§ 35. Motion of Monerisch Fitnes 240. Far to the east of the Oberland and in that interesting part of Switzerland known as the Over Eur gadin, stands a noble group of momaine les in bereit than those of the Oberland, but still of commanding elevation. The group derives its name from its most dominant peak, the Piz Berzina. Is read the place we travel by railway from Basel to Ztritt, and from Zürich to Chur French Coire, whense we pass by 2003gence over either the Albula pass or the Juller prab to the village of Pontresina. Here we are in the in me diate neighbourhood of the Bernina montaize. 241. From Pontresina we may walk or drive along a good coach road over the Bernina pass into Italy. At about an hour above the village you would look from the road into the heart of the mountains, the line of vision passing through a valley, in which is couched a glacier of considerable size. Along its back you would trace a medial moraine, and you could hardly fail to notice how the moraine, from a mere narrow streak at first, widens gradually as it descends, until finally it quite covers the lower end of the glacier. Nor is this an effect of perspective; for were you to stand upon the mountain slopes which nourish the glacier, you would see thence also the widening of the streak of rubbish, though the perspective here would tend to narrow the moraine as it retreats downwards. 242. The in-stream hereed to is the Morteratsci, glacier, the end of which is a short hour's walk fron, the village of Pontresina. We have now to determine its rate of motion and to account for the widening of its media, moraine. 243. In the summer of 184 M. Hirst and myself set out three lines of stakes across the glacier. The first line crossed the ice : the second a good distance lower down, and the third lower still. Even the third line, however, was at a considerable distance above the actual snout of the air. The daily motion of these three lines was as follows: 241. Compare these lines together. You notice the velocity of the first is greater than that of the second, and the velocity of the second greater than that of the third. 245. The lines were permitted to move downwards for 100 hours, at the end of which time the spaces passed over by the points of swiftest motion of the three lines were as follows: 246. Here then is a demonstration that the upper portions of the Morteratsch glacier are advancing on the lower ones. In 1871 the motion of a point on the middle of the glacier near its snout was found to be less than two inches a day! 247. What, then, is the consequence of this swifter march of the upper glacier? Obviously to squeeze this medial moraine longitudinally, and to cause it to spread out laterally. We have here distinctly revealed the cause of the widening of the medial moraine. 248. It has been a question much discussed, whether a glacier is competent to scoop out or deepen the valley through which it moves, and this very Morteratsch glacier has been cited to prove that such is not the case. Observers went to the snout of the glacier, and finding it sensibly quiescent, they concluded that no scooping occurred. But those who contended for the power of glaciers to excavate valleys never stated, or meant to state, that it was the snout of the glacier which did the work. In the Morteratsch glacier the work of excavation, which certainly goes on to a greater or less extent, must be far more effectual high up the valley than at the end of the glacier. § 36. Birth of a Crevasse: Reflections. 249. Preserving the notion that we are working together, we will now entèr upon a new field of enquiry. We have wrapped up our chain, and are turning homewards after a hard day's work upon the Glacier du Géant, when under our feet, as if coming from the body of the glacier, an explosion is heard. Somewhat startled, we look enquiringly over the ice. The sound is repeated, several shots being fired in quick succession. They seem sometimes to our right, sometimes to our left, giving the impression that the glacier is breaking all round us. Still nothing is to be seen. 250. We closely scan the ice, and after an hour's strict search we discover the cause of the reports. They announce the birth of a crevasse. Through a pool upon the glacier we notice air bubbles ascending, and find the bottom of the pool crossed by a narrow crack, from which the bubbles issue. Right and left from this pool we trace the young fissure through long distances. It is sometimes almost too feeble to be seen, and at no place is it wide enough to admit a knife-blade. 251. It is difficult to believe that the formidable fissures among which you and I have so often trodden with awe, could commence in this small way. Such, however, is the case. The great and gaping chasms on and above the ice-falls of the Géant and the Talèfre begin as narrow cracks, which open gradually to crevasses. We are thus taught in an instructive and impressive way that appearances suggestive of very violent action may really be produced by processes so slow as to require refined observations to detect them. In the production of natural phenomena two things always come into play, the intensity of the acting force, and the time during which its acts. Make the intensity great, and the time small, and you have sudden convulsion; but precisely the same apparent effect may be produced by making the intensity small, and the time great. This truth is strikingly illustrated by the Alpine ice-falls and crevasses; and many geological phenomena, which at first sight suggest violent convulsion, may be really produced in the selfsame almost imperceptible way. § 37. Icicles. 252. The crevasses are grandest on the higher névés, where they sometimes appear as long yawning fissures, and sometimes as chasms of irregular outline. A delicate blue light shimmers from them, but this is gradually lost in the darkness of their profounder portions. Over the edges of the chasms, and mostly over the southern edges, hangs a coping of snow, and from this depend like stalactites rows of transparent icicles, 10, 20, 30 feet long. These pendent spears |