the last seven thousand years, and possibly within half that time.

While it is a comfort to think that these catastrophes of ice and flood took place some time ago, it

is disquieting to reflect that what happened in the past may happen again in the future; and the steady fall of the water in our lakes and the St. Lawrence brings it sharply home to us that changes may take piace in our day. When vessels can no longer enter our harbours with full cargoes, the business man begins to think that he may have some interest in changes of water level as well as the geologist. Calculations have been made, showing how many millions of dollars of loss will result from the lowering of the waters a given number of inches, and the prospect

evidence that the St. Lawrence at that point is deepening its channel appreciably. No amount of work dcne in removing obstructions lower down the river can affect the level of Lake Ontario.

Of course, the diverting of a considerable amount of water by the Chicago drainage canal would have its effect on all the lakes and rivers below. Aside from such artificial causes, there is no reason to suppose that their waters are likely to sink below a certain point fixed by the fluctuations of the rain supply of the region as a whole.

Mr. Stupart, Director of the

[ocr errors][merged small][merged small][merged small][merged small][merged small][merged small]

of a further lowering is decidedly disagreeable.

The business man may be reassured, however. There is no prospect, from the scientific side, of any important change within a geologically short time, a few hundred years, for instance; so that corner lots on good business streets in our cities need not be sold hastily at a sacrifice.

In the papers one sees alarmist statements as to the effect of deepening the channels between the lakes and the sea; but this can have no effect unless the deepening takes lace at the immediate outlet of the

in the case of Ontario at the sand Islands. There is no

Meteorological service of Canada, is of opinion that changes in water levels are directly connected with changes in the annual rainfall, which is not likely to vary beyond certain limits; so that the water may be expected presently to rise again.

Changes in the level of the St. Lawrence, such as have disquieted the merchant princes of Montreal, no doubt have their cause in the varying amount of water discharged from the Great Lakes. When water is low in the reservoir, the current that flows from it must be diminished, just as it must rise again when the reservoir is filled.

Lower down on the St. Law

Sir

rence, where the tide flows, and on the coasts of our Maritime Provinces, there are evidences of changes somewhat like those of the lakes, but probably not always produced by the same causes. William Dawson, and others, have described old beaches with marine shells found five or six hundred feet above sea level on the flanks of the mountain at Montreal, as well as here and there along the shores of the lower St. Lawrence; direct proofs that the land once stood that much lower, but has risen to the present height. On the other hand, the Chignecto ship railway excavations have disclosed peat beds buried in the sand many feet below the present tide level, demonstrating a sinking of the land surface in that region.

To discuss the question of how these changes in the relative posi

tion of land and sea are related to the variations in level of the Great Lakes during, and after, the Ice Age, would, however, lead us too far.

That another Ice Age may come, blotting out our cities and leaving only traces of our civilization in obscure interglacial beds; and that other changes in water level may flood the lowlands or leave our ports high and dry and far from lake or sea, is not at all impossible; but probably good Mother Nature will give us a few centuries of warning, so that we may arrange our affairs in time. For the present we may expect the law of averages to hold, so that the years of low water in our lakes and rivers will be balanced by years of higher water in the not distant future.

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small]
« ПредишнаНапред »