Popular Science Monthly/Volume 9/June 1876/The Polar Glaciers II
|THE POLAR GLACIERS.|
THE element of all others most sensitive to the changes and impulses of ever) 7 kind of force is the earth's atmosphere. It is in a state of constant disturbance, and seems to be obedient to no laws or regularity. Yet, unstable as the winds appear, they are really, in their general movements, among the most orderly and effective agents in Nature. This is shown in a remarkable manner by their agency in impelling the great ocean-streams, and therefore their important influence on glacial phenomena. In order to make this evident, it will be necessary to explain in brief the general laws of their circulation.
The earth turns on its axis from west to east, and with it rotates daily the enormous envelope of the atmosphere. The velocity of rotation at the equator is something over 1,000 miles an hour; at thirty degrees distance it is about 150 miles an hour less. In higher latitudes it is still less; and at the poles nothing. Therefore, whenever the air moves north or south on the surface of the earth, it will carry with it a less or greater velocity of rotation than the places it passes over, and will turn into an easterly or westerly wind, according as it approaches or recedes from the equator. In the region of the sun's greatest heat, the air, rarefied and lightened, is continually rising, and cooler currents come in on both sides to take the place of the ascending volume. As these side-currents come from a distance of about thirty degrees from the equator, they have, at starting, an eastward velocity many miles an hour less than the localities they will eventually reach. Consequently they will appear to lag behind in all the course of their progress to the equator—that is, they will have a westerly motion united with their north and south movements. These are the great trade-winds, blowing constantly from the northeast on this side, and the southeast on the other side of the equator.
But the heated air, which has risen in immense volumes in the tropics, spreads out to the north and the south in the upper regions, passes entirely over the trade-winds, and comes down to the earth in the temperate zones. It, however, continues to have the velocity toward the east which it acquired at the equator, and, when it strikes the slower-moving latitudes, it will be traveling much faster than the regions it comes down upon. Hence the westerly winds that prevail almost constantly in the middle latitudes.
This is the normal order of the wind-currents, and that which would prevail with nearly perfect regularity if the world were a uniform globe of water or of land, and equally heated on both sides of the equator. But the continents, and particularly mountain elevations, produce great disturbances—unequal rainfalls and ever-varying atmospheric pressures. When also, from any cause, one of the trade-winds, notably the southern, is increased in its violence, so as to push a tornado-tongue across the dividing line, into the opposite system of winds, there is started one of those cyclones, or great circular storms, which ravage the tropics and whirl through the temperate zones, finally exhausting themselves in the higher latitudes to the eastward.
The southern hemisphere is at the present time colder than the northern, owing primarily to the fact that the winters there are eight days longer than the northern, and the sun, during those seasons, about 3,000,000 miles farther from the earth than during the northern winters. The difference of temperature, therefore, between the warm air that rises at the equator and the cold air that comes in from the south is greater than that on the north side. And, as it is difference of temperature that produces the whole movement of the air-currents, of course the greater strength of that movement must be on the southern side. Hence the larger share of the equatorial current passes over to the south, and the southern trades are much the strongest. In accordance with this theory, it is a matter of observation that the southern trade-winds reach across the equator and into the northern hemisphere in some places ten to fifteen degrees.
In obedience to and perfect accord with this great system of winds, the waters of the oceans move. The strong southeast trades blow up from Southern Africa, cross the equator, and drive the waters of the South Atlantic into the Caribbean Sea. The lighter northeast trades, blowing between North Africa and the West Indies, assist and give direction to this movement, which finally impels through the Straits of Florida a tide of tropical waters a hundred times greater than the outflow of all the rivers in the world. This great flood of thermal waters spreads out in the Northern Atlantic, imparting to Europe a climate corresponding to countries twenty degrees south of it on this side of the ocean. There is, of course, an under-current from the Arctics to the equator, exactly compensating this enormous northward flow of the surface-waters. The same process and effect are repeated in the Pacific Ocean; and the great Japan Stream robs the southern hemisphere, for the benefit of our Pacific States, only in a degree less than does the Gulf Stream for the benefit of Europe.
