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Nature's Miracles: Familiar Talks on Science, Vol. 1.

<h2>CHAPTER XXVII.</h2> <h3>GLACIAL AND PREGLACIAL LAKES AND RIVERS.</h3> Since the recession of the ice, preglacial lakes have been filled up and are now dry land, and river beds have been changed so that new channels have been cut and new lakes have been formed. Even the imagination, that wonderful architect, with all its tendencies to exaggeration, palls in its attempt to give expression in measured quantities to the mighty power exerted by the great glacier or combination of glaciers that existed in comparatively recent times. I say recent times, because even 10,000 years is only a mere point of time when compared with the actual age of our globe. Some years ago, in company with Dr. Wright, author of the "Ice Age in North America," I visited Devil's Lake near Baraboo, Wis. At this point are striking evidences of the work of the ice age. Before the glacial period the Wisconsin River made a detour some miles west of its present channel through the high hills in the region of Baraboo. The hills on each side of Devil's Lake are very precipitous and are formed almost entirely of rocks. The river at that point passed between two of these hills. When the ice flowed down it surrounded these hills, yet did not sweep over their tops, but left great piles of glacial drift, both at the points where the river channel entered the hills and where it emerges from them. The channel between the hills was protected and not filled with the débris. Therefore a deep basin was left, which is kept filled by the watershed furnished by the surrounding hills. This lake recedes many feet during the summer, but it is again filled up by the rains and snows of winter. There is no considerable stream either flowing into or out from it. It is a lake formed by the glaciers, but in a different way from those in the gravel deposits at other parts of southern Wisconsin and northern Illinois. There are hundreds and perhaps thousands of lakes that have been formed in one way or another through the power of glacial action. These smaller inland lakes, so many of which are seen in northern Illinois, southern Wisconsin, and Minnesota, are due almost entirely to the great deposits of glacial drift that have been transported with the ice. Wherever these "kettle holes" are found large bodies of ice have become anchored, while the ice behind it has carried the drift until it is covered over and piled up at the sides. When these ice mountains melted away depressions were left which in some cases have resulted in lakes, and in others simply dry kettle holes. This process has been hinted at in a former chapter, but we give it here as one of the kinds of lakes formed during the glacial period. They are found everywhere that glacial action has prevailed. They are found in great abundance in some parts of New England on the margin of the terminal moraine. These lakes, however, are comparatively insignificant as compared with the great inland seas like Lake Superior and Lake Michigan, that undoubtedly owe their origin largely to the ice age. There are other factors, however, that enter into the formation of the great chain of lakes on the northern boundary of the United States besides those mentioned, that have brought into existence the smaller inland lakes. Glacial lakes may be divided into three classes. Those found in the "kettle holes" of the terminal or medial moraines, and those that are formed by the deposition of the glacial drift, as, for instance, Devil's Lake, and those that are caused by ice forming dams across the valley of a river that lasted only during the ice age. In some lakes of the second class erosion undoubtedly entered into their formation as well as the piling up of glacial drift. In order, however, that we may understand more fully the formation of these greater lakes it will be necessary for us to go back and examine the conditions that seem to have existed before the glacial period. It is a fact well known that continents have periods of elevation and depression. There is abundant evidence that the northern portion of the North American continent was elevated to a much higher level in preglacial times than it occupies now. This is evidenced in very many ways by sounding the depths of old river beds now filled with glacial débris. The old beds show unmistakable evidences of having been worn down to their present level by the action of running water. They also prove to be many feet below the present sea-level. This fact seems to be sufficient to prove the theory of a higher elevation of the North American continent in preglacial times. It should be said here that undoubtedly the constant filling up of the ocean with the drift carried down by the rivers has somewhat raised its level, but hardly to the extent indicated by the old river beds. The question naturally arises, Where did all the dirt come from to fill up these great river beds and change the whole topography of the northern half of the continent? Dr. Wright estimates that there is not less than 1,000,000 square miles of territory in North America covered with glacial débris to an average depth of 50 feet. Of course, the depth varies in different places from a few inches to several hundred feet. Of the carrying power of these great glaciers we will speak more fully in a future chapter. In preglacial