the single road in Glen Gluoy and the highest road in Glen Roy. This
state of things would continue as long as the ice dam was sufficiently
high to dominate the cols at A and B; but when through change of
climate the gradually sinking dam reached, in succession, the levels
of these cols, the water would then begin to flow over the dam instead
of over the cols. Let us suppose the wasting of the ice to continue
until a connection was established between Glen Roy and Glen Glaster,
a common lake would then fill both these glens, the level of which
would be determined by that of the col c, over which the water would
pour for an indefinite period into Glen Spean. During this period the
second Glen Roy road and the highest road of Glen Glaster would be
formed. The ice subsiding still further, a connection would
eventually be established between Glen Roy, Glen Glaster, and the
upper part of Glen Spean. A common lake would fill all three glens,
the level of which would be that of the col D, over which for an
indefinite period the lake would pour its water. During this period
the lowest Glen Roy road, which is common also to Glen Glaster and
Glen Spean, would be formed. Finally, on the disappearance of the ice
from the lower part of Glen Spean the waters would flow down their
respective valleys as they do to-day.
Fig. 7
Reviewing our work, we find three considerable steps to have marked
the solution of the problem of the Parallel Roads of Glen Roy. The
first of these was taken by Sir Thomas Dick-Lauder, the second was the
pregnant conception of Agassiz regarding glacier action, and the third
was the testing and verification of this conception by the very
thorough researches of Mr. Jamieson. No circumstance or incident
connected with this discourse gives me greater pleasure than the
recognition of the value of these researches. They are marked
throughout by unflagging industry, by novelty and acuteness of
observation, and by reasoning power of a high and varied kind. These
pages had been returned 'for press' when I learned that the relation
of Ben Nevis and his colleagues to the vapour-laden winds of the
Atlantic had not escaped Mr. Jamieson. To him obviously the
exploration of Lochaber, and the development of the theory of the
Parallel Roads, has been a labour of love.
Thus ends our rapid survey of this brief episode in the physical
history of the Scottish hills,--brief, that is to say, in comparison
with the immeasurable lapses of time through which, to produce its
varied structure and appearances, our planet must have passed. In the
survey of such a field two things are specially worthy to be taken
into account--the widening of the intellectual horizon and the
reaction of expanding knowledge upon the intellectual organ itself.
At first, as in the case of ancient glaciers, through sheer want of
capacity, the mind refuses to take in revealed facts. But by degrees
the steady contemplation of these facts so strengthens and expands the
intellectual powers, that where truth once could not find an entrance
it eventually finds a home. [Footnote: The formation, connection,
successive subsidence, and final disappearance of the glacial lakes of
Lochaber were illustrated in the discourse here reported by the model
just described, constructed under the supervision of my assistant, Mr.
John Cottrell. Glen Gluoy with its lake and road and the cataract
over its col; Glen Roy and its three roads with their respective
cataracts at the head of Glen Spey, Glen Glaster, and Glen Spean, were
all represented. The successive shiftings of the barriers, which were
formed of plate glass, brought each successive lake and its
corresponding road into view, while the entire removal of the barriers
caused the streams to flow down the glens of the model as they flow
down the real glens of to-day.]
A map of the district, with the parallel roads shown in red, is
annexed.
LITERATURE OF THE SUBJECT.
THOMAS PENNANT.--A Tour in Scotland. Vol. iii. 1776, p. 394. JOHN
MACCULLOCH.--On the Parallel Roads of Glen Roy. Geol. Soc. Trans.
vol. iv. 1817, p. 314.
THOMAS LAUDER DICK (afterwards SIR THOMAS DICK-LAUDER, Bart.)--On the
Parallel Roads of Lochaber. Edin. Roy. Soc. Trans. 1818, vol. ix.
p. 1.
CHARLES DARWIN.--Observations on the Parallel Roads of Glen Roy, and
of the other parts of Lochaber in Scotland, with an attempt to prove
that they are of marine origin. Phil. Trans. 1839, vol. cxxix. p. 39.
SIR CHARLES LYELL.--Elements of Geology. Second edition, 1841.
Louis AGASSIZ.--The Glacial Theory and its Recent Progress--Parallel
Terraces. Edin. New Phil. Journal, 1842, vol. xxxiii. p. 236.
DAVID MILNE (afterwards DAVID MILNE-HOME).--On the Parallel Roads of
Lochaber; with Remarks on the Change of Relative Levels of Sea and
Land in Scotland, and on the Detrital Deposits in that Country. Edin.