A change in the relative strength of the trade-winds, such that the northeast trades would blow across the equator into the southern hemisphere, would entirely reverse the course of the warm ocean-currents, and carry to the southern continents the heat abstracted from the northern. Such a change in the course of ocean-streams has unquestionably followed every change in the glaciation of the hemispheres from astronomical causes. The winds and the water-currents have always helped to increase the difference in temperature which a considerable eccentricity of the earth's orbit must always have produced between the northern and southern halves of our globe. It matters but little which of the two—the ocean-currents or the astronomical causes—have produced the greater effect, since it is certain that they have ever coöperated in one and the same direction.
On all the tropical seas, between the terminal lines of the two trade-winds, there is what is called the belt of calms, a tract averaging from 300 to 500 miles wide, in which, whatever winds there may be, are exceedingly light and unreliable. It is here, as we have seen, that the air and vapor, heated by the vertical rays of the sun, are continually rising and spreading outward in the upper regions. It is a complete dividing line between the climates of the two hemispheres. One may be frigidly cold, while the other is highly heated; the only difference being that the calm belt would be removed farther into the warmer hemisphere. It now ranges from five to ten degrees of latitude on this side of the equator. In this belt of ascending air-currents is carried up the greater part of the moisture which afterward descends as rain or snow far from the equator. Whatever excess of solar heat there may be on the tropics is here absorbed in evaporating water. To vaporize a pound of water, according to Prof. Tyndall, requires as much heat as to raise fifty-five pounds of ice-water to the boiling-point. It is manifest, therefore, that there must have been, during the glacial periods, an enormous amount of sun-power somewhere on the face of the earth to have supplied the vapor that buried one zone and half of another beneath a solid ocean of ice.
These facts effectually do away with all the theories, except the astronomical, which have been advanced by physicists to account for glacial phenomena: one, that our solar system has, during certain ages, passed through a colder region of space; another, that the sun in glacial times for some cause failed to supply his usual quantity of heat; and, as a consequence of either, that the glaciation of both hemispheres occurred at the same time. Equatorial heat is as necessary to a glacial period as polar cold. The one transforms the waters to vapor and elevates it to the cloud-spheres, while the other sends in the cold winds beneath, which compel the vapors to come over to the frozen side and build up the glacier.
The system of the stratified rocks has been called the great geological book, with its uncounted leaves overlying each other. Now, as it is a part of the glacial theory that each of these leaves or strata, at least in greater part, was the work of a glacial period, it is important for us to examine closely and particularly the course and effect of one of these great cycles of 21,000 years or thereabouts. We will take, for example, that one of the Post-tertiary glacial which was of the greatest extent and severity. Ten cycles back—about 210,000 years ago—one of the periods of maximum eccentricity had just commenced, the highest since four times that number of years. The perigee, or nearest approach to the sun, happened then as now, a few days after the winter solstice of our half of the world. It was the great summer of the northern hemisphere. But over the southern hemisphere at this time, almost if not quite to the tropics, extended one vast sheet of ice. It reached far into Brazil, it covered Southern Africa, and lapped over on Australia. The marks are all there, scored on the solid rocks, to show how it crept up the southern slopes of the hills, and how far it pushed its icy arms. In South America at least there is ample proof that the great glacier spanned the southern ocean to reach it; for the furrows on the rock-beds of Patagonia are from the pole toward the equator, whereas in any other case they would have been from the mountains to the sea. With such a state of things at the southern end of the world, with probably miles in depth of ice and sea in its higher latitudes, there could have been but little water left for the opposite northern regions. What is called the Atlantic-cable plateau, between Newfoundland and Ireland, was very possibly the north shore of the Atlantic Ocean; and probably no considerable bodies of water existed anywhere north of that parallel. The present continents were all mountain tablelands, far from the vicinity of evaporating surfaces. Like all such elevated regions not exposed to specially moist winds, they were doubtless dry and arid deserts. However warm may have been the climate of the north temperate and arctic zones during this their great summer, their great elevation and the want of any kind of water-supply must have made them barren of all forms of animal or vegetable life. Consequently there would be, as is notably the case, but few if any traces of this part of the great season left in the geological records, at least above the present seas.
