Dick-Lauder invoked a new agency. He supposed that at a certain point
in the breaking down or waste of his dam, a halt occurred, the barrier
holding its ground at a particular level sufficiently long to dam a
lake rising to the height of, and forming the second road. This point
of weakness was at once detected by Mr. Darwin, and adduced by him as
proving that the levels of the cols did not constitute an essential
feature in the phenomena of the parallel roads. Though not destroyed,
Sir Thomas Dick-Lauder's theory was seriously shaken by this argument,
and it became a point of capital importance, if the facts permitted,
to remove such source of weakness. This was done in 1847 by Mr. David
Milne, now Mr. Milne-Home. On walking up Glen Roy from Roy Bridge, we
pass the mouth of a lateral glen, called Glen Glaster, running
eastward from Glen Roy. There is nothing in this lateral glen to
attract attention, or to suggest that it could have any conspicuous
influence in the production of the parallel roads. Hence, probably,
the failure of Sir Thomas Dick-Lauder to notice it. But Mr.
Milne-Home entered this glen, on the northern side of which the middle
and lowest roads are fairly shown. The principal stream running
through the glen turns at a certain point northwards and loses itself
among hills too high to offer any outlet. But another branch of the
glen turns to the south-east; and, following up this branch, Mr.
Milne-Home reached a col, or watershed, of the precise level of the
second Glen Roy road. When the barrier blocking the glens had been so
far removed as to open this col, the water in Glen Roy would sink to
the level of the second road. A new lake of diminished depth would be
thus formed, the surplus water of which would escape over the Glen
Glaster col into Glen Spean. The margin of this new lake, acting upon
the detrital matter, would form the second road. The theory of Sir
Thomas Dick-Lauder, as regards the part played by the cols, was
re-riveted by this new and unexpected discovery.
I have referred to Mr. Darwin, whose powerful mind swayed for a time
the convictions of the scientific world in relation to this question.
His notion was--and it is a notion which very naturally presents
itself--that the parallel roads were formed by the sea; that this
whole region was once submerged and subsequently upheaved; that there
were pauses in the process of upheaval, during which these glens
constituted so many fiords, on the sides of which the parallel
terraces were formed. This theory will not bear close criticism; nor
is it now maintained by Mr. Darwin himself. It would not account for
the sea being 20 feet higher in Glen Gluoy than in Glen Roy. It would
not account for the absence of the second and third Glen Roy roads
from Glen Gluoy, where the mountain flanks are quite as impressionable
as in Glen Roy. It would not account for the absence of the shelves
from the other mountains in the neighbourhood, all of which 'would
have been clasped by the sea had the sea been there. Here then, and
no doubt elsewhere, Mr. Darwin has shown himself to be fallible; but
here, as elsewhere, he has shown himself equal to that discipline of
surrender to evidence which girds his intellect with such unassailable
moral strength.
But, granting the significance of Sir Thomas Dick-Lauder's facts, and
the reasonableness, on the whole, of the views which he has founded on
them, they will not bear examination in detail. No such barriers of
detritus as he assumed could have existed without leaving traces
behind them; but there is no trace left. There is detritus enough in
Glen Spean, but not where it is wanted. The two highest parallel
roads stop abruptly at different points near the mouth of Glen Roy,
but no remnant of the barrier against which they abutted is to be
seen. It might be urged that the subsequent invasion of the valley by
glaciers has swept the detritus away; but there have been no glaciers
in these valleys since the disappearance of the lakes. Professor
Geikie has favoured me with a drawing of the Glen Spean 'road' near
the entrance to Glen Trieg. The road forms a shelf round a great
mound of detritus which, had a glacier followed the formation of the
shelf, must have been cleared away. Taking all the circumstances into
account, you may, I think, with safety dismiss the detrital barrier as
incompetent to account for the present condition of Glen Gluoy and
Glen Roy.
