Muir Glacier, Alaska, showing crevasses and dust layer on surface of ice.Muir Glacier, Alaska, showing crevasses and dust layer on surface of ice.
We have already noted the fact that the water in the clouds is very commonly in the frozen state; a large part of that fluid which is evaporated from the sea attains the solid form before it returns to the earth. Nevertheless, in descending, at least nine tenths of the precipitation returns to the fluid state, and does the kind of work which we have noted in our account of water. Where, however, the water arrives on the earth in the frozen condition, it enters on a rôle totally different from that followed by the fluid material.
Beginning its descent to the earth in a snowflake, the little mass falls slowly, so that when it comes against the earth the blow which it strikes is so slight that it does no effective work. In the state of snow, even in the separate flakes, the frozen water contains a relatively large amount of air. It is this air indeed, which, by dividing the ice into many flakes that reflect the light, gives it the white colour. This important point can be demonstrated by breaking transparent ice into small bits, when we perceive that it has the hue of snow. Much the same effect is given where glass is powdered, and for the same reason.
As the snowflakes accumulate layer on layer they imbed air between them, so that when the material falls in a feathery shape—say to the depth of a foot—more than nine tenths of the mass is taken up by the air-containing spaces. As these cells are very small, the circulation in them is slight, and so the layer becomes an admirable non-conductor, having this quality for the same reason that feathers have it—i.e., because the cells are small enough to prevent the circulation of the air, so that the heat which passes has to go by conduction, and all gases are very poor conductors. The result is that a snow coating is in effect an admirable blanket. When the sun shines upon it, much of the heat is reflected, and as the temperature does not penetrate it to any depth, only the superficial part is melted. This molten water takes up in the process of melting a great deal of heat, so that when it trickles down into the mass it readily refreezes. On the other hand, the heat going out from the earth, the store accumulated in its superficial parts in the last warm season, together with the small share which flows out from the earth's interior, is held in by this blanket, which it melts but slowly. Thus it comes about that in regions of long-enduring snowfall the ground, though frozen to the depth of a foot or more at the time when the accumulation took place, may be thawed out and so far warmed that the vegetation begins to grow before the protecting envelope of snow has melted away. Certain of the early flowers of high latitudes, indeed, begin to blossom beneath the mantle of finely divided ice.
In those parts of the earth which for the most part receive only a temporary coating of snow the effect of this covering is inconsiderable. The snow water is yielded to the earth, from which it has helped to withdraw the frost, so that in the springtime, the growing season of plants, the ground contains an ample store of moisture for their development. Where the snowfall accumulates to a great thickness, especially where it lodges in forests, the influence of the icy covering is somewhat to protract the winter and thus to abbreviate the growing season.
Where snow rests upon a steep slope, and gathers to the depth of several feet, it begins to creep slowly down the declivity in a manner which we may often note on house roofs. This motion is favoured by the gradual though incomplete melting of the flakes as the heat penetrates the mass. Making a section through a mass of snow which has accumulated in many successive falls, we note that the top may still have the flaky character, but that as we go down the flakes are replaced by adherent shotlike bodies, which have arisen from the partial melting and gathering to their centres of the original expanded crystalline bits. In this process of change the mass can move particle by particle in the direction in which gravity impels it. The energy of its motion, however, is slight, yet it can urge loose stones and forest waste down hill. Sometimes, as in the cemetery at Augusta, Me., where stone monuments or other structures, such as iron railings, are entangled in the moving mass, it may break them off and convey them a little distance down the slope.
So long as the summer sun melts the winter's snow, even if the ground be bare but for a day, the rôle of action accomplished by the snowfall is of little geological consequence. When it happens that a portion of the deposit holds through the summer, the region enters on the glacial state, and its conditions undergo a great revolution, the consequences of which are so momentous that we shall have to trace them in some detail. Fortunately, the considerations which are necessary are not recondite, and all the facts are of an extremely picturesque nature.
Taking such a region as New England, where all the earth is life-bearing in the summer season, and where the glacial period of the winter continues but for a short time, we find that here and there on the high mountains the snow endures throughout most of the summer, but that all parts of the surface have a season when life springs into activity. On the top of Mount Washington, in the White Mountains of New Hampshire, in a cleft known as Tuckerman's Ravine, where the deposit accumulates to a great depth, the snow-ice remains until midsummer. It is, indeed, evident that a very slight change in the climatal conditions of this locality would establish a permanent accumulation of frozen water upon the summit of the mountain. If the crest were lifted a thousand feet higher, without any general change in the heat or rainfall of the district, this effect would be produced. If with the same amount of rainfall as now comes to the earth in that region more of it fell as snow, a like condition would be established. Furthermore, with an increase of rainfall to something like double that which now descends the snow bore the same proportion to the precipitation which it does at present, we should almost certainly have the peak above the permanent snow line, that level below which all the winter's fall melts away. These propositions are stated with some care, for the reason that the student should perceive how delicate may be—indeed, commonly is—the balance of forces which make the difference between a seasonal and a perennial snow covering.
