Stake 1 2 3 4 5 6 7 8 9 10
Inches 5 8 10 9 9 8 6 9 7 6
199. Here our conjecture is still further verified, the rate of
motion being even less than that of the Glacier du Géant.
§ 28. _Motion of Top and Bottom of Glacier._
200. We have here the most ample and varied evidence that the sides
of a glacier, like those of a river, are retarded by friction against
its boundaries. But the likeness does not end here. The motion of a
river is retarded by the friction against its bed. Two observers,
viz. Prof. Forbes and M. Charles Martins, concur in showing the same
to be the case with a glacier. The observations of both have been
objected to; hence it is all the more incumbent on us to seek for
decisive evidence.
201. At the Tacul (near the point _a_ upon the sketch plan, p. 83) a
wall of ice about 150 feet high has already attracted our attention.
Bending round to join the Léchaud the Glacier du Géant is here drawn
away from the mountain side, and exposes a fine section. We try to
measure it top, bottom, and middle, and are defeated twice over. We
try it a third time and succeed. A stake is fixed at the summit of
the ice-precipice, another at 4 feet from the bottom, and a third at
35 feet above the bottom. These lower stakes are fixed at some risk
of boulders falling upon us from above; but by skill and caution
we succeed in measuring the motions of all three. For 24 hours the
motions are:--
Top stake 6 inches.
Middle stake 4½ "
Bottom stake 2⅔ "
202. The retarding influence of the bed of the glacier is reduced to
demonstration by these measurements. The bottom does not move with
half the velocity of the surface.
§ 29. _Lateral Compression of a Glacier._
203. Furnished with the knowledge which these labours and
measurements have given us, let us once more climb to our station
beside the Cleft under the Aiguille de Charmoz. At our first visit
we saw the medial moraines of the glacier, but we knew nothing about
their cause. We now know that they mark upon the trunk its tributary
glaciers. Cast your eye, then, first upon the Glacier du Géant;
realise its width in its own valley, and see how much it is narrowed
at Trélaporte. The broad ice-stream of the Léchaud is still more
surprising, being squeezed upon the Mer de Glace to a narrow white
band between its bounding moraines. The Talèfre undergoes similar
compression. Let us now descend, shake out our chain, measure, and
express in numbers the width of the tributaries, and the actual
amount of compression suffered at Trélaporte.
204. We find the width of the Glacier du Géant to be 5,155 links, or
1,134 yards.
205. The width of the Glacier de Léchaud we find to be 3,725 links,
or 825 yards.
206. The width of the Talèfre we find to be 2,900 links, or 638 yards.
207. The sum of the widths of the three branch glaciers is therefore
2,597 yards.
208. At Trélaporte these three branches are forced through a gorge
893 yards wide, or one-third of their previous width, at the rate of
twenty inches a day.
209. If we limit our view to the Glacier de Léchaud, the facts are
still more astonishing. Previous to its junction with the Talèfre,
this glacier has a width of 825 yards; in passing through the jaws of
the granite vice at Trélaporte, its width is reduced to eighty-eight
yards, or in round numbers to one-tenth of its previous width. (Look
to the sketch on the next page.)
[Illustration: SKETCH-PLAN SHOWING THE MORAINES, _a_, _b_, _c_, _d_,
_e_, OF THE MER DE GLACE.]
210. Are we to understand by this that the ice of the Léchaud is
squeezed to one-tenth of its former _volume_? By no means. It is
mainly a change of _form_, not of volume, that occurs at Trélaporte.
Previous to its compression, the glacier resembles a plate of ice
_lying flat_ upon its bed. After its compression, it resembles a
plate _fixed upon its edge_. The squeezing, doubtless, has deepened
the ice.
§ 30. _Longitudinal Compression of a Glacier._
211. The ice is forced through the gorge at Trélaporte by a pressure
from behind; in fact the Glacier du Géant, immediately above
Trélaporte, represents a piston or a plug which drives the ice
through the gorge. What effect must this pressure have upon the plug
itself? Reasoning alone renders it probable that the pressure will
shorten the plug; that the lower part of the Glacier du Géant will to
some extent yield to the pressure from behind.
212. Let us test this notion. About three-quarters of a mile above
the Tacul, and on the mountain slope to the left as we ascend, we
observe a patch of verdure. Thither we climb; there we plant our
theodolite, and set out across the Glacier du Géant, a line, which we
will call line No. 1 (F F' upon sketch, p. 68).
