Chapter VI: Glaciers (2)

Important corroborative evidence of ice action is generally to be found by inspecting the bed rock below the detritus, which indicates glacial action. Even if it be somewhat decayed, as is apt to be the case where the ice sheet long since passed away, the bed rock is likely to have a warped surface; it is cast into ridges and furrows of a broad, flowing aspect, such as liquid water never produces, which, indeed, can only be created by an ice sheet moving over the surface, cutting its bed in proportion to the hardness of the material. Furthermore, if the bed rock have a firm texture, and be not too much decayed, we almost always find upon it grooves or scratches, channels carved by the stones embedded in the body of the ice, and drawn by its motion over the fixed material. Thus the proof of glacial extension in the last ice epoch is made so clear that accurate maps can be prepared showing the realm of its action. This task is as yet incomplete, although it is already far advanced.

While the study of glaciers began in Europe, inquiries concerning their ancient extension have been carried further and with more accuracy in North America than in any other part of the world. We may therefore well begin our description of the limits of the ice sheets with this continent. Imagining a seafarer to have approached America by the North Atlantic, as did the Scandinavians, and that his voyage came perhaps a hundred thousand years or more before that of Leif Ericsson, he would have found an ice front long before he attained the present shores of the land. This front may have extended from south of Greenland, off the shores of the present Grand Banks of Newfoundland, thence and westward to central or southern New Jersey. This cliff of ice was formed by a sheet which lay on the bottom of the sea. On the New Jersey coast the ice wall left the sea and entered on the body of the continent. We will now suppose that the explorer, animated with the valiant scientific spirit which leads the men of our day to seek the poles, undertook a land journey along the ice front across the continent. From the New Jersey coast the traveller would have passed through central Pennsylvania, where, although there probably detached outlying glaciers lying to the southward as far as central Virginia, the main front extended westward into the Ohio Valley. In southern Ohio a tongue of the ice projected southwardly until it crossed the Ohio River, where Cincinnati now lies, extending a few miles to the southward of the stream. Thence it deflected northwardly, crossing the Mississippi, and again the Missouri, with a tongue or lobe which went far southward in that State. Then again turning to the northwest, it followed in general the northern part of the Missouri basin until it came to within sight of the Rocky Mountains. There the ice front of the main glacier followed the trend of the mountains at some distance from their face for an unknown extent to the northward. In the Cordilleras, as far south as southern Colorado, and probably in the Sierra Nevada to south of San Francisco, the mountain centres developed local glaciers, which in some places were of very great size, perhaps exceeding any of those which now exist in Switzerland. It will thus be seen that nearly one half of the present land area of North America was beneath a glacial covering, though, as before noted, the region about the Gulf of Mexico may have swayed upward when the northern portion of the land was borne down by the vast load of ice which rested upon it. Notwithstanding this possible addition to the land, our imaginary explorer would have found the portion of the continent fit for the occupancy of life not more than half as great as it is at present.

In the Eurasian continent there was no such continuous ice sheet as in North America, but the glaciers developed from a number of different centres, each moving out upon the lowlands, or, if its position was southern, being limited to a particular mountain field. One of these centres included Scandinavia, northern Germany, Great Britain about as far south as London, and a large part of Ireland, the ice covering the intermediate seas and extending to the westward, so that the passage of the North Atlantic was greatly restricted between this ice front and that of North America. Another centre, before noted, was formed in the Alps; yet another, of considerable area, in the Pyrenees; other less studied fields existed in the Apennines, in the Caucasus, the Ural, and the other mountains of northern Asia. Curiously enough, however, the great region of plains in Siberia does not appear to have been occupied by a continuous ice sheet, though the similar region in North America was deeply embedded in a glacier. Coincident with this development of ice in the eastern part of the continent, the ice streams of the Himalayan Mountains, some of which are among the greatest of our upland glaciers, appear to have undergone but a moderate extension. Many other of the Eurasian highlands were probably ice-bound during the last Glacial period, but our knowledge concerning these local fields is as yet imperfect.

