=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. [Illustration: +Fig.+ 244.—Bowlder on ice pinnacle. Forno glacier, Switzerland. (Reid.)] [Illustration: +Fig.+ 245.—Dust-wells. Igloodahomyne glacier, North Greenland.] [Illustration: +Fig.+ 246.—Disposition of débris in ice. North Greenland glacier. (Libbey.)] [Illustration: +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). TEMPERATURE, WASTE, AND DRAINAGE.