Popular Science Monthly/Volume 49/June 1896/How the Great Lakes Were Built
|HOW THE GREAT LAKES WERE BUILT.|
By J. W. SPENCER, Ph. D., F. G. S.
THE framing of the continent was a work of great antiquity Upon that foundation the plains and mountains were slowly built, and out of them the valleys have since been carved. The last touch in the completion of the continent has been the making of the lakes. The work is geologically new, and the knowledge of how the lakes were produced is only a few years old—or about a decade and a half since the students have been seriously attempting to disentangle the complex history of the lakes, and from the maze of disorderly speculation to bring together an orderly assemblage of scattered facts and events. To have partially accomplished this effort, it required tedious waiting for the discovery of connecting links which were not always obtained in their logical order; and it was often necessary to learn how to look for them, and so the footsteps had to be retraced many times before the lost trails were recovered. Many new things have been learned in studying the history of the lakes, but the most striking physical changes have been during the period immediately preceding and reaching into modern times.
High Continental Altitude of Former Times.—In very ancient times the lake district formed a great plateau at a considerable altitude above the sea, with some bordering mountains or high lands. Those ancient plains have since been molded into rolling hills and broad valleys, and the mountains have been worn down to almost plains themselves. When mountain ridges are close upon the sea or adjacent low plains, at so slight an elevation that the streams are all sluggish, then, aided by chemical action, the rains and streams are always washing down the elevated lands, first making ravines and valleys, and then enlarging them into broad plains with low hills, for the level has been reached below which the agents of destruction can scarcely affect the slightly elevated lands, as is illustrated in Figs. 1 and 2.
If the plains were always to remain at low altitudes, increasing slightly in elevation in proceeding landward, above the drainage basins, and with the high lands gone or going, the country would become monotonous without any bold reliefs or the possibility of the formation of any deep valleys. While such finished conditions may often occur over large districts, yet such is not the character of very great regions, as Nature seldom allows the completion of these processes, for with the wearing down of one area another quarter is elevated by internal forces, until new plateaus rise in bold relief. The rains gather into streams and cut out gorges and valleys with their forms depending upon the character of the rocks and the length of time that the erosion is in progress. The gorges and valleys grow in length, like the Niagara cañon, until the slopes of the streams become so gentle that they can not deepen their channels any more. After that stage, the only work of the river is to carry away the rocks dissolved or washed from the sides of the valleys, which are thus widened into broad flats, and in their later stages great plains. Such a formation of base planes of erosion is illustrated in Fig. 2.
Applying this process of denudation to the lake region, it becomes evident that the land must have stood high enough above the sea for the rivers to remove the débris washed into them by the millions of little streams—that is to say, the continent was sufficiently high for the excavation of the deepest valleys now beneath the lake waters. As the sea was very distant from the lakes, much farther than now, and the upper lake basins were still farther inland, the altitude of the continent must have been even greater than the depth of the deepest lake basin below the sea level. On the other hand, the slope of the land must have been gentle, with the elevation just high enough to allow the drainage of the valleys, without the production of canons through them, and to enable the streams to widen them into broad, rolling hills and plains scores or hundreds of miles wide. (See Figs. 13 and 14.) The necessary altitude may have varied from time to time, but the duration of the proper conditions was very long. The elevation was not merely high enough to allow the reduction of the table lands to a depth, of five hundred feet below sea level, as is shown in the Ontario basin at this day, and the upper lakes to nearly as great a depth (Lake Erie alone being shallow, but with deep buried channels running through it), but high enough to allow for the necessary slope down the St. Lawrence Valley, not merely to the present gulf, but to the edge of the continent, some eight hundred miles from the present outlet of Lake Ontario. In short, the lake region was elevated more than twelve hundred feet higher than now, which amount itself is indicated by the soundings of the Gulf of St. Lawrence.
While the lake district was thus moderately elevated for long ages, there was an extraordinary altitude of the continent lasting for a comparatively short time, as is seen in the drowned valleys near the coastal margin of the continent; but this elevation did not last long enough for the great canons to be cut back to Lake Ontario.
