The transporting power of a glacier is' not determined by its velocity, at least so far as the material carried on its surface is concerned. This is because the rocks may be regarded as floating bodies with reference to the ice, and thus a rock weighing many tons is carried with as much ease as a grain of sand. The masses of material transported by a glacier are known as moraines. The moraines which are carried on the top of the glacier are derived from the cliffs and peaks which overhang the ice, and the action of frost and land-slips is continually showering down earth, sand, and rocks of all sizes, from small blocks up to masses the size of houses. This material is heaped up along the sides of the glacier in disorderly array, and here forms the lateral moraines. When a glacier is composed of branch streams, it will have a corresponding number of medial moraines (see Fig. 65), in the middle of the glacier. When two branches unite, their coalesced lateral moraines form a single medial moraine.

Fig. 65. - Edge of the Greenland ice-sheet, with a glacier descending from it. The dark line is a medial moraine. (Photograph by Libbey).

The quantity of material thus carried on the top of the glacier depends upon the amount of rock surface which extends above the level of the ice and is subject to the action of the ice and the atmosphere. In the Alps, where the glaciers flow in deep ravines, the moraines are large, and some of the great Alaskan glaciers have their lower reaches so covered with rubbish, that the ice is visible only in the crevasses. In Greenland, on the contrary, the inland ice-cap has very little material on its surface, because only scattered nunataks rise above it.

Fig. 73. - Front of Bowdoin Glacier, Greenland. The dark bands are made by englacial drift. (Photograph by Libbey).

The bottom part of the glacier is a confused mass of ice, stones, etc., and this debris is the ground moraine, which is to be regarded, not as so much material pushed along between the ice and the rocky bed, but as an integral part of the glacier. At the foot or end of the glacier is the terminal moraine (see Fig. 62), where all the materials carried are dumped in a promiscuous heap, except so much as is swept away by the stream of water. Besides the moraines proper, there is a certain amount of englacial drift, carried in the body of the ice. This is derived from debris that comes from the surface, but does not work its way entirely to the bottom, as well as from that which gathers upon the surface of the snow or neve and is covered up by subsequent snowfalls. The materials carried by a glacier are as characteristic as the marks left upon the rocks over which the ice has flowed. Aside from the substances swept along by the subglacial stream, the various fragments are not rounded and water-worn, as is the sediment of rivers. The moraines on the top of the ice (lateral and medial) are little or not at all abraded, but are deposited as angular blocks and fragments. The ground moraine, on the other hand, is abraded in the peculiar way already described.

Fig. 62. - A hanging glacier, Cascade Pass, Wash. Note the terminal moraine and the crevasses. (U. S. G. S).

In all this work of glacial denudation the process is entirely mechanical, - chemical decomposition plays no part in it.

Certain other forms of transportation by ice may be conveniently mentioned here.

Ground Ice forms in rivers and ponds on the bottom, freezing around stones and boulders, and when broken up by thaws, this ice may float for long distances, carrying with it burdens far greater than the stream which transports the ice could carry unassisted. The shores of the St. Lawrence River are fringed with lines of large boulders which have thus been brought down.

Lake Ice produces some curious effects in northern regions. When the lake is covered with cakes of ice as the result of an early thaw, refreezing, by expanding the water between the floating blocks, causes the ice to press strongly upon the shore. In case the lake beach is covered with boulders, the push of the ice heaps up the boulders into a ring wall.

Coast Ice

In Arctic regions the shallow water along the coast is frozen in winter into a broad shelf of ice called the ice-foot. In the spring land-slips cover the ice with debris, while the bottom is studded with stones and pebbles. When the ice-foot is broken up in summer, part of it is drifted away and transports its load of rock for long distances. Other parts are worked backward and forward by the waves and tides, scoring the rocks of the coast and grinding and polishing the fragments of rock frozen in the ice, in much the same fashion as glacial pebbles are scored and ground. Over comparatively limited areas the marks of coast ice often have a deceptive resemblance to those left by glaciers.

Icebergs

When a glacier enters the sea, it ploughs along the bottom until the buoyant power of the water breaks off great fragments of it, which float away as icebergs. These are often of gigantic size, veritable islands of ice, and huge as they appear, only about one-ninth of their bulk is above water. As icebergs are derived from glaciers, they carry away whatever debris the parent glacier had upon or within it.