Hypotheses in science, though apparently transcending experience, are
in reality experience modified by scientific thought and pushed into
an ultra experiential region. At the time that he wrote, Sir Thomas
Dick-Lauder could not possibly have discerned the cause subsequently
assigned for the blockage of these glens. A knowledge of the action
of ancient glaciers was the necessary antecedent to the new
explanation, and experience of this nature was not possessed by the
distinguished writer just mentioned. The extension of Swiss glaciers
far beyond their present limits, was first made known by a Swiss
engineer named Venetz, who established, by the marks they had left
behind them, their former existence in places which they had long
forsaken. The subject of glacier extension was subsequently followed
up with distinguished success by Charpentier, Studer, and others. With
characteristic vigour Agassiz grappled with it, extending his
observations far beyond the domain of Switzerland. He came to this
country in 1840, and found in various places indubitable marks of
ancient glacier action. England, Scotland, Wales, and Ireland he
proved to have once given birth to glaciers. He visited Glen Roy,
surveyed the surrounding neighbourhood, and pronounced, as a
consequence of his investigation, the barriers which stopped the glens
and produced the parallel roads to have been barriers of ice. To Mr.
Jamieson, above all others, we are indebted for the thorough testing
and confirmation of this theory.
And let me here say that Agassiz is only too likely to be misrated and
misjudged by those who, though accurate within a limited sphere, fail
to grasp in their totality the motive powers invoked in scientific
investigation. True he lacked mechanical precision, but he abounded
in that force and freshness of the scientific imagination which in
some sciences, and probably in some stages of all sciences, are
essential to the creator of knowledge. To Agassiz was given, not the
art of the refiner, but the instinct of the discoverer, and the
strength of the delver who brings ore from the recesses of the mine.
That ore may contain its share of dross, but it also contains the
precious metal which gives employment to the refiner, and without
which his occupation would depart.
Let us dwell for a moment upon this subject of ancient glaciers. Under
a flask containing water, in which a thermometer is immersed, is
placed a Bunsen's lamp. The water is heated, reaches a temperature of
212°, and then begins to boil. The rise of the thermometer then
ceases, although heat continues to be poured by the lamp into the
water. What becomes of that heat? We know that it is consumed in the
molecular work of vaporization. In the experiment here arranged, the
steam passes from the flask through a tube into a second vessel kept
at a low temperature. Here it is condensed, and indeed congealed to
ice, the second vessel being plunged in a mixture cold enough to
freeze the water. As a result of the process we obtain a mass of ice.
That ice has an origin very antithetical to its own character. Though
cold, it is the child of heat. If we removed the lamp, there would be
no steam, and if there were no steam there would be no ice. The mere
cold of the mixture surrounding the second vessel would not produce
ice. The cold must have the proper material to work upon; and this
material--aqueous vapour--is, as we here see, the direct product of
heat.
It is now, I suppose, fifteen or sixteen years since I found myself
conversing with an illustrious philosopher regarding that glacial
epoch which the researches of Agassiz and others had revealed. This
profoundly thoughtful man maintained the fixed opinion that, at a
certain stage in the history of the solar system, the sun's radiation
had suffered diminution, the glacial epoch being a consequence of this
solar chill. The celebrated French mathematician Poisson had another
theory. Astronomers have shown that the solar system moves through
space, and 'the temperature of space' is a familiar expression with
scientific men. It was considered probable by Poisson that our
system, during its motion, had traversed portions of space of
different temperatures; and that, during its passage through one of
the colder regions of the universe, the glacial epoch occurred.
Notions such as these were more or less current everywhere not many
years ago, and I therefore thought it worth while to show how
incomplete they were. Suppose the temperature of our planet to be
reduced, by the subsidence of solar heat, the cold of space, or any
other cause, say one hundred degrees. Four-and-twenty hours of such a
chill would bring down as, snow nearly all the moisture of our
atmosphere. But this would not produce a glacial epoch. Such an
epoch would require the long-continued generation of the material from
which the ice of glaciers is derived. Mountain snow, the nutriment of
glaciers, is derived from aqueous vapour raised mainly from the
tropical ocean by the sun. The solar fire is as necessary a factor in
the process as our lamp in the experiment referred to a moment ago.