times the watershed of the Mississippi and of the great rivers east of the Alleghany Mountains, the Susquehanna and Hudson, extended probably farther north than it does to-day. The larger portion of the drainage area that now finds an outlet through the River St. Lawrence at one time undoubtedly drained off through the Mississippi Valley into the Gulf and the Valley of the Mohawk into that of the Hudson. It is supposed by those who have made this branch of geology a study that prior to the glacial period a river flowed down through Lake Superior, which connected with Lake Michigan at a point near its present outlet at Sault Ste. Marie, the channel of the river passing down through what is now the bottom of Lake Michigan, which had an outlet at the head of the lake near Chicago and flowed off into the Mississippi River. All of the lake bottoms of this great chain, with the exception of Lake Erie, are now below sea-level. The reason for this exception will appear further on. Before the ice age there was supposed to be no connection between Lake Michigan and Lake Huron, as there is now, through the Straits of Mackinac. Another preglacial river had its rise in the region of Lake Huron and flowed through an old river bed extending from the Georgian Bay in a southeasterly direction through the province of Ontario, and emptied into the present Lake Ontario. From Lake Ontario there is an old river bed running through the Valley of the Mohawk which empties into the Hudson at Troy. Neither of these two rivers, having their sources in the north, found an outlet through the present St. Lawrence River. During the time of the glacial period there is evidence that there was more than one center of snow and ice accumulation and each of these great centers probably had several subcenters. This theory has color given to it by the directions of movement shown by the glacial drift. The rounded appearance of bowlders was caused by the grinding action of the ice. These bowlders, when they were first torn from their rocky beds by the irresistible power of ice pressure, were rough and jagged in shape, the same as any rock would be, torn from a quarry by a blast. They have been smoothed and rounded by rubbing against the moving ice and against each other in the progress of their long journey from their original homes. Where their home was the geologist can immediately tell upon examination. It is only necessary then to examine the bowlders of any particular locality to determine the direction of the ice flow at that point. There seem to have existed centers of ice accumulation to the north of all of the great lakes. And when they had grown to a sufficient height they joined at their edges, making one grand glacier, the movements of which were the resultant of the combined pressure exerted by these great centers of power, so that all of North America north of the line of the terminal moraine, with the exception of a small area (heretofore noted) chiefly in Wisconsin, became covered with one vast sheet of ice. The glacier north of Lake Superior widened out the old river bed by a process of erosion to its present width. There may have existed something of a lake in preglacial times, through which the river ran, but it undoubtedly owes its present width to the grinding action of the irresistible icebergs and the piling up of débris on the shores. The river bed was filled up by a glacial drift at the point of its present outlet until the lake was raised in its level much higher than that of Lake Michigan. Another glacier plowed down through Lake Michigan, widening it out to its present dimensions, while the glacial drift was deposited at what is now the head of the lake, filling up the old outlet and thus making a great dam. The damming up of these great water courses was another cause for increasing the width of these lakes. In a similar way Lake Erie was formed. It is supposed, however, that this lake is entirely the product of glacial action, as there is no evidence of an old river bed in its bottom; besides, it is much shallower than the other lakes. The same action that formed Lake Erie filled up the old river bed running through the province of Ontario, so that when the ice receded Lake Erie became the new channel for the old river. The same process filled up the Valley of the Mohawk to more than 100 feet in depth and also raised the Valley of the Hudson. This caused the new channel to be made through the Niagara River and a new route to the ocean for the drainage of all the chain of lakes through the St. Lawrence. It will be seen that the bottoms of all of these great lakes to a certain extent were worn out by the action of running water, except Erie. The great glaciers widened them out, and in the case of Lake Erie scooped it out. At the same time it built great dams across the outlets which raised the surface of the water to a much higher level and caused them to form new outlets, thus changing the whole face of the country over which the ice drifted. The glaciated region of North America is among the most productive in the world, and in many respects presents a most pleasing landscape. Other lakes besides these mentioned have been formed during the ice period through blocking the course of a river by the ice itself. Dr. Wright, during the time he traced out the line of the terminal moraine, discovered that the ice sheet crossed the Ohio River at a point near