There are phenomena in Nature which give the clue to so many of its
mysteries that their correct interpretation leads at once to the
broadest generalizations and to the rapid advance of science in new
directions. The explanation of one very local and limited problem may
clear up many collateral ones, since its solution includes the answer to
a whole set of kindred inquiries. The "parallel roads" of Glen Roy offer
such a problem. For half a century they have been the subject of patient
investigation and the boldest speculation. To them natural philosophers
have returned again and again to test their theories, and until they are
fully understood no steady or permanent advance can be made in the
various views which they have suggested to different observers. The
theory of the formation of lakes by barriers, presented by McCulloch and
Sir T. Lauder-Dick, that of continental upheavals and subsidences,
advocated by Sir Charles Lyell and Charles Darwin, that of inundations
by great floods, maintained by Professor H. D. Rogers and Sir George
Mackenzie, that of glacial action, brought forward by myself, have been
duly discussed with reference to this difficult case; all have found
their advocates, all have met with warm opposition, and the matter still
remains a mooted point; but the one of all these theories which shall
stand the test of time and repeated examination and be eventually
accepted will explain many a problem besides the one it was meant to
solve, and lead to farther progress in other directions.
I propose here to reconsider the facts of the case, and to present anew
my own explanation of them, now more than twenty years old, but which I
have never had an opportunity of publishing in detail under a popular
form, though it appeared in the scientific journals of the day.
Before considering, however, the phenomena of Glen Roy, or the special
glacial areas scattered over Scotland and the other British Isles, let
us see what general evidence we have that glaciers ever existed at all
in that realm. The reader will pardon me, if, at the risk of repetition,
I sum up here the indications which, from our knowledge of glaciers as
they at present exist, must be admitted, wherever they are found, as
proof of their former existence. Such a summary may serve also as a
guide to those who would look for glacial traces where they have not
hitherto been sought.
In the first place, we have to consider the singular abrasion of the
surfaces over which the glacier has moved, quite unlike that produced by
the action of water. We have seen that such surfaces, wherever the
glacier-marks have not been erased by some subsequent action, have
several unfailing characteristics: they are highly polished, and they
are also marked with scratches or fine _striæ_, with grooves and deeper
furrows. Where best preserved, the smooth surfaces are shining; they
have a lustre like stone or marble artificially polished by the combined
friction and pressure of some harder material than itself until all its
inequalities have been completely levelled and its surface has become
glossy. Any marble mantel-piece may serve as an example of this kind of
glacier-worn surface.
The levelling and abrading action of water on rock has an entirely
different character. Tides or currents driven powerfully and constantly
against a rocky shore, and bringing with them hard materials, may
produce blunt, smooth surfaces, such as the repeated blows of a hammer
on stone would cause; but they never bring it to a high polish, because,
the grinding materials not being held steadily down, in firm, permanent
contact with the rocky surfaces against which they move, as is the case
with the glacier, but, on the contrary, dashed to and fro, they strike
and rebound, making a succession of blows, but never a continuous,
uninterrupted pressure and friction. The same is true of all the marks
made on rocky shores against which loose materials are driven by
water-currents. They are separate, disconnected, fragmentary; whereas
the lines drawn by the hard materials set in the glacier, whether light
and fine or strong and deep, are continuous, often unbroken for long
distances, and rectilinear. Indeed, we have seen[A] that we have beneath
every glacier a complete apparatus adapted to all the results described
above. In the softer fragments ground to the finest powder under the
incumbent mass we have a polishing paste; in the hard materials set in
that paste, whether pebbles, or angular rocky fragments of different
sizes, or grains of sand, we have the various graving instruments by
which the finer or coarser lines are drawn. Not only are these lines
frequently uninterrupted for a distance of many yards, but they are also
parallel, except when some change takes place in the thickness of the
ice, which may slightly modify the trend of the mass, or where lines in
a variety of directions are produced by the intermittent action of
separate glaciers running successively at different angles over the same
surfaces. The deeper grooves sometimes present a succession of short
staccato touches, just as when one presses the finger vertically along
some surface where the resistance is sufficient to interrupt the action
without actually stopping it,--a kind of grating motion, showing how
firmly the instrument which produced it must have been held in the
moving mass. No currents or sudden freshets carrying hard materials with
them, even moving along straight paths down hill-sides or
mountain-slopes, have ever been known to draw any such lines. They could
be made only by some instrument held fast as in a vice by the moving
power. Something of the kind is occasionally produced by the drag of a
wheel grating over rocks covered with loose materials.
