The Elements of Geology; Adapted to the Use of Schools and Colleges

by Justin R. Loomis

The friction of the glacier, at its edges and along its bed, separates more or less of the rock over which it moves; and hence there is always a layer of mud and pebbles under the glacier, and a line of loose fragments, called a lateral moraine, at the sides. When two glaciers unite, the two lateral moraines, thus brought together, come to the surface, forming a medial moraine, and show the line of junction sometimes for miles.

The friction of the glacier on the bed of rock, assisted by the layer of pebbles, will wear down the prominent portions, and everywhere polish the surface. Fragments of rocks may be frozen into the glacier at all depths. Those which lie near the lower surface of the glacier would, by slight melting of that surface, project downward so as to act as a graver’s tool on the rock over which it passes. Hence, when the extremity of the glacier has receded beyond its ordinary limit, the surface of rock exposed is found, upon examination, to be polished, striated, and occasionally grooved an inch or two deep.

Since the waste is almost wholly superficial, earthy matter, which was at first concealed in the mass of the glacier, is continually coming to view, as the surface melts and runs off. Thus, none of the freight of the glacier is left along its course, but all is carried to its terminus and discharged there. Hence, at the lower extremity of the glacier there is always an embankment of earth, pebbles, and boulders. If the glacier recedes a few yards at one season of the year, and leaves its earthy fragments scattered over this surface, they will be pushed forward into a ridge, as the glacier again advances. This ridge is called a terminal moraine, and consists wholly of substances which have been separated from the mountain mass, often at the highest beginnings of the glacier. At the terminus of all the Alpine glaciers, there is a series of these moraines (a a a, [Fig. 77]) marking the successive limits of the glacier in former times.

There is a ridge of boulders on the north side of the Swiss valley, near the base of the Jura Mountains, resembling a terminal moraine. These boulders consist of several groups, distinguished by peculiarities of structure and composition; and each group lies opposite to the particular Alpine valley which now furnishes the same kind of fragments. It has been thought that, at a former period of more severe climate, the Swiss valley was filled in part with ice, and that the present glaciers extended across it to the Jura Mountains.

It is found that the polished and striated surfaces of the rocks in the Alpine valleys are precisely like the surface of the rock, which has not been exposed to atmospheric influences, in the north of Europe and America. It has been proposed to extend the glacier theory, and account for these phenomena by supposing that the north polar regions were, at the ice period, capped with a glacier-mass, extending as far south as the drift phenomena appear.

It is not to be doubted that the phenomena of polished surfaces and transported materials in the immediate vicinity of the Alps, and near other high mountains, are correctly referred to glacial action. This theory has therefore solved, in part, one of the most difficult problems in geology; but there is great difficulty in extending it so as to account for the drift phenomena in general. If the motion depends upon gravitation only, the origin must have a much greater elevation than the terminus, which would not be the case in the great glacier supposed to extend southward from the Arctic regions. Elevation of temperature, it has been thought, might account for the movement of the mass southward.

2. Icebergs.—In very high latitudes, the ice, which makes out from the land into the sea during the cold season, suffers but little waste at any time. This sheet of ice continues to increase in breadth and thickness, by congelation, from year to year. The spray and the snows of each succeeding year will also add to the mass. It thus accumulates to the height of several hundred yards. It will also reach down a good many feet below the surface of the sea, and will extend back on the land, or lie heaped up against a precipitous escarpment, and firmly frozen to it.

After a certain amount of extension over the sea, the accumulated weight of the ice and snow would tend to depress it, and break it loose from the shore. The waves would tend to the same result, and would act at greater mechanical advantage, as its extension from the shore becomes greater. Hence, it would ultimately become separated from the shore, and float in the water.

At its commencement, the earth, pebbles and rocks, which may lie along the shore, and as far down into the sea as the congelation extends, are frozen into it. In many situations its mass would be increased by avalanches while it remained attached to the land, and these would supply also masses of earth and rocks, as they do to glaciers. When it becomes loosened from the shore, it will break off, and carry with it some of the earthy portions of the coast, or the less firmly fixed masses of rock from the escarpment against which it formed. Thus every iceberg becomes freighted, more or less, with earth and rocks. This has almost uniformly been found to be the case, when they have been landed upon by ships’ crews and examined.