The Atlantic Monthly/Volume 14/Number 81/Ice-Period in America

by Jean Louis Rodolphe Agassiz

ICE-PERIOD IN AMERICA.

In the autumn of 1846, six years after my visit to Great Britain in search of glaciers, I sailed for America. When the steamer stopped at Halifax, eager to set foot on the new continent so full of promise for me, I sprang on shore and started at a brisk pace for the heights above the landing. On the first undisturbed ground, after leaving the town, I was met by the familiar signs, the polished surfaces, the furrows and scratches, the line-engraving of the glacier, so well known in the Old World; and I became convinced of what I had already anticipated as the logical sequence of my previous investigations, that here also this great agent had been at work, although it was only after a long residence in America, and repeated investigations of the glacial phenomena in various parts of the country, that I fully understood the universality of its action. A detailed description of these appearances could hardly be more than a monotonous repetition of my statements respecting their existence in other regions; but the peculiar configuration of this continent, as compared with the more mountainous countries of Europe and Asia, has led to some modifications of the same phenomena here, worthy of special notice.

Thus far, the traces of ancient glaciers in America have been studied only east of the Rocky Mountains; little is known of the glaciers still remaining in the high mountain-ranges dividing the eastern part of the continent from California, still less respecting any indications of their former extension. There can be little doubt that such traces exist, and as soon as the so-called parks between Pike's Peak and Long Peak are explored, we may hope for information on this point. Indeed, the investigation may be spoken of as already undertaken; for among the exploring parties now on their way to that region are some intelligent observers who will not fail to make this point a subject of special study. But it is well known that the usual characteristic marks of glaciers extend over the whole surface of the land in the eastern half of the continent, from the Atlantic shores to the States west of the Mississippi, and from the Arctic Sea to the latitude of the Ohio, in its middle course, while within the range of the Alleghanies they stretch as far south as Georgia and Alabama. In no other region where these traces have been observed do they extend over such wide tracts of country in unbroken continuity, this being of course owing to the level character of the land itself.

The continent of North America, east of the Rocky Mountains, is, indeed, an immense uniform plain, intersected from east to west only by the ranges of low hills running in the direction of the St. Lawrence and the Canadian lakes, and from northeast to southwest by the Alleghany range stretching from Alabama to New England, where it trends towards the Canadian hills in the ridges known as the Green and White Mountains. This coast-range has a short slope towards the Atlantic, and a long one in the direction of the great Mississippi valley. With the exception of some higher points of the Alleghany range, the surface of this whole plain is glacier-worn from the Arctic regions to about the fortieth degree of northern latitude, the glacier-marks trending from north to south, with occasional slight inclinations to the east or west, according to the minor inequalities of the surface. There is, however, no decided modification of their general trend in consequence of the range of hills intersecting them at right angles for nearly the whole width of the continent between latitudes forty-six and fifty; indeed, the Canadian, or, as they are sometimes called, the Laurentian Hills, formed a no more powerful barrier to the onward progress of the immense fields of ice covering the continent than did the small hummocks, or roches moutonnées, in the Swiss valleys to the advance of the Alpine glaciers. In fact, these low hills may be considered as a succession of roches moutonnées, trending in a continuous ridge from east to west, over which the masses of northern ice have moved unimpeded to the latitude of the Ohio.

Owing to the absence of high mountain-ranges over this vast expanse of land, the glacial phenomena of America are not grouped about special centres of dispersion, or radiating from them, as in Europe. During the greatest extension of the ice-fields, there were but few prominent peaks rising above them, and dropping here and there huge boulders on their surface, to be transported to great distances without losing their rough angular character. And when the temperature under which these vast frozen masses had been formed rose again, the wasting ice must have yielded first on its southern boundary, gradually and uniformly retreating to the Arctic regions, without breaking up into distinct glacial regions, separated from one another, each with its local distribution of erratic boulders and glacier-marks radiating from circumscribed areas on higher levels, as they occur everywhere in Europe. It is true that there are a few localities within the Alleghany range, on the Green and White Mountains, and in parts of Maine, where it is evident that local glaciers have had a temporary existence; but even throughout this eastern coast-range the elevation of the mountains is so slight, and their trend so uniform in a northeasterly and southwesterly direction through twenty degrees of latitude, that the localization of the phenomena is less marked than in Norway, Great Britain, or Switzerland. In short, the ice of the great glacial period in America moved over the continent as one continuous sheet, overriding nearly all the inequalities of the surface. Thus the peculiar physical character of the country gives a new aspect to the study of glacial evidences here. The polished surfaces stretch continuously over hundreds and hundreds of miles; the rectilinear scratches, grooves, and furrows are unbroken for great distances; the drift spreads in one vast sheet over the whole land, consisting of an indiscriminate medley of clays, sands, gravels, pebbles, boulders of all dimensions, and so uniformly mixed together that it presents hardly any difference in its composition, whether we examine it in New England, New York, Pennsylvania, Ohio, Michigan, Indiana, Illinois, Wisconsin, in Iowa beyond the Mississippi, in the more northern Territories, or in Canada.