It has been said that grounded ice or icebergs floating along a rocky
shore might produce similar marks; but they will chiefly be at the level
of high-water mark, and, if grounded, they will trend in various
directions, owing to the rocking or rotating movement of the iceberg. It
has also been urged, that, without admitting any general glacier-period,
icebergs and floating ice from more northern latitudes might account for
the extensive transportation of the loose materials scattered in a
continuous sheet over a large portion of the globe. There can be no
doubt that an immense amount of _débris_ of all sorts is carried to
great distances by floating ice; where their presence is due to this
cause, however, they are everywhere stranded along the shore or dropped
to the sea-bottom. Large boulders are frequently left by the ice along
the New-England coast, and we shall trace them hereafter among the
sand-dunes of Cape Cod. But before it can be admitted that the
drift-phenomena, and the polished and engraved surfaces with which they
are everywhere intimately associated, are owing to floating ice or
icebergs, it must be shown that all these appearances have been produced
by some agency moving from the sea-board towards the land, and extending
up to the very summits of the mountains, or else that all the countries
exhibiting glacial phenomena have been sunk below the ocean to the
greatest height at which glacier-marks are found, and have since
gradually emerged to their present level. Now, though geologists are
lavish of immersions when something is to be accounted for which they
cannot otherwise explain, and a fresh baptism of old Mother Earth is
made to wash away many obstacles to scientific theories, yet the common
sense of the world will hardly admit the latter assumption without
positive proof, and all the evidence of the kind we have, at the period
under consideration, indicates only a comparatively slight change of
relative level between sea and land within a narrow belt along the
shores; and even this is shown to be posterior, not anterior, to the
glacial phenomena. As to the supposition that the motion proceeded from
the sea towards the land, all the facts are against it, since the whole
trend of these phenomena is from inland centres toward the shore,
instead of being from the coast upward.
Certainly, no one familiar with the facts could suppose that floating
ice or icebergs had abraded, polished, and furrowed the bottom of narrow
valleys as we find them worn, polished, and grooved by glaciers. And it
must be remembered that this is a theory founded not upon hypothesis,
but upon the closest comparison. I have not become acquainted with these
marks in regions where glaciers no longer exist, and made a theory to
explain their presence. I have, on the contrary, studied them where they
are in process of formation. I have seen the glacier engrave its lines,
plough its grooves and furrows in the solid rock, and polish the
surfaces over which it moved, and was familiar with all this when I
found afterwards appearances corresponding exactly to those which I had
investigated in the home of the present glaciers. I could therefore say,
and I think with some reason, that "this also is the work of the glacier
acting in ancient times as it now acts in Switzerland."
There is another character of glacial action distinguishing it from any
abrasions caused by water, even if freighted with a large amount of
loose materials. On any surface over which water flows we shall find
that the softer materials have yielded first and most completely. Hard
dikes will be left standing out, while softer rocks around them are worn
away,--furrows will be eaten into more deeply,--fissures will be
widened,--clay-slates will be wasted,--while hard sandstone or limestone
and granite will show greater resistance. Not so with surfaces over
which the levelling plough of the glacier has passed. Wherever softer
and harder rocks alternate, they are brought to one outline; where dikes
intersect softer rock, they are cut to one level with it; where rents or
fissures traverse the rock, they do not seem to have been widened or
scooped out more deeply, but their edges are simply abraded on one line
with the adjoining surfaces. Whatever be the inequality in the hardness
of the materials of which the rock consists, even in the case of
pudding-stone, the surface is abraded so evenly as to leave the
impression that a rigid rasp has moved over all the undulations of the
land, advancing in one and the same direction and levelling all before
it.
Among the inequalities of the glacier-worn surfaces which deserve
especial notice, are the so-called "_roches moutonnées_." They are
knolls of a peculiar appearance, frequent in the Alps, and first noticed
by the illustrious De Saussure, who designated them by that name,
because, where they are numerous and seen from a distance, they resemble
the rounded backs of a flock of sheep resting on the ground. These
knolls are the result of the prolonged abrasion of masses of rocks
separated by deep indentations wide enough to be filled up by large
glaciers, overtopping the summits of the intervening prominences, and
passing over them like a river, or like tide-currents flowing over a
submerged ledge of rock. It is evident that water rushing over such
sunken hills or ledges, adapting itself readily to all the inequalities
over which it flows, and forming eddies against the obstacles in its
course, will scoop out tortuous furrows upon the bottom, and hollow out
rounded cavities against the walls, acting especially along preëxisting
fissures and upon the softer parts of the rock,--while the glacier,
moving as a solid mass, and carrying on its under side its gigantic file
set in a fine paste, will in course of time abrade uniformly the angles
against which it strikes, equalize the depressions between the prominent
masses, and round them off until they present those smooth bulging
knolls known as the "_roches moutonnées_" in the Alps, and so
characteristic everywhere of glacier-action. A comparison of any
tide-worn hummock with such a glacier-worn mound will convince the
observer that its smooth and evenly rounded surface was never produced
by water.
