_ALPINE SCULPTURE._
To the physical geologist the conformation of the Alps, and of
mountain-regions generally, constitutes one of the most interesting
problems of the present day. To account for this conformation, two
hypotheses have been advanced, which may be respectively named the
hypothesis of _fracture_ and the hypothesis of _erosion_. Those who
adopt the former maintain that the forces by which the Alps were
elevated produced fissures in the earth’s crust, and that the valleys
of the Alps are the tracks of these fissures. Those who hold the latter
hypothesis maintain that the valleys have been cut out by the action
of ice and water, the mountains themselves being the residual forms
of this grand sculpture. To the erosive action here indicated must be
added that due to the atmosphere (the severance and detachment of rocks
by rain and frost), as affecting the forms of the more exposed and
elevated peaks.
I had heard it stated that the Via Mala was a striking illustration of
the fissure theory--that the profound chasm thus named, and through
which the Hinter-Rhein now flows, could be nothing else than a crack
in the earth’s crust. To the Via Mala I therefore went in 1864 to
instruct myself by actual observation upon the point in question.
The gorge commences about a quarter of an hour above Tusis; and, on
entering it, the first conclusion is that it must be a fissure. This
conclusion in my case was modified as I advanced. Some distance up the
gorge I found upon the slopes to my right quantities of rolled stones,
evidently rounded by water-action. Still further up, and just before
reaching the first bridge which spans the chasm, I found more rolled
stones, associated with sand and gravel. Through this mass of detritus,
fortunately, a vertical cutting had been made, which exhibited a
section showing perfect stratification. There was no agency in the
place to roll these stones, and to deposit these alternating layers of
sand and pebbles, but the river which now rushes some hundreds of feet
below them. At one period of the Via Mala’s history the river must have
run at this high level. Other evidences of water-action soon revealed
themselves. From the parapet of the first bridge I could see the solid
rock 200 feet above the bed of the river scooped and eroded.
It is stated in the guide-books that the river, which usually runs
along the bottom of the gorge, has been known almost to fill it during
violent thunderstorms; and it may be urged that the marks of erosion
which the sides of the chasm exhibit are due to those occasional
floods. In reply to this, it may be stated that even the existence of
such floods is not well authenticated, and that if the supposition
were true, it would be an additional argument in favour of the cutting
power of the river. For if floods operating at rare intervals could
thus erode the rock, the same agency, acting without ceasing upon the
river’s bed, must certainly be competent to excavate it.
I proceeded upwards, and from a point near another bridge (which of
them I did not note) had a fine view of a portion of the gorge. The
river here runs at the bottom of a cleft of profound depth, but so
narrow that it might be leaped across. That this cleft must be a crack
is the impression first produced; but a brief inspection suffices to
prove that it has been cut by the river. From top to bottom we have
the unmistakable marks of erosion. This cleft was best seen by looking
downwards from a point near the bridge; but looking upwards from the
bridge itself, the evidence of aqueous erosion was equally convincing.
The character of the erosion depends upon the rock as well as upon the
river. The action of water upon some rocks is almost purely mechanical;
they are simply ground away or detached in sensible masses. In other
cases the action is chemical as well as mechanical. Water, in passing
over limestone, charges itself with carbonate of lime without damage to
its transparency; the rock is _dissolved_ in the water; and the gorges
cut by water in such rocks often resemble those cut in the ice of
glaciers by glacier streams. To the solubility of limestone is probably
to be ascribed the fantastic forms which peaks of this rock usually
assume, and also the grottos and caverns which interpenetrate limestone
formations. A rock capable of being thus dissolved will expose a smooth
surface after the water has quitted it; and in the case of the Via Mala
it is the polish of the surfaces, and also the curved hollows scooped
in the sides of the gorge, which assure us that the chasm has been the
work of the river.
About four miles from Tusis, and not far from the little village of
Zillis, the Via Mala opens into a plain bounded by high terraces,
evidently cut by water. It occurred to me the moment I saw it that
the plain had been the bed of an ancient lake; and a farmer, who was
my temporary companion, immediately informed me that such was the
tradition of the neighbourhood. This man conversed with intelligence,
and as I drew his attention to the rolled stones, which rest not only
above the river, but above the road, and inferred that the river must
have been there to have rolled those stones, he saw the force of the
evidence perfectly. In fact, in former times, and subsequent to the
retreat of the great glaciers, a rocky barrier crossed the valley at
this place, damming the river which came from the glaciers higher up.
