MOVEMENT OF THE GLACIERS.
"There is a multitude of facts which would seem to necessitate the belief that the substance of glaciers enjoys a kind of ductility, which permits it to mould itself to the locality which it occupies, to grow thin, to swell and to narrow itself like a soft paste."
And this, true of the Alpine passes, is true also of the Greenland valleys. A great frozen flood is pouring down the east and west slopes of the Greenland continent; and, as in the Alps, what is gained in height by one year's freezing is lost by the downward flow of the mobile mass.
And this movement is not embarrassed by any obstacle. The lower chains of hills do not arrest it, for it moulds itself to their form, sweeps through every opening between them, or overtops them. Valleys do not interfere with its onward march, for the frozen stream enters them, and levels them with the highest hills. It heeds not the precipice, for it leaps over it into the plain below,—a giant, frozen waterfall. Winter and summer are to it alike the same. It moves ever forward in its irresistible career,—a vast, frozen tide swelling to the ocean. It pours through every outlet of the coast ranges, down every ravine and valley, overriding every impediment, grinding and crushing over the rocks; and at length it comes upon the sea. But here it does not stop. Pushing back the water, it makes its own coast line; and, moving still onward, accommodating itself to every inequality of the bed of the sea, as it had before done to the surface of the land, filling up the wide bay or fiord, expanding where it expands, narrowing where it narrows, swallowing up the islands in its slow and steady course, it finally reaches many miles beyond the original shore-line.
And now it has attained the climax of its progress.
When, long ages ago, after pouring over the sloping land, it finally reached the coast and looked down the bay which it was ultimately to fill up, its face was many hundreds of feet high. Gradually it sank below the line of waters as it moved outward, and finally its front has almost wholly disappeared.
FORMATION OF ICEBERGS.
In a former chapter I have mentioned that a block of fresh-water ice floating in sea water rises above the surface to the extent of one eighth of its weight and bulk, while seven eighths of it are below the surface. The cause of this is too well known to need more than a passing explanation. Every school-boy is aware that water, in the act of freezing, expands, and that in the crystal condition fresh water occupies about one tenth more space than when in a fluid state; and hence, when ice floats in the fresh water from which it was formed, one tenth of it is exposed above, while the remaining nine tenths are beneath the surface. When this same fresh-water ice (which it will be remembered is the composition of the glacier) is thrown into the sea, the proportion of that above to that below being changed from one and nine to one and seven, is due to the greater density of the sea-water, caused by the salt which it holds in solution.
FORMATION OF ICEBERGS.
Now it will be obvious that, as the glacier continues to press further and further into the sea, the natural equilibrium of the ice must ultimately become disturbed,—that is, the end of the glacier is forced further down into the water than it would be were it free from restraint, and at liberty to float according to the properties acquired by congelation. The moment that more than seven eighths of its front are below the water line, the glacier will, like an apple pressed down by the hand in a pail of water, have a tendency to rise, until it assumes its natural equilibrium. Now it will be remembered that the glacier is a long stream of ice, many miles in extent, and, although the end may have this tendency to rise, yet it is, for a time, held down firmly by the continuity of the whole mass. At length, however, as the end of the glacier buries itself more and more in the water, the tendency to rise becomes stronger and stronger, and finally the force thus generated is sufficient to break off a fragment, which, once free, is buoyed up to the level that is natural to it. This fragment may be a solid cube half a mile through, or even of much greater dimensions. The disruption is attended with a great disturbance of the waters, and with violent sounds which may be heard for many miles; but, floating now free in the water, the oscillations which the sudden change imparted to it gradually subside; and, after acquiring its natural equilibrium, the crystal mass drifts slowly out to sea with the current, and is called an Iceberg.[6]