Besides their peculiar form, the _roches moutonnées_ present all the
characteristic features of glacier-action in their polished surfaces
accompanied with the straight lines, grooves, and furrows above
described. But there are two circumstances connected with these knolls
deserving special notice. They frequently present the glacial marks only
on one side, while the opposite side has all the irregularities and
roughness of a hill-slope not acted upon by ice. It is evident that the
polished side was the one turned towards the advancing glacier, the side
against which the ice pressed in its onward movement,--while it passed
over the other side, the lee side as we may call it, without coming in
immediate contact with it, bridging the depression, and touching bottom
again a little farther on. As an additional evidence of this fact, we
frequently find on the lee side of such knolls accumulations of the
loose materials which the glacier carries with it. It is only, however,
when the knolls are quite high, and abrupt enough to allow any rigid
substance to bridge over the space in its descent from the summit to the
surface below, that we find these conditions; when the knolls are low
and slope gently downward in every direction, they present the
characteristic glacier-surfaces equally on all sides. This circumstance
should be borne in mind by all who investigate the traces of
glacier-action; for this inequality in the surfaces presented by the
opposite sides of any obstacle in the path of the ice is often an
important means of determining the direction of its motion.
The other characteristic peculiarity of these _roches moutonnées_
consists in the direction of the glacier-scratches, which ascend the
slope to its summit in a direct line on one side, while they deviate to
the right and left on the other sides of the knoll, more or less
obliquely according to its steepness. Occasionally, large boulders may
be found perched on the very summit of such prominences. Their position
is inexplicable by the supposition of currents as the cause of their
transportation. Any current strong enough to carry a boulder to such a
height would of course sweep it on with it. This phenomenon finds,
however, an easy explanation in the glacial theory. The thickness of
such a sheet of ice is of course less above such a hill or mound than
over the lower levels adjoining it. Not only will the ice melt,
therefore, more readily at this spot, but, as ice is transparent to
heat, the summit of the prominence will become warmed by the rays of the
sun, and will itself facilitate the melting of the ice above it. On the
breaking up of the ice, therefore, such a spot will be the first to
yield, and allow the boulders carried on the back of the glacier to fall
into the hollow thus formed, where they will rest upon the projecting
rock left uncovered. This is no theoretical explanation; there are such
cases in Switzerland, where holes in the ice are formed immediately
above the summit of hills or prominences over which the glacier passes,
and into which it drops its burdens. Of course, where the ice is
constantly renewed over such a spot by the onward progress of the
glacier, these materials may be carried off again; but if we suppose
such a case to occur at the breaking up of the glacier-period, when the
ice was disappearing forever from such a spot, it is easy to account for
the poising of these large boulders on prominent peaks or ledges.
The appearances about the _roches moutonnées_, especially the straight
scratches and grooves on the side up which the ice ascended, have led to
a mistaken view of the mode in which large boulders are transported by
ice. It has been supposed, by those who, while they accepted the glacial
theory, were not wholly conversant with the mode of action of glaciers,
that, in passing through the bottom of a valley, for instance, the
glacier would take up large boulders, and, carrying them along with it,
would push them up such a slope and deposit them on its summit. It is
true that large boulders may sometimes be found in front of glaciers
among the materials of their terminal moraines, and may, upon any
advance of the glacier, be pushed forward by it. But I know of no
example of erratic boulders being carried to considerable distances and
raised from lower to higher levels by this means. All the angular
boulders perched upon prominent rocks must have fallen upon the surface
of the glacier in the upper part of its course, where rocky ledges rise
above its surface and send down their broken fragments. The surface of
any boulder carried under the ice, or pushed along for any distance at
its terminus, would show the friction and pressure to which it had been
subjected. In this connection it should be remembered that in the case
of large glaciers low hills form no obstacle to their onward progress,
especially when the glacier is thick enough to cover them completely,
and even to rise far above them. The _roches moutonnées_ about the
Grimsel show that hills many hundred feet high have been passed over by
the great glacier of the Aar, when it descended as far as Meyringen,
without having seemingly influenced its onward progress.
But in enumerating the evidences of glacier-action, we have to remember
not only the effects produced upon the surface of the ground by the ice
itself, but also the deposits it has left behind it. The loose materials
scattered over the face of the earth may point as distinctly to the
source of their distribution as does the character of the rocky surfaces
on which they rest indicate the different causes of abrasion. In
characteristic localities the loose materials deposited by glaciers may
readily be recognized at first sight, and distinguished from water-worn
pebbles; nor is it difficult to distinguish both from loose materials
resulting from the decomposition of rocks on the spot,--the latter
always agreeing with the rocks on which they rest, while the
decomposition to which they owe their separation from the solid rock is
often still going on. Such _débris_ are found everywhere about
disintegrating rocks, and they constantly mingle with the loose
fragments brought from a distance by various agencies. They are found
upon and among the glacier-worn pebbles, especially where the latter
have themselves been disturbed since their accumulation. They are also
found among water-worn pebbles, wherever the rocky beds of our rivers or
the rocky bluffs of our sea-shores crumble down. In investigating the
character of loose materials transported from greater or less distances,
either by the agency of glaciers or by water-currents, it is important
at the very outset to discriminate between these deposits of older date
and the local accessions mingling with them.
