Chapter 15

[DX]Onpage 106and sequel I have summarised the reasons which lead me to discard the Inter-Glacial epoch, and to look upon the whole Glacial period as constituting a grand unity with minor episodes. It does not yet seem to me that the duality of the period is proved. On the contrary, Mr. Kendall’s chapter on the Glacial phenomena of Great Britain strongly confirms my view.

On the part of many the theory here provisionally adopted will be regarded with disfavour by reason of a disinclination to supposing any great recent changes of level in the continental areas. So firmly established do the continents appear to be, that it seems like invoking an inordinate display of power to have them exalted for the sake of producing a Glacial period. Due reflection, however, will make it evident that within certain limits the continents are exceedingly unstable, and that they have displayed this instability to as great an extent in recentgeological times as they have done in any previous geological periods. When one reflects, also, upon the size of the earth, a continental elevation of 3,000 or 4,000 feet upon a globe whose diameter is more than 40,000,000 feet is an insignificant trifle. On a globe one foot in diameter it would be represented by a protuberance of barely one thousandth of an inch. A corresponding wrinkle upon a large apple would require a magnifying-glass for its detection. Moreover, the activity of existing volcanoes, the immense outflows of lava which have taken place in the later geological periods, together with the uniform increase of heat as we penetrate to deeper strata in the crust of the earth—all point to a condition of the earth’s interior that would make the elevations of land which we have invoked for the production of the Glacial period easily credible. Physicists do not, indeed, now hold to the entire fluidity of the earth’s interior, but rather to a solid centre, where gravity overcomes the expansive power of heat, and maintains solidity even when the heat is intense. But between the cooling crust of the earth’s exterior and a central solid core there is now believed to be a film where the influences of heat and of the pressure of gravity are approximately balanced, and the space is occupied by a half-melted or viscous magma, capable of yielding to a slow pressure, and of moving in response to it from one portion of the enclosed space to another where the pressure is for any cause relieved.

As a result of prolonged enquiries respecting the nature of the forces at work both in the interior and upon the exterior of the earth, and of a careful study of the successive changes marking the geological period, we are led to believe that the continental elevations necessary to produce the phenomena of the Glacial period are not only entirely possible but easily credible, and in analogy with the natural progress of geological history. In the first place, it is easy to see that two causes are in operationto produce a contraction of the earth’s volume and a shortening of its diameter. Heat is constantly being abstracted from the earth by conduction and radiation, but perhaps to a greater extent through ceaseless volcanic eruptions which at times are of enormous extent. It requires but a moment’s thought to see that contraction of the volume of the earth’s interior means that the hardened exterior crust must adjust itself by wrinkles and folds. For a long period this adjustment might show itself principally in gentle swells, lifting portions of the continents to a higher level, accompanied by corresponding subsidence in other places. This gradually accumulating strain would at length be relieved along some line of special weakness in the crust by that folding process which has pushed up the great mountain systems of the world.

Careful study of the principal mountain systems shows that all the highest of them are of late geological origin. Indeed, the latter part of the Tertiary period has been the great mountain-building epoch in the earth’s history. The principal part of the elevation of the Andes and the Rocky Mountains has taken place since the middle of the Tertiary period. In Europe there is indubitable evidence that the Pyrenees have been elevated eleven thousand feet during the same period, and that the western Alps have been elevated thirteen thousand feet in the same time. The Carpathians, the western Caucasus, and the Himalayas likewise bear explicit evidence to the fact that a very considerable portion of their elevation, amounting to many thousand feet, has been effected since the middle of the Tertiary period, while a considerable portion of this elevation of the chiefest mountain systems of the world has occurred in what would be called post-Tertiary time—that is, has been coincident with a portion of the Glacial period.

The Glacial period, however, we suppose to have been brought about, not by the specific plications in the earth’scrust which have produced the mountain-chains, but by the gentler swells of larger continental areas whose strain was at last relieved by the folding and mashing together of the strata along the lines of weakness now occupied by the mountain systems. The formation of the mountains seems to have relieved the accumulating strain connected with the continental elevations, and to have brought about a subsequent subsidence.