Five thousand years pass, and the perigee has advanced to meet the vernal equinox. The spring season is now the shortest of all; but, as the autumnal is correspondingly lengthened, the average climate is about that of the present time. But it is the season of the great thaw—the breaking-up time—of the southern hemisphere, and the waters are returning to fill the northern ocean-beds. Imperceptibly a permanent white cap begins to fasten itself to the heights of the boreal zone, to extend its outline, and to increase its depth. Slowly the lands are being submerged and the oceans broaden out, till there comes a time when land and water are equalized in the two hemispheres, and the climates are substantially alike.
Another 5,000 years pass, and the perigee now coincides with the summer solstice of the northern hemisphere. This is the position there of greatest cold: the winters are twenty-eight days longer than the summers; and the extra days are in great part those of the briefest sunshine. Besides this, the earth is 10,500,000 miles farther from the sun in winter than in summer. According to the most careful calculations, the temperature of extreme northern regions would be lowered 50°, and the mean annual range would be fully 60° below zero. This in all probability would carry the isothermal line of Labrador, South Greenland, and Iceland (32° Fahr.),down to Charleston and the Gulf of Mexico. The late Prof. Agassiz found ice-marks as far south as this, though it can hardly be supposed that the permanent glacier extended so far. There are, however, abundant signs of the permanent ice-layer all over the State of New York, and both east and west of it. The same distinguished authority was wont to claim in his lectures that all the beautiful north and south lakes of Western New York—the Cayuga, the Seneca, the Canandaigua—were ploughed out of the solid rock and walled around with their clay and gravel hills by advancing and retreating glaciers. The rocky summits of New England are found to be grooved and scored all over their sides and tops with markings always in nearly a north and south direction. They have been traced on Mount Washington to within 300 feet of the highest point. There can be no doubt that at the time we are writing of, about 200,000 years ago, there was one solid ice-stratum of immense thickness—Agassiz said from two to three miles—slowly being pushed from the northward by the power of freezing water, over all of New England and the lake States.
Again the perigee proceeds to meet the autumnal equinox. The winter and the summer seasons have again become equal in length; and the sun is just half its time on the north side of the equator. The great ice-shroud is now being gradually withdrawn. Where it abuts on deep waters, enormous icebergs are broken off and float away to the south, carrying bowlders and soil and whatever it may have picked up in its slow course down to the sea. Where it terminates in shallow waters or on the land, its effect is to produce such an arrangement and diversity of soils and such a peculiar outline of country as no other agency could ever have brought about. So different is the nature and work of the great polar glacier from anything with which we are familiar at the present day, that it has seemed to me to require a few words of more particular description.
As is well known, the glacier is an accumulation of many winters' snows consolidated by pressure into a clear blue ice. In this condition it manifests the peculiar property of viscous bodies—it is in continual slow motion in the direction of least resistance. Whether it is by the expansion produced by the repeated thawing and freezing of water in its interstices, as Agassiz claimed, or whether by the pressure of the mass and glacial regelation, which is the constant freezing together of ice-surfaces in contact, after breaking under unequal pressures, or crashing against obstacles, which is the theory of Prof. Tyndall, or whether by both causes combined, certain it is that large bodies of ice not only flow like a heavy lava-stream, conforming themselves to all inequalities of the surface, but they also scrape along in solid mass, as if pushed by some irresistible force from behind. Mountain-glaciers show both motions. But the great polar glacier, extending over comparatively level surfaces, seems to have been pushed bodily outward from its fixed polar base, and to have moved almost entirely under the mighty impulse of expansion. The parallel scratches and furrows which, in our hemisphere, mount straight up the north sides of mountains; the worn and rounded appearance of those sides and of the summits, as compared with the rough, unsmoothed southern slopes; the erratic blocks, or some peculiar specimens like the native copper of Lake Superior, carried almost directly south for scores or hundreds of miles, over heights, and even over arms of the sea—all show conclusively that the great glacier pushed its meridional course over all obstacles and to long distances.
