Fragments of Earth Lore: Sketches & Addresses Geological and Geographical

by James Geikie

We may now consider another form of the “earth-movement hypothesis.” It has frequently been suggested that our glacial phenomena may have been caused by the submergence of the Isthmus of Panama, and the deflection of the Equatorial Current into the Pacific. But it may be doubted whether a submergence of that isthmus, unless very extensive indeed, would result in more than a partial escape of Atlantic water into the Pacific basin. The Counter Current of the Pacific which now strikes against the isthmus might even sweep into the Caribbean Sea, and join the Equatorial on its way to the Gulf of Mexico. But putting that consideration aside, what evidence have we that the Isthmus of Panama was submerged during the glacial epoch? None whatsoever, it may be replied. It is only a pious opinion. Considerable movements of elevation and depression of the islands in the Caribbean Sea would seem to have taken place at a comparatively recent date, but those movements may quite well belong to Pliocene times. Whether they be of Pliocene or Pleistocene age, however, no one has yet proved that the Isthmus of Panama was sufficiently submerged, either at the one time or the other, to permit the escape of the Atlantic Equatorial into the Pacific basin. But let it be supposed that the isthmus has become so deeply submerged that the Equatorial Current is wholly deflected, and that no Gulf Stream issues through the Straits of Florida to temper the climate of higher latitudes. What would result from such an unhappy change? Can any one conversant with the geographical distribution of the glacial phenomena imagine that the conditions of the Glacial period could be thus reproduced? Norway might indeed become a second south Greenland, and perennial snow and ice might appear in the mountainous tracts of the British Islands. The climate of Hudson’s Bay and the surrounding lands might be experienced in the Baltic and its neighbourhood, and what are now the temperate latitudes of Europe, north of the 50th parallel, would possibly approach Siberia in character. But surely these changes are not comparable to the conditions of the Glacial period. The absence of a Gulf Stream would not sensibly affect the climate of south-eastern Europe and Asia, and could not have the smallest influence on that of the Pacific coast-lands of North America.

Yes, but if we conceive the submergence of the Isthmus of Panama to coincide with great elevation of northern lands, would not such geographical conditions bring about a glacial epoch comparable to that of Pleistocene times? It is hard to see how they could. No doubt the climate of all those regions that would be affected by the withdrawal of the Gulf Stream alone would become still more deteriorated if they stood some 3000 feet higher than now. A vast area in the north-west of Europe would certainly be uninhabitable, but it is for the advocates of the “earth-movement hypothesis” to explain why those inhospitable regions should necessarily be covered with an ice-sheet. For the production of great snow-fields and continental ice-sheets, considerable precipitation, no less than a low temperature, is requisite. Under the conditions we have been imagining, however, precipitation would probably be much less than it is at present. But to whatever extent north-west Europe might be glaciated, it is obvious that the geographical revolutions referred to could have little influence on the climate of south-eastern Europe, not to mention central and eastern Asia. Nor could they possibly influence the climate of the Pacific coast-lands of North America. And yet, as is well known, the climate of all those regions was more or less profoundly affected during the Glacial period. To account for the widespread evidences of glaciation by means of elevation it would therefore seem necessary to infer that all the affected areas were in Pleistocene times uplifted en masse into the Arctic zone that stretches above our heads. Now it seems easier to believe that the snow-line was lowered by several thousand feet than that the continents were elevated to the same extent. Glaciation, as we have seen, was developed in the same directions and over the same areas as we should expect it to be were the snow-line to be generally depressed. To put it in another way, were the snow-line by some means or other to be lowered over Europe, Asia, and North America, then, with sufficient precipitation, great ice-fields and glaciers would reappear in the very regions which they visited during Pleistocene times. Neither elevation nor depression of the land would be required to bring about such a result. Certain advocates of the “earth-movement hypothesis,” however, do not maintain that all the glaciated areas were uplifted at one and the same time. The glaciation of the Alps, they think, may have taken place earlier or later than that of north-western Europe, while the ice-period of the Rocky Mountains may not have coincided with that of eastern North America. It is not impossible, they suppose, that the glaciation of the Himalayas may have been caused by an uplifting of that great chain, quite independent of similar earth-movements in other places. It can be demonstrated, however, that the glaciation of the Alps and of northern Europe were contemporaneous, and the facts go far to prove that the glaciers of the Rocky Mountains and the inland-ice of north-east America likewise co-existed. At all events all the old glacial accumulations of our hemisphere are of Pleistocene age, and it is for the advocates of the hypothesis under review to prove that they are not contemporaneous. Their doubts on the subject probably arise from the simple fact that they are well aware how highly improbable or even impossible it is that all those glaciated lands could have been pushed up within the snow-line at one and the same time.