As soon as the snow outlasts the summer, the region which it occupies is sterilized to life. From the time the snow begins to hold over the warm period until it finally disappears, that field has to be reckoned out of the habitable earth, not only to man, but to the lowliest organisms.[6]
If the snow in a glaciated region lay where it fell, the result would be a constant elevation of the deposit year by year in proportion to the annual excess of deposition over the melting or evaporation of the material. But no sooner does the deposit attain any considerable thickness than it begins to move in the directions of least resistance, in accordance with laws which the students of glaciers are just beginning to discern. In small part this motion is accomplished by avalanches or snow slides, phenomena which are in a way important, and therefore merit description.Immediately after a heavy snowfall, in regions where the slopes are steep, it often happens that the deposit which at first clung to the surface on which it lay becomes so heavy that it tends to slide down the slope; a trifling action, the slipping, indeed, of a single flake, may begin the movement, which at first is gradual and only involves a little of the snow. Gathering velocity, and with the materials heaped together from the junction of that already in motion with that about to be moved, the avalanche in sliding a few hundred feet down the slope may become a deep stream of snow-ice, moving with great celerity. At this stage it begins to break off masses of ice from the glaciers over which it may flow, or even to move large stones. Armed with these, it rends the underlying earth. After it has flowed a mile it may have taken up so much earth and material that it appears like a river of mud. Owing to the fact that the energy which bears it downward is through friction converted into heat, a partial melting of the mass may take place, which converts it into what we call slush, or a mixture of snow and water. Finally, the torrent is precipitated into the bottom of a valley, where in time the frozen water melts away, leaving only the stony matter which it bore as a monument to show the termination of its flow.
It was the good fortune of the writer to see in the Swiss Oberland one very great avalanche, which came from the high country through a descent of several thousand feet to the surface of the Upper Grindelwald Glacier. The first sign of the action was a vague tremor of the air, like that of a great organ pipe when it begins to vibrate, but before the pulsations come swiftly enough to make an audible note. It was impossible to tell when this tremor came, but the wary guide, noting it before his charge could perceive anything unusual, made haste for the middle of the glacier. The vibration swelled to a roar, but the seat of the sound amid the echoing cliffs was indeterminable. Finally, from a valley high up on the southern face of the glacier,there leaped forth first a great stone, which sprang with successive rebounds to the floor of ice. Then in succession other stones and masses of ice which had outrun the flood came thicker and thicker, until at the end of about thirty seconds the steep front of the avalanche appeared like a swift-moving wall. Attaining the cliffs, it shot forth as a great cataract, which during the continuance of the flow—which lasted for several minutes—heaped a great mound of commingled stones and ice upon the surface of the glacier. The mass thus brought down the steep was estimated at about three thousand cubic yards, of which probably the fiftieth part was rock material. An avalanche of this volume is unusual, and the proportion of stony matter borne down exceptionally great; but by these sudden motions of the frozen water a large part of the snow deposited above the zone of complete melting is taken to the lower valleys, where it may disappear in the summer season, and much of the erosion accomplished in the mountains is brought about by these falls.
In all Alpine regions avalanches are among the most dreaded accidents. Their occurrence, however, being dependent upon the shape of the surface, it is generally possible to determine in an accurate way the liability of their happening in any particular field. The Swiss take precaution to protect themselves from their ravages as other folk do to procure immunity from floods. Thus the authorities of many of the mountain hamlets maintain extensive forests on the sides of the villages whence the downfall may be expected, experience having shown that there is no other means so well calculated to break the blow which these great snowfalls can deliver, as thick-set trees which, though they are broken down for some distance, gradually arrest the stream.
As long as the region occupied by permanent snow is limited to sharp mountain peaks, relief by the precipitation of large masses to the level below the snow line is easily accomplished, but manifestly this kind of a discharge canonly be effective from a very small field. Where the relief is not brought about by these tumbles of snow, another mode of gravitative action accomplishes the result, though in a more roundabout way, through the mechanism of glaciers.
We have already noted the fact that the winter's snow upon our hillsides undergoes a movement in the direction of the slope. What we have now to describe in a rather long story concerning glaciers rests upon movements of the same nature, though they are in certain features peculiarly dependent on the continuity of the action from year to year. It is desirable, however, that the student should see that there is at the foundation no more mystery in glacial motion than there is in the gradual descent of the snow after it has lain a week on a hillside. It is only in the scale and continuity of the action that the greatest glacial envelope exceeds those of our temporary winters—in fact, whenever the snow falls the earth it covers enters upon an ice period which differs only in degree from that from which our hemisphere is just escaping.