Imbedding in its under surface the grit and gravel on which it froze, this mountain grindstone grated and ground the solid rocks over which it passed into the various materials of soil. Sand and gravel were the products from granitic rocks and sandstones, clay from the slates and shales, and loam from the softer lime-rocks. But the most striking effects which the polar glacier produced were the long ridges of gravel and bowlder-clay hills which it scraped up as it advanced, and left at the end of its journey, or at each halting-place of its retreat. For it must be borne in mind that the glacier was still pushing southward all the time that it was, on the whole, retreating. These terminal moraines are either the promiscuous gatherings of clay and bowlders and earths of all kinds, or, if they have been subjected to the sorting influence of moving waters, they are gravel hills with sandy bases, and clay flats extending usually to the southward of them. They run in somewhat parallel courses easterly and westerly, sometimes hundreds of miles. Great numbers of these concentric ridges may be counted in Western New York, between the long Lake Ontario ridge and the lake hills of the south part of the State. Several cross the New England States, one running along the coast of Maine, and westerly through the White Mountains. In addition to these are the lateral moraines, running in an opposite direction. These were, some of them, pushed out at the sides by outstretching arms of the glacier; others were formed by streams running down through breaks or fiords in the melting ice-sheet. So extensive and so marked are the traces of the great polar glacier over all middle latitudes, both north and south, that it may truly be called the great landscape-gardener of the temperate zones.
But it is natural to conclude that, if there has been one glacial era caused by astronomical cycles, there must also have been others in earlier geological times. And, as we turn back the pages of the great earth-book, we find therein recorded the evidences of the vicissitudes of climate which we thus anticipate, but, if we mistake not, in continually-lessening force and extent the farther back we go. For, long ages previous to the recent glacial epoch, through all the Tertiary era, the fossil plants and animals indicate the prevalence of a warm and genial climate over the greater part of the globe. Then come the chalk-beds of the Cretaceous period, in which are frequently found water-worn blocks of granite and aggregations of pebbles, proving that then, as now, the iceberg floated down from the north over seas that were quietly depositing the chalk-shells. Still older is found a long series of secondary strata, the Oölite, the Lias, and the Trias, which were deposited in at least sub-tropical climates. They are the burial-grounds of the enormous saurian reptiles that once had an age all to themselves in the world's chronology. Their remains have been found within a thousand miles of the north-pole, thus proving that warm seas covered every zone.
Between the great divisions of Secondary and Primary in geology, there lies a stratum found only in the higher half of the latitudes, and known as the Permian or New Red Sandstone. The scanty life-forms found in it, and the coarse grit and angular bowlders of which it is composed, evince the well-known glacial action. Geologists generally think that there elapsed between these great divisions a very long period of time in which, excepting the sandstone, but little was done one way or another to build up the crust of the earth or leave a mark in its records. This doubtless indicates periods of very small eccentricity. Such periods did occur, according to Mr. Croll's calculations, immediately before and after the great eccentricity of 850,000 years ago, in which we may perhaps conjecture the New Red Sandstone to have been formed.
Previous to this age were the long Carboniferous periods, during all of which a warm and moist climate prevailed over all lands that have yet been explored. Below the coal-measures are found again the grits and bowldery conglomerates of the Old Red Sandstone, which, with great paucity of organic remains, would imply the alternations of somewhat glacial climates. The Silurian, Cambrian, and Laurentian systems preceded the Old Red in the order named, and reach back to the dawn of life on the earth. These formations are of vast thickness, and were deposited at the bottom of warm seas in all parts of the world.
It cannot be denied that, as we go back in the geologic records, we find more and more the evidences of greater heat and a more equable climate. It is certain that the astronomical relations which we have pointed out—the revolutions of the orbital points and the alternations of great and small eccentricity—have never ceased to exist. Therefore, if the world had been subjected to only the same solar heat in ancient as in recent periods, there must have been repeated glacial epochs; and we should find the bowlder, and the unsorted drift, and the scratched and polished rocks, all through the stone presentations. But very few, if any, such evidences have been found.