In the southern hemisphere the lands are of less extent and, on the whole, less studied than in the northern realm. Here and there where glaciers exist, as in New Zealand and in the southern part of South America, observant travellers have noticed that these ice fields have recently shrunk away. Whether the time of greatest extension and of retreat coincided with that of the ice sheets in the north is not yet determined; the problem, indeed, is one of some difficulty, and may long remain undecided. It seems, however, probable that the glaciers of the southern hemisphere, like those in the north, are in process of retreat. If this be true, then their time of greatest extension was probably the same as that of the ice sheets about the southern pole. From certain imperfect reports which we have concerning evidences of glaciation in Central America and in the Andean district in the northern part of South America, it seems possible that at one time the upland ice along the Cordilleran chain existed from point to point along that system of elevations, so that the widest interval between the fields of permanent snow with their attendant glaciers did not much exceed a thousand miles.

Observing the present gradual retreat of those ice remnants which remain mere shreds and patches of the ancient fields, it seems at first sight likely that the extension and recession of the great glaciers took place with exceeding slowness. Measured in terms of human life, in the manner in which we gauge matters of man's history, this process was doubtless slow. There are reasons, however, to believe that the coming and going were, in a geological sense, swift; they may have, indeed, been for a part of the time of startling rapidity. Going back to the time of geological yesterday, before the ice began its development in the northern hemisphere, all the evidence we can find appears to indicate a temperate climate extending far toward the north pole. The Miocene deposits found within twelve degrees, or a little more than seven hundred miles, of the north pole, and fairly within the realm of lowest temperature which now exists on the earth, show by the plant remains which they contain that the conditions permitted the growth of forests, the plants having a tolerably close resemblance to those which now freely develop in the southern portion of the Mississippi Valley. Among them there are species which had the habit of retaining their broad, rather soft leaves throughout the winter season. The climate appears, in a word, to have been one where the mean annual temperature must have been thirty degrees or more higher than the present average of that realm. Although such conditions near the sea level are not inconsistent with the supposition that glaciers existed in the higher mountains of the north, they clearly deny the possibility of the realm being occupied by continental glaciers.

Although the Pliocene deposits formed in high latitudes have to a great extent been swept away by the subsequent glacial wearing, they indicate by their fossils a climatal change in the direction of greater cold. We trace this change, though obscurely, in a progressive manner to a point where the records are interrupted, and the next interpretable indication we have is that the ice sheet had extended to somewhere near the limits which we have noted. We are then driven to seek what we can concerning the sojourn of the ice on the land by the amount of wearing which it has inflicted upon the areas which it occupied. This evidence has a certain, though, as we shall see, a limited value.

When the students of glacial action first began the great task of interpreting these records, they were led to suppose that the amount of rock cutting which was done by the ice was very great. Observing what goes on, in the manner we have noted, beneath a valley glacier such as those of Switzerland, they saw that the ice work went on rapidly, and concluded that if the ice remained long at work in a region it must do a vast deal of erosion. They were right in a part of their premises, but, as we shall see, probably in another part wrong. Looking carefully over the field where the ice has operated, we note that, though at first sight the area appears to have lost all trace of its preglacial river topography, this aspect is due mainly to the irregular way in which the glacial waste is laid down. Close study shows us that we may generally trace the old stream valleys down to those which were no larger than brooks. It is true that these channels are generally and in many places almost altogether filled in with rubbish, but a close study of the question has convinced the writer, and this against a previous view, that the amount of erosion in New England and Canada, where the work was probably as great as anywhere, has not on the average exceeded a hundred feet, and probably was much less than that amount.

Even in the region north of Lake Ontario, over which the ice was deep and remained for a long time, the amount of erosion is singularly small. Thus north of Kingston the little valleys in the limestone rocks which were cut by the preglacial streams, though somewhat encumbered with drift, remain almost as distinct as they are on similar strata in central Kentucky, well south of the field which the ice occupied. In fact, the ice sheet appears to have done the greatest part of its work and to have affected the surface most in the belt of country a few hundred miles in width around the edges of the sheet. It was to be expected that in a continental glacier, as in those of mountain valleys, the most of the _débris_ should be accumulated about the margin where the materials dropped from the ice. But why the cutting action should be greatest in that marginal field is not at first sight clear. To explain this and other features as best we may, we shall now consider the probable history of the great ice march in advance and retreat, and then take up the conditions which brought about its development and its disappearance.

