Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/645

Rh GLACIER A glacier usually protrudes into a valley far below the ' li1nit of perpetual snow, and terminates amidst a wilderness of stones borne down upon its surface and deposited by its fusion. This earthy and rocky rubbish is termed moraine matter, and has already been described (GEOLOGY, p. 281). Lying in front of the lower end of a glacier, it marks in a characteristic and certain manner the greatest limit of extension which the glacier has at any one time attained. Sometimes a glacier is seen to have withdrawn very far within its old limits, leaving a prodigious barren waste of stones in advance of it, which, being devoid of soil, nourishes not one blade of grass. At other times the glacier pushes forward its margin beyond the limit which it has ever reached (at least within the memory of man), tears up the ground with its icy ploughshare, and shoves f or- ward the yielding turf in wrinkled folds, uprooting trees, nnving vast rocks, and scattering the walls of dwelling- houses in fragments before its irresistible onward march.‘ The lower end of a glacier is usually steep,—son1etimes ' with a dome-shaped unbroken outline, more frequently broken up by intersecting cracks into prismatic masses which the continued action of the sun and rai11 sharpen into pyramids, often assuming (as in the glacier of Bossons at Chamouni) grotesque or beautiful forms. From a vault in the green—blue ice, more or less perfectly formed each sum- mer, the torrent issues which represents the natural drain- age of the valley, derived partly from land springs, partly from the fusion of the ice. The united or crevassed condi- tion of the glacier generally depends almost entirely on the slope of its bed. If it incline rapidly, numerous transverse fissures are formed from the imperfect yielding of the ice during its forced descent along its uneven channel. These cracks often extend for lumdreds of yards, and may be hundreds of feet in depth; but their greatest depth is not accurately known, since they are rarely quite vertical. I11 many cases, however, the crevasses are comparatively few in number, and the glacier may be readily traversed in all directions. This is especially the case if a glacier of con- siderable dimensions meets with any contraction in its course. The ice is embayed and compressed, and its slope lessens, just as iii the case of a river when it nears a similar contraction preceding a fall. Such level and generally traversable spaces may be found about the middle regions of the )[er de Glace, the lower glacier of Grindelwald, the lower glacier of the Aar, and in many other cases. The last-named gla.cier is perhaps the most remarkably even and accessible of any in Switzerland. The slope of its surface is in many places only 3’. The Pasterzen glacier in C-arinthia is even less inclined. It is in such portions of a glacier that we commonly ﬁnd internal cascades, or “ moulins.” These arise from the surface water being collected into a considerable mass by a long course over its unbroken surface, and then precipitated with violence into the first fissure it meets with. The descending cascade keeps open its channel, which ﬁnally loses the form of a ﬁssure, presenting that of an open shaft, often of immense depth. Nearly connected in their origin with the internal cas- cades are the “ gravel cones,” occasionally seen on the sur- face of glaciers, which appear to be formed in this way. A considerable amount of earthy matter derived by the superﬁcial water—runs from the moraine accumulates in heaps in the inequalities of the ice, or at the bottom of the “ moulins.” As the glacier surface wastes by the action of the sun and rain, these heaps are brought to the surface, or 1 ‘Sue-h a sudden and disastrous increase took place in many of the glaciers of Switzerland and Savoy in 1818 (occasioning the catastrophe of the Val de Bagnes), and in those of the Bergenstift in .'orway about 1740. The retreat of a glacier far within its 01-1 in. raines i- well exemplilieel in most of the _;lar:’_ers of the latter countrv. and especially in that of .’ygaar-1. ', the striking phenomenon of “glacier tables.” 627 rather the general surface is depressed to their level. If the earthy mass be considerable, the ice beneath is protected from the radiation of the sun and from the violent washing of the rain ; it at length protrudes above the general level of the glacier, and ﬁnally forms a cone which appears to be entirely composed of gravel, but is in fact ice at the heart, with merely a protecting cover of earthy matter. These singular cones are very well seen on the glacier of the Aar, but on most others they are comparatively rare. The similar protective action of large stones detached from the moraines and lying on the surface of the ice often produces Stones of, any considerable size almost invariably stand upon a slightly elevated pillar of ice; but when they are broad and ﬂat 1 they occasionally attain a height of 6 and even of 12 feet ' above the general level. The superﬁcial waste of a glacier is thus a very important phenomenon. Owing to it the body of the ice has its vertical thickness rapidly diminished during the heats of sunnner, and, as we have already intimated, the lower end of a glacier l1as its position determined by the amount of this waste. Suppose a glacier to move along its bed at the rate of 300 feet per annum, and imagine (merely for the sake of illustration) its yearly superﬁcial waste to be 20 feet; then the thickness of the glacier will diminish by 20 feet for every 300 feet of its length, or at the rate of 360 feet per mile, so that the longitudinal section of a. glacier has the form of a wedge; and however enormous its original thickness, after a certain course we must at length come to the thin end of the wedge, and that the more rapidly as the causes of melting increase towards the lower extremity. These causes are indeed so various that it is difficult to estimate them with accuracy. “To have (1) the direct solar heat, (2) the contact of warm air, and (3) the washing of rain. All these causes act on the surface and produce the “ ablation ” of the surface. Besides these, the ice of the glacier wastes somewhat beneath by the contact of the soil and the washing of the inferior streams. This may be called its “subsidence.” Further, the natural slope of the rocky bed of the glacier causes any point of the surface to stand absolutely lower each day in con- sequence of the progressive motion. These three causes united produce the “ geometrical depression ” of the sur- face. Principal J. D. Forbes showed how the several effects may usually be distinguished by observation. During the height of sunnner, near the .Iontanvert, he found the daily average ablation to be 3'62 inches, the daily subsidence to be 1-63 inches. SeVen—tenths of the geo- metrical depression are due therefore to the former cause, and three-tenths to the latter. This is a very large amount, and it is certain that during the colder period of the year, and whilst the glacier is covered with snow, the subsidence is not only suspended, but the glacier recruits in thickness a portion of its waste during the seasons of summer and autumn. To this subject we shall again return. The middle region of the great glaciers of the Alps extends I from the level of about 6000 to 8000 feet above the sea. The inclination is usually there most moderate——say from 2._1.-° to 6°. But this is not invariably the case. Beyond 8000 feet we reach the snow-line. The snow-line is a fact ' as definite on the surface of a glacier as 011 that of a moun- I gr-.1duall_v from the state of ice to the state of snow. tain, only in the former case it occurs at a somewhat lower level. It cannot be too distinctly understood that the fresh snow annually disappears from the glacier proper. Where it ceases entirely to melt, it of course becomes in- corporated with the glacier. We have therefore arrived at the region where the glacier forms; everywhere below it only um-tes. This snowy region of the glacier is called in French m’-2-6, in German _ﬁr'/2. As we ascend the glacier it paiscs ' he