In Europe, boulders of large dimensions do not often occur within the drift, but are usually resting above it with their sharp angles and rough surfaces unchanged, having travelled evidently upon the glacier and not under it. But such large boulders, polished and scratched like the smaller pebbles, are to be found everywhere imbedded in American drift, while the angular fragments of rock resting above these triturated masses are comparatively rare.^[1] It is evident from this that the ice overtopped the rocky inequalities of the land, and that the detached fragments remaining beneath the icy covering underwent the same action from friction and pressure to which the whole mass of drift was subjected. The distribution of the few angular boulders scattered over the country no doubt began when some of the higher portions of the land had emerged from the mass of snow and ice; and they are most frequent in New England, where the mountain-elevation is greatest.

The mineralogical character of the loose materials forming the American drift leaves no doubt that the whole movement, with the exception of a few local modifications easily accounted for by the lay of the land, was from north to south, all the fragments not belonging to the localities where they occur being readily traced to rocks in situ to the north of their present resting-places. The farther one journeys from their origin, the more extraordinary does the presence of these boulders become. It strikes one strangely to find even in New England fragments of rock from the shores of Lake Superior; but it is still more impressive to meet with masses of northern rock on the prairies of Illinois or Iowa. One may follow these boulders to the fortieth degree of latitude, beyond which they become more and more rare, while the finer drift alone extends farther south.

It is not only, however, by tracking the boulders back to their origin in the North that we ascertain the starting-point of the whole mass; we have another kind of evidence to this effect, already alluded to in the description of the roches moutonnées. Wherever the natural surface of any hill, having a steep southern slope, is exposed, the marks are always found to be very distinct on the northern side and entirely wanting on the southern one, showing, that, as in the case of many of the roches moutonnées in Switzerland, the mass moved up the northern slope, forcing its way against it, grinding and furrowing the northern face of the hill as it moved over it, but bridging the opposite side in its descent without coming into contact with it.^[2] This is true, not only of hills, but of much slighter obstacles which presented themselves in the path of the ice. Even pebbles imbedded in masses of pudding-stone, but rising sometimes above the level of the general surface, often have their northern side polished and scratched, while the southern one remains untouched.

Moraines are not wanting to complete the chain of evidence respecting the ancient existence of glaciers in this country, although we cannot expect to find them here so frequently as in Europe, where the many local glaciers in circumscribed valleys afforded special facilities for the building up of these lateral and transverse walls. Over the broad expanse of the United States, on the contrary, with such slight variations of level, the disappearance of the ice at its breaking-up would naturally be more complete and continuous than in a country intersected by frequent mountain-chains, where the ice would linger in the higher valleys long after it had disappeared from the plains below. Yet it is evident that here also in certain localities the boundary-line of the ice underwent oscillations, pausing here and there long enough to collect mounds of the same character as those spanning the valleys of Switzerland and Great Britain. We have several of these mounds in our immediate vicinity. The Waverley Oaks, so well known to all lovers of fine trees in our community, stand on an ancient moraine, and there are others in the neighborhood of the Blue Hills. In the southeastern parts of Maine, also, I have observed very well-defined moraines. In Vermont, the valley of the Winooski River retains ample traces of the local glacier by which it was formerly filled; and, indeed, throughout the Alleghany range, in its northeastern as well as its southern extension, we have various evidences of localized glaciers, which must have outlived the general ice-period for a longer or shorter time.

