Part 26 (1/2)
It is usual for visitors to the Montanvert to descend to the glacier, and to be led by their guides to the edges of the creva.s.ses, where, being firmly held, they look down into them; but those who have only made their acquaintance in this way know but little of their magnitude and beauty in the more disturbed portions of glaciers. As might be expected, they have been the graves of many a mountaineer; and the skeletons found upon the glacier prove that even the chamois itself, with its elastic muscles and admirable sureness of foot, is not always safe among the creva.s.ses. They are grandest in the higher ice-regions, where the snow hangs like a coping over their edges, and the water trickling from these into the gloom forms splendid icicles. The Gorner Glacier, as we ascend it towards the old Weissthor, presents many fine examples of such creva.s.ses; the ice being often torn in a most curious and irregular manner. You enter a porch, pillared by icicles, and look into a cavern in the very body of the glacier, enc.u.mbered with vast frozen bosses which are fringed all round by dependent icicles. At the peril of your life from slipping, or from the yielding of the stalact.i.tes, you may enter these caverns, and find yourself steeped in the blue illumination of the place. Their beauty is beyond description; but you cannot deliver yourself up, heart and soul, to its enjoyment.
There is a strangeness about the place which repels you, and not without anxiety do you look from your ledge into the darkness below, through which the sound of subglacial water sometimes rises like the tolling of distant bells. You feel that, however the cold splendours of the place might suit a purely spiritual essence, they are not congenial to flesh and blood, and you gladly escape from its magnificence to the suns.h.i.+ne of the world above.
[Sidenote: BIRTH OF A CREVa.s.sE.]
From their numbers it might be inferred that the formation of creva.s.ses is a thing of frequent occurrence and easy to observe; but in reality it is very rarely observed. Simond was a man of considerable experience upon the ice, but the first creva.s.se he ever saw formed was during the setting out of one of our lines, when a narrow rent opened beneath his feet, and propagated itself through the ice with loud cracking for a distance of 50 or 60 yards. Creva.s.ses always commence in this way as mere narrow cracks, which open very slowly afterwards. I will here describe the only case of creva.s.se-forming which has come under my direct observation.
On the 31st of July, 1857, Mr. Hirst and myself, having completed our day's work, were standing together upon the Glacier du Geant, when a loud dull sound, like that produced by a heavy blow, seemed to issue from the body of the ice underneath the spot on which we stood. This was succeeded by a series of sharp reports, which were heard sometimes above us, sometimes below us, sometimes apparently close under our feet, the intervals between the louder reports being filled by a low singing noise. We turned hither and thither as the direction of the sounds varied; for the glacier was evidently breaking beneath our feet, though we could discern no trace of rupture. For an hour the sounds continued without our being able to discover their source; this at length revealed itself by a rush of air-bubbles from one of the little pools upon the surface of the glacier, which was intersected by the newly-formed creva.s.se. We then traced it for some distance up and down, but hardly at any place was it sufficiently wide to permit the blade of my penknife to enter it. M. Aga.s.siz has given an animated description of the terror of his guides upon a similar occasion, and there was an element of awe in our own feelings as we heard the evening stillness of the glacier thus disturbed.
[Sidenote: MECHANICAL ORIGIN.]
With regard to the mechanical origin of the creva.s.ses the most vague and untenable notions had been entertained until Mr. Hopkins published his extremely valuable papers. To him, indeed, we are almost wholly indebted for our present knowledge of the subject, my own experiments upon this portion of the glacier-question being for the most part ill.u.s.trations of the truth of his reasoning. To understand the fissures in their more complex aspects it is necessary that we should commence with their elements. I shall deal with the question in my own way, adhering, however, to the mechanical principles upon which Mr. Hopkins has based his exposition.
[Ill.u.s.tration: Fig. 25. Diagram explanatory of the mechanical origin of Creva.s.ses.]
Let A B, C D, be the bounding sides of a glacier moving in the direction of the arrow; let _m_, _n_ be two points upon the ice, one, _m_, close to the r.e.t.a.r.ding side of the valley, and the other, _n_, at some distance from it. After a certain time, the point _m_ will have moved downwards to _m'_, but in consequence of the swifter movement of the parts at a distance from the sides, _n_ will have moved in the same time to _n'_. Thus the line _m n_, instead of being at right angles to the glacier, takes up the oblique position _m' n'_; but to reach from _m'_ to _n'_ the line _m n_ would have to stretch itself considerably; every other line that we can draw upon the ice parallel to _m' n'_ is in a similar state of tension; or, in other words, the sides of the glacier are acted upon by an oblique pull towards the centre. Now, Mr. Hopkins has shown that the direction in which this oblique pull is strongest encloses an angle of 45 with the side of the glacier.
[Sidenote: LINE OF GREATEST STRAIN.]
[Ill.u.s.tration: Fig. 26. Diagram showing the line of Greatest Strain.]
What is the consequence of this? Let A B, C D, Fig. 26, represent, as before, the sides of the glacier, moving in the direction of the arrow; let the shading lines enclose an angle of 45 with the sides. _Along_ these lines the marginal ice suffers the greatest strain, and, consequently _across_ these lines and at right angles to them, the ice tends to break and to form _marginal creva.s.ses_. The lines, _o p_, _o p_, mark the direction of these creva.s.ses; they are at right angles to the line of greatest strain, and hence also enclose an angle of 45 with the side of the valley, _being obliquely pointed upwards_.