A lake was thus formed which poured its waters over the barrier. Two
actions were here at work, both tending to obliterate the lake--the
raising of its bed by the deposition of detritus, and the cutting of
its dam by the river. In process of time the cut deepened into the
Via Mala; the lake was drained, and the river now flows in a definite
channel through the plain which its waters once totally covered.
From Tusis I crossed to Tiefenkasten by the Schien Pass, and thence
over the Julier Pass to Pontresina. There are three or four ancient
lake-beds between Tiefenkasten and the summit of the Julier. They are
all of the same type--a more or less broad and level valley-bottom,
with a barrier in front through which the river has cut a passage, the
drainage of the lake being the consequence. These lakes are sometimes
dammed by barriers of rock, sometimes by the moraines of ancient
glaciers.
An example of this latter kind occurs in the Rosegg valley, about
twenty minutes below the end of the Rosegg glacier, and about an hour
from Pontresina. The valley here is crossed by a pine-covered moraine
of the noblest dimensions: in the neighbourhood of London it might
be called a mountain. That it is a moraine, the inspection of it
from a point on the Surlei slopes above it will convince any person
possessing an educated eye. Where, moreover, the interior of the mound
is exposed, it exhibits moraine-matter--detritus pulverised by the
ice, with boulders entangled in it. It stretched quite across the
valley, and at one time dammed the river up. But now the barrier is
cut through, the stream having about one-fourth of the moraine to its
right, and the remaining three-fourths to its left. Other moraines of a
more resisting character hold their ground as barriers to the present
day. In the Val di Campo, for example, about three-quarters of an hour
from Pisciadello, there is a moraine composed of large boulders, which
interrupt the course of a river and compel the water to fall over them
in cascades. They have in great part resisted its action since the
retreat of the ancient glacier which formed the moraine. Behind the
moraine is a lake-bed, now converted into a meadow, which is quite
level, and rests on a deep layer of mould.
At Pontresina a very fine and instructive gorge is to be seen. The
river from the Morteratsch glacier rushes through a deep and narrow
chasm which is spanned at one place by a stone bridge. The rock is not
of a character to preserve smooth polishing; but the larger features of
water-action are perfectly evident from top to bottom. Those features
are in part visible from the bridge, but still better from a point a
little distance from the bridge in the direction of the upper village
of Pontresina. The hollowing out of the rock by the eddies of the water
is here quite manifest. A few minutes’ walk upwards brings us to the
end of the gorge; and behind it we have the usual indications of an
ancient lake, and terraces of distinct water origin.
From this position the genesis of the gorge is clearly revealed. After
the retreat of the ancient glacier, a transverse ridge of comparatively
resisting material crossed the valley at this place. Over the lowest
part of this ridge the river flowed, rushing steeply down to join
at the bottom of the slope the stream which issued from the Rosegg
glacier. On this incline the water became a powerful eroding agent, and
finally cut its channel to its present depth.
Geological writers of reputation assume at this place the existence
of a fissure, the ‘washing out’ of which resulted in the formation
of the gorge. Now no examination of the bed of the river ever proved
the existence of this fissure; and it is certain that water can cut
a channel through unfissured rock--that cases of deep cutting can be
pointed out where the clean bed of the stream is exposed, the rock
which forms the floor of the river not exhibiting a trace of fissure.
An example of this kind occurs near the Bernina Gasthaus, about two
hours from Pontresina. A little way below the junction of the two
streams from the Bernina Pass and the Heuthal the river flows through a
channel cut by itself, and 20 or 30 feet in depth. At some places the
river-bed is covered with rolled stones; at other places it is bare,
but shows no trace of fissure. The abstract power of water (if I may
use the term) to cut through rock is demonstrated by such instances.
But if water be competent to form a gorge without the aid of a fissure,
why assume the existence of such in cases like that at Pontresina?
It seems far more philosophical to accept the simple and impressive
history written on the walls of those gorges by the agent which
produced them.