Doubtless, also, correlated subsidences and elevations of the earth’s crust have been aided by the transfer of the sediment from continental to oceanic areas, and, as already suggested, during the Glacial period by the transfer of water evaporated from the surface of the ocean to the ice-fields of the glaciated area. For example, present erosive agencies are lowering the level of the whole Mississippi basin from the Alleghanies to the Rocky Mountains at the rate of a foot in five thousand years. All this sediment removed is being transferred to the ocean-bed. Present agencies, therefore, if not counteracted, would remove the whole continent of America (whose average elevation above the sea is only 748 feet) in less than four million years; while the great rivers which descend in all directions from the central plateau of Asia are transferring sediment to the ocean from two to four times as fast as the Mississippi is, and the Po is transferring it from the Alps to the Adriatic fully seven times as fast as the Mississippi is from its basin to the Gulf of Mexico. This rapid transfer of sediment from the continents to the ocean is producing effects in disturbing the present equilibrium of the earth’s crust, which are too complicated for us fully to calculate; but it is by no means improbable that when accumulating for a considerable length of time, the ultimate results may be very marked and perhaps sudden in their appearance.

The same may also be said of the accumulation of ice during the Glacial period. The glaciated areas of NorthAmerica and Europe combined comprise about six million square miles. At a moderate estimate, the ice was three-quarters of a mile deep. Here, therefore, there would be between four and five million cubic miles of water, which had first relieved the ocean-beds of the pressure of its weight, and then concentrated its force over the elevated areas of the northern hemisphere. This disturbance of the equilibrium, by the known transfer of force from one part of the earth’s crust to another, certainly gives much plausibility to the theory of Jamieson, Winchell, Le Conte, and Upham, that the Glacial period partly contained in itself its own cure, and by the weight of its accumulated weight of ice helped to produce that depression over the glaciated area which at length rendered the accumulation of ice there impossible.

This general view of the known causes in operation during the Glacial period will go far towards answering an objection that has probably before this presented itself to the reader’s mind. It seems clear that the Glacial period in the southern hemisphere has been nearly contemporaneous with that of the northern. The Glacial period proper of the southern hemisphere is long since passed. The existing glaciers of New Zealand, of the southern portion of the Andes Mountains, and of the Himalaya Mountains are but remnants of those of former days. In the light of the considerations just presented, it would not seem improbable that the same causes should produce these similar effects in the northern and the southern hemisphere contemporaneously. At any rate, it would not seem altogether unlikely that the pressure of ice during the climax of the Glacial period upon the northern hemisphere (which, as we have seen, there is reason to believe aided in the depression of the continent to below its present level in the latter part of the Glacial period) should have contributed towards the elevation of mountains in other parts of the world, and so to thetemporary enlargement of the glaciers about their summits.

Nor are we wholly without evidence that these readjustments of land-level which have been carried on so Vigorously since the middle of the Tertiary period are still going on with considerable though doubtless with diminished rapidity. There has been a re-elevation of the land in North America since the Glacial period amounting to 230 feet upon the coast of Maine, 500 feet in the vicinity of Montreal, from 1,000 to 1,600 feet in the extreme northern part of the continent, and in Scandinavia to the extent of 600 feet. In portions of Scandinavia the land is now rising at the rate of three feet in a century. Other indications of even the present instability of the earth’s surface occur in numbers too numerous to mention.[DY]

[DY]For a convincing presentation of the views here outlined, together with abundant references to literature, see Mr. Warren Upham’s Appendix to the author’s Ice Age in North America.

But, while we are increasingly confident that the main causes of the Glacial period have been changes in the relative relation of land-levels connected with diversion of oceanic currents, it is by no means impossible, as Wallace[DZ]and others have suggested, that these were combined with the astronomical causes urged by Drs. Croll and Geikie. By some this combination is thought to be the more probable, because of the extreme recentness of the close of the Glacial period, as shown by the evidence which will be presented in the following chapter. The continuance of glaciers in the highlands of Canada, down to within a few thousand years of the present time, coincides in a remarkable manner with the last occurrence of the conditions favourable to glaciation upon Mr. Croll’s theory, which took place about eleven thousand years ago.