The lake basins are simply fragments of the old valleys of the St. Lawrence River and its tributaries. These normal but ancient depressions have since been obstructed so as not to allow a free drainage and are thus turned into lake basins with the district further depressed, partially below sea level. The manner in which these things were accomplished is now our theme.
Drowned and Buried Valleys.—Fragments of the ancient valleys which existed in the lake region are discovered by the
Fig. 3.—Section across Lake Ontario from Point Peter to Pultneyville, showing the submerged valley with the bounding escarpment.
soundings in the lakes. Throughout or across some of them great, broad channels, resembling old land valleys, such as are seen in every country, extend, and are bounded on one side or another by the steep slopes of some drowned mountain or escarpment, three hundred or four hundred feet high. Such a valley occurs in the Ontario basin, of which Fig. 3 is a cross-section. An equally good example may be seen in Lake Huron and other lakes. But at the surface these drowned valleys do not appear connected. What do they mean? We shall see.
In the lake district wells have been sunk for considerable depths for water, oil, and gas. On the now level plains the borings have often penetrated great depths of loose rock and dirt deposits before reaching the solid strata, yet, perhaps in proximity, the bedded rocks appear near the surface of the country. These depressions are portions of ancient valleys which have been filled often to depths of five hundred feet, and in some cases probably to a thousand feet. By chains of borings the buried valleys may be traced. Their general course is frequently shown by the surface features; but without the borings their great depth would not be suspected. Thus the Dundas Valley may be taken as an example. It is situated at the head of Lake Ontario and bounded by mountain walls, but is also deeply buried by drift, as shown in Fig. 4. Some of the filled valleys are chiefly occupied with bowlder clay; in other cases with both till and stratified materials; so that their burial is not always alike. Not merely have many of the old valleys been filled with the sweepings of the highlands, but they have been further obscured by the submergence of the district beneath the modern lake waters.
The Course of the Ancient St. Lawrence compared with that of a Modern River.—In Seeking for the explanation of the drowned and buried valleys, discoveries have been made showing that some of them can be connected, and thus is the change in the course of the ancient Laurentian (so named to distinguish the old water way from the modern) River established. The modern St. Lawrence River is characterized by the most remarkable system of lakes in the world. The basins are very deep, and not mere expansions of a river having no noteworthy depth. In the soundings of the lakes and in the buried valleys the connecting links of a great chain of evidence are welded together, showing that the ancient water way did not pursue the present eccentric course, but was an ordinary river valley of large size, yet its course was not everywhere coincident with that of the modern stream.
From the northern portion of the Michigan basin the channel of the ancient Laurentian River is more or less buried beneath
Fig. 5.—Map showing Course of the Ancient St. Lawrence and its Tributaries.
drift and also submerged in extending by way of Mackinac Straits to the Huron basin, across which its course is plainly marked at the foot of an escarpment from three hundred and fifty to four hundred and fifty feet high, and the whole somewhat further submerged. Again it passes through the narrows across the broken mountain ridge into Georgian Bay, where the deep channel skirts the foot of another high escarpment. The old water way across these lakes is shown on map (Fig. 5).
From Georgian Bay the ancient channel is buried below drift deposits to a known depth of seven hundred feet, and almost certainly the drift reaches to a depth of one thousand feet beneath the highest obstructing ridges. The course of the channel passes through Lake Simcoe and enters the Ontario Valley about twenty miles east of Toronto, where the deep trench is made known by the soundings in the lake. The buried valley was broad and comparable to the portions through the lakes. On its western side, but some miles away, it is paralleled by the “mountain” or Niagara escarpment, which reaches to more than fifteen hundred feet above the sea. On the eastern side of the valley the plains are underlaid by solid rock, although these are often covered by drift ridges. Between these rocky boundaries the drift has been penetrated to great depths in many places, yet in the center of the channel the bottom of the filling has never been reached.
