CHAPTER III. Astronomical Considerations

ASTRONOMICAL CONSIDERATIONS

The Great Ice Age—Extension of glaciation—Croll’s ingenious hypothesis—Inadequacy of the explanation—The ice caps of Mars—De Geer’s discovery of the annual layers of glacial clay in Sweden—Application to the measurement of time—Sederholm’s observations in Finland—Cyclic sedimentation in the Cretaceous strata of Colorado—Its correlation by Gilbert with an astronomical time unit.

In the last chapter mention was made of Kelvin’s work on tidal friction and of Sir G. Darwin’s speculations as to the early history of the earth-moon system. Their conclusions depended very largely on a cosmogony which has failed to meet with general acceptance. A very different appeal to astronomical causes was made by Croll in his famous attempt to account for the anomalous conditions of the glacial period. Here we may also consider the correlation by Baron De Geer and Mr. G. K. Gilbert of certain unusual phases of sedimentation with the seasonal and climatic changes brought about respectively by the earth’s motion and its periodic fluctuations. All of these investigations find a common basis in their direct reference to astronomical considerations.

In the course of its history the earth has undoubtedly passed through several periods of extreme cold, periods when Arctic conditions swept down from their polar strongholds and invaded the temperate zones and even the tropics. Ice and snow have left their traces in many a grooved and striated surface, and in gently rounded outlines the landscape often betrays their former presence. Erratics and perched blocks, terminal and lateral moraines, lakes and alluvial terraces, U-shaped and hanging valleys all bear witness to the wide extension of the vanished glaciers of the past. Relics of glaciation have been preserved in the Cambrian or pre-Cambrian rocks of Norway, China, Australia and South Africa. Still more remarkable are the records of a Permian ice age found in the rocks of South America, South Africa, India and Australia. No unequivocal evidence of glaciation in later periods is forthcoming until the advent of the Pleistocene. The Great Ice Age through which the earth has so recently passed was not, however, a single epoch of glacial extension. Four episodes in the climatic cycle can be recognised—a genial episode, a period of falling temperature and glacial advance, a glacial episode, and finally, a period of rising temperature and glacial retreat. That this cycle has been four times repeated is the testimony of the Eastern Alps as interpreted by Prof. Penck. In the British area the ebb and flow of temperature has not left so clear a record, and a more continuous and persistent glaciation appears to be indicated.

At the time of the maximum advance of the ice, all northern Europe lay buried beneath an immense ice sheet, which was fed by enormous glaciers slowly creeping down from the uplands. Curiously enough, Siberia, which in parts experiences the coldest winter in the world, is not now and was not then covered by the ice-field. Farther south the ice collected in the Pyrenees, Alps, Caucasus and in the great Asiatic ranges and descended in all directions far below the level of the present snow-line. In Canada and the United States the ice mantle proceeded from three great centres—from Labrador along the Atlantic coast, from the Keewatin district by Hudson’s Bay and from the Cordilleras along the Pacific coast. In the tropics and in the southern hemisphere the story is the same. Down the slopes of Kenia, Kilima N’jaro and Ruwenzori, the towering peaks of Central East Africa, the glaciers descended 5000 feet below their present termination. Kosciusko in New South Wales bears conspicuous traces of a former ice-cap which completely shrouded all but the highest peaks of the plateau. From the highlands of Tasmania and New Zealand, and from the Cordilleras and Andes of Patagonia and Chili the evidences of a prolonged glaciation are equally clear.

No satisfactory theory of climate has yet been propounded which affords an adequate explanation of this universal lowering of the snow-line. Whether or not the temperature fluctuations were synchronous all over the earth is difficult to decide, but that a colder climate characterised the southern no less than the northern hemisphere cannot be doubted. The causes to which appeal has been made in accounting for climatic changes are of three classes,—astronomical, geographical and atmospheric. In general the various tendencies will be opposed to one another and by their interference prevent the attainment of universal extremities of climate. Occasionally, however, circumstances may arise when their joint action will lead in a single direction. A wide extension of tropical or polar conditions would then be expected to follow.

Of the three contributory causes, only one, the astronomical, bears any determinable relation with time. The theory that secular variation of terrestrial climates results from the changing eccentricity of the earth’s orbit, was first proposed by Adhémar. In 1868 James Croll greatly elaborated and extended this theory, presenting it with such a formidable array of quantitative data and yet in such an attractive form that it exerted a considerable influence on the geological thought of the day. Not only did it seem to offer an acceptable means of escape from the maze of difficulties by which the problem was shrouded, but in addition it promised a faithful chronology fixing the date and duration of the Ice Age with almost ideal precision.

