Part 30 (1/2)
[Sidenote: MOULINS EXPLAINED.]
But although this portion of the glacier is free from those long-continued and permanent strains which, having once rent the ice, tend subsequently to widen the rent and produce yawning creva.s.ses, it is not free from local strains sufficient to produce _cracks_ which penetrate the glacier to a great depth. Imagine such a crack intersecting such a glacier-rivulet as we have described. The water rushes down it, and soon scoops a funnel large enough to engulf the entire stream. The moulin is thus formed, and, as the ice moves downward, the sides of the crack are squeezed together and regelated, the seam which marks the line of junction being in most cases distinctly visible. But as the motion continues, other portions of the glacier come into the same state of strain as that which produced the first crack; a second one is formed across the stream, the old shaft is forsaken, and a new one is hollowed out, in which for a season the cataract plays the thunderer. I have in some cases counted the forsaken shafts of six old moulins in advance of an active one. Not far from the Grand Moulin of the Mer de Glace in 1857 there was a second empty shaft, which evidently communicated by a subglacial duct with that into which the torrent was precipitated. Out of the old orifice issued a strong cold blast, the air being manifestly impelled through the duct by the falling water of the adjacent moulin.
These shafts are always found in the same locality; the portion of the Mer de Glace to which I have referred is never without them. Some of the guides affirm that they are motionless; and a statement of Prof. Forbes has led to the belief that this was also his opinion.[A] M. Aga.s.siz, however, observed the motion of some of these shafts upon the glacier of the Aar; and when on the spot in 1857, I was anxious to decide the point by accurate measurements with the theodolite.
My friend Mr. Hirst took charge of the instrument, and on the 28th of July I fixed a single stake beside the Grand Moulin, in a straight line between a station at Trelaporte and a well-defined mark on the rock at the opposite side of the valley. On the 31st, the displacement of the stake amounted to 50 inches, and on the 1st of August it had moved 74-1/2 inches--the moulin, to all appearance, occupying throughout the same position with regard to the stake. To render this certain, moreover we subsequently drove two additional stakes into the ice, thus enclosing the mouth of the shaft in a triangle. On the 8th of August the displacements were measured and gave the following results:--
Total Motion.
First (old) stake 198 inches.
Second (new) do. 123 ”
Third 124 ”
[Sidenote: MOTION OF THE MOULINS.]
The old stake had been fixed for 11 days, and its daily motion--_which was also that of the moulin_--averaged 18 inches a day. Hence the moulins share the general motion of the glacier, and their apparent permanence is not, as has been alleged, a proof of the semi-fluidity of the glacier, but is due to the breaking of the ice as it pa.s.ses the place of local strain.
[Sidenote: DEPTH OF ”GRAND MOULIN” SOUGHT.]
Wis.h.i.+ng to obtain some estimate as to the depth of the ice, Mr. Hirst undertook the sounding of some of the moulins upon the Glacier de Lechaud, making use of a tin vessel filled with lumps of lead and iron as a weight. The cord gave way and he lost his plummet. To measure the depth of the Grand Moulin, we obtained fresh cord from Chamouni, to which we attached a four-pound weight. Into a cavity at the bottom of the weight we stuffed a quant.i.ty of b.u.t.ter, to indicate the nature of the bottom against which the weight might strike. The weight was dropped into the shaft, and the cord paid out until its slackening informed us that the weight had come to rest; by shaking the string, however, and walking round the edge of the shaft, the weight was liberated, and sank some distance further. The cord partially slackened a second time, but the strain still remaining was sufficient to render it doubtful whether it was the weight or the action of the falling water which produced it.
We accordingly paid out the cord to the end, but, on withdrawing it, found that the greater part of it had been coiled and knotted up by the falling water. We uncoiled, and sounded again. At a depth of 132 feet the weight reached a ledge or protuberance of ice, and by shaking and lifting it, it was caused to descend 31 feet more. A depth of 163 feet was the utmost we could attain to. We sounded the old moulin to a depth of 90 feet; while a third little shaft, beside the large one, measured only 18 feet in depth. We could see the water escape from it through a lateral ca.n.a.l at its bottom, and doubtless the water of the Grand Moulin found a similar exit. There was no trace of dirt upon the b.u.t.ter, which might have indicated that we had reached the bed of the glacier.
