Fig. 423.—Small glacier table upon the surface of the Great Aletsch glacier in 1908.
Isolated and large rock slabs, as the season advances, may come to form the capping of an ice pedestal which they overhang and are known as glacier tables ([Fig. 423]). Such tables the sun attacks more upon one side than upon the other, so that the slab inclines more and more to the south and may eventually slip down until its edges rest against the glacier surface. Rounded bowlders, which less frequently become perched upon ice pedestals, may, from a similar process, slide down upon the southern side and leave a pyramid of ice furrowed upon this side and known as an ice pyramid.
Fine dirt when scattered over the glacier surface is, on the other hand, most effective in lowering its level by melting. Use was made of this knowledge to lower the great drifts of snow which had to be removed each season during the construction of the new Bergen railway of southern Norway. Each dirt particle, being warmed throughout by the sun’s rays, melts its way rapidly into the glacier surface until the dust well which it has formed is so deep that the slanting rays of the sun no longer reach it. When the dirt particles are near together, the thin walls which separate the dust wells are attacked from the sides in the warm air of summer days, thus producing from a patch of dirt upon the glacier surface a bath tub ([Fig. 424 d]). At night the water which fills these basins is frozen to form a lining of ice needles projecting inward from the wall, and this, repeated in succeeding nights, may entirely close the basin with water ice and produce the familiar glacier star ([Fig. 424 c]).
Fig. 424.—Effects of differential melting and subsequent refreezing upon the glacier surface. a, dust wells; b, glacier tub produced by melting about a group of scattered dust particles; c, glacier star produced when the inclosed water of the glacier well has frozen in successive nights; d, “bath tub.”
If the dirt upon the glacier surface, instead of being scattered, is so disposed as to make a patch completely covering the ice to the thickness of an inch or more, the effect is altogether different. Protecting as it now does the ice below, a local ice hillock rises upon its site as the surrounding surface is lowered, and as this grows in height its declivities increase and a portion of the dirt slides down the side. The final product of this shaping is an almost perfectly conical ice hill encased in dirt and known as a débris, sand, or dirt cone ([Fig. 425]). The novice in glacier study is apt to assume that these black cones contain only dirt, but is rudely awakened to the reality when he attempts to kick them to pieces. Both glacier tubs and débris cones may assume large dimensions; as, for example, in Alaska, where they may be properly described as lakes and hills.
Fig. 425.—Dirt cone and one with its casing in part removed. Victoria glacier (after Sherzer).
A patch of hard and dense snow which is less easily melted than that upon which it rests may lead to the formation of snow cones upon the glacier surface similar in size and shape to the better known débris cones. Such cones of snow have, with doubtful propriety, been designated “penitents”, for it is pretty clear that the interesting bowed snow figures, which really resemble penitents and which were first described from the southern Andes under the name of nieves penitentes, are of somewhat different character.