Laplace established the fact that the ellipticity of the earth’s orbit is subject to periodic oscillations between certain limits. Sometimes the path is nearly circular but at other times it becomes a more flattened ellipse. If the earth were the only planet, its orbit would suffer no change; that it does is due to the attractions of the sister planets. Nevertheless, the orbit is essentially stable, and the yearly journey always occupies the same time. Formulæ were devised by Leverrier from which it was possible to calculate with some accuracy, the actual value of the eccentricity at any given period in past or future within a few million years of the present. Croll utilised these formulæ to compute the dates of maximum and minimum eccentricity for the past three million years. He found three important periods when that factor was specially high, betraying a type of orbit more than usually flattened. These were:

(a) from 2,500,000 to 2,600,000 years ago. (b) ” 720,000 to 980,000 ” ” (c) ” 80,000 to 240,000 ” ”

It was to the last of these that the Glacial Period was assigned.

When the earth is at perihelion, i.e. in that part of its orbit which lies nearest to the sun, it enjoys a more generous radiation than falls upon it at aphelion, when it is farthest from the sun. But this alone does not determine the time of summer and winter. At present the northern winter and southern summer occur when the earth is in perihelion. As is well known, this is owing to the inclination of the earth’s axis and the blanketing effect of the atmosphere. In the northern hemisphere, the greater thickness of air presented to the sun’s rays during the winter keeps out more heat than is gained by the relative closeness to the sun. However, this condition is not permanent. As discovered by Hipparchus in the year 134 B.C., the positions of summer and winter and of the equinoxes on the ecliptic are subject to a slow forward movement. In 26,000 years they make the complete circuit, and so in the course of time the relation of the seasons to perihelion is slowly altered.

The precession of the equinoxes was shown by Newton to be a dynamical consequence of the spinning of the earth about a tilted axis. While this change is progressing, the position of perihelion is also shifting, and the resultant period is thereby reduced on the average to 21,000 years. Thus, in about 10,000 years from now the northern hemisphere will enjoy summer at perihelion. At the same time, the northern winter will occur at aphelion. The cold season will then be longer and more severe than now, and the annual accumulation of snow correspondingly increased. If, in addition, we suppose the orbital eccentricity to approach its maximum value, the northern hemisphere would then be in the grip of winter for nearly four months of the year, and the cold would become still more bitter than before. As a slight compensation for the rigours of the winter, the summer, though short, would be very much hotter. It was extremes such as these, accompanied perhaps by favourable geographical conditions, that Croll postulated for his glacial period. He believed that the snow and ice which would collect during a long frosty winter would successfully resist the evaporative powers of the summer, and that permanent snow-fields would therefore arise. Once started, the snowy mantle would tend to continue. A great deal of heat could be absorbed without raising the temperature above freezing-point, and the result of evaporation would be the creation of a thick blanket of fog, an effective agent in guarding the ice against the ravages of the summer radiance.

A peculiarity of Croll’s hypothesis is that glacial epochs could not exist in both hemispheres at the same time. South of the Equator the summer would be longer than it is at present, and the winter would not only be short, but also comparatively mild. The climate would be that of a genial interglacial period. The alternation of glacial epochs between north and south is a necessary consequence of Croll’s view, but when the facts are examined they are found to be unfavourable to this assumption. In Sweden the last period of extension of the ice appears to have been synchronous with that in New South Wales, the close of each being dated, if our present time-scale is reliable, at 15,000 to 20,000 years ago. Both date and coincidence are decisive against the theory. Although Croll’s hypothesis achieved a great popularity, geologists were not wanting who considered the suggested causes to be utterly inadequate to produce so radical a change of climate. It happened that during the penultimate period of extreme eccentricity, which began nearly a million years ago, the astronomical conditions were more favourable to glaciation than they have been since. The question therefore arose why no traces had been preserved in the deposits of that time. Indeed, since the Cambrian, many hundreds of glacial periods should have come and gone. Had this been the case, and corroborative evidence sufficiently convincing, the earth’s chronology would have been written boldly in its rocks. But Croll’s brave attempt to number the ages was unsuccessful, and his attractive theory no longer holds the field.

At maximum: full extent of white.

At minimum: inner circle.

NORTH POLAR CAP.]