I am unwilling to weary my readers by dwelling upon appearances identical with those already described; but I may state, for the guidance of those who wish to investigate these traces for themselves, that any recently uncovered ledge of rock in our neighborhood, the surface of which has not been altered by atmospheric agencies, presents the glacier-worn surfaces with the characteristic striæ and furrows. These marks may be traced everywhere, even to the sea-shore, not only down to the water's edge, but beneath it, where-ever the harder rocks have resisted the action of the tides and retain their original character. In our granitic regions intersected by innumerable trap dikes, as, for instance, at Nahant, the smooth surface of many of the rocks, where sienite and trap have been evenly levelled, shows that the same inexorable saw, cutting alike through hard and soft materials, has passed over them. In the hills of pudding-stone in the neighborhood of Roxbury, we have quartz pebbles cut down to the same level with the softer paste in which they lie imbedded with pebbles of sandstone, clay-slate, gneiss, and limestone. In the limestone regions of Western New York and Northern Ohio, about the neighborhood of Buffalo and Cleveland, the flat surfaces of the limestone are most uniformly polished, furrowed, and scratched, the furrows often exhibiting that staccato grating action described in a former article. I have observed the same traces in the vicinity of Milwaukee and Iowa City, and we know, from the accounts given by Arctic travellers of their overland expeditions, that these peculiar appearances of the surface are characteristic of the rocks in those regions, wherever they are not disintegrating under the influence of the present atmospheric agents.

Upon these surfaces, through the whole expanse of the country, rests the drift, having everywhere the characteristic composition of glacier-drift, and nowhere that of an aqueous stratified deposit, except when afterwards remodelled by the action of water. But of this stratified drift I shall have occasion to speak more in detail hereafter. There is, however, one circumstance, of frequent occurrence along our New-England shores, requiring special explanation, because it is generally misunderstood. Along our sea-shore, and even within the harbor of Boston, at the base of the harbor-islands, as well as at the outlet of our larger Atlantic streams, numbers of boulders are found of considerable size; and this fact is often adduced as showing the power of water to transport massive fragments of rock to great distances, the mineralogical character of these boulders being frequently such as to show that they cannot have originated in the neighborhood of their present resting-places. But a careful examination of the surrounding country, and a comparison of the nature and level of the drift on the mainland with those of the same deposits on the harbor-islands, (a series of evidence to be given with more detail in a future article,) suggest a different explanation of these phenomena. The sheet of drift was once more continuous and extensive than it is now, and the localities in which we find these crops of boulders are spots where the tide has eaten into the drift, wearing away the finer materials, or the paste in which the larger fragments were imbedded, and allowing them to fall to the bottom, or where the same result has been produced by the action of rivers cutting their way through the drift, and thus finding an outlet to the sea. In short, instead of showing the power of currents to carry along heavy fragments, these stranded boulders prove, on the contrary, the inability of water to produce any such effect, since it is evident that the tides washing against the shore, or the rivers rushing down to the sea, were equally incapable of bearing off the weightier materials, and allowed them to drop to the bottom, while they readily swept away the lighter ones. Such localities compare with the surrounding drift much as the bottom of a gravel-pit which has been partially worked compares with its banks. Look into any gravel-pit, a portion of which has already been carted away. At its bottom a number of larger stones and boulders are usually lying, too heavy for the cart, and therefore left upon the spot. Fragments of the same size and character, and equally numerous, will be seen protruding at various heights from the sides, where they are imbedded in the general mass of the drift. As soon as the work progresses a little farther, and the finer materials are removed, these boulders will also drop out, and lie as thickly scattered over the surface of the ground, as they now do in that portion of the bottom where the pit has been completely opened and the gravel removed. We shall see hereafter how these boulders, derived from the land-drift and scattered along the coast, may be distinguished from those cast ashore by icebergs.

Notwithstanding the number of facts thus far collected respecting glacial phenomena in America, certainly forming in their combination a very strong chain of evidence, the scientific world has, nevertheless, been slow to admit the possibility of the former existence of glaciers over such a wide, unbroken expanse of level land. This backwardness is, no doubt, partly due to the fact, that, as glaciers have hitherto been studied in mountainous countries, their presence has been supposed to imply the presence of mountains, this impression being strengthened by the downward and onward movement of existing glaciers, so long supposed to be exclusively due to the slopes along which all modern glaciers advance. Were it true that glaciers move solely or mainly on account of the sloping bottom on which they rest, and that they can advance only on an inclined plane, all the phenomena concerning drift, polished and furrowed surfaces, boulders, etc., in America, would hardly justify us in assuming a moving sheet of ice as their cause. But we have seen that the phenomena of glaciers, like those of currents, are in great part meteorological. The Gulf-Stream does not flow toward the English shore because the ocean-bottom slopes eastward; nor does the cold current of Baffin's Bay run down-hill when it pours its icy waters southward upon our northeast coast. Their course is determined by laws of temperature, and so have we also seen that the motion of glaciers is mainly determined by conditions of temperature, although, in this case, an internal mechanical action is combined with external influences; and while it is true that glaciers, as they now exist, are dependent upon the shape of the valleys in lofty mountain-chains, yet under different geographical conditions the same phenomena may be produced over level, open countries.

