CHAPTER VI

ICE-RECORDS

ICE in various ways alters the surface of the land. By disintegrating and eroding even the most durable rocks, and by removing loose materials and piling them up elsewhere, it greatly modifies the details of a landscape. As it assumes various forms, so it accomplishes its work with considerable diversity. The action of frost upon soil and bare surfaces of rock has already (p. 13) been described. We have now to consider the action of frozen rivers and lakes, snow and glaciers, which have each their own characteristic style of operation, and leave behind them their distinctive contribution to the geological history of the earth.

the cake of ice When this cake

Frozen Rivers and Lakes. In countries with a severe winter climate, the rivers and lakes are frozen over, and that covers them may be more than two feet thick. is broken up in early summer, large masses of it are driven ashore, tearing up the littoral boulders, gravel, sand, or mud, and pushing them to a height of many feet above the ordinary level of the water. When the ice melts, huge heaps of detritus are found to have been piled up by it, which remain as enduring monuments of its power. Not only so, but large fragments of the ice that has been formed along shore and has enclosed blocks of stone, gravel, and sand, are driven away and may travel many miles before they melt and drop their freight of stones. On the St. Lawrence and on the coast of Labrador, there is a constant transportation of boulders by this means. Further, besides freezing over the surface, the water not infrequently forms a loose spongy kind of ice on the bottom (Anchor-ice, Ground-ice) which encloses stones and gravel, and carries them up to the surface where it joins the cake of ice there. This bottom-ice is formed abundantly on some parts of the Canadian rivers. Swept down by the current, it accumulates against the bars or banks, or is pushed over the upper ice, and from time to time gathers into temporary barriers, the bursting of which may cause destructive floods. In the river St. Lawrence, banks and islets have been to a large extent worn down by the grating of successive ice-rafts upon them.

Snow. On level or gently inclined ground, whence snow disappears merely by melting or evaporation, it exercises, while it remains, a protective influence upon the soil and vegetation, shielding them from the action of frost. On slopes of sufficient declivity, however, the sheet of snow acquires a tendency to descend by gravitation, as we may often see on house-roofs in winter. In many cases, it creeps or slides down the side of a hill or valley, and in so doing pushes forward bare soil, loose stones, or other objects lying on the surface. By this means, the debris of weathered rock in exposed situations is gradually thrust down-hill and the rock is bared for further disintegration. But where the declivities are steep enough to allow the snow to break off in large sheets and to rush rapidly down, the most striking changes are observable. Such descending masses are known as Avalanches. Varying from 10 to 50 feet or more in thickness and several hundred yards broad and long, they sweep down the mountain sides with terrific force, carrying away trees, soil, houses, and even large blocks of rock. The winter of 1884-85 was especially remarkable for the number of avalanches in the valleys of the Alps, and for the enormous loss of life and property which they caused. In such mountain ground, not only are declivities bared of their trees, soil, and boulders, but huge mounds of debris are piled up in the valleys below. Frequently, also, such a quantity of snow, ice, and rubbish is thrown across the course of a stream as to dam back the water, which accumulates until it overflows or sweeps away the barrier. In another but indirect way, snow may powerfully affect the surface of a district where, by rapid melting, it so swells the rivers as to give rise to destructive floods.

While, therefore, the influence of snow is on the whole to protect the surface of the land, it shows itself in mountainous regions singularly destructive, and leaves as chief memorials of this destructiveness the mounds and rough heaps of earth and stones that mark where the down-rushing avalanches have come other kind of geological agent. The changes which they produce on the surface of the land may be divided into two parts: (1) the transport of materials from the high grounds to lower levels, and (2) the erosion of their beds.