Throughout the Ontario Valley the Laurentian Kiver flowed at the foot of a high escarpment now submerged (see Figs. 3, 5, 11). At the eastern end of Lake Ontario the channel turned toward the present outlet of the lake and then down what is now the modern course of the St. Lawrence to the sea. The origin of the barrier across the present outlet of Lake Ontario will be noticed later.
One of the great tributaries was the Huronian River, crossing the southern portion of Michigan, as shown upon the map (Fig. 5), and extending through Saginaw Bay to join the Laurentian River farther north. The Superior outlet is supposed to have crossed the upper peninsula of Michigan and joined the branch draining from the northern end of what is now Lake Michigan.
The now shallow Erie basin was then a portion of a plain across which the ancient Erigan River flowed in a valley two hundred feet or more in depth. One of the buried and submerged tributaries at Cleveland was described by Dr. J. S. Newberry, others by Dr. T. Sterry Hunt, and those near Buffalo by Dr. J. Pohlman. From the Erie basin the Erigan River crossed by a channel about forty miles west of the Niagara River, which did not then exist, and passed down the Dundas Valley (Fig. 1) into the head of the Ontario basin, and farther eastward joined the Laurentian River (Fig. 5). All the features of the ancient and drowned valleys are those characterizing ancient topography; that is to say, without the boldnesses and abruptnesses of youthful features and without great waterfalls, although rapids must have existed.
Reversals of Ancient Rivers in Pennsylvania and New York.—The great changes in the water way of the Laurentian River had their counterpart in the highlands to the south of the lakes where the ancient streams were tributary to the Laurentian, in place of to the modern Ohio and Susquehanna Rivers. Among the more notable changes, the Alleghany (discovered by Mr. J. F. Carll) flowed to the Erie basin, as did also the upper Ohio (suggested by the writer, and further explained by Dr. P. Max Foshay and Mr. F. Leverett). These and other streams now reversed were tributaries of the Erigan River. In New York the upper Susquehanna and some tributaries descended through the “finger lakes” to the Laurentian River as it passed through the Ontario basin. All the old streams coming from the highlands south of the lake basins flowed through broad, V-shaped valleys, of ancient form, although of considerable depth. These valleys became filled with drift which turned the waters of the Ohio and Susquehanna Rivers to the south. This reversal in the drainage has been further assisted by the recent northward tilting of the land, to be explained later.
How the Ancient Valleys were Obstructed.—The Laurentian Valley and its tributaries were completed before the ice age. Indeed, the high elevation of the continent during the culmination of that period did not last long enough for the deepening of the channels of the main valley, as they could scarcely be affected until a great cañon had been excavated from the continental margin for eight hundred miles to the Ontario basin, which was not the case.
As for the modification of the ancient topography by glacial action, it could have been only slight, and does not appear to have been more than the sweeping of loose geological dust into the valleys, or on to the highlands to the south. The absence of any great plow is shown by the direction of the scratches on the rock surfaces, which lines are everywhere at great angles to the walls and sides of the lake basins, and nowhere parallel to them, as must have been the case if the valleys had been plowed out by ice in any form. This crucial test and many other features had not been applied fifteen years ago, when the writer commenced these researches. Now this fancy of closet geologists has vanished before the application of facts. Yet the work of the ice age was complex, and it is immaterial to the study of the lakes how it was performed. In one way only does it come within the limit of this subject, and that is in the phenomena of the ancient valleys being filled by drift, whether stratified or not. It was this filling of the old channels with drift that closed the ancient drainage of the Laurentian Valley, which at a later date gave rise to the lake basins. But the barriers of the lakes were further exaggerated by the tilting of the land, which will be noted later. The closing of the old water ways ends the history of the ancient Laurentian River. When the river began to flow again, the lacustrine epoch was established.
Submergence and Re-elevation of the Lake District.—After the obstruction of the valleys with drift, the whole lake region was submerged; but this depression is best treated of in the rising of the land which has brought the evidence to view.