Ice is in many ways the most remarkable substance with which the physicist has to deal, and among its eminent peculiarities is that it expands in freezing, while the rule is that substances contract in passing from the fluid to the solid state. On this account frozen water acts in a unique manner when subjected to pressure. For each additional atmosphere of pressure--a weight amounting to about fifteen pounds to the square inch--the temperature at which the ice will melt is lowered to the amount of sixteen thousandths of a degree centigrade. If we take a piece of ice at the temperature of freezing and put upon it a sufficient weight, we inevitably bring about a small amount of melting. Where we can examine the mass under favourable conditions, we can see the fluid gather along the lines of the crystals or other bits of which the ice is composed. We readily note this action by bringing two pieces of ice together with a slight pressure; when the pressure is removed, they will adhere. The adhesion is brought about not by any stickiness of the materials, for the substance has no such property. It is accomplished by melting along the line of contact, which forms a film of water, that at once refreezes when the pressure is withdrawn. When a firm snowball is made by even pressing snow, innumerable similar adhesions grow up in the manner described. The fact is that, given ice at the temperature at which it ordinarily forms, pressure upon it will necessarily develop melting.

The consequences of pressure melting as above described are in glaciers extremely complicated. Because the ice is built into the glacier at a temperature considerably below the freezing point, it requires a great thickness of the mass before the superincumbent weight is sufficient to bring about melting in its lower parts. If we knew the height at which a thermometer would have stood in the surface ice of the ancient glacier which covered the northern part of North America, we could with some accuracy compute how thick it must have been before the effect of pressure alone would have brought about melting; but even then we should have to reckon the temperature derived from the grinding of the ice over the floor and the crushing of rocks there effected, as well as the heat which is constantly though slowly coming forth from the earth's interior. The result is that we can only say that at some depth, probably less than a mile, the slowly accumulating ice would acquire such a temperature that, subjected to the weight above it, the material next the bottom would become molten, or at least converted into a sludgelike state, in which it could not rub against the bottom, or move stones in the manner of ordinary glaciers.

As fast as the ice assumed this liquid or softened state, it would be squeezed out toward the region where, because of the thinning of the glacier, it would enter a field where pressure melting did not occur. It would then resume the solid state, and thence journey to the margin of the ice in the ordinary manner. We thus can imagine how such a glacier as occupied the northern part of this continent could have moved from the central parts toward its periphery, as we can not do if we assume that the glacier everywhere lay upon the bed rock. There is no slope from Lake Erie to the Ohio River at Cincinnati. Knowing that the ice moved down this line, there are but two methods of accounting for its motion: either the slope of the upper surface to the northward was so steep that the mass would have been thus urged down, the upper parts dragging the bottom along with them, or the ice sheet for the greater part of its extent rested upon pressure-molten water, or sludge ice, which was easily squeezed out toward the front. The first supposition appears inadmissible, for the reason that the ice would have to be many miles deep at Hudson Bay in order that its upper surface should have slope enough to overcome the rigidity of the material and bring about the movement. We know that any such depth is not supposable.

The recent studies in Greenland supply us with strong corroborative evidence for the support of the view which is here urged. The wide central field of that area, where the ice has an exceeding slight declivity, and is unruptured by crevices, can not be explained except on the supposition that it rests on pressure-molten water. The thinner section next the shore, where the glacier is broken up by those irregular movements which its wrestle with the bottom inevitably induces, shows that there it is in contact with the bed rock, for it behaves exactly as do the valley glaciers of like thickness.

The view above suggested as to the condition of continental glaciers enables us to explain not only their movements, but the relatively slight amount of wearing which they brought about on the lands they occupied. Beginning to develop in mountain regions, or near the poles on the lowlands, these sheets, as soon as they attained the thickness where the ice at their bottom became molten, would rapidly advance for great distances until they attained districts where the melting exceeded the supply of frozen material. In this excursion only the marginal portion of the glacier would do erosive work. This would evidently be continued for the greatest amount of time near the front or outer rim of the ice field, for there, we may presume, that for the longest time the cutting rim would rest upon the bed rock of the country. As the ice receded, this rim would fall back; thus in the retreat as in the advance the whole of the field would be subjected to a certain amount of erosion. On this supposition we should expect to find that the front of a continental glacier, fed with pressure-molten water from all its interior district, which became converted into ice, would attain much warmer regions than the valley streams, where all the flow took place in the state of ice, and, furthermore, that the speed of the going on the margin would be much more rapid than in the Alpine streams. These suppositions are well borne out by the study of existing continental ice sheets, which move with singular rapidity at their fronts, and by the ancient glaciers, which evidently extended into rather warm fields. Thus, when the ice front lay at the site of Cincinnati, at six hundred feet above the sea, there were no glaciers in the mountains of North Carolina, though those rise more than five thousand feet higher in the air, and are less than two hundred miles farther south. It is therefore evident that the continental glacier at this time pushed southward into a comparatively warm country in a way that no stream moving in the manner of a valley glacier could possibly have done.

