Читать книгу Walks and Talks in the Geological Field - Alexander Winchell - Страница 13

Perhaps it is best to pause at once and contemplate a fuller sketch of some living glaciers. We indulged in a little speculation about the cause of the Drift. We argued that glaciers must perform a work pretty nearly such as the Drift required; and I cited you to Alpine glaciers as actually exemplifying this kind of work. But come, now, let us take a closer look at Alpine glaciers. The scenes are abundant in picturesque interest as well as instruction.

About fifty miles from Geneva lies the “vale of Chamonix”—the classic valley of classic glaciers. Its axis lies nearly east and west, and the Arve, taking its rise from the east, flows through the length of the valley, and bends north to the Lake of Geneva. On the north, the valley is bounded by the sharp pinnacled Aiguilles Rouges (A-ghee-Roosj); on the south rises the stupendous mass of the Mont Blanc (Blahnc) range, nearly sixteen thousand feet above sea level. The rounded summit of the monarch mountain is silver white with perpetual snow. On one shoulder rises the Dome du Goûter, and on the other the Aiquille de Goûter (A-ghee-du-Goó-tay). For three thousand feet below the summit, compact snow covers the surface to an unknown depth. In one region below the Aiguille de Goûter, may be seen a long perpendicular cliff of snow left by a slide. It looks like a vast entablature to the glittering dome. This is said to be fifteen hundred feet in height. At the foot of the final dome stretches a fathomless crevasse, in which a number of persons have been lost. This is the "Grande Crevasse," and for a long time it prevented all successful approach to the mountain’s summit. Sometimes a temporary bridge is stretched across by drifting snow. Occasionally it becomes sufficiently solid to serve for a passage over, but it is always treacherous, and once precipitated an English lady and her companion to a depth from which they were never recovered.

From the Grande Crevasse stretches a gentle slope called the Grand Plateau at an elevation of thirteen thousand feet. This is covered with granular névé. Along its lower limit the snow-mass is broken into tumultuous confusion, and the passage over it is difficult and dangerous. Below this is the Little Plateau, ten thousand feet above sea-level; and then come other broken belts of snowy precipices. Now, the upper limits of two glaciers are reached in the downward flow of the ice. This common ice-field is a scene of grand confusion. The mountain slope beneath the ice-sheet presents many irregularities of pitch, and many projecting bosses. Over all these the ice-stream flows toward the lower level. In one place, nine thousand feet above sea-level, a vast pinnacled mass of rock rises some hundreds of feet above the ice. This divides the wide stream, but the parts completely coalesce again around the lower side. In other places, the underlying inequalities break the sheet by fractures large and small. Some of these crevasses extend up the general slope, and others are transverse. The ice-mass is therefore broken into innumerable prismatic fragments. The tremendous mashing together which they experience through the movements of the flow, squeeze numbers of them out of their places; and they stand as huge pyramids and columns ten, twenty, and forty feet above the general surface. The columnar forms are called séracs. The afternoon sun acts on them, and some are sharpened to a point; others are worked out at the sides, and stand with broad flat caps. Finally they tumble down or waste away, while new ones rise in other places. Though the ice is continually shattered by crevassing, the fissures are continually closing together, when changes in underlying configuration permit. Two fractured surfaces pressed tightly unite again as one mass; and a patch shivered into ten thousand fragments becomes solid and transparent under the lateral squeezing to which it may become subjected. So, to whatever extent the ice-sheet may be shattered, it is continually healing, and tends to return to the condition of a sound and solid mass. Thus the tourist, picking his way among the séracs, and jumping the bottomless chasms, hears frequently the detonation of some new split, which is echoed back from the red pinnacles of Mont Maudit, which rises on his left. These themselves hurl down rocky fragments to keep alive the watchfulness of the traveler, and place material on the back of the glacier to be borne gradually but steadily down toward the valley.

The common glacier-field just mentioned strikes the sharp upper limit of a mountain salience, which slopes down to the valley of Chamonix, and separates two mountain valleys. This prominent dividing point is the Aiguille de la Tour. As the common ice-mass impinges against it, the ice parts to the right and left like a river. Down the western valley flows the ice-stream known as the Glacier de Taconnay. Down the eastern valley flows the greater stream known as the Glacier des Bossons, having the little village of Bossons at its foot. Another valley lies still farther west, and the common ice-field of Mont Blanc fills it with a stream known as Glacier de la Gria.

These three glaciers descend to the valley on the west of the pretty village of Chamonix. On the east are three others. The nearest is the celebrated Mer de Glace, the lower part of which is called the Glacier des Bois, with the little village of Bois at its foot. The snowy eastern slope of Mont Blanc and Mont Maudit (Mo-deé) feeds an enormous glacier which, to an observer from the valley of Chamonix, lies behind the pinnacled summits of Charmoz and Midi. This is the Glacier du Géant, and it forms the western tributary of the Mer de Glace. Into the head of the Mer de Glace comes the Glacier de Léchaud (La-shó), fed by the snow-fields of the Grandes Jorasses. On the east, the Léchaud is reinforced by the broad triangular Glacier de Talèfre (Tah-lefr’), in the midst of which, at an elevation of 9,143 feet, is the Jardin, an island of land-surface, walled in on all sides by lofty mountains, and adorned in August with a display of several species of Alpine flowers.