Occasionally we may have also to distinguish between all these deposits
and the _débris_ brought down by land-slides, or by sudden freshets
transporting to a distance a vast amount of loose materials which are
neither ice-worn nor water-worn. At Rossberg, for instance, in the
Canton of Schwitz, the land-slide which buried the village of Goldau
under a terrific avalanche, and filled a part of the Lake of Lauertz,
spread an immense number of huge boulders across the valley, some of
which even rolled up the opposite side to a considerable height. Many of
these boulders might easily be mistaken for erratic boulders, were not
the aggregate of these loose materials traceable to the hills from which
they descended. In this case water had no part in loosening or bringing
down this mass of fragments. They simply rolled from the declivity, and
stopped when they had exhausted the momentum imparted to them by their
weight. In the case of the _débâcle_ of Bagnes, above Martigny, in a
valley leading to the St. Bernard, the circumstances were very
different. A glacier, advancing beyond its usual limits and rising
against the opposite mountain-slope, dammed up the waters of the torrent
and caused a lake to be formed. The obstruction gave way in the course
of time, and the waters of the lake rushed out, carrying along with
them huge boulders and a mass of loose materials of all sorts, and
scattering them over the plain below. Such an accumulation of _débris_
differs from the pebbles and loose fragments found in river-beds. The
comparatively short distance over which they are carried, and the
suddenness of the transportation, allow no time for the abrasion which
produces the smooth surfaces of water-worn pebbles or the polished and
scratched surfaces of glacier-worn ones. In the latter case, we have
seen that the pebbles, being so set in the ice as to expose only one
side, may be only partially polished, while others, more loosely held
and turning in their sockets, may receive the same high polish on every
side. In such a case the lines will intersect one another, in
consequence of the different position in which the stone has been held
at different times. No such appearances exist in the water-worn pebbles:
their blunt surfaces, smoothed and rounded uniformly by the action of
the water in which they have been rolled or tossed about, present
everywhere the same aspect.
The correlation between these different loose materials and the position
in which they are found helps us also to detect their origin. The loose
materials bearing glacier-marks are always found resting upon surfaces
which have been worn, abraded, and engraved in the same manner, while
the water-worn pebbles are everywhere found resting upon rocks the
abrasion of which may be traced to water. It is true that in some
localities, as, for instance, in the gravel-pit of Mount Auburn, near
Cambridge, large masses of glacier-worn pebbles alternate with
beach-shingle; but it is easy to show that there was here a glacier
advancing into the sea, crowding its front moraine and the materials
carried under it over and into the shingle washed up by the waves upon
the beach. Not infrequently, also, river-pebbles may be found among
glacial materials. This is especially the case where, after the
disappearance of large glaciers, rivers have occupied their beds.
Examples of this kind may be seen in all the valleys of the Alps.
But, besides the special character of the individual fragments, the true
origin of any accumulation of glacier-_débris_, commonly called drift,
may be detected by the total absence of stratification, so essential a
feature in all water-deposits. This absence of stratification throughout
its mass is, after all, the great and important characteristic of the
drift; and though I have alluded to it before, I reiterate it here, as
that which distinguishes it from all like accumulations under water. I
may be pardoned for dwelling upon this point, because the great
controversy among geologists respecting the nature and origin of the
sheet of loose materials scattered over a great part of the globe turns
upon it. The _débris_ of which the drift consists are thrown together
pell-mell, without any arrangement according to size or weight, larger
and smaller fragments being mixed so indiscriminately that the heaviest
materials may be on the very summit of the mass, and the lightest at the
bottom in immediate contact with the underlying rock, or the larger
pieces may stand at any level in the mass of finer ones. Impalpable
powder, coarse sand, rounded, polished, and scratched fragments of every
size are mixed together in a homogeneous paste, in which the larger
materials are imbedded, to use a homely, but expressive comparison, like
raisins and currants in a pudding. The adhesive paste holding all these
fragments together is, no doubt, the result of the friction to which the
whole was subjected under the glacier, and which has worked some of the
softer materials into a kind of cement.
The mode of aggregation of water-worn materials is very different.
Examine the shingle along our beaches: we find it so distributed as to
show that the fading tide-wave has carried the lighter materials farther
than the heavier ones, and the successive deposits exhibit an imperfect
cross-stratification resulting from changes in the height of the tide
and the direction of the wind. Moreover, in any materials collected
under water we find the heavier ones at the bottom, the lighter on the
top. It is true that large angular boulders may occasionally be found
resting upon beach-shingle, but their presence in such a connection is
easily explained. They may have been dropped there by floating icebergs,
or have fallen from crumbling drift-cliffs.
I should add, in speaking of drift-materials, that, while we find the
large angular boulders resting above them, we occasionally find boulders
of unusual size mingled with them; but, when this is the case, such
massive fragments are more or less rounded, polished, and marked in the
same way as the smaller pebbles, or as the surfaces over which the
glacier has passed. This is important to remember, because, when we
examine the drift in countries where the ice, during the glacier-period,
overtopped nearly all the mountains, so that few fragments could fall
from them upon its surface, we find scarcely any angular boulders, while
the drift is interspersed with larger fragments of this character,
carried under the ice, instead of on its back. Another distinction
between water-worn deposits and drift consists in the fact that the
former are washed clean, while the latter always retains the mud
gathered during its journey and spread throughout its mass.