The continental glaciers manage in many cases to convey detritus from a great distance. Thus, when the ice sheet advanced southwardly from the regions north of the Great Lakes, they conveyed quantities of the _débris_ from that section as far south as the Ohio River. In part this rubbish was dragged forward by the ice as the sheet advanced; in part it was urged onward by the streams of liquid water formed by the ordinary process of ice melting. Such subglacial rivers appear to have been formed along the margins of all the great glaciers. We can sometimes trace their course by the excavation which they have made, but more commonly by the long ridges of stratified sand and gravel which were packed into the caverns excavated by these subglacial rivers, which are known to glacialists as _eskers_, or as serpent kames. In many cases we can trace where these streams flowed up stream in the old river valleys until they discharged over their head waters. Thus in the valley of the Genesee, which now flows from Pennsylvania, where it heads against the tributaries of the Ohio and Susquehanna, to Lake Ontario, there was during the Glacial epoch a considerable river which discharged its waters into those of the Ohio and the Susquehanna over the falls at the head of its course.

The effect of widespread glacial action on a country such as North America appears to have been, in the first place, to disturb the attitude of the land by bearing down portions of its surface, a process which led to the uprising of other parts which lay beyond the realm of the ice. Within the field of glaciation, so far as the ice rested bodily on the surface, the rocks were rapidly worn away. A great deal of the _débris_ was ground to fine powder, and went far with the waters of the under-running streams. A large part was entangled in the ice, and moved forward toward the front of the glacier, where it was either dropped at the margin or, during the recession of the glacier, was laid upon the surface as the ice melted away. The result of this erosion and transportation has been to change the conditions of the surface both as regards soil and drainage. As the reader has doubtless perceived, ordinary soil is, outside of the river valleys, derived from the rock beneath where it lies. In glaciated districts the material is commonly brought from a considerable distance, often from miles away. These ice-made soils are rarely very fertile, but they commonly have a great endurance for tillage, and this for the reason that the earth is refreshed by the decay of the pebbles which they contain. Moreover, while the tillable earth of other regions usually has a limited depth, verging downward into the semisoil or subsoil which represent the little changed bed rocks, glacial deposits can generally be ploughed as deeply as may prove desirable.

The drainage of a country recently affected by glaciers is always imperfect. Owing to the irregular erosion of the bed rocks, and to the yet more irregular deposition of the detritus, there are very numerous lakes which are only slowly filled up or by erosion provided with drainage channels. Though several thousand years have passed by since the ice disappeared from North America, the greater part of the area of these fresh-water basins remains, the greater number of them, mostly those of small size, have become closed.

Where an ice stream descends into the sea or into a large lake, the depth of which is about as great as the ice is thick, the relative lightness of the ice tends to make it float, and it shortly breaks off from the parent mass, forming an iceberg. Where, as is generally the case in those glaciers which enter the ocean, a current sweeps by the place where the berg is formed, it may enter upon a journey which may carry the mass thousands of miles from its origin. The bergs separated from the Greenland glaciers, and from those about the south pole, are often of very great size; sometimes, indeed, they are some thousand feet in thickness, and have a length of several miles. It often happens that these bergs are formed of ice, which contains in its lower part a large amount of rock _débris_. As the submerged portion of the glacier melts in the sea water, these stones are gradually dropped to the bottom, so that the cargo of one berg may be strewed along a line many hundred miles in length. It occasionally happens that the ice mass melts more slowly in those parts which are in the air than in its under-water portions. It thus becomes top-heavy and overturns, in which case such stony matter as remains attains a position where it may be conveyed for a greater distance than if the glacier were not capsized. It is likely, indeed, that now and then fragments of rock from Greenland are dropped on the ocean floor in the part of the Atlantic which is traversed by steamers between our Atlantic ports and Great Britain.

Except for the risks which they bring to navigators, icebergs have no considerable importance. It is true they somewhat affect the temperature of sea and air, and they also serve to convey fragments of stone far out to sea in a way that no other agent can effect; but, on the whole, their influence on the conditions of the earth is inconsiderable.