I believe that circumstances similar to those determining the more rapid advance of the glaciers from higher to lower levels at that point where the alternate thawing and freezing, the infiltration of water and consequent expansion of the ice under frost, are greatest, would also determine the motion of a large body of ice from north to south, since it would be along its southern limits that these conditions would prevail; while the great reservoir of snow at the north would correspond to the upper troughs of the present glaciers, from which their lower ranges are constantly fed. The change of snow into ice is owing to alterations of temperature, to partial melting and subsequent freezing, constantly renewed,—and also to the sinking of the mass upon itself in consequence of its own weight, the lower portions being thus forced out by the pressure of the superincumbent ice. Upon an inclined plane the movement consequent upon these changes will of course be downward; but what would be the result, if a field of snow many thousand feet thick, corresponding, except in its greater bulk, to the accumulations by which the present glaciers are caused, were stretched over an extensive level surface? The moisture from the upper superficial layers would permeate the larger mass as it now does the smaller one, trickling down into its lower portions, while the pressure from above would render the bottom hard and compact, changing it gradually into ice. If this should take place under climatic conditions which would keep the whole as a mass in a frozen state, the pressure from above would force out the lower ice in every direction beyond its original circumscription, thus enlarging the area covered by it, while the whole would subside in its bulk. Let us for a moment assume that such an accumulation of snow takes place around the northern and southern poles, stretching thence over the northern and southern hemispheres to latitude forty, and that this field of snow acquires a thickness of from twelve to fifteen thousand feet. Such a mass would subside upon itself in consequence of its own weight; it would be transformed into ice with greater or less rapidity and completeness, according to the latitude determining the surrounding climatic influences and the amount of moisture falling upon it as rain or dew, the alternations of temperature being of course more frequent and greater along its outer limit. In proportion as, with the rising of the temperature, these alternations became more general, a packing of the mass would begin, corresponding to that observed in the glacial valleys of Switzerland, though here the action would not be intensified by lateral pressure; an internal movement of the whole mass would be initiated, and the result could be no other than a uniform advance in a southerly direction from the Arctic toward the more temperate latitudes in Europe, Asia, and North America, and from the Antarctic toward South America, the Cape of Good Hope, and Van Diemen's Land. But we need not build up a theoretical case in order to form an approximate idea of the great ice-sheet stretching over the northern part of this continent during the glacial period. It would seem that man was intended to decipher the past history of his home, for some remnants or traces of all its great events are left as a key to the whole. Greenland and the Arctic regions hold all that remains of the glacial period in North America. Their shrunken ice-fields, formidable as they seem to us, are to the frozen masses of that secular winter but as the patches of snow and ice lingering on the north side of our hills after the spring has opened; let us expand them in imagination till they extend over half the continent, and we shall have a sufficiently vivid picture of this frozen world. And a temperature which would bring the climate of Greenland down to the fortieth degree of latitude would not only render the field of ice far more extensive, but thousands of feet thicker than it is at present. The physical configuration of Greenland also confirms the possibility of a glacial period in America, for there we have at this moment a wide expanse of land unbroken by mountains, over which a uniform sheet of ice moves southward, with occasional variations of its trend according to the undulations of the surface. The interesting accounts of Dr. Rink show that in reality Greenland is a miniature picture of the ice-period. The immense number of icebergs breaking off and floating southward every summer gives us some idea of the annual waste and renewal of the ice. How can we doubt, that, when, under the same latitude, Norway, Sweden, Scotland, England, and Ireland were covered by sheets of ice many thousand feet in height, the ice-fields of Greenland must have shared in the same climatic influences, and have been much thicker and far more extensive than they are at present?