Numerous cases might be pointed out, varying in magnitude, but all
identical in kind, of barriers which crossed valleys and formed
lakes having been cut through by rivers, narrow gorges being the
consequence. One of the most famous examples of this kind is the
Finsteraarschlucht in the valley of Hasli. Here the ridge called the
Kirchet seems split across, and the river Aar rushes through the
fissure. Behind the barrier we have the meadows and pastures of Imhof
resting on the sediment of an ancient lake. Were this an isolated
case, one might reasonably conclude that the Finsteraarschlucht was
produced by an earthquake, as some suppose it to have been; but when
we find it to be a single sample of actions which are frequent in the
Alps--when probably a hundred cases of the same kind, though different
in magnitude, can be pointed out--it seems quite unphilosophical to
assume that in each particular case an earthquake was at hand to form
a channel for the river. As in the case of the barrier at Pontresina,
the Kirchet, after the retreat of the Aar glacier, dammed the waters
flowing from it, thus forming a lake, on the bed of which now stands
the village of Imhof. Over this barrier the Aar tumbled towards
Meyringen, cutting, as the centuries passed, its bed ever deeper, until
finally it became deep enough to drain the lake, leaving in its place
the alluvial plain, through which the river now flows in a definite
channel.[20]
[20] For further observations see p. 256.
But the broad view taken by the advocates of the fracture theory is,
that the valleys are the tracks of primeval fissures produced by the
upheaval of the land, and the cracks across the barriers to which I
have referred are in reality portions of the great cracks which formed
the valleys. Such an argument, however, would virtually concede the
theory of erosion as applied to the _valleys_ of the Alps. The narrow
gorges, often not more than twenty or thirty feet across, sometimes
even narrower, frequently occur at the bottom of broad valleys. Such
fissures might enter into the list of accidents which gave direction to
the real erosive agents which scooped the valley out; but the formation
of the valley, as it now exists, could no more be ascribed to it than
the motion of a railway train could be ascribed to the finger of the
engineer which turns on the steam.
These deep gorges occur, I believe, for the most part in limestone
strata; and the effects which the merest driblet of water can produce
on such rocks are quite astonishing. It is not uncommon to meet chasms
of considerable depth produced by small streams the beds of which are
dry for a large portion of the year. Right and left of the larger
gorges such secondary chasms are usually to be found. The idea of
_time_ must, I think, be more and more included in our reasonings on
these phenomena. Happily, the marks which the rivers have, in most
cases, left behind them, and which refer, geologically considered, to
actions of yesterday, give us ground and courage to conceive what may
be effected in geologic periods. Thus the modern portion of the Via
Mala throws light upon the whole. Near Bergün, in the valley of the
Albula, there is also a little Via Mala, which is not less significant
than the great one. The river flows here through a profound limestone
gorge; but to the very edges of the gorge we have the evidences of
erosion. The most striking illustration of water-action upon limestone
rock which I have ever witnessed is, I think, furnished by the gorge at
Pfäffers. Here the traveller passes along the side of the chasm midway
between top and bottom. Whichever way he looks, backwards or forwards,
upwards or downwards, towards the sky or towards the river, he meets
everywhere the irresistible and impressive evidence that this wonderful
fissure has been sawn through the mountain by the waters of the Tamina.
I have thus far confined myself to the consideration of the gorges
formed by the cutting through of the rock-barriers which frequently
cross the valleys of the Alps; as far as I have examined them they are
the work of erosion. But the larger question still remains, To what
action are we to ascribe the formation of the valleys themselves? This
question includes that of the formation of the mountain-ridges, for
were the valleys wholly filled, the ridges would disappear. Possibly no
answer can be given to this question which is not beset with more or
less of difficulty. Special localities might be found which would seem
to contradict every solution which refers the conformation of the Alps
to the operation of a single cause.
Still the Alps present features of a character sufficiently definite
to bring the question of their origin within the sphere of close
reasoning. That they were in whole or in part once beneath the sea will
not be disputed; for they are in great part composed of sedimentary
rocks which required a sea to form them. Their present elevation
above the sea is due to one of those local changes in the shape of
the earth which have been of frequent occurrence throughout geologic
time, and which in some cases have depressed the land, and in others
caused the sea-bottom to protrude beyond its surface. Considering the
inelastic character of its materials, the protuberance of the Alps
could hardly have been pushed out without dislocation and fracture; and
this conclusion gains in probability when we consider the foldings,
contortions, and even reversals in position of the strata in many parts
of the Alps. Such changes in the position of beds which were once
horizontal could not have been effected without dislocation. Fissures
would be produced by these changes; and such fissures, the advocates of
the fracture theory contend, mark the positions of the valleys of the
Alps.
Imagination is necessary to the man of science, and we could not reason
on our present subject without the power of presenting mentally a
picture of the earth’s crust cracked and fissured by the forces which
produced its upheaval. Imagination, however, must be strictly checked
by reason and by observation. That fractures occurred cannot, I
think, be doubted, but that the valleys of the Alps are thus formed is
a conclusion not at all involved in the admission of dislocations. I
never met with a precise statement of the manner in which the advocates
of the fissure theory suppose the forces to have acted--whether they
assume a general elevation of the region, or a local elevation of
distinct ridges; or whether they assume local subsidences after a
general elevation, or whether they would superpose upon the general
upheaval minor and local upheavals.
