=The marginal rock islands.=—From its margin the ice surface rises so steeply as to be climbed only with difficulty, but this gradient steadily diminishes until at a distance of between seventy-five and a hundred miles its slope is less than two degrees. Where crossed by Nansen near latitude 64° N. the broad central area of ice was so nearly level as to appear to be a plain. As we pass across the irregular ice margin in the direction of the interior, larger and larger proportions of the land’s surface are submerged, until only projecting peaks rise above the ice as islands which are described as _nunataks_ (Fig. 302). Though not a universal observation, it has been often noted that the absorption of the sun’s rays by rock masses projecting through the snow results in a radiation of the heat and a lowering by melting of the surrounding snow and ice. For this reason nunataks are often surrounded by a deep trench due to a melting of the snow. Such a depression in the ice surface about the margin of a nunatak, from its resemblance to a trench about an ancient castle, has been designated a _moat_ (Fig. 303). For the same reason, the outlet tongues of ice which descend in deep fjords between walls of rock are melted away from the walls and a lateral stream of water is sometimes found to flow between ice and rock (pl. 13 B). [Illustration: FIG. 302.—Edge of the Greenland inland ice, showing the nunataks diminishing in size toward the interior. The lines upon the ice are medial moraines starting from nunataks (after Libbey).] =Rock fragments which travel with the ice.=—Rock surfaces which are exposed to the atmosphere are in high latitudes broken down through the freezing of water within their crevices. The fragments resulting from this rending process fall upon the glacier surface and are carried forward as passengers in the direction of the ice margin. They are either visible as long and narrow ridges or trains following the directions of the steepest slope (Fig. 302), or they become buried under fresh falls of snow and only again become visible where summer melting has lowered the glacier surface in the vicinity of its margin. These longitudinal trains of rock fragments upon the glacier surface always have their starting point at the lower margin of one of the nunataks, and are known as _medial moraines_ (Fig. 301, p. 273, and Fig. 302). Inside the zone of nunataks the glacier surface is, however, clear of rock débris except where dust has been blown on by the wind, and this extends for a few miles only. The material of the medial moraines is a collection of angular blocks whose surfaces are the result of frost rending, for in their travel above the ice they are subjected to no abrading processes. [Illustration: FIG. 303.—Moat surrounding a nunatak in Victoria Land (after Scott).]