It may be asked, how it is known that the glacier carries this powerful apparatus on its sides and bottom, when they are hidden from sight. I answer, that we might determine the fact theoretically from certain known conditions respecting the conformation of the glacier; to which I shall allude presently; but we need not resort to this kind of evidence, since we have ocular demonstration of the truth. Here and there on the sides of the glacier it is possible to penetrate between the walls and the ice to a great depth, and even to follow such a gap to the very bottom of the valley, and everywhere do we find the surface of the ice fretted as I have described it, with stones of every size, from the pebble to the boulder, and also with sand and gravel of all sorts, from the coarsest grain to the finest, and these materials, more or less firmly set in the ice, form the grating surface with which, in its onward movement down the Alpine valleys, it leaves everywhere unmistakable, traces of its passage.
We come now to the moraines, those walls of loose materials built by the glaciers themselves along their road. They have been divided into three classes, namely, lateral, medial, and terminal moraines. Let us look first at the lateral ones; and to understand them we must examine the conformation of the glacier below the névé, where it assumes the character of pure compact ice. We have seen that the fields of snow, where the glaciers have their origin, are level, and that lower down, where these masses of snow begin to descend toward the narrower valley, they follow its trough-like shape, sinking toward the centre and sloping upward against the sides, so that the surface of the glacier, about the region of the névé, is slightly concave. But lower down in the glacier proper, where it is completely transformed into ice, its surface becomes convex, for the following reason: The rocky walls of the valley, as they approach the plain, partake of its higher temperature. They become heated by the sun during the day in summer, so that the margins of the glacier melt rapidly in contact with them. In consequence of this, there is always in the lower part of the glacier a broad depression between the ice and the rocky walls, while, as this effect is not felt in the centre of the glacier, it there retains a higher level. The natural result of this is a convex surface, arching upward toward the middle, sinking toward the sides. It is in these broad, marginal depressions that the lateral moraines accumulate; masses of rock, stones, pebbles, dust, all the fragments, in short, which become loosened from the rocky walls above, fall into them, and it is a part of the materials so accumulated which gradually work their way downward between the ice and the walls, till the whole side of the glacier becomes studded with them. It is evident, that, when the glacier runs in a northerly or southerly direction, both the walls will be affected by the sun, one in the morning, the other in the afternoon, and in such a case the sides will be uniform, or nearly so. But when the trend of the valley is from east to west, or from west to east, the northern side only will feel the full force of the sun; and in such a case, only one side of the glacier will be convex in outline, while the other will remain nearly on a level with the middle. The large masses of loose materials which accumulate between the glacier and its rocky walls and upon its margins form the lateral moraines. These move most slowly, as the marginal portions of the glacier advance at a much slower rate than its centre.
The medial moraines arise in a different way, though they are directly connected with the lateral moraines. It often happens that two smaller glaciers unite, running into each other to form a larger one. Suppose two glaciers to be moving along two adjoining valleys, converging toward each other, and running in an easterly or westerly direction; at a certain point these two valleys open into a single valley, and here, of course, the two glaciers must meet, like two rivers rushing into a common bed. But as glaciers consist of a solid, and not a fluid, there will be no indiscriminate mingling of the two, and they will hold their course side by side. This being the case, the lateral moraine on the southern side of the northernmost glacier and that on the northern side of the southernmost one must meet in the centre of the combined glaciers. Such are the so-called medial moraines formed by the junction of two lateral ones. Sometimes a glacier may have a great number of tributaries, and in that case we may see several such moraines running in straight lines along its surface, all of which are called medial moraines in consequence of their origin midway between two combining glaciers. The glacier of the Aar represented in the wood-cut below affords a striking example of a large medial moraine. It is formed by the junction of the glaciers of the Lauter-Aar, on the right-hand side of the wood-cut, and the Finster-Aar, on the left; and the union of their inner lateral moraines, in the centre of the diagram, forms the stony wall down the centre of the larger glacier, called its medial moraine. This moraine at some points is not less than sixty feet high. We have here an effect similar to that of the glacier-tables and the sand-pyramids. The wall protects the ice beneath it, and prevents it from sinking at the same rate as the surrounding surface, while its heated surface increases the melting of the adjacent surfaces of ice, thus forming longitudinal depressions along the medial moraines, in which the largest rivulets and the most conspicuous sand-pyramids, the deepest wells and the finest waterfalls, are usually met with. As the medial moraines rest upon that part of the glacier which moves fastest, they of course advance much more rapidly than the lateral moraines.
Glacier of the Aar.
The terminal moraines consist of all the débris brought down by the glacier to its lower extremity. In consequence of the more rapid movement of the centre of the glacier, it always terminates in a semicircle at its lower end, where these materials collect, and the terminal moraines, of course, follow the outline of the glacier. The wood-cut below represents the terminal moraine of the glacier of Viesch.
Sometimes, when a number of cold summers have succeeded each other, preventing the glacier from melting in proportion to its advance, the accumulation of materials at its terminus becomes very considerable; and when, in consequence of a succession of warm summers, it gradually melts and retreats from the line it has been occupying, a large semicircular wall is left, spanning the valley from side to side, through which the stream issuing from the glacier may be seen cutting its way. It is important to notice that such terminal moraines may actually span the whole width of a valley, from side to side, and be interrupted only where watercourses of sufficient power break through them. To suppose that such transverse walls of loose materials could be thrown across a valley by a river were to suppose that it could build dams across its bed while it is flowing. Such transverse or crescent-shaped moraines are everywhere the work of glaciers.
All these moraines are the land-marks, so to speak, by which we trace the height and extent, as well as the progress and retreat, of glaciers in former times. Suppose, for instance, that a glacier were to disappear entirely. For ages it has been a gigantic ice-raft, receiving all sorts of materials on its surface as it travelled onward, and bearing them along with it; while the hard particles of rock set in its lower surface have been polishing and fashioning the whole surface over which it extended. As it now melts, it drops its various burdens on the ground; boulders are the mile-stones marking the different stages of its journey, the terminal and lateral moraines are the framework which it erected around itself as it moved forward, and which define its boundaries centuries after it has vanished, while the scratches and furrows it has left on the surface below show the direction of its motion.
All the materials which reach the bottom of the glacier, and are moving under its weight, so far as they are not firmly set in the ice must be pressed against one another, as well as against the rocky bottom, and will be rounded off, polished, and scratched, like the rock itself over which they pass. The pebbles or stones set fast in the ice will be thus polished and scratched, however, only over the surface exposed; but, as they may sometimes move in their socket, like a loosely mounted stone, the different surfaces may in turn undergo this process, and in the end all the loose materials under a glacier become more or less polished, scratched, and grooved. These marks exhibit also the peculiarity so characteristic of the grooves and scratches on the bed and walls of the valley: they are rectilinear, trending in the direction in which the superincumbent mass advances, though, of course, owing to the changes in the position of the pebbles or boulders, they may cross each other in every direction on their surface.