Notwithstanding the absence of lofty mountain-chains in America, we are not wholly without the means of measuring the thickness of the ice-sheet, by comparing it, as in Europe, with some of our highest elevations. The slopes of the Alleghany range, wherever they have been examined, are glacier-worn to the very top, with the exception of a few points; but these points are sufficient to give us data for the comparison. Mount Washington, for instance, is over six thousand feet high, and the rough, unpolished surface of its summit, covered with loose fragments, just below the level of which glacier-marks come to an end, tells us that it lifted its head alone above the desolate waste of ice and snow. In this region, then, the thickness of the sheet cannot have been much less than six thousand feet, and this is in keeping with the same kind of evidence in other parts of the country; for, wherever the mountains are much below six thousand feet, the ice seems to have passed directly over them, while the few peaks rising to that height are left untouched. And while we can thus sink our plummet from the summit to the base of Mount Washington and measure the thickness of the mass of ice, we have a no less accurate indication of its extension in the undulating line marking the southern termination of the drift. I have shown that the moraines mark the oscillations of the glaciers in Europe. Where such accumulations of loose materials took place at its terminus, there we know the glacier must have held its ground long enough to allow time for the collection of these débris. In the same way we may trace the southern border of our ancient ice-sheet on this continent by the limit of the boulders; beyond that line it evidently did not advance as a solid mass, since it ceased to transport the heavier materials. But as soon as the outskirts of the ice began to yield and to flow off as water, the lighter portions of the drift were swept onward; and hence we find a sheet of finer drift-deposit, sand and gravel more or less distinctly stratified, carried to greater or less distances, and fading into the Southern States, where it mingles with the most recent river-deposits.

One naturally asks, What was the use of this great engine set at work ages ago to grind, furrow, and knead over, as it were, the surface of the earth? We have our answer in the fertile soil which spreads over the temperate regions of the globe. The glacier was God's great plough; and when the ice vanished from the face of the land, it left it prepared for the hand of the husbandman. The hard surface of the rocks was ground to powder, the elements of the soil were mingled in fair proportions, granite was carried into the lime regions, lime was mingled with the more arid and unproductive granite districts, and a soil was prepared fit for the agricultural uses of man. I have been asked whether this inference was not inconsistent with the fact that a rich vegetation preceded the ice-period,—a vegetation sufficiently abundant to sustain the tropical animals then living throughout the temperate regions. But the vegetation which has succeeded the ice-period is of a different character, and one that could not have flourished on a soil that would nourish a more tropical growth. The soil we have now over the temperate zone is a grain-growing soil,—one especially adapted to those plants most necessary to the higher domestic and social organizations of the human race. Therefore I think we may believe that God did not shroud the world He had made in snow and ice without a purpose, and that this, like many other operations of His Providence, seemingly destructive and chaotic in its first effects, is nevertheless a work of beneficence and order.

 

In the next article, in order to put the reader in possession of the glacial question as it stands at present, I shall say something of the possible causes of this extraordinary accumulation of snow,—though all such explanations are thus far mere suggestions,—and shall also give some more precise estimates of the changes of temperature involved in the history of the glacial period, before proceeding to the consideration of the effects produced by the breaking-up of the ice, as shown in our stratified lowland drift, and in our estuaries and river-terraces.

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1. The greater proportion of large, rounded boulders in the American drift, as compared with the European, is a singular fact not fully met by the above explanation; since, while the number of mountain-peaks rising above the ice in Europe would account for the frequency of large angular fragments transported upon its surface, there would seem to be no reason why the drift, carried along by a mass of ice having the same thickness in both continents, should not contain as many rounded masses in one as in the other. The facts, however, are as I have stated them, and the difference may be due partly to the broken character of the ground over which the drift must have passed in Europe, subjecting it to a more violent process of friction and grinding than in America, and partly to the use that has been made of the drift-boulders during so many centuries for building-purposes in the Old World, the drift-boulders being naturally taken first, because they are more easily reached, while the angular ones are frequently perched on almost inaccessible spots. Indeed, the stone fences in both countries tell us the use to which many of the rounded boulders have been put, and the ground in many parts of the United States has already been cleared to a great extent of its rocky fragments for this and like purposes. In the course of time they will, no doubt, disappear from the surface of this country, as they have done from that of Europe.
2. Fuller descriptions of these polished hills may be found in my work on Lake Superior.