Beyond the Mer de Glace is the Glacier of Argentière—a fine long river of ice, almost equal to the Mer de Glace itself. The bright village of Argentière lies at its foot. At the very head of the valley of Chamonix comes down from the same direction, the Glacier du Tour. Thus six glaciers descend into the valley, and each contributes its torrent of muddy water to create and swell the Arve. This grand series of ice-rivers and the more majestic mass of the mountains, with their swelling domes and sky-piercing pinnacles, may be contemplated as a panorama from the summits which overlook the valley from the north, and put the spectator face to face before the stupendous Mont Blanc range. No person can gaze on this spectacle from the Flégère, which faces the Mer de Glace, or from the Brévent, which faces directly the Glacier des Bossons and Mont Blanc, without feeling a sympathy with Coleridge in his “Hymn in the Vale of Chamonix:”

“Ye ice-falls! ye that from the mountain’s brow Adown enormous ravines slope amain, Torrents, methinks, that heard a mighty voice, And stopped at once amid their maddened plunge! Motionless torrents! silent cataracts! Who made you glorious as the gates of heaven Beneath the keen full moon? Who bade the sun Clothe you with rainbow? Who with living flowers Of loveliest blue spread garlands at your feet? God! Let the torrents like a shout of nations Answer, and let the ice-plains echo, God!”

The conception of a glacier as a frozen cataract is suggestive and truthful. When, from the Montanvert, overlooking the Mer de Glace, De Saussure contemplated the sea of ice, he received an impression thus recorded: “Its surface resembles that of a sea which has become suddenly frozen—not during a tempest, but at the instant when the wind has subsided, and the waves, although very high, have become blunted and rounded. These great waves are nearly parallel to the length of the glacier, and intersected by transverse crevasses, the interior of which appears blue, while the ice is white on its external surface.” Farther down, in the narrower Glacier des Bois, the séracs and needles bristle over the surface in mighty uplifts and fearful confusion.

The crevasses really run in any direction, according to the nature of the underlying surface. In length they vary from twenty feet to a mile. The downward direction is originally vertical, but as the surface of the glacier moves more rapidly than the deeper portions, the transverse crevasse assumes, after a while, an inclination which gives it a dip up the valley. Its depth may be ten or a hundred, or two hundred feet; and its width, which is a few inches at first, may grow to fathoms. Forbes measured a crevasse at the base of the Glacier du Géant, which had a breadth of not less than 1,214 feet. The two walls generally approach each other downward, and we may sometimes safely descend to the bottom. The wall-ice is absolutely immaculate, with a greenish blue transparency. Down in the crevasse we hear the rills coursing through the substance of the glacier, and sometimes the central torrent rumbling along the bottom. The surface of the glacier is white and granular, from the action of the sun. Pools of water rest here and there—pure, cool, and refreshing—and numerous rills flow over the surface, discharging themselves through crevasses and perforations in the ice-mass, into some subglacial stream.

Each of these great glaciers is bordered by a moraine, or long ridge of material thrown off the surface in the course of ages, and pushed up by the movements of the ice. It consists of clay and rounded bowlders. It is completely unstratified, and resembles precisely the till at the bottom of the Drift. These lateral moraines at the present epoch, tower fifty to eighty feet above their glaciers. The ice, for centuries, has been in process of shrinkage. Such masses of débris could never have been raised by the existing glaciers. Other attestation of a former higher stage of the glaciers is seen in the smoothed and striated rock-slopes which bound the glacier valleys. These surfaces remind us of the smoothed and striated rocks underneath the till in America. The records of the glaciers may be traced on these smoothed slopes, two or three hundred feet above the present ice-surfaces.

At the foot of each glacier is a terminal moraine, which is continuous with the two lateral moraines. Among the Chamonix glaciers, this moraine is half a mile or more below the termination of the ice, showing to what extent the glaciers have diminished in length. These remote moraines were left in 1817 and 1826. The “chief of guides” at Chamonix remembers the occasion, and narrated to me a number of memorable incidents. The plain between the moraine and the foot of the glacier is strewn with bowlders. Many descend on the surface of the ice or imbedded in its mass. One sees them frequently precipitated from the foot of the Glacier des Bois to the plain below. The diminution of the glaciers appears to be a persistent phenomenon, and not dependent on climatic fluctuations of short period. There must be either a continuous diminution of cold or of precipitation.

All parts of the glacier mass move continually downward. In the Glacier des Bossons the amount of the movement has been determined by means of a catastrophe. In 1820, eight persons were buried in the Grande Crevasse at the foot of the dome of Mont Blanc. In 1861, their remains began to appear in the ice near the termination of the glacier. In forty years they had traveled 26,000 to 29,000 feet, or 680 feet a year. As they were buried 200 feet beneath the surface, it appears that 200 feet had been melted from the top of the glacier in the same interval. The Mer de Glace, as shown by Forbes, moves past Montanvert at the rate of 822 feet per annum. Near the foot of the Glacier des Bois the motion is 209 feet a year. The lower Glacier of the Aar, which was the scene of Agassiz’s observations, moves downward at an average rate of 250 feet per annum. Hugi’s hut according to Agassiz, had been carried 5,900 feet in thirteen years. A record bottled up by Hugi, stated that it had traveled 197 feet in three years and 2,345 feet in nine years. The great continental glacier would not have traveled at rates so rapid; but if it moved 200 feet a year, the time required to transport a bowlder 250 miles would be 6,600 years.

These interesting Chamonix glaciers are but the stumps of what they have been. Once they were noble tributaries of a greater glacier which filled the valley of Chamonix. Out of this valley it passed along the valley of the Arve, all the way to Geneva. As we ride along the highway, the rocky bounding walls rise on either hand, smoothed and scored after the same fashion as the rock-walls of the valley of the Mer de Glace. Evidently, the Chamonix glaciers have long been in process of shrinkage. Evidently, they once existed under an enormous development. When that period was passing, we may well believe our northern states were extensively glaciated, and a work was in progress very nearly like that which we have already reasoned out. With these facts before us, we shall be prepared to appreciate the picture of continental glaciation that will be presented as we trace the later history of the world.