At maximum: full extent of white.

At minimum: disappears entirely.

SOUTH POLAR CAP.

MARS.]

A gigantic experiment illustrating the very conditions which Croll postulated has been recently shown to us by Prof. Lowell. For his working model we must look to the sky and carefully watch the changing seasons of Mars. The eccentricity of the orbit of Mars is much higher than that of the earth could ever have been, and moreover, the southern winter falls near aphelion. The conditions for a permanent ice-cap over the south polar regions are therefore ideal. The actual facts are surprising, and on the accompanying plate Prof. Lowell has depicted them admirably. During the winter a large snow-cap collects around the south pole, but in summer it is entirely dispersed. Around the north pole the winter snow-cap is less extensive, but all through the summer it never quite disappears. While the long southern winter undoubtedly makes possible a greater accumulation of snow, the hot short summer more than compensates by its superior powers of evaporation. A permanent and widely extended ice-field evidently could not originate. This demonstration of Croll’s hypothesis in actual practice does not present an altogether complete analogy to terrestrial conditions. The surface temperature of Mars and the thermal properties of its atmosphere may be widely different from ours, and the excessive rate at which the polar caps diminish in the spring clearly indicates the comparative thinness of the deposit. But in spite of these differences, the remarkable conclusion stands unassailed—that the evaporative power of the short hot summer of the south exceeds that of the long but cooler summer of the north.

Directions of Glacial Movement in Southern Scandinavia.]

It was stated above that since the culmination of the last glacial epoch more than 15,000 years have elapsed. Of the several methods which have been employed to determine this period only one can be dealt with here. The present genial climate has not greatly varied during the last 7000 years. As we trace back the record of temperature still farther a gradual fall can be discerned, accompanied by a wide extension of the ice. Fig. 9 depicts the directions of movement of the last continental glacier of Scandinavia and its southern boundary across Jutland and the Baltic provinces. As it retreated, it left the terminal moraine known as the Baltic ridge, and its southern limit was gradually pushed back till it extended to what is now the coast of Scania. From this point its recession has been followed in great detail by De Geer, who has made a careful study of the deposits which, extending from the Baltic up to the Scandinavian ice-shed, mark the progress of its annual retreat. Upon his observations he has founded a system of geological chronology which is of the greatest importance in that it marks the first effort towards absolute accuracy.

Each spring and summer, as the glacier thawed, a great deal of sand and clay was set free and carried away in suspension by the numerous streams which flowed from under the melting ice. The coarser material, on reaching the sea, settled down almost at once, but the finest particles of clay, able to remain in suspension much longer, were not completely deposited. Then came the autumn and winter, and the freezing of the streams. The sea received no further supply of sediment, and the load of fine mud slowly settled on the sea-bottom to form a thin layer of pure clay, sharply differentiated from the coarser bed below. The following year the glacier retreated a few hundred feet to the north, and the material then liberated was sorted out as before and again deposited in two well-marked seasonal layers. As this process continued year after year the area of deposit moved northwards with the ice, and the annual layers of sediment thus became superimposed one upon the other like wedge-shaped tiles on a roof. The width of each bed is generally less than thirty miles, for even the finest mud cannot be traced beyond that distance. This being the case, no vertical section contains all the layers, and it is rarely that more than a hundred can be counted in one place. The total thickness of the recessional deposit seldom exceeds thirty feet.

De Geer successfully attempted the difficult task of counting the annual bands of glacial clay deposited throughout the period of retreat from the Scanian coast. Fortunately, any given set of layers can be traced from one locality to another, and as each ribbon of sediment dies out the higher beds are followed up in the same way until the whole series has been examined from bottom to top. The late-glacial beds number about 5000, and the time which has elapsed since the ice border reached the eastern coast of Scania is therefore 12,000 years. The time of recession from the Baltic ridge to the Scanian coast remains to be estimated. Much of the record is hidden beneath the waters of the Baltic, and in North Germany De Geer’s method has not yet been applied. It appears, however, that the withdrawal of the ice was not uniform. It began slowly and reluctantly, but towards the north became more rapid. In the region of Stockholm the retreat was five times as fast as in Scania. We may therefore assume that in the still earlier stages the time taken was considerably more than that required for a retreat over an equal distance in Scania. The latter would have occupied rather more than 2500 years, and on this basis Sollas provisionally accepts 5000 years as the period during which the ice front was driven back to the south coast of Sweden. In this way the whole interval which has passed since the culmination of the last glacial episode is determined to be greater than 15,000 years, with 17,000 years as a probable value.