Nothing is easier than to calculate the exact amount of heat expended
by the sun in the production of a glacier. It would, as I have
elsewhere shown, [Footnote: 'Heat a Mode of Motion,' fifth edition,
chap. vi: Forms of Water, sections 55 and 56.] raise a quantity of
cast iron five times the weight of the glacier not only to a white
heat, but to its point of fusion. If, as I have already urged,
instead of being filled with ice, the valleys of the Alps were filled
with white-hot metal, of quintuple the mass of the present glaciers,
it is the heat, and not the cold, that would arrest our attention and
solicit our explanation. The process of glacier making is obviously
one of distillation, in which the fire of the sun, which generates the
vapour, plays as essential a part as the cold of the mountains which
condenses it. [Footnote: In Lyell's excellent 'Principles of Geology,'
the remark occurs that 'several writers have fallen into the strange
error of supposing that the glacial period must have been one of
higher mean temperature than usual.' The really strange error was the
forgetfulness of the fact that without the heat the substance
necessary to the production of glaciers would be wanting.]
It was their ascription to glacier action that first gave the parallel
roads of Glen Roy an interest in my eyes; and in 1867, with a view to
self-instruction, I made a solitary pilgrimage to the place, and
explored pretty thoroughly the roads of the principal glen. I traced
the highest road to the col dividing Glen Roy from Glen Spey, and,
thanks to the civility of an Ordnance surveyor, I was enabled to
inspect some of the roads with a theodolite, and to satisfy myself
regarding the common level of the shelves at opposite sides of the
valley. As stated by Pennant, the width of the roads amounts
sometimes to more than twenty yards; but near the head of Glen Roy the
highest road ceases to have any width, for it runs along the face of a
rock, the effect of the lapping of the water on the more friable
portions of the rock being perfectly distinct to this hour. My
knowledge of the region was, however, far from complete, and nine
years had dimmed the memory even of the portion which had been
thoroughly examined. Hence my desire to see the roads once more
before venturing to talk to you about them. The Easter holidays of
1876 were to be devoted to this purpose; but at the last moment a
telegram from Roy Bridge informed me that the roads were snowed up.
Finding books and memories poor substitutes for the flavour of facts,
I resolved subsequently to make another effort to see the roads.
Accordingly last Thursday fortnight, after lecturing here, I packed
up, and started (not this time alone) for the North. Next day at noon
my wife and I found ourselves at Dalwhinnie, whence a drive of some
five-and-thirty miles brought us to the excellent hostelry of Mr.
Macintosh, at the mouth of Glen Roy.
We might have found the hills covered with mist, which would have
wholly defeated us; but Nature was good-natured, and we had two
successful working days among the hills. Guided by the excellent
ordnance map of the region, on the Saturday morning we went up the
glen, and on reaching the stream called Allt Bhreac Achaidh faced the
hills to the west. At the watershed between Glen Roy and Glen Fintaig
we bore northwards, struck the ridge above Glen Gluoy, came in view of
its road, which we persistently followed as long as it continued
visible. It is a feature of all the roads that they vanish before
reaching the cola over which fell the waters of the lakes which formed
them. One reason doubtless is that at their upper ends the lakes were
shallow, and incompetent on this account to raise wavelets of any
strength to act upon the mountain drift. A second reason is that they
were land-locked in the higher portions and protected from the
south-westerly winds, the stillness of their waters causing them to
produce but a feeble impression upon the mountain sides. From Glen
Gluoy we passed down Glen Turrit to Glen Roy, and through it
homewards, thus accomplishing two or three and twenty miles of rough
and honest work.
Next day we thoroughly explored Glen Glaster, following its two roads
as far as they were visible. We reached the col discovered by Mr.
Milne-Home, which stands at the level of the middle road of Glen Roy.
Thence we crossed southwards over the mountain _Creag Dhubh_, and
examined the erratic blocks upon its sides, and the ridges and mounds
of moraine matter which cumber the lower flanks of the mountain. The
observations of Mr. Jamieson upon this region, including the mouth of
Glen Trieg, are in the highest degree interesting. We entered Glen
Spean, and continued a search begun on the evening of our arrival at
Roy Bridge--the search, namely, for glacier polishings and markings.
We did not find them copious, but they are indubitable.