213. About a quarter of a mile lower down we find a practicable
couloir on the mountain side; we ascend it, reach a suitable
platform, plant our instrument, and set out a second line, No. 2 (G
G' upon sketch). We must hasten our work here, for along this couloir
stones are discharged from a small glacier which rests upon the slope
of Mont Tacul.
214. Still lower down by another quarter of a mile, which brings us
near the Tacul, we set out a third line, No. 3 (H H' upon sketch),
across the glacier.
215. The daily motion of the centres of these three lines is as
follows:--
Inches Distances asunder
No. 1 50·55 }
} 545 yards.
No. 2 15·43 }
} 487 "
No. 3 12·75 }
216. The first line here moves five inches a day more than the
second; and the second nearly three inches a day more than the third.
The reasoning is therefore confirmed. The ice-plug, which is in round
numbers one thousand yards long, is shortened by the pressure exerted
on its front at the rate of about eight inches a day.
217. A river descending the Valley du Géant would behave in
substantially the same fashion. It would have its motion on
approaching Trélaporte diminished, and it would pour through the
defile with a velocity greater than that of the water behind.
§ 31. _Sliding and Flowing. Hard Ice and Soft Ice._
218. We have thus far confined ourselves to the measurement and
discussion of glacier motion; but in our excursions we have noticed
many things besides. Here and there, where the ice has retreated
from the mountain side, we have seen the rocks fluted, scored, and
polished; thus proving that the ice had slidden over them and ground
them down. At the source of the Arveiron we noticed the water rushing
from beneath the glacier charged with fine matter. All glacier rivers
are similarly charged. The Rhone carries its load of matter into the
Lake of Geneva; the rush of the river is here arrested, the matter
subsides, and the Rhone quits the lake clear and blue. The Lake of
Geneva, and many other Swiss lakes, are in part filled up with this
matter, and will, in all probability, finally be obliterated by it.
219. One portion of the motion of a glacier is due to this bodily
sliding of the mass over its bed.
220. We have seen in our journeys over the glacier streams formed by
the melting of the ice, and escaping through cracks and _crevasses_
to the bed of the glacier. The fine matter ground down is thus washed
away; the bed is kept lubricated, and the sliding of the ice rendered
more easy than it would otherwise be.
221. As a skater also you know how much ice is weakened by a thaw.
Before it actually melts it becomes rotten and unsafe. Test such ice
with your penknife: you can dig the blade readily into it, or cut the
ice with ease. Try good sound ice in the same way: you find it much
more resistant. The one, indeed, resembles soft chalk; the other hard
stone.
222. Now the Mer de Glace in summer is in this thawing condition. Its
ice is rendered soft and yielding by the sun; its motion is thereby
facilitated. We have seen that not only does the glacier slide over
its bed, but that the upper layers slide over the under ones, and
that the centre slides past the sides. The softer and more yielding
the ice is, the more free will be this motion, and the more readily
also will it be forced through a defile like Trélaporte.
223. But in winter the thaw ceases; the quantity of water reaching
the bed of the glacier is diminished or entirely cut off. The
ice also, to a certain depth at least, is frozen hard. These
considerations would justify the opinion that in winter the glacier,
if it moves at all, must move more slowly than in summer. At all
events, the summer measurements give no clue to the winter motion.
224. This point merits examination. I will not, however, ask you to
visit the Alps in mid-winter; but, if you allow me, I will be your
deputy to the mountains, and report to you faithfully the aspect of
the region and the behaviour of the ice.
§ 32. _Winter on the Mer de Glace._
225. The winter chosen is an inclement one. There is snow in London,
snow in Paris, snow in Geneva; snow near Chamouni so deep that the
road fences are entirely effaced. On Christmas night--nearly at
mid-night--1859, your deputy reaches Chamouni.
226. The snow fell heavily on December 26; but on the 27th, during a
lull in the storm, we turn out. There are with me four good guides
and a porter. They tie planks to their feet to prevent them from
sinking in the snow; I neglect this precaution and sink often to the
waist. Four or five times during our ascent the slope cracks with an
explosive sound, and the snow threatens to come down in avalanches.[D]
[D] Four years later, viz. in the spring of 1863, a mighty climber
and noble guide and companion of mine, named Johann Joseph Bennen,
was lost, through the cracking and subsequent slipping of snow on
such a slope.
The freshly-fallen snow was in that particular condition which causes
its granules to adhere, and hence every flake falling on the trees
had been retained there. The laden pines presented beautiful and
often fantastic forms.