Icebergs in certain cases afford interesting indices as to the motion of oceanic currents, which, though moving swiftly at a depth below the surface, do not manifest themselves on the plain of the sea. Thus in the region about Greenland, particularly in Davis Strait, bergs have been seen forcing their way southward at considerable speed through ordinary surface ice, which was either at rest or moving in the opposite direction. The train of these bergs, which moves upward from the south polar continent, west of Patagonia, indicates also in a very emphatic way the existence of a very strong northward-setting current in that part of the ocean.

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We have now to consider the causes which could bring about such great extensions of the ice sheet as occurred in the last Glacial period. Here again we are upon the confines of geological knowledge, and in a field where there are no well-cleared ways for the understanding. In facing this problem, we should first note that those who are of the opinion that a Glacial period means a very cold climate in the regions where the ice attained its extension are probably in error. Natural as it may seem to look for exceeding cold as the cause of glaciation, the facts show us that we can not hold this view. In Siberia and in the parts of North America bordering on the Arctic Sea the average cold is so intense that the ground is permanently frozen--as it is, for instance, in the Klondike district--to the depth of hundreds of feet, only the surface thawing out during the warm summers. All this region is cold enough for glaciers, but there is not sufficient snowfall to maintain them. On the other hand, in Greenland, and in a less though conspicuous degree in Scandinavia, where the waters of the North Atlantic somewhat diminish the rigour of the cold, and at the same time bring about a more abundant snowfall, the two actions being intimately related, we have very extensive glaciers. Such facts, which could be very much extended, make it clear that the climate of glacial periods must have been characterized by a great snowfall, and not by the most intense cold.

It is evident that what would be necessary again to envelop the boreal parts of North America with a glacial sheet would not be a considerable decrease of heat, but an increase in the winter's contribution of frozen water. Even if the heat released by this snowfall elevated the average temperature of the winter, as it doubtless would in a considerable measure, it would not melt off the snow. That snowfall tends to warm the air by setting free the heat which was engaged in keeping the water in a state of vapour is familiarly shown by the warming which attends an ordinary snowstorm. Even if the fall begin with a temperature of about 0° Fahr., the air is pretty sure to rise to near the freezing point.

It is evident that no great change of temperature is required in order to bring about a very considerable increase in the amount of snowfall. In the ordinary succession of seasons we often note the occurrence of winters during which the precipitation of snow is much above the average, though it can not be explained by a considerable climatal change. We have to account for these departures from the normal weather by supposing that the atmospheric currents bring in more than the usual amount of moisture from the sea during the period when great falls of snow occur. In fact, in explaining variations in the humidity of the land, whether those of a constant nature or those that are to be termed accidental, we have always to look to those features which determine the importation of vapour from the great field of the ocean where it enters the air. We should furthermore note that these peculiarities of climate are dependent upon rather slight geographic accidents. Thus the snowfall of northern Europe, which serves to maintain the glaciation of that region, and, curiously enough, in some measure its general warmth, depends upon the movement of the Gulf Stream from the tropics to high latitudes. If by any geographical change, such as would occur if Central America were lowered so as to make a free passage for its waters to the westward, the glaciers of Greenland and of Scandinavia would disappear, and at the same time the temperature of those would be greatly lowered. Thus the most evident cause of glaciation must be sought in those alterations of the land which affect the movement of the oceanic currents.

Applying this principle to the northern hemisphere, we can in a way imagine a change which would probably bring about a return of such an ice period as that from which the boreal realm is now escaping. Let us suppose that the region of not very high land about Bering Strait should sink down so as to afford the Kuro Siwo, or North Pacific equivalent of our Gulf Stream, an opportunity to enter the Arctic Sea with something like the freedom with which the North Atlantic current is allowed to penetrate to high latitudes. It seems likely that this Pacific current, which in volume and warmth is comparable to that of the Atlantic, would so far elevate the temperature of the arctic waters that their wide field would be the seat of a great evaporation. Noting once again the fact that the Greenland glaciers, as well as those of Norway, are supplied from seas warmed by the Gulf Stream, we should expect the result of this change would be to develop similar ice fields on all the lands near that ocean.

