Crevasses and séracs.—Prominent surface indications of glacier movement are found in the open cracks or crevasses, which are the marks of its yielding to tensional stresses. Crevasses are apt to run either directly across the glacier, wherever there is a steep descent upon its bed, or diagonally, running in from the margin and directed up-glacier (r, r, r, of [Fig. 416]), though they occasionally run longitudinally with the glacier when there is a rock terrace at the side of the valley beneath the ice. The diagonal crevasses at the glacier margin are due to the more sluggish movement where the ice is held back by friction upon the walls of the valley, as will be clear from [Fig. 416]. The square a has by this movement been distorted into the lozenge a´, so that the line xy has been extended into x´y´, with the obvious tendency to open cracks in the direction ss.
Every glacier surface below its névé is marked by steps or terraces, which are well understood to overlie corresponding steps of the cascade stairway to be seen in all vacated glacier valleys ([plate 19]). The steep risers of these steps are usually marked by parallel crevasses which cross the glacier. Under the rays of the sun, which strike them more from one side than from the other, the slices into which the ice is divided are transformed into sharpened blades and needles which are known as séracs ([Fig. 401], [p. 376], and [Fig. 417]).
Fig. 417.—Transverse crevasses at the fall below a glacier step transformed by unsymmetrical melting into séracs.
The numerous crevasses tell us that the ice is many times wrenched apart during its journey down the glacier. This has been illustrated by somewhat grewsome incidents connected with accidents to Alpinists, but as they illustrate in some measure both the mode and the rate of motion of Swiss glaciers, they are worthy of our consideration.
Bodies given up by the Glacier des Bossons.—In the year 1820, during one of the earlier ascents of Mont Blanc, three guides were buried beneath an avalanche near the Rochers Rouges in the névé of the Glacier des Bossons ([Fig. 418]). In 1858 Dr. Forbes, who had measured the rate of flow of a number of Alpine glaciers, predicted that the bodies of the victims of this accident would be given up by the glacier after being entombed from thirty-five to forty years. In the year 1861, or forty-one years after the disaster, the heads of the three guides, separated from their bodies, with some hands and fragments of clothing, appeared at the foot of the Glacier des Bossons, and in such a state of preservation that they were easily recognized by a guide who had known them in life. Inasmuch as these fragments of the bodies had required forty-one years to travel in the ice the three thousand meters which separate the place of the accident from the foot of the glacier, the rate of movement was twenty centimeters, or eight inches, per day.
Fig. 418.—View of the Glacier des Bossons upon the slopes of Mont Blanc showing the position of accidents to Alpinists and the place of reappearance of their bodies.
Fig. 419.—Lines of flow upon the surface of the Hintereisferner glacier in the Alps (after Hess).