[DZ]See Island Life, chapters viii and ix.

CHAPTER X.

THE DATE OF THE GLACIAL PERIOD.

In approaching the subject of glacial chronology, we are compelled to recognise at the outset the approximate character of all our calculations. Still, we shall find that there are pretty well-defined limits of time beyond which it is not reasonable to place the date of the close of the Glacial period; and, where exact figures cannot be determined, it may yet be of great interest and importance to know something near the limits within which our speculations must range.

For many years past Mr. Croll’s astronomical theory as to the cause of the Glacial period has been considered in certain circles as so nearly established that it has been adopted by them as a chronological table in which to insert a series of supposed successive Glacial epochs which are thought to have characterised not merely the Quaternary epoch but all preceding geological eras. What we have already said, however, respecting the weakness of Mr. Croll’s theory is probably sufficient to discredit it as a chronological apparatus. We will therefore turn immediately to the more tangible evidences bearing upon the subject.

The data directly relating to the length of time which separates the present from the Glacial period are mainly connected with two classes of facts:

1. The amount of erosion which has been accomplished by the river systems since the Glacial period; and 2. Theamount of sedimentation which has taken place in lakes and kettle-holes. We will consider first the evidence from erosion.

Fig. 103.—Diagram of eccentricity and precession: Abscissa represents time and ordinates, degrees of eccentricity and also of cold. The dark and light shades show the warmer and colder winters, and therefore indicate each 10,500 years, the whole representing a period of 300,000 years.

The gorge below Niagara Falls affords an important chronometer for measuring the time which has elapsed since a certain stage in the recession of the great North American ice-sheet. As already shown, the present Niagara River is purely a post-glacial line of drainage;[EA]the preglacial outlet to Lake Erie having been filled up by glacial deposits, so that, on the recession of the ice, the lowest level between Lake Erie and Lake Ontario was in the line of the trough of the present outlet. But, from what has already been said, it also appears that the Niagara River did not begin to flow until considerably after the ice-front had withdrawn from the escarpment at Queenston, where the river now emerges from its cañon to the low shelf which borders Lake Ontario. For a considerable period afterwards the ice continued to block up the easterly and northerly outlets through the valleys of the Mohawk and of the St. Lawrence, and held the water in front of the ice up to the level of the passes leading into the Mississippi Valley. Niagara River, of course, was not born until these ice-barriers on the east and northeast melted away sufficiently to allow the drainage to take its natural course.

[EA]See above,p. 200et seq.

Fig. 104.—Map of the Niagara River below the falls, showing the buried channel from the whirlpool to St. Davids. Small streams,a,b,c, fall into the main gorge over a rocky escarpment. No rock appears in the channel atd, but the rocky escarpment reappears ate.

Of these barriers, that across the Mohawk Valley doubtless gave way first. This would allow the confluent waters of this great glacial lake to fall down to the level of the old outlet from the basin of Lake Ontario into the Mohawk Valley, in the vicinity of Home, N. Y. The moment, however, that the water had fallen to this level, the plunging torrents of Niagara would begin their work; and the gorge extending from Queenston up to the present falls is the work done by this great river since that point of time in the Glacial period when the ice-barrier across the Mohawk Valley broke away.

The problem is therefore a simple one. Considering the length of this gorge as the dividend, the object is to find the rate of annual recession; this will be the divisor. The quotient will be the number of years which have elapsed since the ice first melted away from the Mohawk Valley. We are favoured in our calculation by the simplicity of the geologic arrangement.

The strata at Niagara dip slightly to the south, but not enough to make any serious disturbance in the problem. That at the surface, over which the water now plunges, consists of hard limestone, seventy or eighty feet in thickness, and this is continuous from the falls to the face of the escarpment at Queenston, where the river emerges from the gorge. Immediately underneath this hard superficial stratum there is a stratum of soft rock, of about the same thickness, which disintegrates readily. As a consequence, the plunging water continually undermines the hard stratum at the surface, and prepares the way for it to fall down, from time to time, in huge blocks,which are, in turn, ground to powder by the constant commotion in which they are kept, and thus the channel is cleared ofdébris.