Where the reader is so fortunate as to be able to visit a region of glaciers, he had best begin his study of their majestic phenomena by ascending to those upper realms where the snow accumulates from year to year. He will there find the natural irregularities of the rock surface in a measure evened over by a vast sheet of snow, from which only the summits of the greater mountains rise. He may soon satisfy himself that this sheet is of great depth, for here and there it is intersected by profound crevices. If the visit is made in the season when snow falls, which is commonly during most of the year, he may observe, as before noted in our winter's snow, that the deposit, though at first flaky, attains at a short distance below the surface a somewhat granular character, though the shotlike grains fall apart when disturbed. Yet deeper, ordinarily a few feet below the surface, these granules are more or less cemented together; the mass thus loses the quality ofsnow, and begins to appear like a whitish ice. Looking down one of the crevices, where the light penetrates to the depth of a hundred feet or more, he may see that the bluish hue somewhat increases with the depth. A trace of this colour is often visible even in the surface snow on the glacier, and sometimes also in our ordinary winter fields. In a hole made with a stick a foot or more in depth a faint cerulean glimmer may generally be discerned; but the increased blueness of the ice as we go down is conspicuous, and readily leads us to the conclusion that the air, to which, as we before noted, the whiteness of the snow is due, is working out of the mass as the process of compaction goes on. In a glacial district this snow mass above the melting line is called thenévé.
Remembering that the excess of snow beyond the melting in anévédistrict amounts, it may be, to some feet of material each year, we easily come to the conclusion that the mass works down the slope in the manner which it does even where the coating is impermanent. This supposition is easily confirmed: by observing the field we find that the sheet is everywhere drawing away from the cliffs, leaving a deep fissure between thenévéand the precipices. This crevice is called by the German-Swiss guides theBergschrund. Passage over it is often one of the most difficult feats to accomplish which the Alpine explorer has to undertake. In fact, the very appearance of the surface, which is that of a river with continuous down slopes, is sufficient evidence that the mass is slowly flowing toward the valleys. Following it down, we almost always come to a place where it passes from the upper valleys to the deeper gorges which pierce the skirts of the mountain. In going over this projection the mass of snow-ice breaks to pieces, forming a crowd of blocks which march down the slope with much more speed than they journeyed when united in the higher-lying fields. In this condition and in this part of the movement the snow-ice forms what are called theseracs, or curds, as the word means in the French-Swissdialect. Slipping and tumbling down the steep slope on which theseracsdevelop, the ice becomes broken into bits, often of small size. These fragments are quickly reknit into the body of ice, which we shall hereafter term the glacier, and in this process the expulsion of the air goes on more rapidly than before, and the mass assumes a more transparent icelike quality.
The action of the ice in the pressures and strains to which it is subjected in joining the main glacier and in the further part of its course demand for their understanding a revision of those notions as to rigidity and plasticity which we derive from our common experience with objects. It is hard to believe that ice can be moulded by pressure into any shape without fracturing, provided the motion is slowly effected, while at the same time it is as brittle as ice to a sudden blow. We see, however, a similar instance of contrasted properties in the confection known as molasses candy, a stick of which may be indefinitely bent if the flexure is slowly made, but will fly to pieces like glass if sharply struck. Ice differs from the sugary substance in many ways; especially we should note that while it may be squeezed into any form, it can not be drawn out, but fractures on the application of a very slight tension. The conditions of its movement we will inquire into further on, when we have seen more of its action.
Entering on the lower part of its course, that where it flows into the region below the snow line, the ice stream is now confined between the walls of the valley, a channel which in most cases has been shaped before the ice time, by a mountain torrent, or perhaps by a slower flowing river. In this part of its course the likeness of a glacial stream to one of fluid water is manifest. We see that it twists with the turn of the gorge, widens where the confining walls are far apart, and narrows where the space is constricted. Although the surface is here and there broken by fractures, it is evident that the movement of the frozen current, though slow, is tolerably free. By placing stakesin a row across the axis of a glacier, and observing their movement from day to day, or even from hour to hour if a good theodolite is used for the purpose, we note that the movement of the stream is fastest in the middle parts, as in the case of a river, and that it slows toward either shore, though it often happens, as in a stream of molten water, that the speediest part of the current is near one side. Further observations have indicated that the movement is most rapid on the surface and least at the bottom, in which the stream is also riverlike. It is evident, in a word, that though the ice is not fluid in strict sense, the bits of which it is made up move in substantially the manner of fluids—that is, they freely slip over each other. We will now turn our attention to some important features of a detailed sort which glaciers exhibit.
