« ZurückWeiter »
observed them; the scratches, furrows, grooves, arc always rectilinear, trending in the direction in which the glacier is moving, and most distinct on that side of the surface-inequalities facing the direction of the moving mass, while the lee-side remains mostly untouched.
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 thr 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 i below the ne"cc', where it assumes the character of pure compact ice. We have seen that the fields of snow, where the glaciers have their origin, arc 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 new, 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 walla and upon its margms 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 j unction 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 arc 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 junc
tion of the glaciers of the Lauter-Aar, on the right-hand side of the wood-cut, and the Finstcr-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 Snest 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.
The terminal moraines consist of all the debris 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
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, Bo 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 tho 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.
As the larger materials are pressed onward with the finer ones, that is, with the •and, gravel, and mud accumulated at the bottom of the glacier, the component parts of this underlying bed of deT/ris will be mixed together without any reference to their size or weight. The softest mud and finest sand may be in immediate contact with the bottom of the valley, while larger rocks and pebbles may be held in the ice above; or their position may be reversed, and the coarser materials may rest below, while the finer ones are pressed between them or overlying them. In short, the whole accumulation of loose debris under the glacier, resulting from the trituration of all kinds of angular fragments reaching the lower surface of the ice, presents a sort of paste in which coarser and lighter materials •re impacted without reference to bulk or
weight. Those fragments which are most polished, rounded, grooved, or scratched, have travelled longest under the glacier, and are derived from the hardest locks, which have resisted the general crushing and pounding for a longer time. The masses of rock on the upper surface of the glacier, on the contrary, are carried along on its back without undergoing any such friction. Lying side by side, or one above another, without being subject to pressure from the ice, they retain, both in the lateral and medial moraines, and even in the terminal moraines, their original size, their rough surfaces, and their angular form. Whenever, therefore, a glacier melts, it is evident that the lower materials will be found covered by the angular surfacematerials now brought into immediate contact with the former in consequence of the disappearance of the intervening ice. The most careful observations and surveys have shown this everywhere to be the case; wherever a large tract of glacier has disappeared, the moraines, with their large angular boulders, are found resting upon this bottom layer of rounded materials scattered through a paste of mud and sand.
We shall see hereafter how far we can follow these traces, and what they tell us of the past history of glaciers, and of the changes the climates of our glob* have undergone.
A CHRISTMAS STORY.
Sometime in the year 1856, a family named Yarrow moved into the neighborhood where I then lived, and rented a small house with a bit of ground attached to it, on one of the rich bottom-farms lying along tho eastern shore of the Ohio. The mother, two or three children, and their dog Ready made up the quiet household: not one to attract notice from any cause. People soon knew Martha Yarrow, — all that was in her. She was Western- and farm-born; whatever Nature had given her of good or bad, therefore, thrust itself out at once with pungent directness.
The family supported themselves by selling their poultry and vegetables to the hucksters, leading an eventless life enough, until the change occurred, some five years after they came into the neighborhood, of which I am going to tell you.
I called it a Christmas Story, not so much because it happened on a Christmas, as because the meaning of it seemed suited to that day; and I thought, too, that nobody grows tired of Christmas stones, especially if he chance to have been born in one of those families where the day is kept in tho old fashion: it roots itself so deep, that memory, in whatever quaint superstition, or homely affection for mother or brother, or unreasoning trust in God, may outlive our childhood, and underlie our older years. And surely that is as just, as wise a thing, — to strip off for a child the smirched trading - dress of one day at least, and Bend it down through tho long procession of the years with its true face bared, to waken in him a live sense of man's love and God's love. Some one, perhaps, had done this for this woman, Mrs. Yarrow, long ago; for, let the months before and after be bare as they chose, she kept this day of Christmas with a
feverish anxiety, more eager than her children even to make every moment warm and throb with pleasure, and enjoying them herself, to their last breath, with the whole zest of a nervous, strongblooded nature. Yet she may have had another reason for it.
The evening before the Christmas of which we write, she had gone out to the well with her son before closina the house
o for the night .
"There 's no danger of thaw before morning, Jem?" — looking anxiously up into the night, aa they rested the bucket on the curb.
"Thaw! there 'a a woman's notion for you! Why, tho very crow is frozen out of the cocks yonder!"—stretching his arms, and clapping his hollow chest, as if he were six feet high. "No, we 'll not have a thaw, little woman."
The children often called her that, in a fond, protecting way; but it sounded most oddly from Jem, he was such a weak, swaggering sparrow of a little chap. He stretched his hands as high as he could reach up to her hips, and smoothed her linsey dress down: if it had been her face, the touch could not have been more tender.
"You don't think of the luck we always have. Why, it could n't rain on Christmas for you or me, mother I"
She laughed, nodding several times.
".Well, that is sure, Jem," stopping to look into the lean, emphatic little face, and to pass her hand over the tow-colored hair.
Somehow, the bond between mother and son was curiously strong to-night. It was always so on Christmas. At other times they were much like two children in companionship, but Christmas never came without bringing a vague sense of cowering close together as though some danger stood near them. There