Some Salient Points in the Science of the Earth

by Sir John William Dawson

In passing through the straits in July, I have seen great numbers of bergs, some low and flat-topped, with perpendicular sides, others convex or roof-shaped, like great tents pitched on the sea; others rounded in outline or rising into towers and pinnacles. Most of them were of a pure dead white, like loaf sugar, shaded with pale bluish green in the great rents and recent fractures. One of them seemed as if it had grounded and then overturned, presenting a flat and scored surface covered with sand and earthy matter.

At present we wish to regard the icebergs of Belle-Isle in their character of geological agents. Viewed in this aspect, they are in the first place parts of the cosmical arrangements for equalizing temperature, and for dispersing the great accumulations of ice in the Arctic regions, which might otherwise unsettle the climatic and even the static equilibrium of our globe, as they are believed by some imaginative physicists and geologists to have done in the so-called glacial period. If the ice islands in the Atlantic, like lumps of ice in a pitcher of water, chill our climate in spring, they are at the same time agents in preventing a still more serious secular chilling which might result from the growth without limit of the Arctic snow and ice. They are also constantly employed in wearing down the Arctic land, and aided by the great northern current from Davis's Straits, in scattering stones, boulders and sand over the banks along the American coast. Incidentally to this work, they smooth and level the higher parts of the sea bottom, and mark it with furrows and striæ indicative of the direction of their own motion.

When we examine a chart of the American coast, and observe the deep channel and hollow submarine valleys of the Arctic current, and the sandbanks which extend parallel to this channel from the great bank of Newfoundland to Cape Cod, we cannot avoid the conclusion that the Arctic current and its ice have great power both of excavation and deposition. On the one hand, deep hollows are cut out where the current flows over the bottom, and on the other, great banks are heaped up where the ice thaws and the force of the current is abated. I have been much struck with the worn and abraded appearance of stones and dead shells taken up from the banks off the American coast, and am convinced that an erosive power comparable to that of a river carrying sand over its bed, and materially aided by the grinding action of ice, is constantly in action under the waters of the Arctic current.[159] The unequal pressure resulting from this deposition and abrasion is not improbably connected with the slight earthquakes experienced in Eastern America, and also with the slow depression of the coast; and if we go back to that earliest of all geological periods when the Laurentian rocks of Sir Wm. Logan, constituting the Labrador coast and the Laurentide Hills, were alone above water, we may even attribute in no small degree to the Arctic current of that old time the heaping up of those thousands of feet of deposits which now constitute the great range of the Alleghany and Appalachian mountains, and form the breast bone of the American continent. In those ancient times also large stones were floated southward, and enter into the composition of very old conglomerates.

[159] At the time when this was written I had only studied stones brought up accidentally by fishermen and others from the banks of Newfoundland and elsewhere. At a later date Murray of the Challenger has given more ample material. He states that the bottom in the Labrador current, 100 miles from land, was found to be blue mud with 60 per cent, of sand and stones; and mentions a block of syenite weighing 490 lbs. taken up in 1,340 fathoms, and stones and pebbles of quartzite, limestone, dolomite, mica schist and serpentine, one of which was glaciated. This is the modern boulder clay produced by Greenland glaciers and the field ice of Baffin's Bay and the Labrador coast.

But such large speculations might soon carry us far from Belle-Isle, and to bring us back to the American coast and to the domain of common things, we may note that a vast variety of marine life exists in the cold waters of the Arctic current, and that this is one of the reasons of the great and valuable fisheries of Labrador, Newfoundland and Nova Scotia, regions in which the sea thus becomes the harvest field of much of the human population. On the Arctic current and its ice also floats to the southward the game of the sealers of St. John and the whalers of Gaspé.

We may now proceed to connect these statements as to the distribution of icebergs, with the glaciated condition of our continents, with the remarkable fact that the same effects now produced by the ice and the Arctic current in the Strait of Belle-Isle and the deep-current channel off the American coast, are visible all over the North American and European land north of forty degrees of latitude, and that there is evidence that the St. Lawrence valley itself was once a gigantic Belle-Isle, in which thousands of bergs worked perhaps for thousands of years, grinding and striating its rocks, cutting out its deeper parts, and heaping up in it quantities of northern débris. Out of this fact of the so-called glaciated condition of the surface of our continents has, however, arisen one of the great controversies of modern geology. While all admit the action of ice in distributing and arranging the materials which constitute the last coating which has been laid upon the surface of our continents, some maintain that land glaciers have done the work, others, that sea-borne ice has been the main agent employed. As in some other controversies, the truth seems to lie between the extremes. Glaciers are slow, inactive, and limited in their sphere. Floating ice is locomotive and far-travelled, extending its action to great distances from its sources. So far, the advantages are in favour of the flotation. But the work which the glacier does is done thoroughly, and, time and facilities being given, it may be done over wide areas. Again, the iceberg is the child of the glacier, and therefore the agency of the one is indirectly that of the other. Thus, in any view we must plough with both of these geological oxen, and the controversy becomes like that old one of the Neptunists and Plutonists, which has been settled by admitting both water and heat to have been instrumental in the formation of rocks.