Again, for a warm and exuberant climate to extend into the arctic zone, there was necessary one of those great summers of considerable eccentricity, without the excessive drainage, which an unusually large accumulation of ice in the opposite hemisphere would necessitate. Each summer cycle of coal forests, or of reptile monsters, implies, not only a long visit, and a high evaporating power of the sun, but also the addition, to the opposite polar regions, of a weight of ice only sufficient to draw the waters from a small part of the low and flat lands of the warmer hemisphere. We have seen that periods of warm, perhaps even tropical climates in polar latitudes, intervened between the great winters of the last glacial epoch. But they have left scarcely a trace in the strata. They were the nearest approach possible, with the sun-power of recent times, to the conditions which of old brought out such a profusion of animal and vegetable life. But the only result in the later periods was, that the earth was unbalanced. All the waters were either turned into ice, or were following after it toward one of the poles. One side of the world was a frozen waste, while the other was a burning waste.
I think we cannot avoid the conclusion that the sun shone with a far intenser power on the Carboniferous swamps and the Oölitic shoals than on the gravel-hills of the Drift; that the oceans of early times were wider and warmer than now, and circulated more freely between the tropics and the polar seas; and that the heated and moisture-laden atmosphere retained the heat and equalized the temperature between the equator and the poles far more than at present.
With these conditions, that is, with a greater sun-power and a considerable eccentricity of the earth's orbit, I can conceive a rational explanation, that which I have not yet seen in the books, of the formation of the coal-layers, alternated as they always are with marine deposits. These alternations are sometimes very numerous. There are as many as sixty distinct veins of considerable thickness, one over another, in the coal-mines of South Wales, as also of Nova Scotia. There must have been, in that case, sixty periods of dry land, each of sufficient duration to grow many forests, and each followed by a long-continued submergence, in order that each layer should become fossilized, and buried beneath a shale or a limestone, which could only have formed in the depths of a quiet sea. The books say there were so many upheavals, and a like number of subsidences, alternating with each other. As if Old Earth had bent her back, for her load of pit-coal, threescore times among the Welsh hills, and again as many more at Halifax. It is a far more reasonable explanation, that each considerable layer of coal indicates a cycle of long summers, and the withdrawal of a moderate depth of the oceans from one hemisphere to the other, by reason of moderate accumulations of ice in polar latitudes, and the return, again, of the waters after 10,500 years. In this way, and in no other that I can conceive of, can be fairly explained the constant mixture and alternations of terrestrial and marine relics, all through the fossil-bearing formations, and the hundreds, if not thousands of different and distinct strata which are found lying one above another.
Whoever, even cursorily, studies the phenomena of geology, must be impressed with the enormous length of time it has taken to arrange the terrestrial substructure, and prepare it for the higher forms of life. Even the comparatively recent period of the Bowlder Clay, which laid out the grounds of the present area of civilization, dates back for its commencement, as we have seen, probably 200,000 years. If it might be assumed that the Permian or New Red Sandstone was formed during the next previous period of extraordinary eccentricity, which was 850,000 years ago, then the Devonian or Old Red Sandstone would come in, very appropriately, at the next anterior era of extraordinary focal distance, which occurred 2,500,000 years back. The Carboniferous period, which came between these two, could not have been formed in less than 1,000,000 years, as most geologists concede; and by calculations previously indicated, those sixty Welsh layers of coal, if there are that many, divided off by marine deposits of considerable thickness, would have consumed 1,250,000 years.
The average thickness of all the strata that lie above the Old Red Sandstone is not far from two miles. But this formation is itself, in many places, two miles thick. And the lower Primary systems will add at least ten miles to the vertical measure of the fossil-bearing rocks. It is estimated that "the fossiliferous beds in Great Britain, as a whole, are more than 70,000 feet in thickness;" and many that are there wanting, or nearly so, elsewhere expand into beds of immense depth. There are certainly fifteen miles deep of strata to be accounted for—the slow accretions of the ages—mainly ocean-sediment that has come down from the wear and washings of the solid rocks. It would be by no means a bold assumption to say that 20,000,000 years had elapsed since the eozoön first built its reefs in the warm Laurentian seas.