227. After five hours and a half of arduous work the Montanvert was
attained. We unlocked the forsaken auberge, round which the snow
was reared in buttresses. I have already spoken of the complex play
of crystallising forces. The frost figures on the window-panes of
the auberge were wonderful: mimic shrubs and ferns wrought by the
building power while hampered by the adhesion between the glass and
the film in which it worked. The appearance of the glacier was very
impressive; all sounds were stilled. The cascades which in summer
fill the air with their music were silent, hanging from the ledges
of the rocks in fluted columns of ice. The surface of the glacier
was obviously higher than it had been in summer; suggesting the
thought that while the winter cold maintained the lower end of the
glacier jammed between its boundaries, the upper portions still moved
downwards and thickened the ice. The peak of the Aiguille du Dru
shook out a cloud-banner, the origin and nature of which have been
already explained (84). (See _Frontispiece_.)
[Illustration: SNOW-LADEN PINE-TREE.]
228. On the morning of the 28th this banner was strikingly large and
grand, and reddened by the light of the rising sun, it glowed like a
flame. Roses of cloud also clustered round the crests of the Grande
Jorasse and hung upon the pinnacles of Charmoz. Four men, well roped
together, descended to the glacier. I had trained one of them in
1857, and he was now to fix the stakes. The storm had so distributed
the snow as to leave alternate lengths of the glacier bare and
thickly covered. Where much snow lay great caution was required, for
hidden crevasses were underneath. The men sounded with their staffs
at every step. Once while looking at the party through my telescope
the leader suddenly disappeared; the roof of a crevasse had given
way beneath him; but the other three men promptly gathered round and
lifted him out of the fissure. The true line was soon picked up by
the theodolite; one by one the stakes were fixed until a series of
eleven of them stood across the glacier.
229. To get higher up the valley was impracticable; the snow was
too deep, and the aspect of the weather too threatening; so the
theodolite was planted amid the pines a Little way below the
Montanvert, whence through a vista I could see across the glacier.
The men were wrapped at intervals by whirling snow-wreaths which
quite hid them, and we had to take advantage of the lulls in the
wind. Fitfully it came up the valley, darkening the air, catching the
snow upon the glacier, and tossing it throughout its entire length
into high and violently agitated clouds, separated from each other
by cloudless spaces corresponding to the naked portions of the ice.
In the midst of this turmoil the men continued to work. Bravely and
steadfastly stake after stake was set, until at length a series of
ten of them was fixed across the glacier.
230. Many of the stakes were fixed in the snow. They were four feet
in length, and were driven in to a depth of about three feet. But
that night, while listening to the wild onset of the storm, I thought
it possible that the stakes and the snow which held them might be
carried bodily away before the morning. The wind, however, lulled.
We rose with the dawn, but the air was thick with descending snow.
It was all composed of those exquisite six-petaled flowers, or
six-rayed stars, which have been already figured and described (§
9). The weather brightening, the theodolite was planted at the end
of the first line. The men descended, and, trained by their previous
experience, rapidly executed the measurements. The first line was
completed before 11 A. M. Again the snow began to fall, filling
all the air. Spangles innumerable were showered upon the heights.
Contrary to expectation, the men could be seen and directed through
the shower.
231. To reach the position occupied by the theodolite at the end of
our second line, I had to wade breast-deep through snow which seemed
as dry and soft as flour. The toil of the men upon the glacier in
breaking through the snow was prodigious. But they did not flinch,
and after a time the leader stood behind the farthest stake, and
cried, _Nous avons fini_. I was surprised to hear him so distinctly,
for falling snow had been thought very deadening to sound. The work
was finished, and I struck my theodolite with a feeling of a general
who had won a small battle.
232. We put the house in order, packed up, and shot by glissade down
the steep slopes of _La Filia_ to the vault of the Arveiron. We found
the river feeble, but not dried up. Many weeks must have elapsed
since any water had been sent down from the surface of the glacier.
But at the setting in of winter the fissures were in a great measure
charged with water; and the Arveiron of to-day was probably due to
the gradual _drainage_ of the glacier. There was now no danger of
entering the vault, for the ice seemed as firm as marble. In the
cavern we were bathed by blue light. The strange beauty of the place
suggested magic, and put me in mind of stories about fairy caves
which I had read when a boy. At the source of the Arveiron our winter
visit to the Mer de Glace ends; next morning your deputy was on his
way to London.
§ 33. _Winter Motion of the Mer de Glace._
233. Here are the measurements executed in the winter of 1859:--
Line No. I.