One of the proofs most convenient for reference, is a great rounded
rock by the roadside, 1,000 yards east of the milestone marked
three-quarters of a mile from Roy Bridge. Farther east other cases
occur, and they leave no doubt upon the mind that Glen Spean was at
one time filled by a great glacier. To the disciplined eye the aspect
of the mountains is perfectly conclusive on this point; and in no
position can the observer more readily and thoroughly convince himself
of this than at the head of Glen Glaster. The dominant hills here are
all intensely glaciated.
But the great collecting ground of the glaciers which dammed the glens
and produced the parallel roads, were the mountains south and west of
Glen Spean. The monarch of these is Ben Nevis, 4,370 feet high. The
position of Ben Nevis and his colleagues, in reference to the
vapour-laden winds of the Atlantic, is a point of the first
importance. It is exactly similar to that of Carrantual and the
Macgillicuddy Reeks in the south-west of Ireland. These mountains
are, and were, the first to encounter the south-western Atlantic
winds, and the precipitation, even at present, in the neighbourhood of
Killarney, is enormous. The winds, robbed of their vapour, and
charged with the heat set free by its precipitation, pursue their
direction obliquely across Ireland; and the effect of the drying
process may be understood by comparing the rainfall at Cahirciveen
with that at Portarlington. As found by Dr. Lloyd, the ratio is as 59
to 21--fifty-nine inches annually at Cahirciveen to twenty-one at
Portarlington. During the glacial epoch this vapour fell as snow, and
the consequence was a system of glaciers which have left traces and
evidences of the most impressive character in the region of the
Killarney Lakes. I have referred in other places to the great glacier
which, descending from the Reeks, moved through the Black Valley, took
possession of the lake-basins, and left its traces on every rock and
island emergent from the waters of the upper lake. They are all
conspicuously glaciated. Not in Switzerland itself do we find clearer
traces of ancient glacier action.
What the Macgillicuddy Reeks did in Ireland, Ben Nevis and the
adjacent mountains did, and continue to do, in Scotland. We had an
example of this on the morning we quitted Roy Bridge. From the bridge
westward rain fell copiously, and the roads were wet; but the
precipitation ceased near Loch Laggan, whence eastward the roads were
dry. Measured by the gauge, the rainfall Fort William is 86 inches,
while at Laggan it is only 46 inches annually. The difference between
west and east is forcibly brought out by observations at the two ends
of the Caledonian Canal. Fort William at the south-western end has,
as just stated, 86 inches, while Culloden, at its north-eastern end,
has only 24. To the researches of that able and accomplished
meteorologist, Mr. Buchan, we are indebted for these and other data of
the most interesting and valuable kind.
Adhering to the facts now presented to us, it is not difficult to
restore in idea the process by which the glaciers of Lochaber were
produced and the glens dammed by ice. When the cold of the glacial
epoch began to invade the Scottish hills, the sun at the same time
acting with sufficient power upon the tropical ocean, the vapours
raised and drifted on to these 'northern mountains were more and more
converted into snow. This slid down the slopes, and from every
valley, strath, and corry, south of Glen Spean, glaciers were poured
into that glen. The two great factors here brought into play are the
nutrition of the glaciers by the frozen material above, and their
consumption in the milder air below. For a period supply exceeded
consumption, and the ice extended, filling Glen Spean to an
ever-increasing height, and abutting against the mountains to the
north of that glen. But why, it may be asked, should the valleys
south of Glen Spean be receptacles of ice at a time when those north
of it were receptacles of water? The answer is to be found in the
position and the greater elevation of the mountains south of Glen
Spean. They first received the loads of moisture carried by the
Atlantic winds, and not until they had been in part dried, and also
warmed by the liberation of their latent heat, did these winds touch
the hills north of the Glen.