Fig. 105.—Section of strata along the Niagara gorge from the falls to the lake: 1, 3, strata of hard rock; 2, 4, of soft rock.

Below these two main strata there is considerable variation in the hardness of the rock, as shown in the accompanying diagram, where 3 and 5 are hard strata separated by a soft stratum. In view of this fact it seems probable that, for a considerable period in the early part of the recession, instead of there being simply one, there was a succession of cataracts, as the water unequally wore back through the harder strata, numbered 5, 3, and 1; but, after having receded half the distance, these would cease to be disturbing influences, and the problem is thus really the simple one of the recession through the strata numbered 1 and 2, which are continuous. So uniform in consistency are these throughout the whole distance, that the rate of recession could never have been less than it is now. We come, therefore, to the question of the rapidity with which the falls are now receding.

In 1841 Sir Charles Lyell and Professor James Hall (the State Geologist of New York) visited the falls together, and estimated that the rate of recession could not be greater than one foot a year, which would make the time required about thirty-five thousand years. But Lyell thought this rate was probably three times too large; sothat he favoured extending the time to one hundred thousand years. Before this the eminent French geologist Desor had estimated that the recession could not have been more than a foot in a century, which would throw the beginning of the gorge back more than three million years. But these were mere guesses of eminent men, based on no well-ascertained facts; while Mr. Bakewell, an eminent English geologist, trusting to the data furnished him by the guides and the old residents of Niagara, had, even then, estimated that the rate of recession was as much as three feet a year, which would reduce the whole time required to about ten thousand years.

But the visit of Lyell and Hall in 1841 led to the beginning of more accurate calculations. Professor Hall soon after had a trigonometrical survey of the falls made, from which a map was published in the State geological report. From this and from the monuments erected, we have had since that time a basis of comparison in which we could place absolute confidence.

In recent years three surveys have been made: the first by the New York State Geologists, in 1875; and the third by Mr. R. S. Woodward, the mathematician of the United States Geological Survey, in 1886. The accompanying map shows the outlines of the falls at the time of these three measurements, from 1842 to 1886. According to Mr. Woodward, “the length of the front of the Horseshoe Fall is twenty-three hundred feet. Between 1842 and 1875 four and a quarter acres of rock were worn away by the recession of the falls. Between 1875 and 1886 a little over one acre and a third disappeared in a similar manner, making in all, from 1842 to 1886, about five and a half acres removed, and giving an annual rate of recession of about two feet and a half per year for the last forty-five years. But in the central parts of the curve, where the water is deepest, the Horseshoe Fall retreated between two hundred and two hundred and seventy-five feet in the eleven years between 1875 and 1886.”

Fig. 106.—Map showing the recession of the Horseshoe Falls since 1842, as by survey mentioned in the text (Pohlman). (by courtesy of the American Institute of Mining Engineers.)

It will be perceived that the recession in the centre of the Horseshoe is very much more rapid than that nearer the margin; yet this rate at the centre is more nearly the standard of calculation than is that near the margin, for the gorge constantly tends to enlarge itself below the falls, and so gradually to bring itself into line with the full-formed channel. Taking all things into account, Mr. Woodward and the other members of the Geological Survey thought it not improbable that the average rate of actual recession in the Horseshoe Fall was as great as five feet per annum; and that, if we can rely upon the uniformity of the conditions in the past, seven thousand years is as long a period as can be assigned to its commencement.