In the midst of these controversies a geologist resident in Great Britain or Canada should have some certain doctrine as to the question whether at that period, geologically recent, which we call the Pleistocene period, the land was raised to a great height above the sea, and covered like Greenland with a mantle of perpetual ice, or whether it was, like the strait of Belle-Isle and the banks of Newfoundland, under water, and annually ground over by icebergs, or whether, as now seems more probable, it was in part composed of elevated ridges covered with snow and sending down glaciers, and partly depressed under the level of ice-laden straits and seas.

A great advocate of the glacier* theory has said that we cannot properly appreciate his view without exploring thoroughly the present glaciers of Greenland and ascertaining their effects. This I have not had opportunity to do, but I have endeavoured to do the next best thing by passing as rapidly as possible from the icebergs of Belle-Isle to the glaciers of Mont Blanc, and by asking the question whether Canada was in the Pleistocene period like the present Belle-Isle or the present Mont Blanc, or whether it partook of the character of both? and taking advantage of these two most salient points in order to elicit a reply.

Transporting ourselves, then, to the monarch of the Alps, let us suppose we stand upon the Flegere, a spur of the mountains fronting Mont Blanc, and commanding a view of the entire group. From this point the western end of the range presents the rounded summit of Mont Blanc proper, flanked by the lower eminences of the Dome and Aiguille de Gouté, which rise from a broad and uneven plateau of nevé or hard snow, sending down to the plain two great glaciers or streams of ice, the Bossons and Tacony glaciers. Eastward of Mont Blanc the nevé or snow plateau is penetrated by a series of sharp points of rock or aiguilles, which stretch along in a row of serried peaks, and then give place to a deep notch, through which flows the greatest of all the glaciers of this side of Mont Blanc, the celebrated Mer de Glace, directly in front of our standpoint. To the left of this is another mass of aiguilles, culminating in the Aiguille Verte. This second group of needles descends into the long and narrow Glacier of Argentiere, and beyond this we see in the distance the Glacier and Aiguille de Tour. As seen from this point, it is evident that the whole system of the Mont Blanc glaciers originates in a vast mantle of snow capping the ridge of the chain, and extending about twenty miles in length, with a breadth of about five miles. This mass of snow being above the limits of perpetual frost, would go on increasing from year to year, except so far as it might be diminished by the fall of avalanches from its sides, were it not that its plasticity is sufficient to enable the frozen mass to glide slowly down the valleys, changing in its progress into an icy stream, which, descending to the plain, melts at its base and discharges itself in a torrent of white muddy water. The Mont Blanc chain sends forth about a dozen of large glaciers of this kind, besides many smaller ones. Crossing the valley of Chamouni, and ascending the Montanvert to a height of about 6,000 feet, let us look more particularly at one of these glaciers, the Mer de Glace. It is a long valley with steep sides, about half a mile wide, and filled with ice, which presents a general level or slightly inclined surface, traversed with innumerable transverse cracks or crevasses, penetrating apparently to the bottom of the glacier, and with slippery sloping edges of moist ice threatening at every step to plunge the traveller into the depths below. Still the treacherous surface is daily crossed by parties of travellers, apparently without any accident. The whole of the ice is moving steadily along the slope on which it rests, at the rate of eight to ten inches daily—the rate of motion is less in winter and greater in summer; and farther down, where the glacier goes by the name of the Glacier du Bois, and descends a steeper slope, its rapidity is greater; and at the same time by the opening of immense crevasses its surface projects in fantastic ridges and pinnacles. The movements and changes in the ice of these glaciers are in truth very remarkable, and show a mobility and plasticity which at first sight we should not have been prepared to expect in a solid like ice.[160] The crevasses become open or closed, curved upwards or downwards, perpendicular or inclined, according to the surface upon which the glacier is moving, and the whole mass is crushed upward or flattens out, its particles evidently moving on each other with much the same result as would take place in a mass of thick mud similarly moving. On the surface of the ice there are a few boulders and many stones, and in places these accumulate in long irregular bands indicating the lines of junction of the minor ice streams coming in from above to join the main glacier. At the sides are two great mounds of rubbish, much higher than the present surface of the glacier. They are called the lateral moraines, and consist of boulders, stones, gravel and sand, confusedly intermingled, and for the most part retaining their sharp angles. This mass of rubbish is moved downward by the glacier, and with the stones constituting the central moraine, is discharged at the lower end, accumulating there in the mass of detritus known as the terminal moraine.