Climate and Time in Their Geological Relations / A Theory of Secular Changes of the Earth's Climate

by James Croll

It will not do for the advocates of the iceberg theory to assume, as they have hitherto done, that, as a matter of course, the sea-bottom is being striated and grooved by means of icebergs. They must prove that. They must either show that, as a matter of fact, icebergs are actually efficient agents in striating the sea-bottom, or prove from mechanical principles that they must be so. The question must be settled either by observation or by reason; mere opinion will not do.

The Amount of Material transported by Icebergs much exaggerated.—The transporting of boulders and rubbish, and not the grinding and striating of rocks, is evidently the proper function of the iceberg. But even in this respect I fear too much has been attributed to it.

In reading the details of voyages in the arctic regions one cannot help feeling surprised how seldom reference is made to stones and rubbish being seen on icebergs. Arctic voyagers, like other people, when they are alluding to the geological effects of icebergs, speak of enormous quantities of stones being transported by them; but in reading the details of their voyages, the impression conveyed is that icebergs with stones and blocks of rock upon them are the exceptions. The greater portion of the narratives of voyages in arctic regions consists of interesting and detailed accounts of the voyager’s adventures among the ice. The general appearance of the icebergs, their shape, their size, their height, their colour, are all noticed; but rarely is mention made of stones being seen. That the greater number of icebergs have no stones or rubbish on them is borne out by the positive evidence of geologists who have had opportunities of seeing icebergs.

Mr. Campbell says:—“It is remarkable that up to this time we have only seen a few doubtful stones on bergs which we have passed.... Though no bergs with stones on them or in them have been approached during this voyage, many on board the Ariel have been close to bergs heavily laden.... A man who has had some experience of ice has never seen a stone on a berg in these latitudes. Captain Anderson, of the Europa, who is a geologist, has never seen a stone on a berg in crossing the Atlantic. No stones were clearly seen on this trip.”[149] Captain Sir James Anderson (who has long been familiar with geology, has spent a considerable part of his life on the Atlantic, and has been accustomed to view the iceberg as a geologist as well as a seaman) has never seen a stone on an iceberg in the Atlantic. This is rather a significant fact.

Sir Charles Lyell states that, when passing icebergs on the Atlantic, he “was most anxious to ascertain whether there was any mud, stones, or fragments of rocks on any one of these floating masses; but after examining about forty of them without perceiving any signs of frozen matter, I left the deck when it was growing dusk.”[150] After he had gone below, one was said to be seen with something like stones upon it. The captain and officers of the ship assured him that they had never seen a stone upon a berg.

The following extract from Mr. Packard’s “Memoir on the Glacial Phenomena of Labrador and Maine,” will show how little is effected by the great masses of floating ice on the Labrador coast either in the way of grinding and striating the rocks, or of transporting stones, clay, and other materials.

“Upon this coast, which during the summer of 1864 was lined with a belt of floe-ice and bergs probably two hundred miles broad, and which extended from the Gulf of St. Lawrence at Belles Amours to the arctic seas, this immense body of floating ice seemed directly to produce but little alteration in its physical features. If we were to ascribe the grooving and polishing of rocks to the action of floating ice-floes and bergs, how is it that the present shores far above (500), and at least 250 feet below, the water-line are often jagged and angular, though constantly stopping the course of masses of ice impelled four to six miles an hour by the joint action of tides, currents, and winds? No boulders, or gravel, or mud were seen upon any of the bergs or masses of shore-ice. They had dropped all burdens of this nature nearer their points of detachment in the high arctic regions.” ...

“This huge area of floating ice, embracing so many thousands of square miles, was of greater extent, and remained longer upon the coast, in 1864, than for forty years previous. It was not only pressed upon the coast by the normal action of the Labrador and Greenland currents, which, in consequence of the rotatory motion of the earth, tended to force the ice in a south-westerly direction, but the presence of the ice caused the constant passage of cooler currents of air from the sea over the ice upon the heated land, giving rise during the present season to a constant succession of north-easterly winds from March until early in August, which further served to crowd the ice into every harbour and recess upon the coast. It was the universal complaint of the inhabitants that the easterly winds were more prevalent, and the ice ‘held’ later in the harbours this year than for many seasons previous. Thus the fisheries were nearly a failure, and vegetation greatly retarded in its development. But so far as polishing and striating the rocks, depositing drift material, and thus modifying the contour of the surface of the present coast, this modern mass of bergs and floating ice effected comparatively little. Single icebergs, when small enough, entered the harbours, and there stranding, soon pounded to pieces upon the rocks, melted, and disappeared. From Cape Harrison, in lat. 55°, to Caribo Island, was an interrupted line of bergs stranded in 80 to 100 or more fathoms, often miles apart, while others passed to the seaward down by the eastern coast of Newfoundland, or through the Straits of Belle Isle.”[151]

Boulder Clay the Product of Land-ice.—There is still another point connected with icebergs to which we must allude, viz., the opinion that great masses of the boulder clay of the glacial epoch were formed from the droppings of icebergs. If boulder clay is at present being accumulated in this manner, then traces of the boulder clay deposits of former epochs might be expected to occur. It is perfectly obvious that unstratified boulder clay could not have been formed in this way. Stones, gravel, sand, clay, and mud, the ingredients of boulder clay, tumbled all together from the back of an iceberg, could not sink to the bottom of the sea without separating. The stones would reach the bottom first, then the gravel, then the sand, then the clay, and last of all the mud, and the whole would settle down in a stratified form. But, besides, how could the clay be derived from icebergs? Icebergs derive their materials from the land before they are launched into the deep, and while they are in the form of land-ice. The materials which are found on the backs of icebergs are what fell upon the ice from mountain tops and crags projecting above the ice. Icebergs are chiefly derived from continental ice, such as that of Greenland, where the whole country is buried under one continuous mass, with only a lofty mountain peak here and there rising above the surface. And this is no doubt the chief reason why so few icebergs have stones upon their backs. The continental ice of Greenland is not, like the glaciers of the Alps, covered with loose stones. Dr. Robert Brown informs me that no moraine matter has ever been seen on the inland ice of Greenland. It is perfectly plain that clay does not fall upon the ice. What falls upon the ice is stones, blocks of rocks, and the loose débris. Clay and mud we know, from the accounts given by arctic voyagers, are sometimes washed down upon the coast-ice; but certainly very little of either can possibly get upon an iceberg. Arctic voyagers sometimes speak of seeing clay and mud upon bergs; but it is probable that if they had been near enough they would have found that what they took for clay and mud were merely dust and rubbish.

Undoubtedly the boulder clay of many places bears unmistakable evidence of having been formed under water; but it does not on that account follow that it was formed from the droppings of icebergs. The fact that the boulder clay in every case is chiefly composed of materials derived from the country on which the clay lies, proves that it was not formed from matter transported by icebergs. The clay, no doubt, contains stones and boulders belonging to other countries, which in some cases may have been transported by icebergs; but the clay itself has not come from another country. But if the clay itself has been derived from the country on which it lies, then it is absurd to suppose that it was deposited from icebergs. The clay and materials which are found on icebergs are derived from the land on which the iceberg is formed; but to suppose that icebergs, after floating about upon the ocean, should always return to the country which gave them birth, and there deposit their loads, is rather an extravagant supposition.