The only condition in the problem about which there can be much chance of question relates to the constancy of the volume of water flowing in the Niagara channel. Mr. Gilbert had suggested that, as a consequence of the subsidence connected with the closing portions of the Glacial period, the water of the Great Lakes may have been largely diverted from its present outlet in Niagara River and turned northeastward, through Georgian Bay, French River, and Lake Nipissing, into a tributary of the Ottawa River, and so carried into the St. Lawrence below Lake Ontario. Of this theory there is also much direct evidence. A well-defined shore line of rounded pebbles extends, at an elevation of about fifty feet, across the col from Lake Nipissing to the head-waters of the Mattawa, a tributary of the Ottawa; while at the junction with the Ottawa there is an enormous delta terrace of boulders, forming a bar across the main stream just such as would result from Mr. Gilbert’s supposed outlet. But this outlet was doubtless limited to a comparatively few centuries, and Dr. Robert Bell thinks the evidence still inconclusive.[EB]

[EB]See Bul. Geol. Soc. Am., vol. iv, pp. 423-427, vol. v, pp. 620-626.

A second noteworthy glacial chronometer is found in the gorge of the Mississippi River, extending from the Falls of St. Anthony, at Minneapolis, to its junction with the preglacial trough of the old Mississippi, at Fort Snelling, a distance likewise of about seven miles.

Above Fort Snelling the preglacial gorge is occupied by the Minnesota River, and, as we have before stated, extends to the very sources of this river, and is continuous with the southern portion of the valley of the trough of the Red River of the North. Before the Glacial period the drainage of the present basin of the upper Mississippi joined this main preglacial valley, not at Fort Snelling, but some little distance above, as shown upon our map.[EC]This part of the preglacial gorge became partially filled up with glacial deposits, but it can be still traced by the lakelets occupying portions of the old depression, and by the records of wells which have been sunk along the line. When the ice-front had receded beyond the site of Minneapolis, the only line of drainage left open for the water was along the course of the present gorge from Minneapolis to Fort Snelling.

[EC]See above,p. 209.

Here, as at Niagara, the problem is comparatively simple. The upper strata of rock consist of hard limestone, which is underlaid by a soft sandstone, which, like the underlying shale at Niagara, is eroded faster than the upper strata, and so a perpendicular fall is maintained. The strata are so uniform in texture and thickness that, with the present amount of water in the river, the rate of recession of the falls must have been, from the beginning, very constant. If, therefore, the rate can be determined, the problem can be solved with a good degree of confidence.

Fortunately, the first discoverer of the cataract—the Catholic missionary Hennepin—was an accurate observer,and was given to recording his observations for the instruction of the outside world and of future generations. From his description, printed in Amsterdam in 1704, Professor N. H. Winchell is able to determine the precise locality of the cataract when discovered in 1680.

Again, in 1766 the Catholic missionary Carver visited the falls, and not only wrote a description, but made a sketch (found in an account of his travels, published in London in 1788) which confirms the inferences drawn from Hennepin’s narrative. The actual period of recession, however (which Professor Winchell duly takes into account), extends only to the year 1856, at which time such artificial changes were introduced as to modify the rate of recession and disturb further calculations. But between 1680 and 1766 the falls had evidently receded about 412 feet. Between 1766 and 1856 the recession had been 600 feet. The average rate is estimated by Professor Winchell to be about five feet per year, and the total length of time required for the formation of the gorge above Fort Snelling is a little less than eight thousand years, or about the same as that calculated by Messrs. Woodward and Gilbert for the Niagara gorge.

To these calculations of Professor Winchell it does not seem possible to urge any valid objection. It does not seem credible that the amount of water in the Mississippi should ever have been less than now, while during the continuance of the ice in the upper portion of the Mississippi basin the flow of water was certainly far greater than now.

If any one is inclined to challenge Professor Winchell’s interpretation of the facts, even a hasty visit to the locality will suffice to produce conviction. The comparative youth of the gorge from Fort Snelling up to Minneapolis is evident: 1. From its relative narrowness, when compared with the main valley below. This is represented by the shading upon the map. The gorge fromFort Snelling up is not old enough to have permitted much enlargement by the gradual undermining of the superficial strata on either side, which slowly but constantly goes on. 2. From the abruptness with which it merges into the preglacial valley of the Minnesota-Mississippi. The opening at Fort Snelling is not Y-shaped, as in gorges where there has been indefinite time for the operation of erosive agencies. 3. Furthermore, the precipices lining the post-glacial gorge above Fort Snelling are far more abrupt than those in the preglacial valley below, and they give far less evidence of weathering. 4. Still, again, the tributary streams, like the Minnehaha River, which empty into the Mississippi between Fort Snelling and Minneapolis, flow upon the surface, and have eroded gorges of very limited extent; whereas, below Fort Snelling, the small streams have usually either found underground access to the river or occupy gorges of indefinite extent.