An instructive observation bearing upon this point is here to be
noted. Had our visit been in the winter we should have found all the
mountains covered; had it been in the summer we should have found the
snow all gone. But happily it was at a season when the aspect of the
mountains north and south of Glen Spean exhibited their relative
powers as snow collectors. Scanning the former hills from many points
of view, we were hardly able to detect a fleck of snow, while heavy
swaths and patches loaded the latter. Were the glacial epoch to
return, the relation indicated by this observation would cause Glen
Spean to be filled with glaciers from the south, while the hills and
valleys on the north, visited by warmer and drier winds, would remain
comparatively free from ice. This flow from the south would be
reinforced from the west, and as long as the supply was in excess of
the consumption the glaciers would extend, the dams which closed the
glens increasing in height. By-and-by supply and consumption becoming
approximately equal, the height of the glacier barriers would remain
constant. Then, as milder weather set in, consumption would be in
excess, a lowering of the barriers and a retreat of the ice being the
consequence. But for a long time the conflict between supply and
consumption would continue, retarding indefinitely the disappearance
of the barriers, and keeping the imprisoned lakes in the northern
glens. But however slow its retreat, the ice in the long run would be
forced to yield. The dam at the mouth of Glen Roy, which probably
entered the glen sufficiently far to block up Glen Glaster, would
gradually retreat. Glen Glaster and its col being opened, the
subsidence of the lake eighty feet, from the level of the highest to
that of the second parallel road, would follow as a consequence. I
think this the most probable course of things, but it is also possible
that Glen Glaster may have been blocked by a glacier from Glen Trieg.
The ice dam continuing to retreat, at length permitted Glen Roy to
connect itself with upper Glen Spean. A continuous lake then filled
both glens, the level of which, as already explained, was determined
by the col at Makul, above the head of Loch Laggan. The last to yield
was the portion of the glacier which derived nutrition from Ben Nevis,
and probably also from the mountains north and south of Loch Arkaig.
But it at length yielded, and the waters in the glens resumed the
courses which they pursue to-day.
For the removal of the ice barriers no cataclysm is to be invoked; the
gradual melting of the dam would produce the entire series of
phenomena. In sinking from col to col the water would flow over a
gradually melting barrier, the surface of the imprisoned lake not
remaining sufficiently long at any particular level to produce a shelf
comparable to the parallel roads. By temporary halts in the process
of melting due to atmospheric conditions or to the character of the
dam itself, or through local softness in the drift, small
pseudo-terraces would be formed, which, to the perplexity of some
observers, are seen upon the flanks of the glens to-day.
In presence then of the fact that the barriers which stopped these
glens to a height, it may be, of 1,500 feet above the bottom of Glen
Spean, have dissolved and left not a wreck behind; in presence of the
fact, insisted on by Professor Geikie, that barriers of detritus would
undoubtedly have been able to maintain themselves had they ever been
there; in presence of the fact that great glaciers once most certainly
filled these valleys--that the whole region, as proved by Mr.
Jamieson, is filled with the traces of their action; the theory which
ascribes the parallel roads to lakes dammed by barriers of ice has, in
my opinion, a degree of probability on its side which amounts to a
practical demonstration of its truth.
Into the details of the terrace formation I do not enter. Mr. Darwin
and Mr. Jamieson on the one side, and Sir John Lubbock on the other,
deal with true causes. The terraces, no doubt, are due in part to the
descending drift arrested by the water, and in part to the fretting of
the wavelets, and the rearrangement of the stirred detritus, along the
belts of contact of lake and bill. The descent of matter must have
been frequent when the drift was unbound by the rootlets which hold it
together now. In some cases, it may be remarked, the visibility of
the roads is materially augmented by differences of vegetation. The
grass upon the terraces is not always of the same character as that
above and below them, while on heather-covered hills the absence of
the dark shrub from the roads greatly enhances their conspicuousness.
The annexed sketch of a model will enable the reader to grasp the
essential features of the problem and its solution. Glen Gluoy and
Glen Roy are lateral valleys which open into Glen Spean. Let us
suppose Glen Spean filled from v to w with ice of a uniform elevation
of 1,500 feet above the sea, the ice not filling the upper part of
that glen. The ice would thrust itself for some distance up the
lateral valleys, closing all their mouths. The streams from the
mountains right and left of Glen Gluoy would pour their waters into
that glen, forming a lake, the level of which would be determined by
the height of the col at A, 1170 feet above the sea. Over this col
the water would flow into Glen Roy. But in Glen Roy it could not rise
higher than 1150 feet, the height of the col at B, over which it would
flow into Glen Spey.