SURFACE FEATURES.
Topography.—Many of the minor irregularities of the surface of a glacier are the result of crevassing. After the ice is crevassed, the sun’s rays and the air which has been warmed by them penetrate the openings and melt the ice. The melting is most rapid at the top, and decreases downward. The result is that the sections of ice between adjacent crevasses are narrowed into wedges. If there be cross-crevassing, as is common, points instead of wedges result. As the sort of surface shown in [Fig. 239] develops, any débris which was on the ice slides into the crevices, and the upper surface becomes clean.
Where ice is crevassed transversely, and where melting is not rapid, the crevasses may close as the ice moves forward, and the regelation of adjoining faces heals the rents in the surface. Even in this case, however, the surface is likely to be more or less undulating because of the waste on the sides of the crevices before they are closed. After regelation, surface ablation tends to obliterate the protuberances.
The topography of the surface of the ice is affected by other conditions. All parts of the surface of the ice are not equally compact, and the least compact portions melt most rapidly, giving rise to depressions, while the more solid parts occasion protuberances. Both depressions and protuberances may be regular or irregular in form (Figs. [240] and [241]). Undulations of the bed often show themselves in the surface of the ice as suggested by [Fig. 242]. In such cases, ponds or lakelets sometimes accumulate on the surface of the ice. The topography of the ice in such cases seems to show that the ice is forced up slope.
Surface moraines.—The surface of a glacier is often affected by débris of one sort or another, and this also influences its topography. The débris is sometimes disposed in the form of belts or moraines (Figs. [237], [243]). The surface moraines may be lateral, medial, or terminal. A lateral moraine is any considerable accumulation of débris in a belt on the side of a glacier. A medial moraine is a similar accumulation at some distance from the margins, but not necessarily in or near the middle. There may be several medial moraines on one glacier, in which case some of them may be far from the center. In alpine glaciers, the surface terminal moraine is less well-defined; in polar glaciers it often connects two lateral moraines, making a loop roughly concentric with the terminus of the glacier.
Fig. 240.—End of Mount Dana glacier, Cal. Shows irregularities of surface due to crevassing farther up the glacier, and to unequal melting.
Fig. 241.—Shows irregularities due to unequal melting of veined ice. End of small glacier south of Forno hut, Engadine, Switzerland. (Reid.)
Besides the surface moraines, which represent belted aggregations of débris, there may be scattered bowlders and bits of rock of various sizes on the ice, and, in addition to the coarse material, there is often some dust which has been blown upon the ice.
Relief due to surface débris.—The débris on the ice affects its topography by influencing the melting of the subjacent and adjacent ice. The rock débris absorbs heat more readily than the ice. A small and thin piece of stone lying on the ice is warmed through by the sun’s rays, and, melting the ice beneath, sinks, just as a piece of black cloth on snow will sink because of the increased melting beneath it. Though a good absorber of heat, rock is a poor conductor, and so the lower surface of a large mass of stone is not notably warmed. The ice beneath it is protected from the direct rays of the sun, and is therefore melted more slowly than that around it. The result is that the bowlder presently stands on a protuberance of ice ([Fig. 244]). When its pedestal becomes high, the oblique rays of the sun and the warm air surrounding it cause it to waste away, and the capping bowlder falls. In high latitudes, the great obliquity of the rays sometimes allows them to strike under isolated bowlders. In this case, they are warmed from below, and thus aid rather than hinder the melting of the ice.
Fig. 242.—Irregular surface due to uneven bottom. Bowdoin glacier, Inglefield Gulf, North Greenland. The dark patches near the left margin of the glacier are lakelets in basins produced by the upward bending of the ice as it overrides an elevation in its bed. The figure also shows a depressed medial moraine.
The same principles apply to the moraines. A thin bowlder moraine in high latitudes is sometimes sunk below the surface ([Fig. 242]). Usually, however, a medial moraine protects the ice beneath from melting, and occasions the development of a ridge of ice beneath itself. As the ice on either side is then lowered by ablation, the moraine matter of the medial belt tends to slide down on either hand. The same is true of the lateral moraines. So far does this spreading go, that in some cases the lower end of a glacier is completely covered with the débris which has spread from the medial and lateral moraines. Examples of this may be seen in almost any region of abundant, long, alpine glaciers.
Fig. 243.—A Swiss glacier, showing surface moraines, characteristic profile, etc.
Dust-wells.—The wind-blown dust sometimes gives rise to peculiar topographic features of small size. The dust is not distributed by the wind with absolute equality, and the surface drainage of the ice tends to aggregate it. Every dust particle acts like a small stone, and where aggregations of dust occur, they melt their way down into the ice, developing holes or “dust-wells” ([Fig. 245]). These wells rarely reach a depth of more than a few inches, but they may be so numerous that the pedestrian is obliged to watch his steps. This is especially true near the edge of the large ice-caps. It is evident that the depth of these dust wells must be slight, for so soon as they are deep enough to cut off the sun’s rays from the dust at the bottom, the deepening ceases. Other things being equal, they are deeper in low latitudes than in high.
Fig. 244.—Bowlder on ice pinnacle. Forno glacier, Switzerland. (Reid.)
Fig. 245.—Dust-wells. Igloodahomyne glacier, North Greenland.
Fig. 246.—Disposition of débris in ice. North Greenland glacier. (Libbey.)
Fig. 247.—Profile of the lower part of the lateral margin of a glacier. Southeast side of McCormick Bay, North Greenland.
Débris below the surface.—The lower part of a glacier, as well as the upper, carries rock débris. This débris is sometimes so abundant, especially near the ends and edges of the ice, that it is difficult to locate the bottom of the glacier; for between the moving ice which is full of débris, and the stationary débris which is full of ice, there seems to be a nearly complete gradation. The débris in the lower part of arctic glaciers, and to some extent of others, is often disposed in thin sheets sandwiched in between layers of clean ice. These débris sheets are often numerous and usually discontinuous, though groups of such sheets often persist for considerable distances. Débris also occurs to some extent in the ice well above its base. It is sometimes in belts, as seen in section, and sometimes in bunches. These various relations are illustrated by Figs. [227], [229], and [246–249].
Another characteristic of the basal débris-laden part of some glaciers is the foliation of the ice (Figs. [248], [249], etc.). This is especially well shown in the arctic glaciers, the ends and sides of which have steep or vertical faces. The foliation is best developed in the débris zone, though often shown above. The foliation is sometimes minute, consisting of layers of clean ice, an inch or less in thickness, separated by mere films of earthy matter. In extreme cases there are a score or more of laminæ within a foot. Locally, and especially where débris is abundant, the laminæ are much contorted. This is seen both in section (Figs. [248] and [249]) and on the surface ([Fig. 250]).