The above estimates, setting such narrow limits to post-glacial time in America, will seem surprising only to those who have not carefully considered the glacial phenomena of various kinds to be observed all over the glaciated area. As already said, the glaciated portion of North America is a region of waterfalls, caused by the filling up of old channels with glacialdébris, and the consequent diversion of the water-courses. By this means the streams in countless places have been forced to fall over precipices, and to begin anew their work of erosion. Waterfalls abound in the glaciated region because post-glacial time is so short. Give these streams time enough, and they will wear their way back to their sources, as the preglacial streams had done over the same area, and as similar streams have done outside the glaciated region. Upon close observation, it will be found that the waterfalls in America are nearly all post-glacial, and that their work of erosion has been confined to a very limited time. A fair example is to be seen at Elyria, Ohio, in the falls of BlackRiver, one of the small streams which empty into Lake Erie from the south. Its post-glacial gorge, worn in sandstone which overlies soft shale, is only about two thousand feet in length, and it has as yet made no approach toward a V-shaped outlet.

The same impression of recent age is made by examining the outlets of almost any of the lakes which dot the glaciated area. The very reason of the continued existence of these lakes is that they have not had time enough to lower their outlets sufficiently to drain the water off, as has been done in all the unglaciated region. In many cases it is easy to see that the time during which this process of lowering the outlets has been going on cannot have been many thousand years.

The same impression is made upon studying the evidences of post-glacial valley erosion. Ordinary streams constantly enlarge their troughs by impinging against the banks now upon one side and now upon the other, and transporting the material towards the sea. It is estimated by Wallace that nine-tenths of the sedimentary material borne along by rivers is gathered from the immediate vicinity of its current, and goes to enlarge the trough of the stream. Upon measuring the cubical contents of many eroded troughs of streams in the glaciated region, and applying the tables giving the average amount of annual transportation of sediment by streams, we arrive at nearly the same results as by the study of the recession of post-glacial waterfalls.

Professor L. E. Hicks, of Granville, Ohio, has published the results of careful calculations made by him, concerning the valley of Raccoon Creek in Licking County, Ohio.[ED]These show that fifteen thousand years would be more than abundant time for the erosion of the immediate valley adjoining that small stream. I have made and publishedsimilar calculations concerning Plum Creek, at Oberlin, in Lorain County, Ohio.[EE]Like Raccoon Creek, this has its entire bed in glacial deposits, and has had nothing else to do since its birth but to enlarge its borders. The drainage basin of the creek covers an area of about twenty-five square miles. Its main trough averages about twenty feet in depth by five hundred in width, along a distance of about ten miles. From the rate at which the stream is transporting sediment, it is incredible that it could have been at work at this process more than ten thousand years without producing greater results.

[ED]See Baptist Quarterly for July. 1884.

[EE]See Ice Age in North America, p. 469.

Calculations based upon the amount of sediment deposited since the retreat of the ice-sheet point to a like moderate conclusion. When one looks upon the turbid water of a raging stream in time of flood, and considers that all the sediment borne along will soon settle down upon the bottom of the lake into which the stream empties, he can but feel surprised that the “wash” of the hills has not already filled up the depression of the lake. It certainly would have done so had the present condition of things existed for an indefinite period of time.

Naturally, while prosecuting the survey of the superficial geology of Minnesota, Mr. Upham was greatly impressed by the continued existence of the innumerable lakelets that give such a charm to the scenery of that State. Every day’s investigations added to the evidence that the lapse of time since the Ice age must have been comparatively brief, since, otherwise, the rains and streams would have filled these basins with sediment, and cut outlets low enough to drain them dry, for in many instances he could see such changes slowly going forward.[EF]

[EF]Minnesota Geological Report for 1879, p. 73.