If we visit a glacier during the part of the year when the winter snows are upon it, it may appear to have a very uninterrupted surface. But as the summer heat advances, the mask of the winter coating goes away, and we may then see the structure of the ice. First of all we note in all valley glaciers such as we are observing that the stream is overlaid by a quantity of rocky waste, the greater part of which has come down with the avalanches in the manner before described, though a small part may have been worn from the bed over which the ice flows. In many glaciers, particularly as we approach their termination, this sheet of earth and rock materials often covers the ice so completely that the novice in such regions finds it difficult to believe that the ice is under his feet. If the explorer is minded to take the rough scramble, he can often walk for miles on these masses of stone without seeing, much less setting foot on any frozen water. In some of the Alaskan glaciers this coating may bear a forest growth. In general, this material, which is called moraine, is distributed in bands parallel to the sides of the glaciers, and the strips may amount to a half dozen or more. Those on the sides of the ice have evidently been derived from theprecipices which they have passed. Those in the middle have arisen from the union of the moraines formed in two or more tributary valleys.
Fig. 12.—Map of glaciers and moraines near Mont Blanc.Fig. 12.—Map of glaciers and moraines near Mont Blanc.
Where the avalanches fall most plentifully, the stones lie buried with the snow, and only melt out when the stream attains the region where the annual waste of its surface exceeds the snowfall. In this section we can see how the progressive melting gradually brings the rockydébrisinto plain view. Here and there we will find a boulder perched on a pedestal of ice, which indicates a recent down-wearing of the field. A frequent sound in these regions arises from the tumble of the stones from their pedestals or the slipping of the masses from the sharp ridge which is formed by the protection given to the ice through the thick coating of detritus on its surface. These movements of the moraines often distribute their waste over the glacier, so that in its lower part we can nolonger trace the contributions from the several valleys, the whole area being covered by thedébris. At the end of the ice stream, where its forward motion is finally overcome by the warmth which it encounters, it leaves in a rude heap, extending often like a wall across the valley, all the coarse fragments which it conveys. This accumulation, composed of all the lateral moraines which have gathered on the ice by the fall of avalanches, is called the terminal moraine. As the ice stream itself shrinks, a portion of the detritus next the boundary wall is apt to be left clinging against those slopes. It is from the presence of these heaps in valleys now abandoned by glaciers that we obtain some information as to the former greater extent of glacial action.
The next most noticeable feature is the crevasse. These fractures often exist in very great numbers, and constitute a formidable barrier in the explorer's way. The greater part of these ruptures below theseraczone run from the sides of the stream toward the centre without attaining that region. These are commonly pointed up stream; their formation is due to the fact that, owing to the swifter motion in the central parts of the stream, the ice in that section draws away from the material which is moving more slowly next the shore. As before noted, these ice fractures when drawn out naturally form fissures at right angles to the direction of the strain. In the middle portions of the ice other fissures form, though more rarely, which appear to depend on local strains brought about through the irregularity of the surface over which the ice is flowing.
If the observer is fortunate, he may in his journey over the glacier have a chance to see and hear what goes on when crevasses are formed. First he will hear a deep, booming sound beneath his feet, which merges into a more splintering note as the crevice, which begins at the bottom or in the distance, comes upward or toward him. When the sound is over, he may not be able to see a trace of thefracture, which at first is very narrow. But if the break intersect any of the numerous shallow pools which in a warm summer's day are apt to cover a large part of the surface, he may note a line of bubbles rushing up through the water, marking the escape of the air from the glacier, some remnant of that which is imprisoned in the original snow. Even where this indication is wanting, he can sometimes trace the crevice by the hissing sound of the air streams where they issue from the ice. If he will take time to note what goes on, he can usually in an hour or two behold the first invisible crack widen until it may be half an inch across. He may see how the surface water hastens down the opening, a little river system being developed on the surface of the ice as the streams make their way to one or more points of descent. In doing this work they excavate a shaft which often becomes many feet in diameter, down which their waters thunder to the base of the glacier. This well-like opening is called amoulin, or mill, a name which, as we shall see, is well deserved from the work which falling waters accomplish. Although the institution of themoulinshaft depends upon the formation of a crevice, it often happens that as the ice moves farther on its journey its walls are again thrust together, soldered in the manner peculiar to ice, so that no trace of the rupture remains except the shaft which it permitted to form. Like everything else in the glacier, themoulinslowly moves down the slope, and remains open as long as it is the seat of descending waters produced by the summer melting. When it ceases to be kept open from the summer, its walls are squeezed together in the fashion that the crevices are closed.