Stake 1 2 3 4 5 6 7 8 9 10 11
Inches 7 11 14 13 14 14 16 16 12 12 7
Line No. II.
Stake 1 2 3 4 5 6 7 8 9 10
Inches 8 10 14 16 16 16 18 17 15 14
234. Thus the winter motion of the Mer de Glace near the Montanvert
is, in round numbers, half the summer motion.
235. As in summer, the eastern side of the glacier at this place
moved quicker than the western.
§ 34. Motion of the Grindelwald and Aletsch Glaciers.
236. As regards the question of motion, to no other glacier have we
devoted ourselves with such thoroughness as to the Mer de Glace; we
are, however, able to add a few measurements of other celebrated
glaciers. Rear the village of Grindelwald in the Bernese Oberland,
there are two great ice-streams called respectively the Upper and the
Lower Grindelwald glaciers, the second of which is frequently visited
by travellers in the Alps. Across it on August 6, 1860, a series of
twelve stakes was fixed by Mr. Vaughan Hawkins and myself. Measured
on the 8th and reduced to its daily rate, the motion of these stakes
was as follows:--
MOTION OF LOWER GRINDELWALD GLACIER.
Stake 1 2 3 4 5 6 7 8 9 10 11 12
Inches 18 19 20 21 21 21 22 20 19 18 17 14
237. The theodolite was here planted a little below the footway
leading to the higher glacier region, and at about a mile above
the end of the glacier. The measurement was rendered difficult by
crevasses.
238. The largest glacier in Switzerland is the Great Aletsch, to
which further reference shall subsequently be made. Across it on
August 14, 1860, a series of thirty-four stakes was planted by Mr.
Hawkins and me. Measured on the 16th and reduced to their daily rate,
the velocities were found to be as follows:--
MOTION OF GREAT ALETSCH GLACIER.
East
Stake 1 2 3 4 5 6 7 8 9 10 11 12
Inches 2 3 4 6 8 11 13 14 16 17 17 19
Stake 13 14 15 16 17 18 19 20 21 22 23
Inches 19 18 18 17 19 19 19 19 17 17 15
Stake 24 25 26 27 28 29 30 31 32 33 34
Inches 16 17 17 17 17 17 17 17 16 12 12
West
239. The maximum motion here is nineteen inches a day. Probably the
eastern side of the glacier is shallow, the retardation of the bed
making the motion of the eastern stakes inconsiderable. The width of
the glacier here is 9,030 links, or about a mile and a furlong. The
theodolite was planted high among the rocks on the western flank of
the mountain, about half a mile above the Märgelin See.
§ 35. _Motion of Morteratsch Glacier._
240. Far to the east of the Oberland and in that interesting part
of Switzerland known as the Ober Engadin, stands a noble group of
mountains, less in height than those of the Oberland, but still
of commanding elevation. The group derives its name from its most
dominant peak, the Piz Bernina. To reach the place we travel by
railway from Basel to Zürich, and from Zürich to Chur (French Coire),
whence we pass by diligence over either the Albula pass or the Julier
pass to the village of Pontresina. Here we are in the immediate
neighbourhood of the Bernina mountains.
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 ice-stream here referred to is the Morteratsch glacier, the
end of which is a short hour's walk from the village of Pontresina.
We have now to determine its rate of motion and to account for the
widening of its medial moraine.
243. In the summer of 1864 Mr. Hirst and myself set out three lines
of stakes across the glacier. The first line crossed the ice high up;
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 glacier. The daily motion of these three
lines was as follows:--
First Line.
Stake 1 2 3 4 5 6 7 8 9 10 11
Inches 8 12 13 13 14 13 12 12 10 7 5
Second Line.
Stake 1 2 3 4 5 6 7 8 9 10 11
Inches 1 4 6 8 10 11 11 11 11 11 11
Third Line.
Stake 1 2 3 4 5 6 7 8 9 10 11
Inches 1 2 4 5 6 6 7 7 5 5 4
244. 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 down wards 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:
Maximum Motion in 100 Hours.
First line 56 inches.
Second line 45 "
Third line 30 "
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 a 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 enter 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. Eight 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 it 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
constitute one of the most beautiful features of the higher crevasses.
253. How are they produced? Evidently by the thawing of the snow. But
why, when once thawed, should the water freeze again to solid spears?
You have seen icicles pendent from a house-eave, which have been
manifestly produced by the thawing of the snow upon the roof. If we
understand these, we shall also understand the vaster stalactites of
the Alpine crevasses.