Let me, however, advance to another objection. We know that the Glacial period was interrupted by at least one interglacial epoch of temperate and even genial conditions. Two glacial epochs with one protracted interglacial epoch are now generally admitted. How do the supporters of the “earth-movement hypothesis” explain this remarkable succession of climatic changes? Their views as to the cause of glacial conditions we have considered. If we can believe that the glacial phenomena were due to elevation of the land, then we need have no difficulty in understanding how glacial conditions would disappear when the continents again subsided to a lower level. Not only did North America and Europe lose all their early glacial elevation, but by a lucky coincidence the Isthmus of Panama reappeared, and the Gulf Stream resumed its beneficent course into the North Atlantic. This we are to suppose was the cause of the interglacial epoch. But I would point out that the geographical conditions which are thus inferred to have brought about the disappearance of the glacial climate, and to have ushered in the interglacial epoch, are precisely those that now obtain—and, nevertheless, we are not yet in the enjoyment of a climate like that of interglacial times. The strangely equable conditions that permitted the development of the remarkable Pleistocene flora and fauna are not experienced in the Europe of our day. And what about the second glacial epoch? Are we to suppose that once more the lands were greatly uplifted, and that convenient Isthmus of Panama was again depressed? Did the Alps, the Pyrenees, and the plateau of central France—in all of which we have distinct evidence of at least two glacial epochs—did these heights, one may ask, rise up to bring about their earlier glaciation, sink down again to induce interglacial conditions, and once more become uplifted at the succeeding cold epoch, to subside eventually in order to cause a final retreat of their glaciers?

But the climatic changes to be accounted for were in all probability more numerous and complex than those just referred to. Competent observers have adduced unmistakable evidence of three epochs of glaciation in the Alpine Lands of Europe. And we are not without distinct hints that similar changes have taken place in northern and western Europe. Nor in this connection can we ignore the evidence of several interglacial episodes which Mr. Chamberlin and others have detected in the glaciated tracts of North America. Even this is not all, for the upholders of the “earth-movement hypothesis” have still further to account for the climatic oscillations of post-glacial times. If it be hard enough to allow the possibility of one great movement of elevation having affected so enormous an area of our hemisphere, if we find it extremely difficult to believe either that one such widespread movement, or that a multitude of local movements, each more or less independent of the other, could have lifted the glaciated regions successively within reach of the snow-line—we shall yet find it impossible to admit that such remarkable upheavals could be repeated again and again.

We seem driven to conclude, therefore, that the “earth-movement hypothesis” fails to explain the phenomena of Pleistocene times. One cannot deny, indeed, that glaciation might be induced locally by elevation of the land. It is quite conceivable that mountains now below the limits of perennial snow might come to be ridged up to such an extent as to be capable of sustaining snow-fields and glaciers. And such local movements may possibly have happened here and there during the long-continued Pleistocene period. But the glacial phenomena of that period are on much too grand a scale and far too widely distributed to be accounted for in that way. And if the occurrence of even one glacial epoch cannot be thus explained, we may leave the supporters of the “earth-movement hypothesis” to show us what light is thrown by their urim and thummim on the origin of succeeding interglacial and glacial climates.