Forming here and there, and generally in considerable numbers, the crevices of a glacier entrap a good deal of the morainaldébris, which falls through them to the bottom of the glacier. Smaller bits are washed into themoulin, by the streams arising from the melting ice, which is brought about by the warm sun of the summer, and particularly by the warm rains of that season. On those glaciers where, owing to the irregularity of the bottom over which the ice flows, these fractures are very numerous, it may happen that all the detritus brought upon the surface of the glacier by avalanches finds its way to the floor of the ice.
Although it is difficult to learn what is going on at the under surface of the glacier, it is possible directly and indirectly to ascertain much concerning the peculiar and important work which is there done. The intrepid explorer may work his way in through the lateral fissures, and even with care safely descend some of the fissures which penetrate the central parts of a shallow ice stream. There, it may be at the depth of a hundred feet or more, he will find a quantity of stones, some of which may be in size like to a small house held in the body of the ice, but with one side resting upon the bed rock. He may be so fortunate as to see the stone actually in process of cutting a groove in the bed rock as it is urged forward by the motion of the glacier. The cutting is not altogether in the fixed material, for the boulder itself is also worn and scored in the work. Smaller pebbles are caught in the space between the erratic and the motionless rock and ground to bits. If in his explorations the student finds his way to the part of the floor on which the waters of amoulinfall, he may have a chance to observe how the stones set in motion serve to cut the bed rock, forming elongated potholes much as in the case of ordinary waterfalls, or at the base of those shafts which afford the beginnings of limestone caverns.
The best way to penetrate beneath the glacier is through the arch of the stream which always flows from the terminal face of the ice river. Even in winter time every large glacier discharges at its end a considerable brook, the waters of which have been melted from the ice in small part by the outflow of the earth's heat; mainly, however, by the warmth produced in the friction of the ice on itself and on its bottom—in other words, by the conversion of that energy of position, of which we have oftento speak, into heat. In the summer time this subglacial stream is swollen by the surface waters descending through the crevices and themoulinswhich come from them, so that the outflow often forms a considerable river, and thus excavates in the ice a large or at least a long cavern, the base of which is the bed rock. In the autumn, when the superficial melting ceases, this gallery can often be penetrated for a considerable distance, and affords an excellent way to the secrets of the under ice. The observer may here see quantities of the rock material held in the grip of the ice, and forced to a rude journey over the bare foundation stones. Now and then he may find the glacial mass in large measure made up of stones, the admixture extending many feet above the bottom of the cavern, perhaps to the very top of the arch. He may perchance find that these stones are crushing each other where they are in contact. The result will be brought about by the difference in the rate of advance of the ice, which moves the faster the higher it is above the surface over which it drags, and thus forces the stones on one level over those below. Where the waters of the subglacial stream have swept the bed rock clean ofdébrisits surface is scored, grooved, and here and there polished in a manner which is accomplished only by ice action, though some likeness to it is afforded where stones have been swept over for ages by blowing sand. Here and there, often in a way which interrupts the cavern journey, the shrunken stream, unable to carry forward thedébris, deposits the material in the chamber, sometimes filling the arch so completely that the waters are forced to make a detour. This action is particularly interesting, for the reason that in regions whence glaciers have disappeared the deposits formed in the old ice arches often afford singularly perfect moulds of those caverns which were produced by the ancient subglacial streams. These moulds are termedeskers.
If the observer be attentive, he will note the fact that the waters emerging from beneath the considerable glacierare very much charged with mud. If he will take a glass of the water at the point of escape, he will often find, on permitting it to settle, that the sediment amounts to as much as one twentieth of the volume. While the greater part of this detritus will descend to the bottom of the vessel in the course of a day, a portion of it does not thus fall. He may also note that this mud is not of the yellowish hue which he is accustomed to behold in the materials laid down by ordinary rivers, but has a whitish colour. Further study will reveal the fact that the difference is due to the lack of oxidation in the case of the glacial detritus. River muds forming slowly and during long-continued exposure to the action of the air have their contained iron much oxidized, which gives them a part of their darkened appearance. Moreover, they are somewhat coloured with decayed vegetable matter. The waste from beneath the glacier has been quickly separated from the bed rock, all the faces of the grains are freshly fractured, and there is no admixture of organic matter. The faces of the particles thus reflect light in substantially the same way as powdered glass or pulverized ice, and consequently appear white.
A little observation will show the student that this very muddy character of waters emerging from beneath the glacier is essentially peculiar to such streams as we have described. Ascending any of the principal valleys of Switzerland, he may note that some of the streams flow waters which carry little sediment even in times when they are much swollen, while others at all seasons have the whitish colour. A little further exploration, or the use of a good map, will show him that the pellucid streams receive no contributions of glacial water, while those which look as if they were charged with milk come, in part at least, from the ice arches. From some studies which the writer has made in Swiss valleys, it appears that the amount of erosion accomplished on equal areas of similar rock by the descent of the waters in the form of a glacier or in thatof ordinary torrents differs greatly. Moving in the form of ice, or in the state of ice-confined streams, the mass of water applies very many times as much of its energy of position to grinding and bearing away the rocks as is accomplished where the water descends in its fluid state.