Fig. 107.—Section of kettle-hole near Pomp’s Pond, Andover, Massachusetts (see text). (For general view of the situation, seeFig. 30, p. 78.)

Some years ago I myself made a careful estimate of the amount of deposition and vegetable accumulation which had taken place in a kettle-hole near Pomp’s Pond, in Andover, Mass. The diameter of the depression at the rim was 276 feet. The inclination of the sides was such that the extreme depression of the apex of the inverted cone could not have been more than seventy feet; yet the accumulation of peat and sediment only amounted to a depth of seventeen feet. The total amount of material which had accumulated would be represented by a cone ninety-six feet in diameter at the base and seventeen feet at the apex, which would equal only a deposit of about five feet over the present surface of the bottom. It is easy to see that ten thousand years is a liberal allowance of time for the accumulation of five feet of sediment in the bottom of an enclosure like a kettle-hole, for upon examination it is clear that whatever insoluble material gets into a kettle-hole must remain there, since there is no possible way by which it can get out. Now five feet is sixty inches, and if this amount has been six thousand years in accumulating, that would represent a rate of an inch in one hundred years, while, if it has been twelve thousand years in accumulation, the rate will be only one two-hundredth of an inch per year, a film so small as to be almost inappreciable. If we may judge from appearance, the result would not be much different in the case of the tens of thousands of kettle-holes and lakelets which dot the surface of the glaciated region.

In the year 1869 Dr. E. Andrews, of Chicago, made an important series of calculations concerning the rate at which the waters of Lake Michigan are eating into theshores and washing the sediment into deeper water or towards the southern end of the lake. With reference to the erosion of the shores, it appears from the work of the United States Coast Survey that a shoulder, covered with sixty feet of water, representing the depth at which wave-action is efficient in erosion, extends outward from the west shore a distance of about three miles, where the sounding line reveals the shore of the deeper original lake as it appeared upon the first withdrawal of the ice.

From a variety of observations the average rate at which the erosion of the bluffs is proceeding is found to be such that the post-glacial time cannot be more than ten thousand years, and probably not more than seven thousand.

An independent mode of calculating this period is afforded by the accumulations of sand at the south end of the lake, to which it is constantly drifting by the currents of water propelled against the shores by the wind; for the body of water in the lake is moving southward along the shores towards the closed end in that direction, there being a returning current along the middle of the lake. All the railroads approaching Chicago from the east pass through these sand deposits, and few of the observant travellers passing over the routes can have failed to notice the dunes into which the sand has been drifted by the wind. Now, all the material of these dunes and sand-beaches has been washed out of the bluffs to the northward by the process already mentioned, and has been slowly transferred by wave-action to its present position. It is estimated that south of Chicago and Grand Haven, this wave-transported sand amounts to 3,407,451,000 cubic yards. This occupies a belt curving around the south end about ten miles wide and one hundred miles long.

The rate at which the sand is moving southward along the shore is found by observing the amount annually arrested by the piers at Chicago, Grand Haven, andMichigan City. This equals 129,000 cubic yards for a year, which can scarcely be more than one quarter or one fifth of the total amount in motion. At this rate, the sand accumulations at the southern end of the lake would have been produced in a little less than seven thousand years.

“If,” says Dr. Andrews, “we estimate the total annual sand-drift at only twice the amount actually stopped by the very imperfect piers built—which, in the opinion of the engineers, is setting it far too low—and compare it with the capacity of the clay-basin of Lake Michigan, we shall find that, had this process continued one hundred thousand years the whole south end of Lake Michigan, up to the line connecting Chicago and Michigan City, would have been full and converted into dry land twenty-five thousand years ago, and the coast-line would now be found many miles north of Chicago.”[EG]

[EG]Southall’s Recent Origin of Man, p. 502.