In summing up the glacial evidences, I must not omit the moraines,
though I have described them so fully in a previous article that I need
not do more than allude to them here; but any argument for the glacial
theory which did not include these characteristic walls erected by
glaciers would be most imperfect. We need hardly discuss the theory of
currents with reference to the formation of terminal moraines, extending
across the valleys from side to side. Any current powerful enough to
bring the boulders and _débris_ of all sorts of which these walls are
composed to the places where they are found would certainly not build
them up with such regularity, but would sweep them away or scatter them
along the bottom of the valley. That this is actually the case is seen
in the lower course of the valley of the Rhone, where there are no
transverse moraines, while they are frequent and undisturbed in the
upper part of the valley. This is no doubt owing to the fact, that, when
the main glacier had already retreated considerably up the valley, the
lateral glaciers from the chains of the Combin and the Diablerets still
reached the valley of the Rhone at a lower point, and barred the outlet
of the waters from the glaciers above. A lake was thus formed, which,
when the lower glaciers retreated up the lateral valleys, swept away all
the lower transverse moraines, and formed the flat bottom of Martigny.
In this case, the moraines were totally obliterated; but there are many
other instances in which the materials have been only broken up and
scattered over a wider surface by currents. In such remodelled moraines,
the glacier-mud has, of course, been more or less washed away. We have
here a blending of the action of water with that of the glacier; and,
indeed, how could it be otherwise, when the colossal glaciers of past
ages gradually disappeared or retreated to the mountain-heights? The
wasting ice must have occasioned immense freshets, the action of which
we shall trace hereafter, when examining the formation of our
drift-ponds, of our river-beds and estuaries, as well as the
river-terraces standing far above the present water-level.
And now, if it be asked how much of this evidence for the former
existence of glaciers is to be found in Great Britain, I answer, that
there is not a valley in Switzerland where all these traces are found in
greater perfection than in the valleys of the Scotch Highlands, or of
the mountains of Ireland and Wales, or of the lake-region in England.
Not a link is wanting to the chain. Polished surfaces, traversed by
striæ, grooves, and furrows, with a sheet of drift resting immediately
upon them, extend throughout the realm,--the _roches moutonnées_
raise their rounded backs from the ground there as in
Switzerland,--transverse moraines bar their valleys and lateral ones
border them, and the boulders from the hill-sides are scattered over the
plains as thickly as between the Alps and the Jura, and are here and
there perched upon the summits of isolated hills. This being the case,
let us examine a little more closely the local phenomena connected with
the ancient extension of glaciers in this region, and especially the
parallel roads of Glen Roy.
[Illustration:
G. R. Glen Roy.
M. Moeldhu Hill.
S. Spean River.
G. S. Glen Spean.
L. Loch Laggan.
T. Loch Treig.
G. Glen Gloy.
L. O. Loch Lochy.
A. Loch Arkeig.
E. Loch Eil.
N. Ben Nevis.
1,2,3. The three parallel roads.]
Among the Grampian Hills, a little to the northeast of Ben Nevis, lies
the valley of Glen Roy, a winding valley trending in a northeasterly
direction, and some ten miles in length. Across the mouth of this
valley, at right angles with it, runs the valley of Glen Spean, trending
from east to west, Glen Roy thus opening directly at its southern
extremity into Glen Spean. Around the walls of the Glen Roy valley run
three terraces, one above the other, at different heights, like so many
roads artificially cut in the sides of the valley, and indeed they go by
the name of the "parallel roads." These three terraces, though in a less
perfect state of preservation, are repeated for a short distance at
exactly the same levels on the southern wall of the valley of Glen
Spean, just opposite the opening of the Glen Roy valley; that is, they
make the whole circuit of Glen Roy, stop abruptly, on both sides, at its
southern extremity, and reappear again on the opposite wall of Glen
Spean. I should add, however, that all three do not come to this sudden
termination; for the lowest of these terraces turns eastward into the
valley of Glen Spean, following the whole curve of the eastern half of
the valley, while, of the two upper terraces, there is no trace
whatever, nor is there any indication that either of the three ever
existed in the western half of the valley. When I first visited the
region, these phenomena had already been the subject of earnest
discussion among English geologists. The commonly accepted explanation
of the facts was that these terraces marked ancient sea-levels at a time
when the ocean penetrated much farther into the interior, and Glen Roy
and the adjoining valleys were as many fiords or estuaries. And though
the present elevation of the locality made such an interpretation
improbable at first sight, the first or highest of the terraces being
eleven hundred and forty-four feet above the present sea-level, the
second eighty-two feet below the first, and the third and lowest two
hundred and twelve feet below the second, or eight hundred odd feet
above the level of the sea, it was thought that the oscillations of the
land, its alternate subsidences and upheavals, proved by the modern
results of geology to have been so great and so frequent, might account
even for so remarkable a change. There are, however, other objections to
this theory not so easily explained away. There are no traces of organic
life upon these terraces. If they were ancient sea-beaches, we should
expect to find upon them the remains of marine animals, shells,
crustacea, and the like. All the explanations given to lessen the
significance of this absence of organic remains are futile. Again, why
should the lower terrace alone be continued into the eastern end of the
valley of Glen Spean, while there are no terraces at all in its western
part, since both must have been as fully open to the sea as Glen Roy
valley itself? This seemed the more inexplicable since all the terraces
exist on the valley-wall opposite the outlet of Glen Roy, showing that
this sheet of water, wherever it came from, filled the valley itself and
the space between it and the southern wall of Glen Spean, but failed to
spread, on either side of that space, into the eastern and western
extension of Glen Spean. It is evident, that, at the time the water
filled Glen Roy, some obstruction blocked the valley of Glen Spean, both
to the east and west, leaving, however, that space in the centre free
into which Glen Roy opens, while, by the time the water had sunk to the
level of the lowest terrace, one of these barriers, that to the east,
must have been removed, for the lowest terrace, as I have said, is
continuous throughout the eastern part of Glen Spean.[B]
Prepossessed as I was with the idea of glacial agency in times anterior
to ours, these phenomena appeared to me under a new aspect. I found the
bottom of Glen Spean so worn by glacial action as to leave no doubt in
my mind that it must have been the bed of a great glacier, and Dr.