(1) Transport. As a glacier descends its valley, it receives upon its surface the earth, sand, mud, gravel, boulders, and blocks of rock that roll or are washed down from the slopes on either side. Most of this rubbish accumulates on the edges of the glacier, where it is slowly borne to lower levels as the ice creeps downwards. But some of it falls into the crevasses or rents by

which the ice is split, and may either be imprisoned within the glacier, or may reach the rocky floor over which the ice is sliding. The rubbish borne onward upon the surface of the glacier is known as moraine-stuff. The mounds of it running along each side of the glacier form lateral moraines, those on the right-hand side as we look down the length of the valley being the right lateral moraine, those on the other side the left lateral moraine. Where two glaciers unite, the left lateral moraine of the one joins the right lateral moraine of the other, forming what is called a medial moraine that runs down the middle of the united glacier. Where a glacier has many tributaries bearing much moraine-stuff, its surface may be like a bare plain covered with earth and stones, so that, except where a yawning crevasse reveals the clear blue gleam of the ice below, nothing but earth and stones meets the eye. When the glacier melts, the detritus is thrown in heaps upon the valley, forming there the terminal moraine.

Glaciers, like rivers, are subject to variations of level. Even from year to year they slowly sink below their previous limit or rise above it. The glacier of La Brenva, for example, on the Italian side of Mont Blanc, subsided no less than 300 feet in the first half of the present century. One notable consequence of such diminution is that the blocks of rock lying on the edges of a glacier are stranded on the side of the valley, as the ice shrinks

FIG. 23. Perched blocks scattered over ice-worn surface of rock.

away from them. Such Perched Blocks or Erratics (Fig. 23), as they are called, afford an excellent means of noting how much higher and longer a glacier has once been than it is now. Their great size (some of them are as large as good-sized cottages) and their peculiar positions make it quite certain that they could not have been transported by any current of water. They are often poised on the tops of crags, on the very edges of precipices, or on steep slopes where they could never have been left by any flood, even had the flood been capable of moving them. The agent that deposited them in such positions must have been one that acted very quietly and slowly, letting the blocks gently sink into the sites they now occupy. The only agent known to us that could have done this is glacier-ice. We can actually see similar blocks on the glaciers now, and others which have only recently been stranded on the side of a valley from which the ice has sunk. In the Swiss valleys, the scattered ice-borne boulders may be observed by hundreds far above the existing level of the glaciers and many miles beyond where these now end. If the origin of the dispersed erratics is self-evident in a valley where a glacier is still busy transporting them, those that occur in valleys which are now destitute of glaciers can offer no difficulty; they become, indeed, striking monuments that glaciers once existed there.

In

Scattered erratic blocks offer much interesting evidence of the movements of the ice by which they were transported. In a glacier-valley, the blocks that fall upon the ice remain on the side from which they have descended. Hence, if there is any notable difference between the rocks of the two sides, this difference will be recognisable in the composition of the moraines, and will remain distinct even to the end of the glacier. If, therefore, in a district from which the glaciers have disappeared, we can trace up the scattered blocks to their sources among the mountains, we thereby obtain evidence of the actual track followed by the vanished glaciers. The limits to which these blocks are traceable do not, of course, absolutely fix the limits of the ice that transported them. They prove, however, that the ice extended at least as far as they occur, but it may obviously have risen higher and advanced farther than the space within which the blocks are now confined. Europe, some striking examples occur of the use of this kind of evidence. Thus the peculiar blocks of the Valais can be traced all the way to the site of the modern city of Lyons. There can therefore be no doubt that the glacier of the Rhone once extended over all that intervening country and reached at least as far as Lyons, a distance, of not less than 170 miles from where it now ends. Again, from the occurrence of blocks of some of the characteristic rocks of Southern Scandinavia, in Northern Germany, Belgium, and the east of England, we learn that a great sheet of ice once filled up the bed of the Baltic and the North Sea, carrying with it immense numbers of northern erratics. In Britain, where there are now neither glaciers nor snow-fields, the abundant dispersion of boulders from the chief tracts of high ground shows that this country was once in large part buried under ice, like modern Greenland. The evidence for these statements will be more fully given in a later part of this volume (Chapter XXVII).

Besides the moraine-stuff carried along on the surface, loose detritus and blocks of rock are pushed onwards under the ice.

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