It is proper to add a word in answer co an objection which may arise in the reader’s mind, for it will doubtless occur to some to ask why this sand which is washed out by the waves from the bluffs is not carried inward towards the deeper portion of the trough of the lake, thus producing a waste which would partly counteract the forces of accumulation at the south end. The answer is found in the fact that the south end of Lake Michigan is closed, and that the currents set in motion by the wind are such that there is no off-shore motion sufficient to move sand, and, as a matter of fact, dredgings show that the sand is limited to the vicinity of the shore.

By comparing the eroded cliffs upon Michigan and the other Great Lakes with what occurs in similar situations about the glacial Lake Agassiz, we obtain an interesting means of estimating the comparative length of time occupied by the ice-front in receding from the Canadian border to Hudson Bay.

As we have seen, Lake Agassiz occupied a position quite similar in most respects to Lake Michigan. Its longest diameter was north and south, and the same forces which have eroded the cliffs of Lake Michigan and piled up sand-dunes at its southern end would have produced similar effects upon the shores of Lake Agassiz, had its continuance been anywhere near as long as that of the present Lake Michigan has been. But, according to Mr. Upham, who has most carefully surveyed the whole region, there are nowhere on the shores of the old Lake Agassiz any evidence of eroded cliffs at all to be compared with those found upon the present Great Lakes, while there is almost an entire lack of sand deposits about the south end such as characterise the shore of Lake Michigan. “The great tracts of dunes about the south end of Lake Michigan belong,” as Upham well observes, “wholly to beach accumulations, being sand derived from erosion of the western and eastern shores of the lake.... But none of the beaches of our glacial lakes are large enough to make dunes like those on Lake Michigan, though the size and depth of Lake Agassiz, its great extent from north to south, and the character of its shores, seem equally favorable for their accumulation. It is thus again indicated that the time occupied by the recession of the ice-sheet was comparatively brief.”[EH]

[EH]Proceedings of the Boston Society of Natural History, vol. xxiv, p. 454; Upham’s Glacial Lakes in Canada, in Bulletin of the Geological Society of America, vol. ii, p. 248.

From Mr. Upham’s conclusions it would seem that if ten thousand years be allowed for the post-glacial existence of Lake Michigan, one tenth of that period would be more than sufficient to account for the cliffs, deltas, beaches, and other analogous phenomena about Lake Agassiz. In other words, the duration of Lake Agassiz could not have been more than a thousand years, which gives us a measureof the rate at which the recession of the ice-front went on after it had withdrawn to the international boundary. The distance from there to the mouth of Nelson River is about 600 miles. The recession of the ice-front over that area proceeded, therefore, at the average rate of about half a mile per year.

There are many evidences that the main period of glaciation west of the Rocky Mountains was considerably later than that in the eastern part of the continent. A portion of the facts pointing to this conclusion have been well stated by Mr. George F. Becker, of the United States Geological Survey.

“No one,” he says, “who has examined the glaciated regions of the Sierra can doubt that the great mass of the ice disappeared at a very recent period. The immense areas of polished surfaces fully exposed to the severe climate of say from 7,000 to 12,000 feet altitude, the insensible erosion of streams running over glaciated rocks, and the freshness of erratic boulders are sufficient evidence of this. There is also evidence that the glaciation began at no very distant geologic date. As Professor Whitney pointed out, glaciation is the last important geological phenomenon and succeeded the great lava flows. There is also much evidence that erosion has been trifling since the commencement of glaciation, excepting under peculiar circumstances. East of the range, for example, at Virginia City, andesites which there is every reason to suppose preglacial have scarcely suffered at all from erosion, so that depressions down which water runs at every shower are not yet marked with water-courses, while older rocks, even of Tertiary age and close by, are deeply carved. The rainfall at Virginia City is, to be sure, only about ten inches, so that rock would erode only say one third as fast as on the California coast; but even when full allowance is made for this difference, it is clear that these andesites must be much younger than the commencement of glaciation inthe northeastern portion of the continent as usually estimated. So, too, the andesites near Clear Lake, in California, though beyond a doubt preglacial, have suffered little erosion, and one of the masses, Mount Konocti (or Uncle Sam), has nearly as characteristic a volcanic form as Mount Vesuvius.”[EI]

Back to Index Next