Sonic of the more remarkable phenomena of glaciers remain to be uoticed. The surface of the glacier does not long retain the purity of the snow from which it is derived, but is speedily loaded with long ridges of debris called moraines. The moun tains which rise on either side of the valley occupied by the glacier are continually suffer ing loss from the action of the rain, disruption by frost, and the impulse of avalanches. 'The materials thus liberated find their way to the glacier, and form a line of rock and rubbish on its two borders, of greater or less size, dependent on the friability or com pactness of the adjacent mountains. The lateral moraines often reach to a great height, as much as 40 or 50 ft. above the level of the glacier. The whole ridge appears to con sist of debris, but it is really a ridge of ice with a covering of foreign materials, which, by protecting the underlying ice from the heat which they radiate and only partially transmit, leave the moraine as a more and more elevated ridge, while the surface of the glacier is speedily melting. Glacier tables have a similar origin. A large and isolated mass of rock, resting on the glacier, protects the ice below; and as the glacier melts, it leaves the rock poised on the summit of an icy column. As the rays of the sun play on the table all day obliquely, the column is gradually melted from under the rock, until it slips off, and begins to form another table; while the unprotected column speedily melts and disappears. Where two glaciers unite, the trails of rock on the inner margins unite also and form a single ridge, which runs along the middle of the large trunk glacier, and is called a medial moraine. It is evident that the number of the medial moraines must thus depend upon the number of the branch glaciers, and must indeed be invariably one less. The glacier terminates amidst a mass of stones and debris, which having been carried down on its surface, arc finally deposited by its melting at its extremity, forming there a terminal moraine, Sometimes a glacier decreases in size, either withdrawing from the valley, and leaving the terminal moraine as a barren waste of rocks, or melting on its superfices throughout its length, and depositing its lateral moraines as a ridge of debris on either side at some height above it on the mountain. The existence of such collections of rocks is plain evidence of the former position and altitude of glaciers, and even of their former occurrence in coun tries where they are now unknown.
It has been stated, that when the glacier is subjected to tension, the continuity of its parts is destroyed, and fissures called crevasses, are formed. In passing over a brow on the channel, the ice invariably yields; at first, a deep crack is formed, which gradually until a fissure or chasm is produced across the glacier. Tranverse crevasses dis appear when the glacier reaches a level portion of its bed; the pressure bringing the walls again together, the chasm is closed up. Longitudinal crevasses are produced when the glacier escapes from a confined channel, and spreads itself over a wider area. The spreading of the margins causes a tension in the body of the glacier, which yields, and longitudinal fissures are formed. These occasionally rend the terminal front of a glacier. The smaller marginal crevasses are formed from the tension of the ice, pro duced by the normal motion of the glacier being retarded by the friction against the sides of its channel. The motion of the glacier is gradually accelerated from the mar gin inwards, consequently the lines of greatest tension are inclined downwards and towards the center, more or less, in proportion to the rapidity of the motion. Tho
crevasses formed by the yielding of the ice are at right angles to the lines of tension, and consequently point up the glacier.
The reined structure is apparently the result of pressure. The veins consist of blue ice penetrating the white mass of the glacier, and occur either in irregular directions, or producing a regularly laminated structure. The blue veins are portions of ice from which the air-babies have been expelled, and which are consequently more compact than the general substance of the glacier. The pressure is exerted in three directions, produc ing veins which are complementary to the three kinds of crevasses which have just been noticed. When the glacier passes over a level, or perhaps a gently rising channel, trans verse veins are formed; when it is pressed through a narrower channel, longitudinal veins are produced; and the pressure at the margins the retardation of the flow by friction causes the formation of marginal veins in the lines of greatest pressure, that is, at right angles to the marginal crevasses.
The melting of the ice on the surface of the glacier produces streams, whose course. is often broken by crevasses, down which the water descends, finding egress at last through the cavernous mouth at the termination of the glacier, where it issues after being increased by other streams, which have by similar channels reached the bottom, as well as by the melting of the ice from the contact of the earth. The rushing water wears a shaft of greater diameter than the crevasse, and this shaft often remains after the margins of the crevasse have been reunited. In the progress of the glacier, another crevasse intersects the bed of the stream, and down this the water is diverted, leaving the formed shaft or moulin, as it is called. The forsaken moulin has at its base a. quantity of earth and stones collected by the stream from the surface of the glacier, these are gradually raised to the surface by the melting of the glacier, and event ually appear as cones of debris, sometimes rising high on columns of ice under the same. influences as the glacier tables.
Glaciers are not necessarily peculiar to any country or zone, but wherever there are mountains of sufficient height, it may be expected that they may exist. In Europe, they are chiefly confined to the Alps and Norway. Having their origin in the region of perpetual snow, they reach far down into the valleys, the largest pushing themselves furthest down. That of Bossous at Chamouni, which comes from the highest part of Mont Blanc, reaches a point 5,500 ft. below the snow-line, where it is embosomed amongst luxuriant wood, and is almost iu contact with cornfields. Hooker and others have described the glaciers of the Himalaya. Iceland and Spitzbergen also abound in glaciers. it is in such northern localities that the ends of the glaciers, resting on the waters of the ocean, get broken off by transverse crevasses, and float away as icebergs.
It has already been noticed that the former existence of glaciers is indicated by the occurrence of moraines. These have been noticed in various localities in Wales, Eng land, and Scotland. They are referred to the period when the bowlder-clay (q.v.) was deposited; and this, with the sands and gravels which are associated with it, are some- times included under the title glacial deposits.