Earth Features and Their Meaning / An Introduction to Geology for the Student and the General Reader

by William Herbert Hobbs

Fig. 313.—Snow deltas about the margins of the Fan glacier outlet of Greenland (after Chamberlin).

The glacier broom.—During the calm which succeeds to the blizzard, heat is once more abstracted from the surface air layer, and a new outwardly directed engine stroke is begun. The tempest which later develops acts as a gigantic centrifugal broom which sweeps out to the margins of the glacier all portions of the latest snowfall which have not become firmly attached to the ice surface. The sweepings piled up about the margin of continental glaciers have been described as fringing glaciers, or the glacial fringe. The northern coast of Greenland and Grant Land are bordered by a fringe of this nature ([plate 14], and [Fig. 315], [p. 288]). It is by the operation of the glacier broom that the inland ice is given its characteristic shield-like shape ([Fig. 312]). The granular nature of the snow carried by the wind is well brought out by the little snow deltas about the margins of Greenland ice tongues ([Fig. 313]). Obviously because of the presence of the vigorous anticyclone, no snows such as nourish mountain glaciers can be precipitated upon continental glaciers except within a narrow marginal zone, and, as shown by Nansen rock dust from the coastland ribbon and from the nunataks of Greenland, is carried by a few miles inside the western margin, and not at all within the eastern.

Fig. 314.—Sea ice of the Arctic region in lat. 80° 5´ N. and long. 2° 52´ E. (after Duc d’Orleans).

Field and pack ice.—Within polar regions the surface of the sea freezes during the long winter season, the product being known as sea-ice or field-ice ([Fig. 314]). This ice cover may reach a thickness by direct freezing of eight or more feet, and by breaking up and being crowded above and below neighboring fragments may increase to a considerably greater thickness. Ice thus crowded together and more or less crushed is described as pack ice or the pack.

The pack does not remain stationary but is continually drifting with the wind and tide, first in one direction and then in another, but with a general drift in the direction of the prevailing winds. Because of the vast dimensions of the pack, the winds over widely separated parts may be contrary in direction, and hence when currents blow toward each other or when the ice is forced against a land area, it is locally crushed under mighty pressures and forced up into lines of hummocks—the so-called pressure ridges. At other times, when the winds of widely separated areas blow away from each other, the pack is parted, with the formation of lanes or leads of open water.

If seen in bird’s-eye view the lines of hummocks would according to Nansen be arranged like the meshes of a net having roughly squared angles and reaching to heights of 15 to 25, rarely 30, feet above the general surface of the pack. The ice within each mesh of the network is a floe, which at the times of pressure is ground against its neighbors and variously shifted in position. At the margin of the pack these floes become separated and float toward lower latitudes until they are melted.

The drift of the pack.—The discovery of the drift in the Arctic pack is a romantic chapter in the history of polar exploration, and has furnished an example of faith in scientific reasoning and judgment which may well be compared with that of Columbus. The great figure in this later discovery is the Norwegian explorer Fridtjof Nansen, and to the final achievement the ill-fated Jeannette expedition contributed an important part.

The Jeannette carrying the American exploring expedition was in 1879 caught in the pack to the northward of Wrangel Island ([Fig. 315]), and two years later was crushed by the ice and sunk to the northward of the New Siberian Islands. In 1884 various articles, including a list of stores in the handwriting of the commander of the Jeannette, were picked up at Julianehaab near the southern extremity of Greenland but upon the western side of Cape Farewell. Nansen, having carefully verified the facts, concluded that the recovered articles could have found their way to Julianehaab only by drifting in the pack across the polar sea, and that at the longest only five years had been consumed in the transit. After being separated from the pack the articles must have floated in the current which makes southward along the east coast of Greenland and after doubling Cape Farewell flows northward upon the west coast. It was clear that if they had come through Smith Sound they would inevitably have been found upon the other shore of Baffin Bay. In confirmation of this view there was found at Godthaab, a short distance to the northward of Julianehaab ([Fig. 315]), an ornamented Alaskan “throwing stick” which probably came by the same route. Moreover, large quantities of driftwood reach the shores of Greenland which have clearly come from the Siberian coast, since the Siberian larch has furnished the larger quantity.