Buckland fully concurred with me in this impression. Indeed, the face of
the country throughout that region presents not only the glacier-marks
in great perfection, but other evidences of the ancient presence of
glaciers. There are moraines at the lower end of Glen Spean, remodelled,
it is true, by the action of currents, but still retaining enough of
their ancient character to be easily recognized; and some of the finest
examples of the _roches moutonnées_ I have seen in Scotland are to be
found at the entrance of the valley of Loch Treig, a lateral valley
opening into Glen Spean on its southern side, and, as we shall see
hereafter, intimately connected with the history of the parallel roads
of Glen Roy. These _roches moutonnées_ may very fairly be compared with
those of the Grimsel, and exhibit all the characteristic features of the
Alpine ones. One of them, lying on the western side of the valley where
it opens into Glen Spean, is crossed by a trap-dike. The general surface
of the hill, consisting of rather soft mica, has been slightly worn down
by atmospheric agencies, so that the dike stands out some three-quarters
of an inch above it. On the dike, however, the glacier-marks extend for
its whole length in great perfection, while they have entirely
disappeared from the surrounding surfaces, so as to leave the dike thus
standing out in full relief. This is an instructive case, showing how
little disintegration has gone on since the drift-period. All the
currents that have swept over it, all the rains that have beaten upon
it, have not worn away one inch from the original surface of the hill. I
have observed many other _roches moutonnées_ in Scotland, especially
about the neighborhood of Loch Awe, Loch Fyne, and Loch Etive. In fact,
they may be found in almost all the glens of Scotland, in the
lake-region of England, and in the valleys of Wales and Ireland.
Following the glacial indications wherever we could find them in the
country about Glen Roy, it became evident to me that the whole western
range of the Grampian Hills had once been a great centre of glaciers,
that they had come down toward Glen Spean through all the valleys on the
mountain-slopes to the north and south of it, so that this valley had
become, as it were, the great drainage-bed for the masses of ice thus
poured into it laterally, and moving down the valley from east to west
as one immense glacier. It is natural to suppose, that, at the
breaking-up of the great sheet of ice which, if my view of the case is
correct, must have covered the whole country at this time, the ice would
yield more readily in a valley like that of Glen Roy, lying open to the
south and receiving the full force of the sun, than in those on the
opposite side of Glen Spean, opening to the north. At all events, it is
evident that at some time posterior to this universal glacial period,
when the ice began to retreat, Glen Roy became the basin of a glacial
lake such as we now find in the Alps of Switzerland, where occasionally
a closed valley becomes a trough, as it were, into which the water from
the surrounding hills is drained. In such a lake no animals are found,
such as exist in any other sheet of fresh water, and this would account
for the absence of any organic remains on the terraces of Glen Roy. But
at first sight it seemed that this theory was open in one respect to the
same objection as the other. What prevented this sheet of water from
spreading east and west in Glen Spean? If it not only filled Glen Roy,
but extended to the southern side of Glen Spean immediately opposite
the opening of Glen Roy, what prevented it from filling the whole of
that valley also? In endeavoring to answer this question, I found the
solution of the mystery.
The bed of Glen Spean, through its whole extent from east to west, is
marked, as I have said, by glacial action, in rectilinear scratches and
furrows. This westward track of the main glacier is crossed transversely
near the centre of the valley by two other glacier-tracks cutting it at
right angles. Upon tracing these cross-tracks carefully, I became
satisfied, that, after the surrounding ice had begun to yield, after the
masses of ice which descended from the northern and southern slopes of
the mountains into Glen Spean had begun to retreat, and to form local
limited glaciers, two of those lateral glaciers, one coming down from
Ben Nevis on the southwest, the other from Loch Treig on the southeast,
extended farther than the others and stretched across Glen Spean.[C]
These two glaciers for a long time formed barriers across the western
and eastern extension of this valley, damming back the waters which
filled Glen Roy and the central part of Glen Spean.
