At the mouths of ice-tunnels or ice-channels, especially where they end against terminal moraines, sands and gravels are liable to be bunched in quantity, giving rise, after the adjacent ice has melted, to peculiar hills and hollows of the knob-and-basin type. The hills and short ridges are known as kames (see glacial period). Subglacial streams may leave washed and assorted material in their tracks under the ice, and this is sometimes buried under deposits made by the ice itself, so that glacio-fluvial and glacial deposits are interbedded.
Fig. 283.—Delta at Isola, Lake of Sils, Engadine, Switzerland. (Reid.)
ICEBERGS.
When glaciers advance into water, the depth of which approaches their thickness, their ends are broken off ([Fig. 284]), and the detached masses float away as icebergs ([Fig. 285]). Many of the bergs are overturned, or at least tilted, as they set sail. If this does not happen at the outset, it is likely to occur later as the result of the melting and wave-cutting which disturb their equilibrium. The great majority of bergs do not travel far before losing all trace of stony and earthy débris, but the finding of glacial material in dredgings far south of all glaciers shows that they occasionally carry stones far from land.
Fig. 284.—End of Muir glacier, Alaska. (Reid.)
THE INTIMATE STRUCTURE AND THE MOVEMENT OF GLACIERS.
With the preceding account of glaciers in mind, we may return to a closer study of their origin, their intimate structure, and their mode of motion. The key to this study is the thesis that a glacier is a mass of crystalline rock—the purest and simplest type of crystalline rock known—since it is made up of a single mineral of simple composition and rare purity, which never appears in a solid state except in the crystalline form.
The growth and constitution of a glacier.—The origin and history of a glacier is little more than the origin and aggregate history of the crystals that compose it. The fundamental conception of a glacier is therefore best obtained by tracing the growth of its constituent crystals. A basal fact ever to be kept in mind is that water in the solid form is always controlled by crystalline forces. When it solidifies from the vapor of the atmosphere it takes the form of separate crystals ([Figs. 286–291]). Perfect forms are developed only when the flakes fall quietly through a saturated atmosphere which allows them to grow as they descend. Under other conditions, the crystals are imperfect in growth and are mutilated by impact. But however modified, they are always crystals. The molecules are arranged on the hexagonal plan, and, as the expansive power of freezing water shows, the arrangement is controlled by a strong force. Once the definite crystalline arrangement is established, the molecules can be displaced only by relatively great force.