[Sidenote: MARGINAL AND TRANSVERSE CREVa.s.sES.]
This latter result is noteworthy; it follows from the mechanical data that the swifter motion of the centre tends to produce marginal creva.s.ses which are inclined from the side of the glacier towards its source, and not towards its lower extremity. But when we look down upon a glacier thus creva.s.sed, the first impression is that the sides have been dragged down, and have left the central portions behind them; indeed, it was this very appearance that led M. de Charpentier and M.
Aga.s.siz into the error of supposing that the sides of a glacier moved more quickly than its middle portions; and it was also the delusive aspect of the creva.s.ses which led Professor Forbes to infer the slower motion of the eastern side of the Mer de Glace.
The r.e.t.a.r.dation of the ice is most evident near the sides; in most cases, the ice for a considerable distance right and left of the central line moves with a sensibly uniform velocity; there is no dragging of the particles asunder by a difference of motion, and, consequently, a compact centre is perfectly compatible with fissured sides. Nothing is more common than to see a glacier with its sides deeply cut, and its central portions compact; this, indeed, is always the case where the glacier moves down a bed of uniform inclination.
But supposing that the bed is not uniform--that the valley through which the glacier moves changes its inclination abruptly, so as to compel the ice to pa.s.s over a brow; the glacier is then circ.u.mstanced like a stick which we try to break by holding its two ends and pressing it against the knee. The brow, where the bed changes its inclination, represents the knee in the case of the stick, while the weight of the glacier itself is the force that tends to break it. It breaks; and fissures are formed across the glacier, which are hence called _transverse creva.s.ses_.
[Sidenote: GRINDELWALD GLACIER.]
No glacier with which I am acquainted ill.u.s.trates the mechanical laws just developed more clearly and fully than the Lower glacier of Grindelwald. Proceeding along the ordinary track beside the glacier, at about an hour's distance from the village the traveller reaches a point whence a view of the glacier is obtained from the heights above it. The marginal fissures are very cleanly cut, and point nearly in the direction already indicated; the glacier also changes its inclination several times along the distance within the observer's view. On crossing each brow the glacier is broken across, and a series of transverse creva.s.ses is formed, which follow each other down the slope. At the bottom of the slope tension gives place to pressure, the walls of the creva.s.ses are squeezed together, and the chasms closed up. They remain closed along the comparatively level s.p.a.ce which stretches between the base of one slope and the brow of the next; but here the glacier is again transversely broken, and continues so until the base of the second slope is reached, where longitudinal pressure instead of longitudinal strain begins to act, and the fissures are closed as before. In Fig. 27A I have given a sketchy section of a portion of the glacier, ill.u.s.trating the formation of the creva.s.ses at the top of a slope, and their subsequent obliteration at its base.
[Sidenote: COMPRESSION AND TENSION.]
[Ill.u.s.tration: Fig. 27A, B. Section and Plan of a portion of the Lower Grindelwald Glacier.]
Another effect is here beautifully shown, namely, the union of the transverse and marginal creva.s.ses to form continuous fissures which stretch quite across the glacier. Fig. 27B will ill.u.s.trate my meaning, though very imperfectly; it represents a plan of a portion of the Lower Grindelwald glacier, with both marginal and transverse fissures drawn upon it. I have placed it under the section so that each part of it may show in plan the portion of the glacier which is shown in section immediately above it. It shows how the marginal creva.s.ses remain after the compression of the centre has obliterated the transverse ones; and how the latter join on to the former, so as to form continuous fissures, which sweep across the glacier in vast curves, with their convexities turned upwards. The illusion before referred to is here strengthened; the creva.s.ses turn, so to say, _against_ the direction of motion, instead of forming loops, with their convexities pointing downwards, and thus would impress a person unacquainted with the mechanical data with the idea that the glacier margins moved more quickly than the centre.
The figures are intended to convey the idea merely; on the actual slopes of the glacier between twenty and thirty chasms may be counted: also the word ”compression” ought to have been limited to the level portions of the sketch.
[Sidenote: LONGITUDINAL CREVa.s.sES.]
Besides the two cla.s.ses of fissures mentioned we often find others, which are neither marginal nor transverse. The terminal portions of many glaciers, for example, are in a state of compression; the snout of the glacier abuts against the ground, and having to bear the thrust of the ma.s.s behind it, if it have room to expand laterally, the ice will yield, and _longitudinal creva.s.ses_ will be formed. They are of very common occurrence, but the finest example of the kind is perhaps exhibited by the glacier of the Rhone. After escaping from the steep gorge which holds the cascade, this glacier encounters the bottom of a comparatively wide and level valley; the resistance to its forward motion is augmented, while its ability to expand laterally is increased; it has to bear a longitudinal thrust, and it splits at right angles to the pressure [strain?]. A series of fissures is thus formed, the central ones of which are truly longitudinal; but on each side of the central line the creva.s.ses diverge, and exhibit a fan-like arrangement. This disposition of the fissures is beautifully seen from the summit of the Mayenwand on the Grimsel Pa.s.s.