Reading Reference for Chapter XXVII
William H. Hobbs. The Cycle of Mountain Glaciation, Geogr. Jour., vol. 37, 1910, pp. 268-284.
CHAPTER XXVIII
THE GLACIER’S SURFACE FEATURES AND THE DEPOSITS UPON ITS BED
The glacier flow.—The downward flow of the ice within a mountain glacier has been the subject of many investigations and the topic of many heated discussions since the time when Louis Agassiz and his companions set a line of stakes across the Aar glacier and numbered the surface bowlders in preparation for repeated observations. Their first observation was that the line of stakes, which had run straight across the glacier, was distorted into a curve which was convex downstream ([Fig. 416], A´), thus showing that the surface layers have more rapid motion in proportion as they are distant from the side margins. Summarizing these and later studies, it may be stated that the glacier increases its rate of motion from its side margin towards its center line, from its bed upwards towards its surface, and below the névé the velocity is greatest where the fall is greatest and also wherever the cross section diminishes. In all these particulars, then, the ice of the glacier behaves like a stream of water. The average rate of flow of Alpine glaciers varies from a few inches to a few feet per day, and is greater during the warm summer season. The Muir glacier of Alaska has been shown to move at the rate of about seven feet per day.
Fig. 416.—Diagram to illustrate the migrations of lines of stakes crossing a glacier, due to its surface movement, A, original position of lines; A´, later positions; a and a´, original and distorted forms of a square section of the glacier surface near its margin; r, r´, diagonal crevasses.
In traveling from the névé downward to the glacier foot, the snow not only changes into ice, but it undergoes a granulating process with continued increase in the size of the nodules until at the foot of the glacier these may be picked out of the partially melted ice as articulating balls the size of the fist or larger. Glacier ice has therefore a structure quite different from that of lake ice, since the latter is developed in parallel needles perpendicular to the freezing surface.