As the waves beat upon the shores, the beaches, terraces, sea cliffs, sea caves, etc, become characteristic of coast lines. When the water sinks or the land rises, the various stages of the deserted shore remain and record the recession of the waves. The preservation of the old coast lines is often so perfect as to furnish easy identification of their character, as may be seen in Figs. 6 and 7, which represent sections of old beaches. Behind them lagoons often occur, and the entrances of bays are often barred across with beaches, as is shown in Fig. 8.
The resemblance between modern and ancient shores is further illustrated in Figs. 9 and 10, where great bowlder pavements are shown, marking the modern and deserted strands. In valleys, although broad, beaches do not occur, but they are replaced by terraces.
The deserted beaches and terraces in the lake region occur at all altitudes, where such could be preserved. But in order to find the remains of old shore lines continuous over long distances, it is necessary to descend to the levels where the water was more or less confined in the western and central portions of the lake district, for until a recent date there were no barriers toward the northeast sufficiently high to hold the waters of the lakes above tide. In the prenatal lake epoch, such an embayment called for in the last sentence covered two hundred thousand square miles of the lake region, and has been named Warren Gulf. As there was no land barrier to this gulf in the northeastern direction, and for the exclusion of the sea, of which there is no evidence, some have supposed that these waters of the lake district were held at high altitudes by glacial dams for long ages. This hypothesis, notably advocated by Prof. G. K. Gilbert, although based upon negative evidence to explain some difficulties, which are also applicable to southern regions even within the tropics, has retarded the researches into the history of the lakes, and had it been followed would have prevented the discovery of some of the greatest changes in the geography of northeastern America. Indeed, the greater portion of the modern elevation of the region has been
Fig. 9.—Modern Bowlder Pavement on Georgian Bay, east of the end of Blue Mountains of Collingwood, Ontario.
recorded in the tilting of the beach lines recognized by all. But the period of glacial conditions was prior to the lacustrine. which commenced with the submergence of the lake district to sea level.
The Tilting of the Ancient Shore Lines.—The shore lines of ponds, lakes, or seas are alike water levels. In this respect the
Fig. 10.—Ancient Bowlder Pavement of Algonquin Beach, whose crest rises one hundred and eighty-seven feet above Georgian Bay, upon the northeast side of Blue Mountains of Collingwood, Ontario.
elevated coast lines present a striking difference from those now being formed, for the abandoned strands are everywhere tilted toward the northeast (see Figs. 11, 12, 15).
The tilted beach represents the deformation of the Algonquin beach. At the head of Lake Erie the deformation of the old water planes is not over a very few inches in a mile, while it
Fig. 11.—Section of the Lake District from the Highlands of New York to those or the Laurentian Hills North of Lake Huron, along a Line passing through Buffalo and Lake Nipissing. Length of section, four hundred miles; heights given in feet; t is a ridge of drift north of Lake Ontario. The tilted beach represents the Algonquin plain deformed.
increases toward the northeast, so that it amounts to four feet per mile northeast of Lake Huron, and seven feet per mile near the outlet of Lake Ontario and north of the Adirondack Mountains, to which locality the writer himself has traced the deserted shores all the way from the head of Lake Michigan. This gentle deformation of the surface of the country when carried over such long distances gives rise to the great physical reliefs of the mountain regions of the north and east, which were much lower before the lake epoch than now, as is apparent if the tilting be straightened out, as may be seen in the sections given. Furthermore, the character of the river courses at the surface of the country north of the lake regions indicates that even in the ice age the relatively high reliefs north of the lakes did not obtain.
