Popular Science Monthly/Volume 9/September 1876/On the Formation of Lakes
|ON THE FORMATION OF LAKES.|
IT was not until the studies of Agassiz, Forbes, and others, among the Alps of Switzerland, had made us acquainted with the character and action of glaciers, that we could at all understand many of the most curious and interesting features connected with the formation of the multitude of lakes with which we are more or less familiar, and which lend so much beauty and grandeur to the scenery of the world.
As some classification is necessary for the understanding of a series of facts, we will arrange lakes under four heads: 1. Those filling glacier-worn rock-basins; 2. Those confined by banks of sand, gravel, bowlders, etc., or, in one word, by moraines; 3. Those formed by a subsidence of their bottoms, or by the elevation of the country surrounding them, commonly by the secular changes of level to which the crust of our globe is subject; 4. Lakes filling basins formed by volcanic action.
1. Lakes which fill rock-basins are such as are confined on all sides by the common rock of the country, so that in some cases a person can walk entirely around them without stepping off the solid rock; and in all cases they would be found to have a rocky rim inclosing them, were the superficial material removed. How such spoon-shaped depressions could be scooped out, was for a long time an enigma which eluded the search of the most painstaking observers. As facts accumulated, however, it was noticed that the sides and bottoms of such lakes are smoothed, in many cases polished, and almost always covered with grooves and scratches; and also that in their vicinity beds of clay are usually found, intermixed with pebbles and large bowlders which, like the rocky basins, are also smoothed and frequently scratched. It was noticed, too, that the rock from which these bowlders and pebbles had been formed commonly differed from the rocks in place on the shores of the lakes. Thus, throughout New York and Ohio, huge bowlders are common, composed of crystalline rock found in place nowhere nearer than the Canadian Highlands, a hundred miles to the northward; while the peculiar native copper of Northern Michigan is sometimes found mingled with the bowlders and striated stones of the drift far southward in Ohio.
The problem now was to discover what forces in Nature could polish and scratch both rock-surfaces and detached stones, and could also transport masses of rock, tons in weight, far from their native home.
It is well known that the loose stones and pebbles along the seashore are made very smooth and round, and often polished, by the action of the waves. It might be thought from this that the pebbles found on the shores of the lakes, and imbedded in the clays, were fashioned in the same manner. On one occasion, at the Cape of Good Hope, the writer, after wandering for a time along the sloping sandy beach of Table Bay, came suddenly to a little rocky cove exposed to the full swell of the South Atlantic. As each wave broke on the steep, rocky beach and retreated, it was followed by a sharp, rattling sound that could be distinctly heard above the roar of the waves; we noticed, too, that the stones all along the shore were in motion, rolling down the beach, only to be caught up by the next white-capped wave that came in from the ocean, and again carried up the beach, and rolled and pounded against each other by the untiring waters, that were fast reducing them to sand and dust. On examining these water-worn stones, we found them all smoothed and rounded, and often beautifully polished; but in no case could we discover, even with a magnifying-glass, any that were scratched, or in any way marked similarly to the stones which we have so often examined in the clays and hard-pans that cover so great a portion of our Northern States. From this fact, and also from watching the action of the waves on many other coasts, we conclude that the sea tends to smooth and wear away the stones and rocks along its shores, but has no power to cover them with grooves and scratches; and that, instead of wearing the coast into pockets and basins, it tends only to grind down the islands and continents to one uniform level.
Again, we have traversed the deep, picturesque valleys of the southern Alps, where we could see the glaciers glittering on the mountain-sides far up at the head of the valley, and have noticed as we advanced that the rocks became more and more worn and rounded; that in sheltered places, along the sides of the valley, beds of thick plastic clay were to be found; and also that the whole valley was strewed with smoothed and rounded pebbles, together with huge bowlders, many of which were a hundred tons in weight. These were often planed off and grooved, precisely like many of the transported stones that are scattered so plentifully over the hills and valleys of the State of New York; and like them, too, frequently differed in the nature of their material from the rocks of the surrounding cliffs. As we ascended the valley, these peculiarities became more and more strongly marked; while around us the hills and knolls had a rounded and flowing outline, and formed what are known as roches moutonnées, the mountain-peaks that towered above were sharp and angular, and stood out against the clear sky like cathedral-spires.
All these facts have such a marked and intimate connection with the glaciers that still linger on the mountain-side, that no one—who had traversed those valleys, or traced the streams up to the ice-caves, from which many of them spring, turbid and overloaded with silt, at the foot of the glaciers—could doubt that these valleys, with all their peculiar features, owe their existence to the great extension of the glaciers, which in past time flowed from the mountains in great rivers of ice, and carved out those grand valleys to a depth of many thousands of feet in the solid rock. As these ancient glaciers retreated and melted away, they left the indisputable records of their presence throughout the valley.