In the absence of any distinct statement, I will assume the elevation
to be general--that a swelling out of the earth’s crust occurred here,
sufficient to place the most prominent portions of the protuberance
three miles above the sea-level. To fix the ideas, let us consider
a circular portion of the crust, say one hundred miles in diameter,
and let us suppose, in the first instance, the circumference of this
circle to remain fixed, and that the elevation was confined to the
space within it. The upheaval would throw the crust into a state of
strain; and, if it were inflexible, the strain must be relieved by
fracture. Crevasses would thus intersect the crust. Let us now enquire
what proportion the area of these open fissures is likely to bear to
the area of the unfissured crust. An approximate answer is all that
is here required; for the problem is of such a character as to render
minute precision unnecessary.
No one, I think, would affirm that the area of the fissures would be
one-hundredth the area of the land. For let us consider the strain upon
a single line drawn over the summit of the protuberance from a point on
its rim to a point opposite. Regarding the protuberance as a spherical
swelling, the length of the arc corresponding to a chord of 100 miles
and a versed sine of 3 miles is 100.24 miles; consequently the surface
to reach its new position must stretch 0.24 of a mile, or be broken. A
fissure or a number of cracks with this total width would relieve the
strain; that is to say, the sum of the widths of all the cracks over
the length of 100 miles would be 420 yards. If, instead of comparing
the width of the fissures with the length of the lines of tension, we
compared their areas with the area of the unfissured land, we should of
course find the proportion much less. These considerations will help
the imagination to realise what a small ratio the area of the open
fissures must bear to the unfissured crust. They enable us to say, for
example, that to assume the area of the fissures to be one-tenth of
the area of the land would be quite absurd, while that the area of the
fissures could be one-half or more than one-half that of the land would
be in a proportionate degree unthinkable. If we suppose the elevation
to be due to the shrinking or subsidence of the land all round our
assumed circle, we arrive equally at the conclusion that the area of
the open fissures would be altogether insignificant as compared with
that of the unfissured crust.
To those who have seen them from a commanding elevation, it is needless
to say that the Alps themselves bear no sort of resemblance to the
picture which this theory presents to us. Instead of deep cracks with
approximately vertical walls, we have ridges before us running into
peaks, and gradually sloping to form valleys. Instead of a fissured
crust, we have a state of things closely resembling the surface of the
ocean when agitated by a storm. The valleys, instead of being much
narrower than the ridges, occupy the greater space. A plaster cast
of the Alps turned upside down, so as to invert the elevations and
depressions, would exhibit blunter and broader mountains, with narrower
valleys between them, than the present ones. The valleys that exist
cannot, I think, with any correctness of language be called fissures.
It may be urged that they originated in fissures: but even this is
unproved, and, were it proved, would still make the fissures play the
subordinate part of giving direction to the agents which are to be
regarded as the real sculptors of the Alps.
The fracture theory, then, if it regards the elevation of the Alps
as due to the operation of a force acting throughout the entire
region, is, in my opinion, utterly incompetent to account for the
conformation of the country. If, on the other hand, we are compelled
to resort to local disturbances, the manipulation of the earth’s
crust necessary to obtain the valleys and the mountains will, I
imagine, bring the difficulties of the theory into very strong relief.
Indeed an examination of the region from many of the more accessible
eminences--from the Galenstock, the Grauhaupt, the Pitz Languard,
the Monte Confinale--or, better still, from Mont Blanc, Monte Rosa,
the Jungfrau, the Finsteraarhorn, the Weisshorn, or the Matterhorn,
where local peculiarities are toned down, and the operations of the
powers which really made this region what it is are alone brought into
prominence--must, I imagine, convince every physically-minded man
of the inability of any fracture theory to account for the present
conformation of the Alps.
A correct model of the mountains, with an unexaggerated vertical
scale, produces the same effect upon the mind as the prospect from one
of the highest peaks. We are apt to be influenced by local phenomena
which, though insignificant in view of the general question of Alpine
conformation, are, with reference to our customary standards, vast and
impressive. In a true model those local peculiarities disappear; for
on the scale of a model they are too small to be visible; while the
essential facts and forms are presented to the undistracted attention.