The Gulf Epoch.—In the re-elevation of the lake district after the post-glacial submergence, when the continent was high enough to partially inclose a large gulf, already referred to, to which the name of Warren Water has been given, or more correctly Gulf, there were several water connections through the valleys to the south and west of it. Its last stage as one body of water is marked by the Forest beach, as shown on the map (Fig. 13). Upon the further rise of the land the surface of Warren Gulf fell below the level of the Forest beach for a depth of a hundred and fifty feet. The movement was gradual, without striking
Fig. 12.—Section from the Michigan Highlands back of Alpina to the Laurentian Highlands beyond Lake Nipissing. Length of section, three hundred and thirty miles. The tilted beach represents the deformation of the Algonquin beach.
interruptions, for there is no intervening strand, as the water did not remain at one level long enough to leave beaches. Warren water was now broken up into two great gulfs; the one called the Algonquin, occupying the basins of Lakes Superior, Michigan, and Huron, and opening to the northeast, through the strait to the Ottawa Valley, as is shown on the map (Fig. 13); the other was the Lundy Gulf, occupying most of Erie basin, and extending over the Ontario Valley at a great height. In the region of Nipissing Strait the two gulfs united.
From the deserted shores of these waters, which are now tilted up so much to the northeast, free communication between the lake region and Hudson Bay is indicated, for the Laurentian highlands are now rarely more than fifteen hundred feet above the sea and commonly less. Of course, there was free communication to the Atlantic Ocean by way of the St. Lawrence and also southward of the Adirondacks. These various characteristics and changing conditions would require a volume to tell all that we know about them, and this has been partly done in Duration of
Fig. 13.—Map of Warren Water. bounded by Forest Beach, and its Successors. Surveyed shores represented by solid lines, partly surveyed, by broken lines; modern lakes, by dotted.
Niagara Falls and the History of the Great Lakes, by the writer, and published by the Commissioners of Niagara Falls Reservation, under the presidency of Hon. Andrew H. Green.
The Birth of the Great Lakes.—This marks only an episode in the chain of events which are being described, when the waters fell three hundred feet below the Algonquin and Lundy planes. Although the subsidence of the waters was not continuous and left some evidences of temporary pauses, yet the long rest was not reached until they had sunk to the level of the Iroquois beach. By this time the land had risen so high, and, as there were no sufficient barriers, the upper lakes sunk far within their present basins, as is shown on the map (Fig. 14). Still, the waters of these upper lakes discharged by way of the narrow Nipissing Strait.
Lake Erie at this time had its birth, but then it was a very small body of water, as shown on the map. The Niagara district was then covered with a strait expanded into a lakelet, and afterward a river at first without a fall. In the further sinking of the water to the Iroquois level the falls of Niagara commenced their history, and then there was a comparatively long rest, but Ontario was still a gulf, as shown on the map (Fig. 14).
The plane of the Iroquois shore was at identically or nearly the same level as the Nipissing beach (of Taylor) at the outlet of Lake Huron by way of the Ottawa Valley. It is not apparent, and it is theoretically improbable, that the Nipissing River was characterized by more than a gentle slope, for by the time that the land rose high enough to produce a rapid river the water of the upper lakes had changed their outlet into Lake Erie. The proof of these changes rests in the tilting of the beaches, which aggregates several hundred feet (see Fig. 12).
Barrier to Lake Ontario.—Still, the land has continued to rise, and the deformation since the Iroquois episode amounts to more than before that date. The tilting at the head of Lake Ontario becomes an absolute elevation above the sea, amounting to three hundred and sixty-three feet, and at the northeastern corner of the Adirondacks it is fifteen hundred feet, while near the outlet of Lake Ontario it is seven hundred and thirty feet. This warping of the continent is illustrated in Fig. 15, and to it is due the barrier (to a large extent) which retains the waters of the Ontario basin at an elevation of two hundred and forty-seven feet above the sea.
Sinking and Subsequent Growth of the Modern Lakes and Change of Outlets.—The continuing elevation of the continent lowered all the waters of the lake region until their levels depended* upon the rims of the lake basins from which the waters overflowed. The upper lakes were the first to sink far within
Fig. 14.—Map of the Early Lakes. Broken shading represents extension of the early lake epoch; solid shading, a lower stage of Iroquois Gulf before the birth of Lake Ontario; modern lakes, by dotted lines.
their basins (see Fig. 14), but later even the Iroquois Gulf was contracted so as not to occupy even the head of the present Ontario basin (see Fig. 14).