Cincinnati, where there is a great bend to the northward in the river. With the exception of this point and perhaps another point below, the edge of the great ice sheet kept a little north of the Ohio River. At this point, however, the ice seems to have filled the valley from hill to hill, which very naturally would form a great dam or lake in the Ohio Valley. Of course such a lake could not be permanent, because, when the ice melted away, it again opened the channel and allowed the water to flow off. Some years before this discovery was made there were terraces found along the banks of the Ohio River and its tributaries that had been the subject of much speculation. It is well known that by the action of water from rainfall, earth, gravel, and other débris will wash down the side of a hill or mountain until it strikes a water level, and there it will build out a terrace near the level of the water surface. The width of these terraces will be determined by the time the water has stood at that level and the extent and nature of the soil from which the débris comes. The evidences that are cited, pro and con, would fill a small volume, but it is sufficient to say here that the sum of the evidence goes to show that there was an ice dam formed at a point near Cincinnati and that it was maintained for a considerable period of time. Terraces were formed running up the Ohio and its tributaries corresponding to the level that the water must have risen to if the valley were filled up with ice. These facts, taken with the greater fact that the ice sheet actually did cross the Ohio Valley into Kentucky, as is shown by the terminal moraine, seems to prove conclusively the existence of such a lake during the period that the ice rested at its extreme limit. The fact that in some places successive terraces are found does not disprove the theory, because it is more than likely that when the ice receded it did so in successive stages, remaining at different positions for a considerable length of time. There is abundant proof of this in the successive moraines and also in the formation of successive terraces. Some of these terraces could have been formed from other causes. It does not require any great stretch of the imagination to understand how numerous lakes, much larger than any at the present day, may have extended over large portions of the West and Northwest during the period that the ice was receding. The ice did not stand with an even thickness over the surface of the glaciated area, but at some points it moved down in great lobes, which marked the lines of greatest pressure as well as the greatest accumulation. As the ice melted away, the thick bodies of ice might be many, many years in melting, and they might block the outlet to a very extensive drainage area and thus form a great inland sea from the vast amounts of water that would come from the melting ice. All of the region about Winnipeg, in the Red River country, covering great areas of hundreds of miles in extent, is a level plain only lacking the coloring to give to one passing through it the effect of a great unruffled sea. There is no doubt but that all of this region was the bottom of a great lake at some period when the ice was receding. And this accounts for the great depth of black soil that we find in this and other regions. The soil was a water deposit, such as may be found in the bottom of any shallow lake or pond to-day, and thus many thousand years ago provision was made for the fertile areas which to-day are feeding the world with wheat. We can imagine that during this period the water that flowed off through the great Mississippi must have been of enormous volume as compared to the present time. A large portion of the delta of the Mississippi which now is a part of the States of Louisiana and Mississippi was carried down during the ice-melting period. Dr. Wright—as we have before stated—has estimated that there are a million square miles of country that has been covered to an average depth of fifty feet with glacial drift. A very large amount of the earth that was spread over the northern portion of the United States by leveling down hills and mountains in the northern country and scooping out the great lakes has been carried much farther than to the margin of the ice sheet. And I have no doubt but that a great portion of Louisiana and western Mississippi is made of earth carried down largely during the period of melting ice and deposited in this great delta. Imagine the effect that would be produced by the giving way of an ice dam or a great number of them at different periods, that would allow a body of water as large or larger than Lake Michigan to be drained off in a comparatively short time. When we think of it in this light the great delta of the Mississippi is easily accounted for. There are evidences of a great lake in the Red River country of the Northwest that is much larger than any of our greatest lakes. The shores of this lake—the bed of which is now dry land and the heart of a great agricultural region—are well defined and have been surveyed and mapped out. When this great body of water was released it was to the northward. For this reason it was undoubtedly held for a much longer time than some of the lakes to the southward where the ice melted sooner. <hr style="width: 65%;" /> <div style="break-after:column;"></div>

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