The same connection of rounded and striated bowlders (called Fündlinge—wandering children—by the German peasants) with existing glaciers has been observed by Agassiz and others in the Alps of Switzerland. Not only these facts, but the manner in which the glaciers flow down the valleys like great rivers of ice, has been closely observed and measured; they have been seen time and again transporting immense amounts of dirt and stones on their surface, which in time formed part of the terminal moraines at their extremity. The sides and bottoms of the valleys through which they flow are smoothed and covered with scratches made by the pebbles and stones set in the bottom and sides of the glacier, which in their turn were rounded and scratched, often in various directions, caused by their breaking from their matrix, and being reset in a new position.
If we were to place the rounded and scratched stones from the drift ("hard-pan," "hog's-backs," etc.) of New York beside the similar stones broken from their icy fastenings in the bottom of the glacier of Zermatt, we should find them so similar in their markings that no eye could distinguish but that they had made the journey under the glacier side by side.
If we compare the smoothed and striated rocks from the bottoms and shores of Lake Erie, Cayuga Lake, or almost any of our lakes which fill rock-basins, with the rock-surface fresh from under the ice of the Mer de Glace, we shall find them wonderfully similar in their markings. The characters that are engraved upon them are the same.
Not only do we find these markings in connection with the present glaciers, but we find also the rock-basins themselves with the glaciers yet occupying their upper portions, and still at work grinding down the rocks. The best example of this kind, perhaps, in the world, is Lake Wakatipu, in New Zealand, which has a length of seventy miles, and a depth of 1,400 feet. This lake fills a true rock-basin, and bears every indication of having been excavated by the glaciers, which in the past were greatly extended, and have now retreated to the extreme upper end of the valley, while it has no connection with synclinal folds or volcanic fractures.
How can we resist the conclusion, then, that these bowlders, these beds of clay full of smoothed and striated pebbles, and these rock-basins with their sides covered with inscriptions—which we can now read with ease and accuracy if we take the records made by existing glaciers, as the Rosetta Stone—are all the work of glaciers, since the same results are produced at the present day by the action of ice, and by no other agency known?
A clearer idea of the manner in which a flowing glacier wears out a rock-basin can be gathered, perhaps, from the accompanying diagram, where the rock R R is shown, over which passes the glacier G,
which wears its bottom less at the lower end, not only for the reason that the ice is continually wasting away, and growing thinner in the lower portion, but also because the material carried down on the surface of the glacier is deposited at its extremity M, in the form of a terminal moraine, and thus protects the rock beneath from further waste. When the ice of the glacier is melted away, and the terminus retreats up the valley, the basin which it leaves behind it becomes filled with water (from M to G), and thus forms a lake, which maybe a mere pool across which a school-boy can skip a stone, a great inland sea like Lake Erie or Lake Ontario, or a mirror of grandeur like Lake Geneva or Lucerne, in Switzerland, and Lake Wakatipu and Lake Wanaka, in New Zealand.
It may be urged that these beds of clay, with their striated stones and huge bowlders, are found over a large section of our country, and are not confined to the region of the lakes. This is very true; and from it we conclude that where now so many happy homes are scattered, from Maine to the far West, the snows and frozen mists of a great winter once accumulated to many thousand feet in thickness, and formed a great glacier, like that which covers the interior of Greenland at the present day, which flowed southward, grinding down the country and acting as a ploughshare to prepare the land for a new harvest. Gradually this great winter began to pass away, and the spring-time in which we now live, to draw near. As the great glacier retreated northward, it left the country covered with beds of bowlder-clay and strewed with huge erratics from northern regions, which together with other débris form the surface material of all our northern country, where it has not since been swept away or covered by other and more recent deposits. It is often well exposed along our lines of railroads, and may be known at a glance by the great number of worn and rounded stones of all sizes that are scattered promiscuously through it. These evidences of glacial action are found as far southward as Cincinnati and the central portion of New Jersey, showing that here was the border of the icy mantle that was spread over all the northern regions. After this great continental glacier passed away, or had retreated far northward, smaller and detached streams of ice still flowed southward to complete the task of moulding the valleys and lake-basins. It is to these smaller glaciers that we attribute the formation of the multitude of lakes filling rock-basins that are scattered through the northern part of the United States and over the whole of the British possessions, many of which have been hollowed out in nearly horizontal beds of rock in the same manner as lake-basins are now forming under existing glaciers. Nor are the lakes which fill glacier-worn rock-basins confined to our own continent, but they form the most common and grandest lakes of temperate latitudes, which might be called the lake latitudes, so completely are the lakes of the world confined to these regions.
The theory of the glacial origin of certain lakes was first proposed by the distinguished English geologist, Prof. Ramsay, and, after being tested in nearly every glaciated region in the world, is now held, by those best qualified to understand it, as the simple and true history of the formation of many of our lakes.
