When a glacier retires, this earthy and stony debris, where not swept away by the escaping river, is left on the floor of the valley. One remarkable feature of the stones in it is that a large proportion of them are smoothed, polished, and covered with fine scratches or ruts, such as would be made by hard sharp-pointed fragments of stone or grains of sand. These markings run for the most part along the length of each oblong stone, but not infrequently cross each other, and sometimes an older may be noticed partially effaced by a newer set. This peculiar striation is a most characteristic mark of the action of glaciers. The stones under the ice are fixed in the line of least resistance that

FIG. 24.-Stone smoothed and striated by glacier-ice.

is, end on. In this position, under the weight of hundreds of feet of ice, they are pressed upon the floor over which the glacier is travelling. Every sharp edge of stone or grain of sand, pressed along the surface of a block, or over which the block itself is slowly drawn, engraves a fine scratch or a deeper rut. As the block moves onward, it is more and more scratched, losing its corners and edges, and becoming smaller and smoother till, if it travel far enough, it may be entirely ground into sand or mud (Fig. 24).

(2) Erosion. The same process of erosion is carried on upon the solid rocks over which the ice moves. These are smoothed, striated, and polished by the friction of the grains of sand, pebbles, and blocks of stone crushed against them by the slowly creeping mass of ice. Every boss of rock that looks toward the quarter from which the overlying ice is moving is ground away, while those that face to the opposite side are more or less

sharp and unworn. The striation is especially noteworthy. From the fine scratches, such as are made by grains of sand, up to deep ruts like those of cart-wheels in unmended roadways, or to still wider and deeper hollows, all the friction-markings run in a general uniform direction, which is that of the motion of the glacier. Such striated surfaces could only be produced by some agent with rigidity enough to hold the sand-grains and stones in position, and press them steadily onward upon the rocks. A

river polishes the rocks of its channel by driving shingle and sand across them; but the currents are perpetually tossing these materials now to one side, now to another, so that smoothed and polished surfaces are produced, but with nothing at all resembling striation. A glacier, however, by keeping its grinding materials fixed in the bottom of the ice, engraves its characteristic parallel striæ and groovings, as it slowly creeps down the valley. All the surfaces of rock within reach of the ice are smoothed, polished, and striated. Such surfaces present the most unmistakable evidence of glacier-action, for they can be produced by no other known natural agency. Hence, where they occur in glacier

valleys, far above and beyond the present limits of the ice, they prove how greatly the ice has sunk. In regions also where there are now no glaciers, these rock-markings remain as almost imperishable witnesses that glaciers once existed. By means of their evidence, for example, we can trace the march of great icesheets which once enveloped the whole of Scandinavia and lay deep upon nearly the whole of Britain.

The river that escapes from the end of a glacier is always milky or muddy. The fine sand and mud that discolour the water are not supplied by the thawing of the clear ice, nor by the sparkling brooks that gush out of the mountain-slopes, nor by the melting of the snows among the peaks that rise on either side. This material can only come from the rocky floor of the glacier itself. It is the fine sediment ground away from the rocks and loose stones by their mutual friction under the pressure of the overlying ice. It serves thus as a kind of index or measure of the amount of material worn off the rocky bed by the grinding action of the glacier. We can readily see that as this erosion and transport are continually in progress, the amount of material removed in the course of time must be very great. It has been estimated, for example, that the Justedal glacier in Norway removes annually from its bed 2,427,000 cubic feet of sediment. At this rate the amount removed in a century would be enough to fill up a valley or ravine 10 miles long, 100 feet broad, and 40 feet deep.