A minute analysis of the phenomena strengthens the conviction which the
general aspect of the Alps fixes in the mind. We find, for example,
numerous valleys which the most ardent plutonist would not think of
ascribing to any other agency than erosion. That such is their genesis
and history is as certain as that erosion produced the Chines in the
Isle of Wight. From these indubitable cases of erosion--commencing,
if necessary, with the small ravines which run down the flanks of the
ridges, with their little working navigators at their bottoms--we can
proceed, by almost insensible gradations, to the largest valleys of the
Alps; and it would perplex the plutonist to fix upon the point at which
fracture begins to play a material part.
In ascending one of the larger valleys, we enter it where it is wide
and where the eminences are gentle on either side. The flanking
mountains become higher and more abrupt as we ascend, and at length we
reach a place where the depth of the valley is a maximum. Continuing
our walk upwards, we find ourselves flanked by gentler slopes, and
finally emerge from the valley and reach the summit of an open col,
or depression in the chain of mountains. This is the common character
of the large valleys. Crossing the col, we descend along the opposite
slope of the chain, and through the same series of appearances in the
reverse order. If the valleys on both sides of the col were produced
by fissures, what prevents the fissure from prolonging itself across
the col? The case here cited is representative; and I am not acquainted
with a single instance in the Alps where the chain has been cracked in
the manner indicated. The cols are simply depressions, and in the case
of many of them the unfissured rock can be traced from side to side.
The typical instance just sketched follows as a natural consequence
from the theory of erosion. Before either ice or water can exert great
power as an erosive agent, it must collect in sufficient mass. On the
higher slopes and plateaus--in the region of cols--the power is not
fully developed; but lower down tributaries unite, erosion is carried
on with increased vigour, and the excavation gradually reaches a
maximum. Lower still the elevations diminish and the slopes become more
gentle; the cutting power gradually relaxes, until finally the eroding
agent quits the mountains altogether, and the grand effects which it
produced in the earlier portions of its course entirely disappear.
I have hitherto confined myself to the consideration of the broad
question of the erosion theory as compared with the fracture theory;
and all that I have been able to observe and think with reference
to the subject leads me to adopt the former. Under the term erosion
I include the action of water, of ice, and of the atmosphere,
including frost and rain. Water and ice, however, are the principal
agents, and which of these two has produced the greatest effect it
is perhaps impossible to say. Two years ago I wrote a brief note ‘On
the Conformation of the Alps,’[21] in which I ascribed the paramount
influence to glaciers. The facts on which that opinion was founded are,
I think, unassailable; but whether the conclusion then announced fairly
follows from the facts is, I confess, an open question.
[21] Phil. Mag. vol. xxiv. p. 169.
The arguments which have been thus far urged against the conclusion
are not convincing. Indeed, the idea of glacier erosion appears so
daring to some minds that its boldness alone is deemed its sufficient
refutation. It is, however, to be remembered that a precisely similar
position was taken up by many respectable people when the question of
ancient glacier extension was first mooted. The idea was considered
too hardy to be entertained; and the evidences of glacial action were
sought to be explained by reference to almost any process rather than
the true one. Let those who so wisely took the side of ‘boldness’ in
that discussion beware lest they place themselves, with reference to
the question of glacier erosion, in the position formerly occupied by
their opponents.
Looking at the little glaciers of the present day--mere pigmies as
compared to the giants of the glacial epoch--we find that from every
one of them issues a river more or less voluminous, charged with the
matter which the ice has rubbed from the rocks. Where the rocks are
of a soft character, the amount of this finely pulverised matter
suspended in the water is very great. The water, for example, of the
river which flows from Santa Catarina to Bormio is thick with it. The
Rhine is charged with this matter, and by it has so silted up the Lake
of Constance as to abolish it for a large fraction of its length. The
Rhone is charged with it, and tens of thousands of acres of cultivable
land are formed by it above the Lake of Geneva.
In the case of every glacier we have two agents at work--the ice
exerting a crushing force on every point of its bed which bears its
weight, and either rasping this point into powder or tearing it bodily
from the rock to which it belongs; while the water which everywhere
circulates upon the bed of the glacier continually washes the detritus
away and leaves the rock clean for further abrasion. Confining the
action of glaciers to the simple rubbing away of the rocks, and
allowing them sufficient time to act, it is not a matter of opinion,
but a physical certainty, that they will scoop out valleys. But the
glacier does more than abrade. Rocks are not homogeneous; they are
intersected by joints and places of weakness, which divide them into
virtually detached masses. A glacier is undoubtedly competent to root
such masses bodily away. Indeed the mere _à priori_ consideration of
the subject proves the competence of a glacier to deepen its bed.