Evidently the glacier descending from Loch Treig was the first to yield,
for, by the time the Glen Roy lake had sunk to the level of the lowest
terrace, the entrance to the eastern extension of the valley must have
been free, otherwise the water could not have spread throughout that
basin as we find it did; but it would seem that by the time the western
barrier, or the glacier from Ben Nevis, was removed, the sheet of water
was too far reduced to have left permanent marks of its outflow into the
Great Glen, except by disturbing and remodelling the large moraines of
the older Glen Spean glacier. There are faint indications of other
terraces in Glen Roy, even at a higher level than the uppermost parallel
road, owing their origin probably to the short duration of a higher
level of the glacier-lake, when the great general glacier had not yet
been lowered to a more permanent level determined by a limited
circumscription within the walls of the valleys. There are other
terraces in neighboring valleys at still different levels,--in Glen
Gloy, for instance, where the one horizontal road was no doubt formed in
consequence of the damming of the valley by a glacier from Loch Arkeig.
Mr. Darwin has seen another in Glen Kinfillen, which I would explain by
the presence of a glacier in the Great Glen, the marks of which are
particularly distinct about the eastern end of Glen Garry.
The evidence of the ancient presence of glaciers is no less striking in
other parts of the Scotch Highlands. Between the southeastern range of
the Grampian Hills, in Forfarshire and Perthshire, and the opposite
ridge of Sidlaw Hills, stretches the broad valley of Strathmore. At the
time when Glen Spean received the masses of ice from the slopes of the
western Grampian range, the glaciers descended from the valleys on the
southern slope of the southeastern range and from those on the northern
slope of Sidlaw Hills into the capacious bed of the valley which divides
them. The glacial phenomena of this region present a striking
resemblance in their general relations to those of the Alps and the
Jura. The Grampian range on the northern side of Strathmore valley
occupies the same position in reference to that of the Sidlaw Hills
opposite, as does the range of the Alps to that of the Jura, while the
intervening valley may be compared to the plain of Switzerland. As from
the Bernese Oberland and from the valleys of the Reuss and Limmath
gigantic glaciers came down and stretched across the plain of
Switzerland to the Jura, scattering their erratic boulders over its
summit and upon its slopes at the time of their greater extension, and,
as they withdrew into the higher Alpine valleys, leaving them along
their retreating track at the foot of the Jura and over the whole plain,
so did the glaciers from Glen Prossen and parallel valleys on the
Grampian Mountains extend across the valley of Strathmore, dropping
their boulders not only on the slopes and along the base of the Sidlaw
Hills, but scattering them in their retreat throughout the valley, until
they were themselves reduced to isolated glaciers in the higher valleys.
At the same time other glaciers came down from the heights of
Schihallion on the west, and, descending through the valley of the Tay,
joined the great masses of ice in the valley of Strathmore, thus
combining with the eastern ice-field, just as the glacier from Mont
Blanc and the valley of the Rhone formerly combined in the western part
of Switzerland with those of the Bernese Oberland. The relations are
identical, though the geographical position is reversed,--the higher
range, or the Grampian Hills, lying to the north in Scotland, and the
lower one, or the Sidlaw Hills, to the south, while in Switzerland, on
the contrary, the higher range lies to the south and the lower to the
north. I have alluded especially to Glen Prossen because the glacial
marks in that valley are remarkably distinct, the whole bed of the
valley being scratched, polished, and furrowed by the great rasp which
has moved over it, while the concentric moraines at its lower extremity
are very striking. But these signs, so perfectly preserved in Glen
Prossen, recur with greater or less intensity in all the corresponding
valleys, leaving no doubt that the same phenomena existed over the whole
region.
Among the localities of Scotland where the indications of glacial action
are most marked is the region about Stirling. Near Stirling Castle the
polished surfaces of the rocks with their distinct grooves and scratches
show us the path followed by the ice as it moved down in a northeasterly
direction toward the Frith of Forth from the mountains on the northwest.
To the west of Edinburgh, also, there is a broad glacier-track, showing
that here also the ice was ploughing its way eastward to find an outlet
on the shore.
The western slope of the great Scotch range is no less remarkable for
its glacier-traces. The heads of Loch Long, Loch Fyne, Loch Awe, and
Loch Leven everywhere show upon their margins the most distinct glacial
polish and furrows, while from the trend of these marks and the
distribution of the moraines, especially about Ben Cruachan, it is
obvious that in this part of the country the glaciers moved westward and
southward. About Aberdeen, on the contrary, they moved eastward, while
in the vicinity of Elgin they advanced toward the north.