The great deformation of the whole region since the close of the Iroquois episode has from that day to this been slowly raising the northeastern rims of the lake basins so as to cause them to flood more and more the lowlands and valleys at their southwestern extremities, and even to raise the waters so high as to cover some of the deserted shores in those directions. At the same time the waters are leaving their old margins at their northeastern ends, as shown on the map (Fig. 14).
The changes have not been quite simultaneous in the different basins, as the heights of the lake barriers and the rate of terrestrial movements have not been uniform. Thus, in terms of Niagara Falls, it is estimated that the Iroquois Gulf sank below the Iroquois plane about fourteen thousand years ago; but that the
Fig. 15.—Section showing the Tilting of the Iroquois Beach South of Lake Ontario and the St. Lawrence River as far as the Northeast Corner of the Adirondacks.
waters of Lake Huron, which had been emptying by way of the Nipissing Strait for twenty-four thousand years, were turned into Lake Erie only eight thousand years ago. Again, after the waters of the Ontario basin had sunk much below the present western margin of the lakes, they were rising again to near their present height only some three thousand years ago.
Of the absolute amount of rise of the continent we do not know, for the axis of uplift has not been ascertained, but it is evidently in the interior of the continent. The differential rate of elevation varies, being about a foot and a quarter a century in the Niagara district, two feet northeast of Lake Huron, and nearly four feet north of the Adirondacks.
The Future Drainage of the Upper Lakes into the Mississippi River.—With the land rising as at present, it will be only a matter of time until the northeastern rim of Lake Erie will be so high that the drainage must turn into Lake Huron, and thence by way of Lake Michigan and the Chicago Canal into the Mississippi, and Niagara Falls will then end their life history. Some fifteen hundred years ago there was a barrier about a mile north of the present site of the falls that had risen so high in the general regional uplift as to actually cause some of the waters of the upper lakes to overflow where the Chicago Canal is now being built; but, owing to the peculiar buried valley just behind this ridge crossing Niagara River, when the falls had passed the barrier, before the change of outlet of the upper lakes from the Niagara to the Mississippi was completed, the upper lakes were rapidly lowered, and this re-established the life of the Niagara for some time longer. Upon the basis of calculations made it would appear that the change of outlet for the upper lakes from the Niagara to the Mississippi will not be more than another five thousand years hence—to us living a matter of indifference, but only showing how the present days are simply passing events in the history of the lakes. In the meanwhile the waters of Lake Ontario will more and more flood the head of its basin. However, the end of the lakes is so far removed in geological time that, until such great changes in the configuration of the land shall have obtained of which we have no prophetic vision, the lakes will continue to exist.
Age of the Great Lakes.—When the rate of movement of the earth's crust, as determined in the history of Niagara Falls, is applied to the deserted strands of Warren Water and its successsors, it is estimated that since the commencement of the Warren epoch fifty thousand or sixty thousand years have elapsed. This estimate, although based upon the most analytical knowledge obtainable, can, after all, be regarded as only approximate. The time ratio tells us that the lakes are still youthful; although in terms of solar years very old, yet perhaps not older than the human race.
The vicissitudes between the end of the ice age proper and the birth of Warren Water are too little known to enter into any proportional division of time, except that the ice age culminated prior to or at the date of the closing of the old valleys with drift.
The Great Lakes are the most striking feature of the eastern part of the continent, yet what we know of their history has been mostly discovered within the last few years. In this sketch only some of the more important and generalized results have been given. Many of the observations are beyond doubt, but there is plenty of room for students to add to our knowledge and correct our imperfect work. While the history of the lakes can be told with considerable certainty, the attempt at computing their age in terms of solar years has the same fascination, although not so extravagant, as the speculations concerning the antiquity of the earth itself, as the former question probably comes within the human period.
- ↑ The surveys of the deserted shore lines of the lakes have been mostly made by Messrs J. W. Spencer, G. K. Gilbert, A. C. Lawson, and F. B. Taylor.