Taking the case of a glacier 1,000 feet deep (and some of the older
ones were probably three times this depth), and allowing 40 feet of ice
to an atmosphere, we find that on every square inch of its bed such a
glacier presses with a weight of 375 lbs., and on every square yard
of its bed with a weight of 486,000 lbs. With a _vertical_ pressure
of this amount the glacier is urged down its valley by the pressure
from behind. We can hardly, I think, deny to such a tool a power of
excavation.
Before concluding these remarks, I refreshed my memory by a second
reading of the paper of Mr. John Ball, published in the ‘Philosophical
Magazine’ for February 1863. Mr. Ball’s great experience of the Alps
naturally renders everything he writes regarding them interesting. But
though I have attended to the suggestions contained in his paper, I
am unable to see the cogency of his arguments. An inspection of the
map of Switzerland, with reference to the direction of its valleys,
suggests to my mind no objection whatever to the theory of erosion.
The reperusal of his paper assured me that Mr. Ball had paid attention
to the formation of ancient lakes. He deems their beds a prominent
feature of Alpine valleys; and he considers the barriers which dammed
them up, and which were not removed by the ancient glaciers, as ‘a
formidable difficulty in the way of Prof. Tyndall’s bold hypothesis.’
‘Looking at the operation as a whole,’ writes Mr. Ball, ‘it is to me
quite inconceivable that a glacier should be competent to scoop out
valleys a mile or more in depth, and yet be unable to remove the main
inequalities from its own channel.’
To this I reply that a glacier _is_ competent to remove such barriers,
and they probably have been ground down in some cases thousands of
feet. But being of more resisting material than the adjacent rock, they
are not ground down to the level of that rock. Were its bed uniform in
the first instance, the glacier would, in my opinion, _produce_ the
inequalities which Mr. Ball thinks it ought to remove. I have recently
had the pleasure of examining some of these barriers in the company
of Mr. Ball; and to me they represented nothing more than the natural
accidents of the locality. It would, I think, be far more wonderful
to find the rocks of the Alps perfectly homogeneous, than to find
them exhibiting such variations of resistance to grinding down as are
actually observed.
The question of lake-basins is now in competent hands, and on its
merits I will offer no opinion. But I cannot help remarking that the
dams referred to by Mr. Ball furnish a conclusive reply to some of the
arguments which have been urged against Prof. Ramsay’s theory. These
barriers have been crossed by the ice, and many of them present steeper
gradients than Prof. Ramsay has to cope with in order to get his ice
out of his lake-basins. An inspection of the barriers shows that they
were incompetent to embay the ice: they are scarred and fluted from
bottom to top. When it is urged against Prof. Ramsay that a glacier
cannot drop into a hole 2,000 feet deep and get out again, the distance
ought to be stated over which these 2,000 feet have to be distributed.
A depression 2,000 feet deep, if only of sufficient length, would
constitute no material obstacle to the motion of a great glacier.
The retardation of a glacier by its bed has also been referred to as
proving its impotence as an erosive agent; but this very retardation is
in some measure an expression of the magnitude of the erosive energy.
Either the bed must give way, or the ice must slide over itself; and
to make ice slide over itself requires great power. We get some idea
of the crushing pressure which the moving glacier exercises against
its bed from the fact that the resistance, and the effort to overcome
it, are such as to make the upper layers of a glacier move bodily over
the lower ones--a portion only of the total motion being due to the
progress of the entire mass of the glacier down its valley.
The sudden bend in the valley of the Rhone at Martigny has also been
regarded as conclusive evidence against the theory of erosion. ‘Why,’
it has been asked, ‘did not the glacier of the Rhone go straight
forward instead of making this awkward bend?’ But if the valley be
a crack, why did the crack make this bend? The crack, I submit, had
at least as much reason to prolong itself in a straight line as the
glacier had. A statement of Sir John Herschel with reference to another
matter is perfectly applicable here: ‘A crack once produced has a
tendency to run--for this plain reason, that at its momentary limit,
at the point at which it has just arrived, the divellent force on the
molecules there situated is counteracted only by half of the cohesive
force which acted when there was no crack, viz. the cohesion of the
uncracked portion alone’ (‘Proc. Roy. Soc.’ vol. xii. p. 678). To
account then for the bend, the adherent of the fracture theory must
assume the existence of some accident which turned the crack at right
angles to itself; and he surely will permit the adherent of the erosion
theory to make a similar assumption.