Applying the data gathered by Dr. Croll for the Gulf Stream, it seems likely that the average annual temperature induced in the Arctic Sea by the free entrance of the Japan current would be between 20° and 30° Fahr. This would convert this wide realm of waters into a field of great evaporation, vastly increasing the annual precipitation. It seems also certain that the greater part of this precipitation would be in the form of snow. It appears to the writer that this cause alone may be sufficient to account for the last Glacial period in the northern hemisphere. As to the probability that the region about Bering Strait may have been lowered in the manner required by this view, it may be said that recent studies on the region about Mount St. Elias show that during or just after the ice epoch the shores in that portion of Alaska were at least four thousand feet lower than at present. As this is but a little way from the land which we should have to suppose to be lowered in order to admit the Japan current, we could fairly conclude that the required change occurred. As for the cause of the land movement, geologists are still in doubt. They know, however, that the attitudes of the land are exceedingly unstable, and that the shores rarely for any considerable time maintain their position. It is probable that these swayings of the earth's surface are due to ever-changing combinations of the weight in different parts of the crust and the strains arising from the contraction of its inner parts.

In the larger operations of Nature the effects which we behold, however simple, are rarely the products of a single cause. In fact, there are few actions so limited that they can fairly be referred to one influence. It is therefore proper to state that there are many other actions besides those above noted which probably enter into those complicated equations which determine the climatal conditions of the earth. To have these would carry us into difficult and speculative inquiries.

As before remarked, all the regions which have been subjected to glaciation are still each year brought temporarily into the glacial state. This fact serves to show us that the changes necessary to produce great ice sheets are not necessarily of a startling nature, however great the consequences may be. Assuming, then, that relatively slight alterations of climate may cause the ice sheet to come and go, we may say that all the influences which have been suggested by the students of glaciation, and various other slighter causes which can not be here noted, may have co-operated to produce the peculiar result. In this equation geographic change has affected the course of the ocean currents, and has probably been the most influential, or at least the commonest, cause to which we must attribute the extension of ice sheets. Next, alterations of the solar heat may be looked to as a change-bringing action; unfortunately, however, we have no direct evidence that this is an efficient cause. Thirdly, the variations in the eccentricity of the earth's orbit, combined with the precession of the equinoxes and the rotation of the apsides, may be regarded as operative. The last of all, changes in the constitution of the atmosphere, have to be taken into account. To these must be added, as before remarked, many less important actions which influence this marvellously delicate machine, the work of which is expressed in the phenomena assembled under the name of climate.

Evidence is slowly accumulating which serves to show that glacial periods of greater or less importance have been of frequent occurrence at all stages in the history of the earth of which we have a distinct record. As these accidents write their history upon the ground alone, and in a way impermanently, it is difficult to trace the ice times of ancient geological periods. The scratches on the bed rocks, and the accumulations of detritus formed as the ice disappeared, have alike been worn away by the agents of decay. Nevertheless, we can trace here and there in the older strata accumulations of pebbly matter often containing large boulders, which clearly were shaped and brought together by glacial action. These are found in some instances far south of the region occupied by the glaciers during the last ice epoch. They occur in rocks of the Cambrian or Silurian age in eastern Tennessee and western North Carolina; they are also found in India beyond the limits to which glaciers have attained in modern times.

In closing this inadequate account of glacial action, a story which for its complete telling would require many volumes, it is well for the reader to consider once again how slight are the changes of climate which may alternately withdraw large parts of the land from the uses of life, and again quickly restore the fields to the service of plants and animals. He may well imagine that these changes, by driving living creatures to and fro, profoundly affect the history of their development. This matter will be dealt with in the volume concerning the history of organic beings.

When the ice went off from the northern part of this continent, the surface of the country, which had been borne down by the weight of the glacier, still remained depressed to a considerable depth below the level of the sea, the depression varying from somewhere about one hundred feet in southern New England to a thousand feet or more in high latitudes. Over this region, which lay beneath the level of the sea, the glacier, when it became thin enough to float, was doubtless broken up into icebergs, in the manner which we now behold along the coast of Greenland. Where the shore was swept by a strong current, these bergs doubtless drifted away; but along the most of the coast line they appear to have lain thickly grouped next the shores, gradually delivering their loads of stones and finer _débris_ to the bottom. These masses of floating ice in many cases seem to have prevented the sea waves from attaining the shore, and thus hindered the formation of those beaches which in their present elevated condition enable us to interpret the old position of the sea along coast lines which have been recently elevated. Here and there, however, from New Jersey to Greenland, we find bits of these ancient shores which clearly tell the story of that down-sinking of the land beneath the burden of the ice which is such an instructive feature in the history of that period.