Amongst the Archean rocks of N.W. Finland, Prof. Sederholm has found in the Bottnian schists and phyllites primary characters which are strikingly similar to those of the banded glacial clays. Under the microscope their textures are distinctly clastic, and each composite stratum is sharply divided into two thin bands, the coarser one originally of sand and marking the beginning of a new year of deposition, the finer one originally of clay. Sederholm interprets the phenomenon on a uniformitarian basis as indicating that, even in those remote times, there was a marked difference in the seasons. By measuring the thickness of many thousands of annual layers, he finds their mean thickness to be about five inches. The total thickness of the banded phyllites amounts to 10,000 feet and the time they appear to represent is therefore only 24,000 years. This conclusion is probably far from the truth, for, as Sederholm particularly emphasises, it is difficult to know what is meant by the thickness of a deposit, so many arbitrary and misleading elements enter into its determination.

From De Geer’s work, it might have been said at first that a thickness of thirty feet was deposited in a hundred years, but as the deposit was traced over the country, it became equally evident, that with no apparent thickening of the formation, deposition had gone on for 5000 years, and considering the growth northwards from the Baltic ridge, perhaps for 10,000 years. The whole difficulty lies in determining which parts of a formation are strictly contemporaneous. A measurement of thickness is significant only in relation to the immediate area of deposit and to the rate at which it moves landwards or seawards. The thickness of a formation and the maximum thickness of the layers formed in successive years may differ enormously, as De Geer’s researches have made so evident. For the same reason the term rate of deposit is loose and misleading unless it is clear to what it refers.

Another astronomical method of estimating time, though embracing much longer periods, has been applied by Gilbert to certain formations in Colorado. The basin of the Arkansas River is largely occupied by Cretaceous sediments, a succession of which is given in the adjoining diagram. At the four stages marked A, B, C and D the argillaceous shales give place to a calcareous type, and in these there is a regular alternation of thin layers of limestone and of calcareous shale. At A the average thickness of a pair of beds is 1·5 feet, and the number of repetitions is 15. At B the limestones are more massive, but the parting shales are very thin. Here again the average thickness of two adjacent beds is 1·5 feet. The limestones at C are less pure, and each, with its associated layer of shale, amounts on the average to about 2·7 feet. At D the succession is similar.

Vertical Section of the Upper Cretaceous strata of Colorado. Below are the Comanchian or Lower Cretaceous beds; above are the Transition beds to the Eocene.]

To explain the remarkably regular alternation of conditions which determined this uniform rhythm of sedimentation, purely terrestrial causes appear to be insufficient. Upheaval and subsidence of the earth’s crust, and the changing distribution of land and sea, are characterised rather by their irregularity than by any rhythmic sequence. While there is undoubtedly a rough periodicity in earth movements, yet it is discernible only on the broadest scale and is out of all proportion to the requirements of this case. Gilbert therefore suggests an astronomical cause. Of the several cyclic changes to which it is reasonable to appeal, the annual revolution of the earth, and the variation of the eccentricity of its orbit, demand periods which are in the first case too short (one year), and in the second too long (91,000 years). As we have already seen, the relation of the seasons to the position of perihelion repeats itself about every 21,000 years, and this astronomical cycle seems better adapted to meet the case. The climatic changes which accompany the precession of the equinoxes might influence the character of sedimentation in many ways. With a changing circulation of winds and currents, argillaceous material might be transported and deposited at one time and calcareous at another. On land, vegetation might predominate during part of the cycle, and the surface waters would then dissolve more calcium carbonate than during a period when vegetation became sparse. At the same time mechanical erosion would be impeded in the first instance, but would be more active in the second.

It is evident that the shale was deposited more rapidly than the limestone, for when the principal deposit was calcareous the thickness is 1·5 feet, whereas, in the case of the less calcareous beds, it rises to 2·7 feet. In the normal shales a conservative estimate of the equivalent thickness would be 4 feet. Adopting the astronomical time unit of 21,000 years, the rate of deposit would then be of the order:

Limestone, 1 foot in 14,000 years. Shale, 1 foot in 5,000 years.

On this basis the 3900 feet of shale in the Benton, Niobrara and Pierre formations represent about 20 million years, for in this example the term thickness seems to have a definite meaning. If the assumptions are correct, the duration of the whole Cretaceous period must therefore be considerably greater than this.