In arctic and antarctic latitudes, where the land is buried under a vast ice-sheet, which is continually creeping seaward and breaking off into huge masses that float away as icebergs, there must be a constant erosion of the terrestrial surface. Were the ice to retire from these regions, the ground would be found to wear what is called a glaciated surface; that is to say, all the bare rocks would present a characteristic ice-worn aspect, rising into smooth rounded bosses like dolphins' backs (roches moutonnées), and sinking into hollows that would become lake-basins. Everywhere these bare rocks would show the striæ and groovings graven upon them by the ice, radiating generally from the central high grounds, and thus indicating the direction of flow of the main streams of the ice-sheet. Piles of earth, ice-polished stones, and blocks of rock would be found strewn over the country, especially in the valleys and over the plains. These materials would still further illustrate the movements of the ice, for they would be found to be singularly local in character, each district having

supplied its own contribution of detritus.

Thus from a region of red sandstone, the rubbish would be red and sandy; from one of black slate, it would be black and clayey (see Chapter XXVI).

Summary. In this chapter we have seen that Ice in various ways affects the surface of the land and leaves its mark there. Frost, as already explained in Chapter II, pulverises soil, disintegrates exposed surfaces of stone, and splits open bare rocks along their lines of natural joint. On rivers and lakes, the disrupted ice wears down banks and pushes up mounds of sand, gravel, and boulders along the shores. Snow lying on the surface of the land protects that surface from the action of frost and air. In the condition of avalanches, snow causes large quantities of earth, soil, and blocks of rock to be removed from the mountainslopes and piled up on the valleys. In the form of glaciers, ice transports the debris of the mountains to lower levels, bearing along and sometimes stranding masses of rock as large as cottages, which no other known natural agent could transport. Moving down a valley, a glacier wears away the rocks, giving them a peculiar smoothed and striated surface which is thoroughly characteristic. By this grinding action, it erodes its bed and produces a large amount of fine sediment, which is carried away by the river that escapes at the end of the ice-stream. Land-ice thus leaves thoroughly distinctive and enduring memorials of its presence in polished and grooved rocks, in masses of earth, clay, or gravel, with striated stones, and in the dispersal of erratic blocks from principal masses of high ground. These memorials may remain for ages after the ice itself has vanished. By their evidence we know that the present glaciers of the Alps are only a shrunk remnant of the great ice-fields which once covered that region; that the Scandinavian glaciers swept across what is now the bed of the North Sea as far as the mouth of the Thames; and that Scotland, Ireland, Wales, and the greater part of England were buried under great sheets of ice which crept downwards into the North Sea on the one side, and into the Atlantic on the other (Chapter XXVII).

CHAPTER VII

THE MEMORIALS OF THE PRESENCE OF THE SEA

We have now to inquire how the work of the Sea is registered in geological history. This work is broadly of two kinds. In the first place, the sea is engaged in wearing away the edges of the land, and in the second place, being the great receptacle into which all the materials, worn away from the land, are transported, it arranges these materials over its floor, ready to be raised again into land at some future time.

i. Demolition of the Land.—In its work of destruction along the coasts of the land, the sea acts to some extent (though we do not yet know how far) by chemically dissolving the rocks and sediments which it covers. Cast-iron bars, for example, have been found to be so corroded by sea-water as to lose nearly half their strength in fifty years. Doubtless many minerals and rocks are liable to similar attacks.

But it is by its mechanical effects that the sea accomplishes most of its erosion. The mere weight with which ocean-waves fall upon exposed coasts breaks off fragments of rock from cliffs. Masses, 13 tons in weight, have been known to be quarried out of the solid rock by the force of the breakers in Shetland, at a height of 70 feet above sea-level. As a wave may fall with a blow equal to a pressure of 3 tons on the square foot, it compresses the air in every cleft and cranny of a cliff, and when it drops it allows the air instantly to expand again. By this alternate compression and expansion, portions of the cliff are loosened and removed. Where there is any weaker part in the rock, a long tunnel may be excavated, which may even be drilled through to the daylight above, forming an opening at some distance inland from the edge of the cliff. During storms, the breakers rush through such a tunnel, and spout forth from the opening (or blow-hole) in clouds of spray (Fig. 26).