2. The lakes of our second class, those which are confined by banks of gravel, bowlders, etc., owe their origin, like the ones we have been considering, to the action of ice. Lakes of this class are most commonly found in the deep Alpine valleys of mountainous regions, where the material which accumulated on the surface of the glaciers that once flowed through them, in the form of lateral and medial moraines, was carried down and deposited at the extremity of the glacier in what is known as a terminal moraine, which in many cases stretched completely across the valley and marks the place where the terminal face of the glacier was stationary for a considerable period of time before it melted away, and allowed the water to accumulate in the space once filled by the ice. These glacier-built dams are to be met with in all countries which have been subjected to glacial action, and are especially well marked amid the Alps and in Scotland, where they have been most thoroughly studied, on the Scandinavian peninsula, in the Northern States of the Union, and amid the southern Alps of New Zealand. As the bottom of the valley in which such a lake is formed is usually worn deeper by the action of the glacier during the formation of the terminal moraine, this second form of lake-basin is quite often combined with the first.
To this second class also belong the thousands of little lakelets scattered over the Northern States, which are confined on all sides by banks of drift-material, and fill nearly every depression and hollow in the huge banks of glacier-worn débris—known as till, kaims, eskers, etc., scattered so plentifully throughout our Northern country. We have seen many of these pretty little lakelets through New York, Ohio, and westward. Near Plainfield, New Jersey, scores may be passed in a morning's walk. At the latter place they occupy the hollows and dells in the drift, which is there of great thickness, and formed not only from the Triassic sandstone which underlies it, but also to a large extent from the limestone and gneiss found in place only in the northern portion of the State. Intermingled with these are many blocks of the peculiar reddish conglomerate found in situ in Morris County, which show unmistakably the direction from which the drift has traveled. Many of these stones are glacier-worn, and have without doubt been transported from their northern homes by the agency of ice; not in one or two isolated instances, but in sufficient quantity to cover the country for miles in extent. These little lakelets, becoming filled with vegetable matter, form peat-bogs, which promise to become of considerable agricultural value in the future; these peat-bogs not only contain many wonderful things for the eyes of those who are fortunate enough to possess a microscope, but also in them are sometimes found the bones of the huge mastodon, which at no very distant time inhabited this continent.
3. The formation of lakes by a sinking of their bottoms, although at first sight seeming to be the simplest and most common mode of their formation, is really the most unusual. Lake Superior is described as filling one of these depressions, as the rocks on its shores are found to dip toward the centre of the lake, and the basin seems to have been formed by a subsidence at that point, although greatly modified in after-time by the erosion of the ice during the glacial period. The valley into which the Jordan empties is another such region of subsidence.
No description has been given of the newly-discovered lakes of Central Africa, sufficiently accurate to decide to which mode of formation they owe their origin, but, as they are situated in the tropics, it will probably be found that, like Lake Superior, they fill synclinal valleys.
To this category belong also the truly great lakes which existed in our Western country during Tertiary times, which in the lapse of ages became filled with mud and silt, and now form the greater portion of the rich Territories of Nebraska, Dakota, etc. In this region are found, in great numbers, the remains of the huge animals which lived in these ancient lakes, and fed on the luxuriant tropical vegetation that overhung their banks.
The well-known Salt Lake of Utah is another example of a lake filling an area of depression, and was of far greater extent in past time, as is very plainly shown by the lines of ancient terraces which are so sharply drawn between Ogden and Salt Lake City, nearly a thousand feet above the present level of the lake.
4. Lakes of the fourth class, such as owe their formation to volcanic action, are found occupying the bowl-shaped craters of ancient volcanoes, which, as their fires became extinct, furnished convenient reservoirs for the accumulation of water, and in this manner sometimes formed lakes of considerable extent. Streams of lava, also, when they chance to flow in such a manner as to obstruct the drainage of a valley, may serve as a dam, above which the waters soon accumulate and form a lake.
Besides the kinds of lakes which we have enumerated, there are others, which are of rare occurrence and exceptional in their mode of formation; such is the beautiful little lake in Switzerland known as the Märjelen-See, which is formed by the glacier of the Aletsch blocking up the mouth of a tributary valley, and thus forming a wall of ice above which the waters accumulate. This ice-dam breaks away every few years, and allows the complete and rapid drainage of the lake, which often causes great inundations of the valley below. In ancient times a similar ice-dam existed in the valley of Glen Roy, Scotland, as has been shown by Lyell, which, by damming back the waters, formed a lake similar to the Märjelen-See. The waves of this ancient glacial lake chafed and wore its banks, and thus formed terraces at different levels, in the same manner as we often see the little ripples on the pools of water by the wayside cut their soft, muddy banks into terraces, so that, when the water is evaporated by the heat of the sun, their sides are left in a series of little steps; in the same manner, but on a far grander scale, the terraces were formed which are known as the Parallel Roads of Glen Roy, that have gained a world-wide fame both in science and story.
In our own country we sometimes find lakes which owe their existence to the industry of the beavers, who often build their dams in our streams, and sometimes form shallow lakes of considerable extent.
The lakes to which we have devoted the greatest attention, and which are at the same time the most common and the most interesting, are those which fill glacier-worn rock-basins, to which we hope that, our little article will attract the attention of some one who will give us more light on these wonderful pictures, now but imperfectly illuminated.