VII.] ICEBERGS. THE BOULDER CLAY. 99 up and floats away. Ice is lighter than water, and when floating in salt water, about one-ninth projects above its surface. The gigantic masses which detach themselves from these Arctic glaciers are thus carried away by the sea currents which bring the cold water of the polar regions towards the tropical zone; on the coasts of Newfoundland, the current that issues from Baffin's Bay is frequently laden with such floating islands of ice, gradually melting in the warmer air as they proceed on their slow journey, and hence assuming the most fantastic forms. They are termed icebergs. Splinters and blocks of rock, which, in common with the glaciers of warmer latitudes, these Arctic ice-streams carry down, and the mud and sand which, by friction, they have abraded from their rocky beds, are borne away on the floating iceberg, and as the ice gradually melts, are deposited at the bottom of the sea. The midland counties of England, the eastern part of Ireland, and the plains of Northern Germany and Russia, are in many parts covered by a considerable thickness of clay, containing large blocks of stone termed boulders, and hence termed the Boulder Clay. The origin of this clay was at one time a great puzzle to geologists; the more so, that it was observed that the contained boulders are unlike any rock that occurs in the neighbourhood; and appear to have come from mountains many miles distant. In England they seem to have been derived from the mountains of Cumberland and the Highlands; in Germany and Russia from the more distant mountains of Scandinavia; yet they are of such size, that it is manifest no ordinary power could have transported them. At length it was observed that all these mountains, down to very low levels, bore indications of former glacier action; the moraines, roches moutonnées and grooved surfaces, exactly like those so generally observed on the Alps. Here then was the solution of the difficulty. The boulder clay is distinctly a marine formation; for in some places, though rarely, it contains marine shells. When it was being formed, all the lowlands of Northern Europe (and we may add Northern Asia also) 100 ANCIENT ICE-MARKS IN INDIA. [CHAP. were covered by the sea; and icebergs, drifting from the mountains which still remained above water, carried down rocky masses and mud, like the modern icebergs of Baffin's Bay; strewing them, as they melted, over the sea bottom. The period during which this took place is termed the Glacial period, and immediately preceded the existing state of things. Strange as it must seem, and indeed inexplicable with our present knowledge, it would appear at least probable that, at a very distant period of the earth's history, something of the same kind has taken place in tropical India. At the very bottom of that great series of sedimentary rocks, which contains the coal-beds of Rániganj, Chutia Nágpúr and Central India, there is a bed of rock boulders embedded in fine mud, strangely resembling the boulder clay of the British Isles. This resemblance was noticed by Mr. W. T. Blanford as long ago as 1856, and he ventured then to speculate on the possibility of the deposit having been formed by the agency of ice. Two years ago (1872) some boulders were exhumed from this bed by Dr. T. Oldham, polished and marked with grooves and scratches exactly like those brought down by modern glaciers. One of these remarkable blocks is now in the Government Geological Museum of Calcutta, and is certainly not the least interesting object in that magnificent collection. I have hitherto spoken only of those perennial accumulations of ice and snow, that are to be met with on lofty mountains and on the land of the Polar Regions. But in all countries where the winter temperature remains for many days or weeks below the freezing point, snow and ice accumulate; and sometimes large rivers and lakes are frozen over to such a thickness, that they may be traversed by men and vehicles as if they were dry land. The pure transparent ice, many shiploads of which are brought annually to India, is collected from the surface of the American lakes, which are frozen to the depth of several feet every winter. In Canada, Northern Europe, and Northern Asia, the landscape, for many weeks and even months of the winter season, presents the aspect of a vast expanse of snow: the trees, bare of foliage, with the exception of the dark pine forests, are laden on every branch and twig with a snowy burden; and the pines, whose needle-like leaves hold it in larger masses, bend beneath the weight of their pure white canopy. The treeless steppes of Asiatic Russia become a howling wilderness of snow, and when storms arise, travellers are sometimes