There is yet another physical condition of the Pleistocene and post-glacial periods which any adequate explanation must embrace. I refer to the oscillation of sea-level, of which so many proofs are forthcoming. It is very remarkable that almost everywhere throughout the maritime regions of formerly glaciated areas we find evidence of submergence. So commonly is this the case, that geologists have long suspected that the connection between glaciation and submergence might be one of cause and effect. The possible influence of great ice-sheets in disturbing the relative level of land and sea is a question, therefore, of very great importance. It is one, however, which must be solved by physicists. Croll and others have advocated the view that the great accumulations of ice of the Glacial period may have displaced the earth’s centre of gravity, and thus caused the sea to rise upon the glaciated hemisphere. The various results arrived at by physicists are hardly comparable, because each has used different data, but it seems probable that we have in this view a vera causa of oscillations of the sea-level. Another hypothesis would explain the rise of the sea as due to the attractive influence of the great ice-masses, but Dr. Drygalski’s and Mr. Woodward’s elaborate investigations would seem to have demonstrated that this notion does not account for the facts. Yet another speculation has been advanced. Mr. Jamieson has suggested that the mere weight of the ice-sheets would suffice to press down the earth’s crust into a supposed liquid substratum, and this explanation has met with much acceptance. Unfortunately our knowledge of the condition of the earth’s interior is so very limited that we cannot be certain as to how the crust would be affected by the weight of an ice-sheet. No doubt Mr. Jamieson’s hypothesis gives a specious explanation of certain geological phenomena, but if there be no liquid substratum underlying a thin crust it cannot be true. At present the prevalent view of physicists appears to be that the earth is substantially solid. Professor George Darwin has shown that the prominent inequalities of the earth’s surface could not be sustained unless the crust be as rigid as granite for a depth of 1000 miles. “If the earth be solid throughout,” he remarks, “then at 1000 miles from the surface the material must be as strong as granite. If it be fluid or gaseous inside, and the crust 1000 miles thick, that crust must be stronger than granite, and if only 200 or 300 miles in thickness, much stronger than granite.” This conclusion is obviously strongly confirmatory of Sir William Thomson’s view, that the earth is solid throughout. But many geologists find it hard to account for the convolutions of strata and other structural phenomena on the supposition that the earth is entirely solid, and they are inclined, therefore, to adopt the hypothesis of a sub-crust layer of liquid matter. Whether this be actually the condition or not physicists must be left to determine. All that we need note is, that if there be any force in Professor Darwin’s argument, it is obvious that the crust is possessed of great rigidity, and could not be readily deformed by the mere weight of an ice-sheet. According to Dr. Drygalski, however, the presence of an ice-sheet, by reducing the temperature of the underlying crust, would bring about contraction, and in this way cause the surface to sink. When the ice-sheet had disappeared, then free radiation of earth-heat would be resumed, the depressed isogeotherms would rise, and a general warming of the upper portion of the lithosphere would take place. But the space occupied by the depressed section, owing to the spheroidal form of the earth, would be smaller than that which it occupied before sinking had commenced, and consequently when the ice vanished expansion of the crust would follow, and the land-surface would then rise again. The whole question is one for physicists to decide upon, but I may point out that if Drygalski’s explanation be well founded, then it is obvious that it throws no light upon the origin and subsequent disappearance of an ice-sheet. Somehow or other this ice-sheet comes into existence, and the cooling and contracting crust sinks below it; and that depressed condition of the glaciated area must continue so long as the ice-sheet remains unmelted. Re-elevation can only take place when, owing to some other cause or causes, the climate changes and the ice-sheet vanishes.