The effect of the intense ice action above noted is rapidly to wear away the rocks of the valley in which the glacier is situated. This work is done not only in a larger measure but in a different way from that accomplished by torrents. In the case of the latter, the stream bed is embarrassed by the rubbish which comes into it; only here and there can it attack the bed rock by forcing the stones over its surface. Only in a few days of heavy rain each year is its work at all effective; the greater part of the energy of position of its waters is expended in the endless twistings and turnings of its stream, which result only in the development of heat which flies away into the atmosphere. In the ice stream, owing to its slow movement and to the detritus which it forces along the bottom, a vastly greater part of the energy which impels it down the slope is applied to rock cutting. None of the boulders, even if they are yards in diameter, obstruct its motion; small and great alike are to it good instruments wherewith to attack the bed rocks. The fragments are never left to waste by atmospheric decay, but are to a very great extent used up in mechanical work, while the most of the detritus which comes to a torrent is left in a coarse state when it is delivered to the stream; the larger part of that which the glacier transports is worn out in its journey. To a great extent it is used up in attacking the bed rock. In most cases thedébrisin the terminal moraine is evidently but a small part of what entered the ice during its journey from the uplands; the greater part has been worn out in the rude experiences to which it has been subjected.
It is evident that even in the regions now most extensively occupied by glaciers the drainage systems have been shaped by the movement of ordinary streams—in otherwords, ice action is almost everywhere, even in the regions about the poles, an incidental feature in the work of water, coming in only to modify the topography, which is mainly moulded by the action of fluid water. When, owing to climatal changes, a valley such as those of the Alps is occupied by a glacial stream, the new current proceeds at once, according to its evident needs, to modify the shape of its channel. An ordinary torrent, because of the swiftness of its motion, which may, in general, be estimated at from three to five miles an hour, can convey away the precipitation over a very narrow bed. Therefore its channel is usually not a hundredth part as wide as the gorge or valley in which it lies. But when the discharge takes place by a glacier, the speed of which rarely exceeds four or five feet a day, the ice stream because of its slow motion has to fill the trough from side to side, it has to be some thousand times as deep and wide as the torrent. The result is that as soon as the glacial condition arises in a country the ice streams proceed to change the old V-shaped torrent beds into those which have a broad U-like form. The practised eye can in a way judge how long a valley has been subjected to glacial action by the extent to which it has been widened by this process.
In the valleys of Switzerland and other mountain districts which have been attentively studied it is evident that glacial action has played a considerable part in determining their forms. But the work has been limited to that part of the basin in which the ice is abundantly provided with cutting tools in the stone which have found their way to the base of the stream. In the region of thenévé, where the contributions of rocky matter to the surface of the deposit made from the few bare cliffs which rise above the sheet of snow is small, the snow-ice does no cutting of any consequence. Where it passes over the steep at the head of the deep valley into which it drains, and is riven into theseracs, such stony matter as it may have gathered is allowed to fall to the bottom, and so comes into aposition where it may do effective work. From thisseracsection downward the now distinct ice river, being in general below the snow line, has everywhere cliffs, on either side from which the contributions of rock material are abundant. Hence this part of the glacier, though it is the wasting portion of its length, does all the cutting work of any consequence which is performed. It is there that the underrunning streams become charged with sediment, which, as we have noted, they bear in surprising quantities, and it is therefore in this section of the valley that the impress of the ice work is the strongest. Its effect is not only to widen the valley and deepen it, but also to advance the deep section farther up the stream and its tributaries. The step in the stream beds which we find at theseracsappears to mark the point in the course of the glacier where, owing to the falling of stones to its base, as well as to its swifter movements and the firmer state of the ice, it does effective wearing.
There are many other features connected with glaciers which richly repay the study of those who have a mind to explore in the manner of the physicist interested in ice actions the difficult problems which they afford; but as these matters are not important from the point of view of this work, no mention of them will here be made. We will now turn our attention to that other group of glaciers commonly termed continental, which now exist about either pole, and which at various times in the earth's history have extended far toward the equator, mantling over vast extents of land and shallow sea. The difference between the ice streams of the mountains and those which we term continental depends solely on the areas of the fields and the depth of the accumulation. In an ordinary Alpine region thenévédistricts, where the snow gathers, are relatively small. Owing to the rather steep slopes, the frozen water is rapidly discharged into the lower valleys, where it melts away. Both in thenévéand in the distinct glacier of the lower grounds there are, particularly in the latter, projecting peaks, from which quantities of stone are brought down by avalanches or in ordinary rock falls, so that the ice is abundantly supplied with cutting tools, which work from its surface down to its depths.
