B. Lateral stream beside the Benedict glacier outlet, Greenland (after R. E. Peary).
Fig. 301.—Map of a glacier tongue, with dimple showing above and due to indraught of the ice. Umanakfjord, Greenland (after von Drygalski).
Fissures in the ice, the so-called crevasses, are the recognized marks of ice movement, and these are always concentrated at the steep slopes of the ice surface in the neighborhood of its margins. Upon the Greenland ice, crevasses are restricted in their distribution to a zone which extends from seven to twenty-five miles within the ice border.
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]).