Those who advocate the “earth-movement hypothesis” as an explanation of the origin of extensive glaciation have welcomed Mr. Jamieson’s view as harmonising well with their conclusions. They contend, as we have seen, that glacial conditions were induced by an extensive upheaval of the crust in northern latitudes, accompanied by a depression of the Isthmus of Panama. They then proceed to point out that the ice-sheets brought about their own dissolution by pressing down the crust, and introducing with submergence a disappearance of glacial conditions. See now how much they take for granted. In the first place, they assume an amount of pre-glacial or early glacial elevation of northern regions for which not a scrap of evidence can be adduced, while they can give no proof of contemporaneous depression of the Isthmus of Panama. Next, relying on Mr. Jamieson’s hypothesis, they take for granted that the ice-sheets, called into existence by their postulated earth-movements, succeeded in depressing the earth’s surface even below its present level. That is to say, the land, which, according to them, was in glacial times some 3000 feet higher than now, sank down under the weight of its glacial covering for, say, 3600 feet in north-western Europe. In North America, in like manner, all the pre-glacial elevation was lost—the land sinking below its present level for some 200 feet in New England, for 520 feet at Montreal, for 1000 to 1500 feet in Labrador, and for 1000 to 2000 feet in the Arctic regions. Now, even if we concede the reasonableness of Mr. Jamieson’s hypothesis, and admit that a certain degree of deformation may take place under the mere weight of an ice-sheet, it is difficult to believe that the crust can be so readily deformed as the supporters of the “earth-movement hypothesis” seem to imply. If it could yield so readily to pressure, one is at a loss to understand how a great ice-sheet could accumulate—the ice would simply float off as the land subsided. Take the case of north-western Europe. The ice-sheet that covered Scotland did not attain, on the average, 3000 feet in thickness, and yet we are to suppose that it was able to depress the land for some 600 feet below its present level—that is to say, for 3600 feet below its assumed pre-glacial elevation. Either the ice depressed the crust to that remarkable extent, or the land upon which the ice accumulated was not nearly so high as the advocates of the “earth-movement hypothesis” have supposed. But the average I have taken for the thickness of the Scottish ice-sheet is excessive, for it was only in the low-grounds that the mer de glace attained such a depth. A large part of our country, however, is mountainous, and the mountain-tops were, of course, not nearly so thickly mantled with ice as the valleys. And the same to even a larger extent holds good for the Scandinavian peninsula. If we take the thickness of the Scandinavian ice-sheet that coalesced with that of Scotland as 4000 feet, we shall be over the mark. Now, I ask, is it possible to believe that a sheet of ice of that thickness actually pressed down the crust of the earth for not less than 3600 feet? But if we accept the “earth-movement hypothesis,” as it has been recently advocated, that is what we must believe. If we cannot do so, then we cannot accept the assumption of great elevation of the land in pre-glacial and glacial times. Let me put the case shortly: if the glacial marine beds and raised beaches of the Atlantic borders of Europe and North America owe their origin to depression induced by the weight of an ice-sheet, then it is quite certain that at the advent of glacial conditions the land could not have been so highly elevated as the advocates of the “earth-movement hypothesis” suppose. But if we are to accept the notion of great elevation of the land, then we must conclude that the submergence to which the raised beaches testify cannot have been caused by the pressure of ice-sheets.

It is hardly necessary to pursue this particular subject further, but before leaving it, attention may be drawn for a moment to the curious conclusion that the ice-sheets were self-destructive. One is left to guess at what particular stage the sinking process began, but if the earth’s crust were as readily deformed as the extreme views I have been examining would compel one to imply, then depression must have commenced almost immediately with the accumulation of snow and ice. The several ice-sheets must soon have attained their maximum thickness, and their disappearance must have been correspondingly rapid. And yet all the evidence goes to show that a glacial epoch endured for a comparatively long time—for a time sufficient to account for a prodigious amount of rock-erosion, and for the accumulation of vast sheets of glacial débris and fluvio-glacial detritus.[BY]

[BY] It must not be inferred from the above remarks that I deny the possibility of deformation of the crust having been induced by the old ice-sheets. The geological evidence is certainly suggestive of such having been the case. But I much doubt whether the sinking of the surface was brought about by the mere weight of the ice pressing the crust down into a subjacent liquid layer. Dr. Drygalski’s explanation would better account for the geological phenomena, but, according to Rev. Osmond Fisher, it cannot be maintained.