As the glacial accumulation grows in depth there are fewer peaks emerging from it, and the streams which it feeds rise the higher until they mantle over the divides between the valleys. Thus by imperceptible stages valley glaciers pass to the larger form, usually but incorrectly termed continental. We can, indeed, in going from the mountains in the tropics to the poles, note every step in this transition, until in Greenland we attain the greatest ice mass in the world, unless that about the southern pole be more extensive. In the Greenland glacier the ice sheet covers a vast extent of what is probably a mountain country, which is certainly of this nature in the southern part of the island, where alone we find portions of the earth not completely covered by the deep envelope. Thanks to the labours of certain hardy explorers, among whom Nansen deserves the foremost place, we now know something as to the conditions of this vast ice field, for it has been crossed from shore to shore. The results of these studies are most interesting, for they afford us a clew as to the conditions which prevail over a large part of the earth during the Glacial period from which the planet is just escaping, and in the earlier ages when glaciation was likewise extensive. We shall therefore consider in a somewhat detailed way the features which the Greenland glacier presents.
Starting from the eastern shore of that land, if we may thus term a region which presents itself mainly in the form of ice, we find next the shore a coast line not completely covered with ice and snow, but here and there exhibiting peaks which indicate that if the frozen mantle were removed the country would appear deeply intersected with fiords in the manner exhibited in the regions to the south of Greenland or the Scandinavian peninsula. The icecomes down to the sea through the valleys, often facing the ocean for great distances with its frozen cliffs. Entering on this seaward portion of the glacier, the observer finds that for some distance from the coast line the ice is more or less rifted with crevices, the formation of which is doubtless due to irregularities of the rock bottom over which it moves. These ruptures are so frequent that for some miles back it is very difficult to find a safe way. Finally, however, a point is attained where these breaks rather suddenly disappear, and thence inward the ice rises at the rate of upward slope of a few feet to the mile in a broad, nearly smooth incline. In the central portion of the region for a considerable part of the territory the ice has very little slope. Thence it declines toward the other shore, exhibiting the same features as were found on the eastern versant until near the coast, when again the surface is beset with crevices which continue to the margin of the sea.
Although the explorations of the central field of Greenland are as yet incomplete, several of these excursions into or across the interior have been made, and the identity of the observations is such that we can safely assume the whole region to be of one type. We can furthermore run no risk in assuming that what we find in Greenland, at least so far as the unbroken nature of the central ice field is concerned, is what must exist in every land where the glacial envelope becomes very deep. In Greenland it seems likely that the depth of the ice is on the average more than half a mile, and in the central part of the realm the sheet may well have a much greater profundity; it may be nearly a mile deep. The most striking feature—that of a vast unbroken expanse, bordered by a region where the ice is ruptured—is traceable wherever very extensive and presumably deep deposits of ice have been examined. As we shall see hereafter, these features teach us much as to the conditions of glacial action—a matter which we shall have to examine after we have completed our generalsurvey as to the changes which occur during glacial periods.
In the present state of that wonderful complex of actions which we term climate, glaciers are everywhere, so far as our observations enable us to judge, generally in process of decrease. In Switzerland, although the ancients even in Roman days were in contact with the ice, they were so unobservant that they did not even remark that the ice was in motion. Only during the last two centuries have we any observations of a historic sort which are of value to the geologist. Fortunately, however, the signs written on the rock tell the story, except for its measurement in terms of years, as clearly as any records could give it. From this testimony of the rocks we perceive that in the geological yesterday, though it may have been some tens of thousands of years ago, the Swiss glaciers, vastly thickened, and with their horizontal area immensely expanded, stretched over the Alpine country, so that only here and there did any of the sharper peaks rise above the surface. These vast glaciers, almost continually united on their margins, extended so far that every portion of what is now the Swiss Republic was covered by them. Their front lay on the southern lowlands of Germany, on the Jura district of France; on the south, it stretched across the valley of the Po as far as near Milan. We know this old ice front by the accumulations of rockdébriswhich were brought to it from the interior of the mountain realm. We can recognise the peculiar kinds of stone, and with perfect certainty trace them to the bed rock whence they were riven. Moreover, we can follow back through the same evidence the stages of retreat of the glaciers, until they lost their broad continental character and assumed something like their present valley form. Up the valley of any of the great rivers, as, for instance, that of the Rhône above the lake of Geneva, we note successive terminal moraines which clearly indicate stages in the retreat of the ice when for a time it ceased to go backward, or even made a slight temporaryreadvance. It is easily seen that on such occasions the stones carried to the ice front would be accumulated in a heap, while during the time when day by day the glacier was retreating the rock waste would be left broadcast over the valley.