FOOTNOTES:
[A] ”Every year, and year after year, the watercourses follow the same lines of direction--their streams are precipitated into the heart of the glacier by vertical funnels, called 'moulins,' at the very same points.”--Forbes's Fourth Letter upon Glaciers: 'Occ. Pap.,' p. 29.
[Ill.u.s.tration: DIRT-BANDS OF THE MER DE GLACE, AS SEEN FROM A POINT NEAR THE FLeGeRE.
Fig. 35. _To face p. 367._]
DIRT-BANDS OF THE MER DE GLACE.
(26.)
[Sidenote: DIRT-BANDS FROM THE FLEGeRE.]
These bands were first noticed by Prof. Forbes on the 24th of July, 1842, and were described by him in the following words:--”My eye was caught by a very peculiar appearance of the surface of the ice, which I was certain that I now saw for the first time. It consisted of nearly hyperbolic brownish bands on the glacier, the curves pointing downwards, and the two branches mingling indiscriminately with the moraines, presenting an appearance of a succession of waves some hundred feet apart.”[A] From no single point of view hitherto attained can all the Dirt-Bands of the Mer de Glace be seen at once. To see those on the terminal portion of the glacier, a station ought to be chosen on the opposite range of the Brevent, a few hundred yards beyond the Croix de la Flegere, where we stand exactly in front of the glacier as it issues into the valley of Chamouni. The appearance of the bands upon the portion here seen is represented in Fig. 35.
It will be seen that the bands are confined to one side of the glacier, and either do not exist, or are obliterated by the debris, upon the other side. The cause of the acc.u.mulation of dirt on the right side of the glacier is, that no less than five moraines are crowded together at this side. In the upper portions of the Mer de Glace these moraines are distinct from each other; but in descending, the successive engulfments and disgorgings of the blocks and dirt have broken up the moraines; and at the place now before us the materials which composed them are strewn confusedly on the right side of the glacier. The portion of the ice on which the dirt-bands appear is derived from the Col du Geant. They do not quite extend to the end of the glacier, being obliterated by the dislocation of the ice upon the frozen cascade of Des Bois.
[Sidenote: DIRT-BANDS FROM LES CHARMOZ.]
Let us now proceed across the valley of Chamouni to the Montanvert; where, climbing the adjacent heights to an elevation of six or eight hundred feet above the hotel, we command a view of the Mer de Glace, from Trelaporte almost to the commencement of the Glacier des Bois. It was from this position that Professor Forbes first observed the bands.
Fifteen, sixteen, and seventeen years later I observed them from the same position. The number of bands which Professor Forbes counted from this position was eighteen, with which my observations agree. The entire series of bands which I observed, with the exception of one or two, must have been the _successors_ of those observed by Professor Forbes; and my finding the same number after an interval of so many years proves that the bands must be due to some regularly recurrent cause. Fig. 36 represents the bands as seen from the heights adjacent to the Montanvert.
[Ill.u.s.tration: DIRT-BANDS OF THE MER DE GLACE, AS SEEN FROM LES CHARMOZ.
Fig. 36. _To face p. 368._]
I would here direct attention to an a.n.a.logy between a glacier and a river, which may be observed from the heights above the Montanvert, but to which no reference, as far as I know, has. .h.i.therto been made. When a river meets the b.u.t.tress of a bridge, the water rises against it, and, on sweeping round it, forms an elevated ridge, between which and the pier a depression occurs which varies in depth with the force of the current. This effect is shown by the Mer de Glace on an exaggerated scale. Sweeping round Trelaporte, the ice pushes itself beyond the promontory in an elevated ridge, from which it drops by a gradual slope to the adjacent wall of the valley, thus forming a depression typified by that already alluded to. A similar effect is observed at the opposite side of the glacier on turning round the Echelets; and both combine to form a kind of skew surface. A careful inspection of the frontispiece will detect this peculiarity in the shape of the glacier.