The influence of small accidents on the direction of rivers is
beautifully illustrated in glacier streams, which are made to cut
either straight or sinuous channels by causes apparently of the
most trivial character. In his interesting paper ‘On the Lakes of
Switzerland,’ M. Studer also refers to the bend of the Rhine at Sargans
in proof that the river must there follow a pre-existing fissure. I
made a special expedition to the place in 1864; and though I felt that
M. Studer had good grounds for the selection of this spot, I was unable
to arrive at his conclusion as to the necessity of a fissure.
Again, in the interesting volume recently published by the Swiss
Alpine Club, M. Desor informs us that the Swiss naturalists who met
last year at Samaden visited the end of the Morteratsch glacier, and
there convinced themselves that a glacier had no tendency whatever to
imbed itself in the soil. I scarcely think that the question of glacier
erosion, as applied either to lakes or valleys, is to be disposed of so
easily. Let me record here my experience of the Morteratsch glacier.
I took with me in 1864 a theodolite to Pontresina, and while there
had to congratulate myself on the invaluable aid of my friend Mr.
Hirst, who in 1857 did such good service upon the Mer de Glace and its
tributaries. We set out three lines across the Morteratsch glacier,
one of which crossed the ice-stream near the well-known hut of the
painter Georgei, while the two others were staked out, the one above
the hut and the other below it. Calling the highest line A, the line
which crossed the glacier at the hut B, and the lowest line C, the
following are the mean hourly motions of the three lines, deduced from
observations which extended over several days. On each line eleven
stakes were fixed, which are designated by the figures 1, 2, 3, &c. in
the Tables.
_Morteratsch Glacier, Line_ A.
No. of Stake. Hourly Motion.
1 0.35 inch.
2 0.49 „
3 0.53 „
4 0.54 „
5 0.56 „
6 0.54 „
7 0.52 „
8 0.49 „
9 0.40 „
10 0.29 „
11 0.20 „
As in all other measurements of this kind, the retarding influence
of the sides of the glacier is manifest: the centre moves with the
greatest velocity.
_Morteratsch Glacier, Line_ B.
No. of Stake. Hourly Motion.
1 0.05 inch.
2 0.14 „
3 0.24 „
4 0.32 „
5 0.41 „
6 0.44 „
7 0.44 „
8 0.45 „
9 0.43 „
10 0.44 „
11 0.44 „
The first stake of this line was quite close to the edge of the
glacier, and the ice was thin at the place, hence its slow motion.
Crevasses prevented us from carrying the line sufficiently far across
to render the retardation of the further side of the glacier fully
evident.
_Morteratsch Glacier, Line_ C.
No of Stake. Hourly Motion.
1 0.05 inch.
2 0.09 „
3 0.18 „
4 0.20 „
5 0.25 „
6 0.27 „
7 0.27 „
8 0.30 „
9 0.21 „
10 0.20 „
11 0.16 „
Comparing the three lines together, it will be observed that the
velocity diminishes as we descend the glacier. In 100 hours the maximum
motion of the three lines respectively is as follows:
_Maximum Motion in 100 hours._
Line A 56 inches
„ B 45 „
„ C 30 „
This deportment explains an appearance which must strike every observer
who looks upon the Morteratsch from the Piz Languard, or from the new
Bernina Road. A medial moraine runs along the glacier, commencing as
a narrow streak, but towards the end the moraine extending in width,
until finally it quite covers the terminal portion of the glacier.
The cause of this is revealed by the foregoing measurements, which
prove that a stone on the moraine where it is crossed by the line A
approaches a second stone on the moraine where it is crossed by the
line C with a velocity of twenty-six inches per one hundred hours.
The moraine is in a state of longitudinal compression. Its materials
are more and more squeezed together, and they must consequently move
laterally and render the moraine at the terminal portion of the glacier
wider than above.
The motion of the Morteratsch glacier, then, diminishes as we descend.
The maximum motion of the third line is thirty inches in one hundred
hours, or seven inches a day--a very slow motion; and had we run a line
nearer to the end of the glacier, the motion would have been slower
still. At the end itself it is nearly insensible. Now I submit that
this is not the place to seek for the scooping power of a glacier. The
opinion appears to be prevalent that it is the snout of a glacier that
must act the part of ploughshare; and it is certainly an erroneous
opinion. The scooping power will exert itself most where the weight,
and consequently (other things being equal) the motion, is greatest.
A glacier’s snout often _rests upon_ matter which has been scooped
from the glacier’s bed higher up. I therefore do not think that the
inspection of what the end of a glacier does or does not accomplish can
decide this question.