As we go up from the course of the glacial streams we note that the successive moraines have their materials in a progressively less decayed state. Far away from the heap now forming, and in proportion to the distance, the stones have in a measure rotted, and the heaps which they compose are often covered with soil and occupied by forests. Within a few miles of the ice front the stones still have a fresh aspect. When we arrive within, say, half a mile of the moraine now building, we come to the part of the glacial retreat of which we have some written or traditional account. This is in general to the effect that the wasting of the glaciers is going on in this century as it went on in the past. Occasionally periods of heavy snow would refresh the ice streams, so that for a little time they pushed their fronts farther down the valley. The writer has seen during one of these temporary advances the interesting spectacle of ice destroying and overturning the soil of a small field which had been planted in grain.
It should be noted that these temporary advances of the ice are not due to the snowfall of the winter or winters immediately preceding the forward movement. So slow is the journey of the ice from thenévéfield to the end of a long glacier that it may require centuries for the store accumulated in the uplands to affect the terminal portion of the stream. We know that the bodies of the unhappy men who have been lost in the crevices of the glacier are borne forward at a uniform and tolerably computable rate until they emerge at the front, where the ice melts away. In at least one case the remains have appeared after many years in thedébriswhich is contributed to the moraine. On account of this slow feeding of the glacial stream, we naturally may expect to find, as we do, in fact, that a greatsnowfall of many years ago, and likewise a period when the winter's contribution has been slight, would influence the position of the terminal point of the ice stream at different times, according to its length. If the length of the flow be five miles, it may require twenty or thirty years for the effect to be evident; while if the stream be ten miles long, the influence may not be noted in less than threescore years. Thus it comes about that at the present time in the same glacial district some streams may be advancing while others are receding, though, on the whole, the ice is generally in process of shrinkage. If the present rate of retreat should be maintained, it seems certain that at the end of three centuries the Swiss glaciers as a whole will not have anything like their present area, and many of the smaller streams will entirely disappear.
Following the method of the illustrious Louis Agassiz, who first attentively traced the evidence which shows the geologically recent great extension of glaciers by studying the evidence of the action in fields they no longer occupy, geologists have now inspected a large part of the land areas with a view to finding the proofs of such ice work. So far as these indications are concerned, the indications which they have had to trace are generally of a very unmistakable character. Rarely, indeed, does a skilled student of such phenomena have to search in any region for more than a day before he obtains indubitable evidence which will enable him to determine whether or not the field has recently been occupied by an enduring ice sheet—one which survives the summer season and therefore deserves the name of glacier. The indications which he has to consider consist in the direction and manner in which the surface materials have been carried, the physical conditions of these materials, the shape of the surface of the underlying rock as regards its general contour, and the presence or absence of scratches and groovings on its surface. As these records of ice action are of first importance in dealing with this problem, and as they afford excellent subjects for the studyof those who dwell in glaciated regions, we shall note them in some detail.
The geologist recognises several ways in which materials may be transported on the surface of the earth. They may be cast forth by volcanoes, making their journey by being shot through the air, or by flowing in lava streams; it is always easy at a glance, save in very rare instances, to determine whether fragments have thus been conveyed. Again, the detritus may be moved by the wind; this action is limited; it only affects dust, sand, and very small pebbles, and is easily discriminated. The carriage may be effected by river or marine currents; here, again, the size of the fragments moved is small, and the order of their arrangement distinctly traceable. The fragments may be conveyed by ice rafts; here, too, the observer can usually limit the probabilities he has to consider by ascertaining, as he can generally do, whether the region which he is observing has been below a sea or lake. In a word, the before-mentioned agents of transportation are of somewhat exceptional influence, and in most cases can, as explanations of rock transportation, be readily excluded. When, therefore, the geologist finds a country abundantly covered with sand, pebbles, and boulders arranged in an irregular way, he has generally only to inquire whether the material has been carried by rivers or by glaciers. This discrimination can be quickly and critically effected. In the first place, he notes that rivers only in their torrent sections can carry large fragments of rock, and that in all cases the fragments move down hill. Further, that where deposits are formed, they have more or less the form of alluvial deposits. If now the observations show that the rock waste occupying the surface of any region has been carried up hill and down, across the valleys, particularly if there are here and there traces of frontal moraines, the geologist is entitled to suppose—he may, indeed, be sure—that the carriage has been effected by a glacial sheet.
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 thecontinent. 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 tothe 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 intervalbetween 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 thischange, 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 withdrift, 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 thedébrisshould 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. Whena 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 LakeErie 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 frontor 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 thedébrisfrom 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 stratifiedsand and gravel which were packed into the caverns excavated by these subglacial rivers, which are known to glacialists aseskers, 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.