GLACIER TABLES ACCOUNTED FOR.
Almost all glaciers present examples of such tables; but no glacier with which I am acquainted exhibits them in greater number and perfection than the Unteraar glacier, near the Grimsel. Vast masses of granite are thus poised aloft on icy pedestals; but a limit is placed to their exaltation by the following circumstance. The sun plays obliquely upon the table all day; its southern extremity receives more heat than its northern, and the consequence is, that it dips towards the south. Strictly speaking, the plane of the dip rotates a little during the day, being a little inclined towards the east in the morning, north and south a little after noon, and inclined towards the west in the evening; so that, theoretically speaking, the block is a sun-dial, showing by its position the hour of the day. This rotation is, however, too small to be sensible, and hence the dip of the stones upon a glacier sufficiently exposed to the sunlight, enables us at any time to draw the meridian line along its surface. The inclination finally becomes so great that the block slips off its pedestal, and begins to form another, while the one which it originally occupied speedily disappears, under the influence of sun and air. [Fig. 20] represents a typical section of a glacier-table, the sun's rays being supposed to fall in the direction of the shading lines.
TYPE "TABLE."
Stones of a certain size are always lifted in the way described. A considerable portion of the heat which a large block receives is wasted by radiation, and by communication to the air, so that the quantity which reaches the ice beneath is trifling. Such a mass is, of course, a protector of the ice beneath it. But if the stone be small, and dark in colour, it absorbs the heat with avidity, communicates it quickly to the ice with which it is in contact, and consequently sinks in the ice. This is also the case with bits of dirt and the finer fragments of débris; they sink in the glacier. Sometimes, however, a pretty thick layer of sand is washed over the ice from the moraines, or from the mountain-sides; and such sand-layers give birth to ice-cones, which grow to peculiarly grand dimensions on the Lower Aar glacier. I say "grow," but the truth, of course, is, that the surrounding ice wastes, while the portion underneath the sand is so protected that it remains as an eminence behind. At first sight, these sand-covered cones appear huge heaps of dirt, but on examination they are found to be cones of ice, and that the dirt constitutes merely a superficial covering.
Turn we now to the moraines. Protecting, as they do, the ice from waste, they rise, as might be expected, in vast ridges above the general surface of the glacier. In some cases the surrounding mass has been so wasted as to leave the spines of ice which support the moraines forty or fifty feet above the general level of the glacier. I should think the moraines of the Mer de Glace about the Tacul rise to this height. But lower down, in the neighbourhood of the Echelets, these high ridges disappear, and nought remains to mark the huge moraine but a strip of dirt, and perhaps a slight longitudinal protuberance on the surface of the glacier. How have the blocks vanished that once loaded the moraines near the Tacul? They have been swallowed in the crevasses which intersect the moraines lower down; and if we could examine the ice at the Echelets we should find the engulfed rocks in the body of the glacier.
MORAINES ENGULFED AND DISGORGED.
Cases occur, wherein moraines, after having been engulfed, and hidden for a time, are again entirely disgorged by the glacier. Two moraines run along the basin of the Talèfre, one from the Jardin, the other from an adjacent promontory, proceeding parallel to each other towards the summit of the great ice-fall. Here the ice is riven, and profound chasms are formed, in which the blocks and shingle of the moraines disappear. Throughout the entire ice-fall the only trace of the moraines is a broad dirt-streak, which the eye may follow along the centre of the fall, with perhaps here and there a stone which has managed to rise from its frozen sepulchre. But the ice wastes, and at the base of the fall large masses of stone begin to reappear; these become more numerous as we descend; the smaller débris also appears, and finally, at some distance below the fall, the moraine is completely restored, and begins to exercise its protecting influence; it rises upon its ridge of ice, and dominates as before over the surface of the glacier.
TRANSPARENCY OF ICE UNDER THE MORAINES.
The ice under the moraines and sand-cones is of a different appearance from that of the surrounding glacier, and the principles we have laid down enable us to explain the difference. The sun's rays, striking upon the unprotected surface of the glacier, enter the ice to a considerable depth; and the consequence is, that the ice near the surface of the glacier is always disintegrated, being cut up with minute fissures and cavities, filled with water and air, which, for reasons already assigned, cause the glacier, when it is clean, to appear white and opaque. The ice under the moraines, on the contrary, is usually dark and transparent; I have sometimes seen it as black as pitch, the blackness being a proof of its great transparency, which prevents the reflection of light from its interior.