The snout of a glacier is potent to remove anything against which it
can fairly abut; and this power, notwithstanding the slowness of the
motion, manifests itself at the end of the Morteratsch glacier. A
hillock, bearing pine-trees, was in front of the glacier when Mr. Hirst
and myself inspected its end; and this hillock is being bodily removed
by the thrust of the ice. Several of the trees are overturned; and in a
few years, if the glacier continues its reputed advance, the mound will
certainly be ploughed away.
I will here record a few other measurements executed on the Rosegg
glacier: the line was staked out across the trunk formed by the
junction of the Rosegg proper with the Tschierva glacier, a short
distance below the rocky promontory called Agaliogs.
_Rosegg Glacier._
No. of Stake. Hourly Motion.
1 0.01 inch.
2 0.05 „
3 0.07 „
4 0.10 „
5 0.11 „
6 0.13 „
7 0.14 „
8 0.18 „
9 0.24 „
10 0.23 „
11 0.24 „
This is an extremely slowly moving glacier; the maximum hardly amounts
to seven inches a day. Crevasses prevented us from continuing the line
quite across the glacier.
To return to the question of Alpine conformation: it stands, I think,
thus: We have, in the first place, great valleys, such as those of the
Rhine and the Rhone, which we might conveniently call valleys of the
first order. The mountains which flank these main valleys are also cut
by lateral valleys running into the main one, and which may be called
valleys of the second order. When these latter are examined, smaller
valleys are found running into them, which may be called valleys of
the third order. Smaller ravines and depressions, again, join the
latter, which may be called valleys of the fourth order, and so on
until we reach streaks and cuttings so minute as not to merit the name
of valleys at all. At the bottom of every valley we have a stream,
diminishing in magnitude as the order of the valley ascends, carving
the earth and carrying its materials to lower levels. We find that the
larger valleys have been filled for untold ages by glaciers of enormous
dimensions, always moving, grinding down and tearing away the rocks
over which they passed. We have, moreover, on the plains at the feet of
the mountains, and in enormous quantities, the very matter derived from
the sculpture of the mountains themselves.
The plains of Italy and Switzerland are cumbered by the _débris_ of
the Alps. The lower, wider, and more level valleys are also filled to
unknown depths with the materials derived from the higher ones. In
the vast quantities of moraine-matter which cumber many even of the
higher valleys we have also suggestions as to the magnitude of the
erosion which has taken place. This moraine-matter, moreover, can only
in small part have been derived from the falling of rocks _upon_ the
ancient glacier; it is in great part derived from the grinding and the
ploughing-out of the glacier itself. This accounts for the magnitude of
many of the ancient moraines, which date from a period when almost all
the mountains were covered with ice and snow, and when, consequently,
the quantity of moraine-matter derived from the naked crests cannot
have been considerable.
The erosion theory ascribes the formation of Alpine valleys to the
agencies here briefly referred to. It invokes nothing but true causes.
Its artificers are still there, though, it may be, in diminished
strength; and if they are granted sufficient time, it is demonstrable
that they are competent to produce the effects ascribed to them. And
what does the fracture theory offer in comparison? From no possible
application of this theory, pure and simple, can we obtain the slopes
and forms of the mountains. Erosion must in the long run be invoked,
and its power therefore conceded. The fracture theory infers from the
disturbances of the Alps the existence of fissures; and this is a
probable inference. But that they were of a magnitude sufficient to
determine the conformation of the Alps, and that they followed, as the
Alpine valleys do, the lines of natural drainage of the country, are
assumptions which do not appear to me to be justified either by reason
or by observation.
There is a grandeur in the secular integration of small effects implied
by the theory of erosion almost superior to that involved in the idea
of a cataclysm. Think of the ages which must have been consumed in the
execution of this colossal sculpture. The question may, of course, be
pushed to further limits. Think of the ages which the molten earth
required for its consolidation. But these vaster epochs lack sublimity
through our inability to grasp them. They bewilder us, but they fail
to make a solemn impression. The genesis of the mountains comes more
within the scope of the intellect, and the majesty of the operation is
enhanced by our partial ability to conceive it. In the falling of a
rock from a mountain-head, in the shoot of an avalanche, in the plunge
of a cataract, we often see more impressive illustrations of the power
of gravity than in the motions of the stars. When the intellect has
to intervene, and calculation is necessary to the building up of the
conception, the expansion of the feelings ceases to be proportional to
the magnitude of the phenomena.