It thus appears that the whole range of the Grampians formed a great
centre for the distribution of glaciers, and that a colossal ice-field
spread itself over the whole country, extending in every direction
toward the lower lands and the sea-shore. As the glaciers which now
descend through all the valleys of the Alps, along their northern as
well as their southern slopes, and in their eastern as well as their
western prolongation, though limited, in our days, within the
valley-walls, nevertheless once covered the plain of Switzerland and
that of Northern Italy, so did the ice-fields of the Grampians during
the greatest extension of the Scotch glaciers spread over the whole
country. They also were, in course of time, reduced to local glaciers,
circumscribed within the higher valleys of the more mountainous parts of
the country, until they totally disappeared, as those of Switzerland
would also have done, had it not been for the greater elevation of that
country above the level of the sea. Scotland nowhere rises above the
present level of perpetual snow, while in Switzerland the whole Alpine
range has an altitude favorable to the preservation of glaciers. In the
range of the Jura, however, which had at one time its local glaciers
also, but which nowhere now rises above the line of perpetual snow, they
have disappeared as completely as in the Grampian Hills.
It would lead me too far, were I to give here a special account of all
the investigations I made in 1840 upon the distribution of glaciers in
Great Britain. I will therefore only point out a few of the more
distinct areas of distribution. The region surrounding Ben Wyvis formed
such a centre of dispersion from which glaciers radiated, and we have
another in the Pentland Hills about Edinburgh. In Northumberland, the
Cheviot Hills present a glacial centre of the same kind, and in the
Westmoreland Hills we have still another. In the last-named locality,
the glacial tracks can be followed in various directions, some of them
descending toward the northwest from the heights of Helvellyn, others
moving southward toward Ambleside. In Wales the same kind of glacial
distribution has been observed; but, as Professor Ramsay has treated
this subject in full, I would refer my readers to his masterly work for
a further account of the ancient Welch glaciers. In Ireland I had also
opportunities of making extensive local investigations of glacial
action. I observed the centres of distribution in the neighborhood of
Belfast, in the County of Wicklow, and in Cavan.
But nowhere are these phenomena more striking than in Fermanagh County
about the neighborhood of Enniskillen, and more especially in the
immediate vicinity of Florence Court, the seat of the Earl of
Enniskillen. On the northern slope of Ben Calcagh are five valleys lying
parallel with each other and opening into the valley of Loch Nilly,
which runs from east to west at the base of the mountain. A road now
passes through this valley, and, where it crosses the mouth of either of
the five valleys rising towards the mountain-slope, it cuts alternately
through the two horns of a crescent-shaped wall which bars the lower end
of every one of them. These crescent-shaped mounds are so many terminal
moraines, built up by the five glaciers formerly descending through
these lateral valleys into the valley of Loch Nilly. They bore the same
relation to each other as the glaciers de Tour and d'Argentière, the
Glacier des Bois with the Mer de Glace, the Glacier des Bossons and the
Glacier de Taconet, now bear to each other in the valley of Chamouni;
and were it not for the smaller dimensions of the whole, any one
familiar with the tracks of ancient glaciers might easily fancy himself
crossing the ancient moraines at the foot of the northern slope of the
range of Mont Blanc, through which the Arve has cut its channel, the
valley of Chamouni standing in the same relation to Mont Blanc as the
valley of Loch Nilly does to Ben Calcagh.
I have dwelt thus at length on the glaciers of Great Britain because
they have been the subject of my personal investigations. But the Scotch
Highlands and the mountains of Wales and Ireland are but a few of the
many centres of glacial distribution in Europe. From the Scandinavian
Alps glaciers descended also to the shores of the Northern Ocean and the
Baltic Sea. There is not a fiord of the Norway shore that does not bear
upon its sides the tracks of the great masses of ice which once forced
their way through it, and thus found an outlet into the sea, as in
Scotland. Indeed, under the water, as far as it is possible to follow
them through the transparent medium, I have noticed in Great Britain and
in the United States the same traces of glacial action as higher up, so
that these ancient glaciers must have extended not only to the
sea-shore, but into the ocean, as they do now in Greenland. Nor is this
all. Scandinavian boulders, scattered upon English soil and over the
plains of Northern Germany, tell us that not only the Baltic Sea, but
the German Ocean also, was bridged across by ice, on which these masses
of rock were transported. In short, over the whole of Northern Europe,
from the Arctic Ocean to the northern borders of its southern
promontories, we find all the usual indications of glacial action,
showing that a continuous sheet of ice once spread over nearly the whole
continent, while from all the mountain-ranges descended those more
limited glacial tracks terminating frequently in transverse moraines
across the valleys, showing, that, as the general ice-sheet broke up and
contracted into local glaciers, every cluster or chain of hills became a
centre of glacial dispersion, such as the Alps are now, such as the
Jura, the Highlands of Scotland, the mountains of Wales and Ireland, the
Alps of Scandinavia, the Hartz, the Black Forest, the Vosges, and many
others have been in ancient times.
* * * * *
In the next article we shall consider the glacial phenomena as they
exist in America.