overwhelmed and buried in the drifts. But in fine weather, even when thus covered, they are easily traversed. The snow, when beaten hard and frozen, offers a smooth surface; over which vehicles, called sleighs, running (not on wheels, but) on wooden bars shod with iron, are propelled with a speed and ease unattainable on the best metalled road. The rivers, bound fast with a covering of ice, beneath which a dark silent stream carries down the yet unfrozen waters, offer a natural roadway; and wrapped in furs to protect him against the bitter cold, the traveller speeds on from stage to stage; where double windows and the never-extinguished stove keep up a pleasing warmth, and ensure a genial resting place in the heart of the wintry waste. In the spring, with a rising temperature, the snows melt; the rivers, freed from their icy bonds, become gorged and swollen with the accumulated drainage, thus suddenly set free; and overtopping the banks of their summer channels, inundate the surrounding flats. In the sub-arctic and the colder parts of the temperate zone, the spring is therefore the season of floods; and in some degree, such is the case with all rivers and streams that are fed by winter snows. The Indus, which derives its waters almost exclusively from the mountains, and chiefly from their melting snows, is an Indian example of such a river.1 In the Arctic Regions, not only is the land covered with ice and snow, but the sea itself is sometimes frozen. Salt water, however, requires greater cold than fresh water to freeze it; and when it freezes, the ice that is formed is free from salt; the salt being, so to speak, squeezed out in the act of freezing. * See Chapter IX. 102 ICE AND SNOW PROTECTIVE. [CHAP. In all cases in which the sea or deep lakes and rivers are frozen, the ice is formed only on the surface; and with the exception of icebergs, which are of foreign origin, it is rarely very many feet in thickness. In the case of freshwater lakes, this is owing to a peculiar property of water. Pure water, on the point of freezing, is lighter than that which is a little higher in temperature; and when therefore it has cooled down to this latter degree, the very cold water floats and since heat passes through water very slowly, that which is below cools no further until the upper layer is frozen. As a consequence, the greater part of the water below the ice sheet that forms on the surface of a lake remains somewhat warmer than the rest, and the ice sheet first formed thickens but slowly. In rivers this is not the case. The water being in motion has almost the same temperature as the ice formed from it. Moreover, ice-cold water in freezing has to part with a large amount of heat, in the mere act of freezing; and this can only escape upwards through the ice already formed. Sea water, again, behaves in a different manner. As already mentioned, it requires a greater degree of cold to freeze it at all than fresh water, and the ice formed by it is relatively much lighter. It floats therefore, and as in the case of rivers and lakes, by the opposition that it offers to the passage of heat, it checks further freezing. Moreover, in all these cases, snow falling on the surface of ice lies unmelted, and offers even greater resistance to the passage of heat than the ice itself. In the same way, the snow that falls on a land surface, covered with vegetation, protects the plants from the intense cold of the air, while a certain amount of heat reaches them through the solid earth from below. Thus plants which would be killed by frost, may be protected and preserved through a severe winter, by a thick covering of snow; and when it melts under the rays of the spring sun, they are ready to put forth their young leaves and to open out with renewed verdure. From what has been stated above, my readers will understand that, however little we experience their effects in India, ice and snow play a very important part in the VII.] SUMMARY. 103 economy of the world. On high mountains, even in the tropics, and on the low hills of the Polar Regions, ice and frost help in the work of wearing away the rocks and in the form of glaciers and icebergs, they carry down to lower levels, and even to the sea bottom, the waste of the landsurface. Snow becomes destructive only when it becomes compacted into ice; or when, accumulating in vast drifts, it buries men and animals, who may be overtaken by the wintry storm in some unprotected waste. At other times, and more frequently, it subserves a useful and beneficial purpose, in covering and protecting vegetation from destruction by cold; and in affording a smooth surface, over which men and vehicles can travel almost with more facility than on an ordinary road. |