Roy. Soc. Trans. 1847, vol. xvi. p. 395.
ROBERT CHAMBERS.--Ancient Sea Margins. Edinburgh, 1848.
H. D. ROGERS.--On the Parallel Roads of Glen Roy. Royal Inst.
Proceedings, 1861, vol. iii. p. 341.
THOMAS F. JAMIESON.--On the Parallel Roads of Glen Roy, and their
Place in the History of the Glacial Period. Quart. Journal Geol.
Soc. 1863, vol. xix. p. 235.
SIR CHARLES LYELL.--Antiquity of Man. 1863, p. 253.
REV. R. B. WATSON.--On the Marine Origin of the Parallel Roads of Glen
Roy. Quart. Journ. Geol. Soc. 1865, vol. xxii. p. 9.
SIR JOHN LUBBOCK.--On the Parallel Roads of Glen Roy. Quart. Journ.
Geol. Soc. 1867, vol. xxiv. p. 83.
CHARLES BABBAGE.--Observations on the Parallel Roads of Glen Roy.
Quart. Journ. Geol. Soc. 1868, vol. xxiv. p. 273.
JAMES NICOL.--On the Origin of the Parallel Roads of Glen Roy. 1869.
Geol. Soc. Journal, vol. xxv. p. 282.
JAMES NICOL.--How the Parallel Roads of Glen Roy were formed. 1872.
Geol. Soc. Journal, vol. xxviii. p. 237.
MAJOR-GENERAL SIR HENRY JAMES, R.E.--Notes on the Parallel Roads of
Lochaber. 4to. 1874.
********************
IX. ALPINE SCULPTURE.
1864.
TO account for the conformation of the Alps, two hypotheses have been
advanced, which may be respectively named the hypothesis of fracture
and the hypothesis of erosion. The former assumes that the forces by
which the mountains were elevated produced fissures in the earth's
crust, and that the valleys of the Alps are the tracks of these
fissures; while the latter maintains 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. I had heard the Via Mala
cited as a conspicuous illustration of the fissure theory--the
profound chasm thus named, and through which the Hinter-Rhein now
flows, could, it was alleged, be nothing else than a crack in the
earth's crust. To the Via Mala I therefore went in 1864 to instruct
myself upon the point in question.
The gorge commences about a quarter of an hour above Tusis; and, on
entering it, the first impression certainly 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 thunder-storms; 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 on 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. Water, however, 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 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. 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 once 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 mountains 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 were
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 level meadow, which 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 indeed 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 the 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, particularly
when charged with solid matter in suspension, can cut a channel
through unfissured rock. 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 on a small scale 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 fissures 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 Hash. 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 with an apparent show of reason 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.
In 1866 I subjected the Finsteraarschlucht to a close examination. The
earthquake theory already adverted to was then prevalent regarding it,
and I wished to see whether any evidences existed of aqueous erosion.
Near the summit of the Kirchet is a signboard inviting the traveller
to visit the Aarenschlucht, a narrow lateral gorge which runs down to
the very bottom of the principal one. The aspect of this smaller
chasm from bottom to top proves to demonstration that water had in
former ages been there at work. It is scooped, rounded, and polished,
so as to render palpable to the most careless eye that it is a gorge
of erosion. But it was regarding the sides of the great chasm that
instruction was needed, and from its edge nothing to satisfy me could
be seen. I therefore stripped and waded into the river until a point
was reached which commanded an excellent view of both sides of the
gorge. The water was cutting cold, but I was repaid. Below me on the
left-hand side was a jutting cliff which bore the thrust of the river
and caused the Aar to swerve from its direct course. From top to
bottom this cliff was polished, rounded, and scooped. There was no
room for doubt. The river which now runs so deeply down had once been
above. It has been the delver of its own channel through the barrier
of the Kirchet.
But the broad view taken by the advocates of the fracture theory is,
that the valleys themselves follow the tracks of primeval fissures
produced by the upheaval of the land, the cracks across the barriers
referred to being 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 such cracks 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 limestone 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 often 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, and to the very edges of the gorge we have the evidences of
erosion. But the most striking illustration of water-action upon
limestone rock that I have ever seen is the gorge at Pfaeffers. 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 they have been examined by me
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, in some cases depressing the land, and in others
causing 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.