particular course, it is obvious that this could not be the case
with the under portion of the great sheet, the path of which would
be controlled in large measure by the form of the ground over which
the ice moved. The upper strata that overflowed the Outer Hebrides,
as we have seen, were locally deflected again and again by important
obstacles, and it is quite certain that the same would take place with
the deeper portions of the ice-flow.
It is well known that the sea along the inner margin of the Long Island
is very deep. In many places it reaches a depth of 600 feet, and
occasionally the sounding-lead plunges down for upwards of 700 feet.
It would seem, however, that these great depths did not exist before
the advent of the ice-sheet, but that the bottom of the Minch along
the eastern borders of the Long Island was then some 250 or 300 feet
shallower than now, the floor of the sea having since been excavated
in the manner I shall presently describe. It is quite apparent,
therefore, that the long ridge of the Outer Hebrides must have offered
an insuperable obstacle to the direct passage of the bottom-ice out
to the Atlantic. Here was a great wall of rock shooting up from the
floor of the Minch, at a high angle, to a height ranging in elevation
from 400 feet to upwards of 3000 feet. It is simply impossible that
the lower strata of the ice that occupied the bed of the Minch could
climb that precipitous barricade. They were necessarily deflected, one
portion creeping to north-east and another to south-west, but both
hugging the great wall of rock all the way. We see precisely the same
result taking place in the bed of every stream. Let us stand upon an
almost submerged boulder, and note how the water is deflected to right
and left, and we shall observe at the same time that the boulder, by
obstructing the current, forces the water downwards upon the bed of the
stream, the result being that a hollow is dug out in front. Now, in a
similar manner, the ice, squeezed and pressed against the Hebridean
ridge by the steady flow of the great current that crossed the Minch,
necessarily acted with intense erosive force upon its bed. Hence in the
course of time it scooped out a series of broad deep trenches along the
whole inner margin of the Long Island, the amount of the excavation
reaching from 200 to 300 feet. Similar excavated basins occur in like
positions opposite all the precipitous islands of the Inner Hebrides.
Wherever, indeed, the ice-sheet met with any great obstruction to
its flow, there excessive erosion took place, and a more or less
deep hollow was dug out in front of the opposing cliff, or crag,
or precipitous mountain. While, therefore, the upper strata of the
ice-sheet overflowed the Outer Hebrides from south-east to north-west,
the under portions of the same great ice-flow were compelled by the
contour of the ground to creep away to north-east and south-west, until
they could steal round the ridge and so escape outwards to the Atlantic.
This being the case, we have a very simple and obvious explanation of
the absence of Skye rocks in the till of the Long Island. One sees
readily enough that the sub-glacial débris dragged across the Minch
would naturally be carried away to south-west and north-east by the
"under-tow" or deflected ice. It is quite impossible that any Skye
fragments or bits of rock from the mainland could travel over the bed
of the Minch, and then be pushed up the precipitous rock wall of the
Long Island. There is only one place in all the Outer Hebrides where
we might expect to meet with extraneous boulders in the till, and
that is in the north of Lewis, where the land shelves gently into the
sea, and the great rocky ridge terminates. Here the under-strata of
the ice would begin to steal up upon the land, favoured by its gentle
inclination, and in that very place accordingly we meet with a deposit
of till in which are found many boulders of a hard red sandstone, and
some of various porphyries which are quite alien to the Long Island.
Moreover, the till itself in that locality is much more of a clay than
the usual sub-glacial débris in other parts of Lewis, and contains
numerous fragments of sea-shells. All this is quite in keeping with the
other evidence. The extreme north end of Lewis was overflowed by the
under-current that crept up the bed of the Minch, hugging the Hebridean
ridge, and dragging along with it a muddy mass interspersed with the
shells and other marine exuviæ that lay in its path, and numerous
stones, some of which may have come from Skye, while others were
derived from the mainland.
I have already said enough, perhaps, about the abrasion of the
Hebrides, but I may add a few words upon the origin of the freshwater
lakes. Many of these rest in complete rock-basins; others, again, seem
to lie partly upon solid rock and partly upon till; while yet others
appear to occupy mere shallow depressions in the surface of the till.
All of them thus owe their origin to the action of the ice-sheet. As
one might have expected, the great majority lie along the outcrop of
the gneissic strata, which, as a rule, corresponds pretty closely to
the flow of the ice. Hence the general trend of the lakes is from
south-east to north-west. In many cases in fashioning these rock-basins
the ice has merely deepened in an irregular manner previously existing
hollows, which are now, of course, filled with water. In not a few
places, however, the lakes are drawn out in other directions--this
being due usually to changes in the strike or outcrop of the strata.
For example, over a considerable district in the south of Lewis many
lake-hollows extend from south-west to north-east, or at right angles
to the direction of the ice-flow. Such lakes are usually dammed up at
one or both extremities by glacial débris.
Thus most of the features characteristic of the Outer Hebrides owe
their origin directly or indirectly to the action of that great sheet
of ice which swept over the islands during what is called the Glacial
Period. And there is no region in northern Europe where the immensity
of the abrading agent can be more vividly realised. From a study of
the phenomena there exhibited we for the first time obtain a definite
idea of the surface-slope, and are able to plumb the old ice-sheet, and
ascertain with some approach to accuracy its exact thickness. In the
deeper parts of the area, between the mainland and the Long Island, its
thickness was not less than 3800 feet. Of course this great depth of
ice could not have been derived exclusively from the snow that fell on
the mountains of the north-west Highlands. Doubtless the precipitation
took place over its whole surface, just as is the case in Greenland and
over the Antarctic continent. The winter cold must have been excessive,
but the precipitation necessary to sustain such a mass of ice implies
great evaporation; in other words, the direct heat of the sun _per
diem_ in summer-time was probably considerably in excess of what it is
now in these latitudes. The west and south-west winds must have been
laden with moisture, the greater portion of which would necessarily
fall in the form of snow. We see something analogous to this taking
place in the Antarctic regions at the present day. That quarter of the
globe has its summer in perihelion, and, therefore, must be receiving
then more heat _per diem_ than our hemisphere does in its summer
season, which, as every one knows, happens when the earth is furthest
removed from the sun. But, notwithstanding this, the summer of the
Antarctic continent is cold and ungenial--the presence of the great
ice-sheet there cooling the air and causing most of the moisture to
fall as snow. Paradoxical as it may seem, therefore great summer heat
is almost, if not quite, as necessary as excessive winter cold for the
production and maintenance of a wide continental glacier.
III.
When we last took a peep at the Outer Hebrides we found those luckless
islands all but obliterated under an immense sheet of ice extending
from the mainland out into the Atlantic. How far west the great glacier
spread itself we cannot as yet positively say; but if the known slope
of its surface between the north-west Highlands and the Long Island
continued, as there is every reason to believe it would, then it is
extremely probable that the ice flowed out to the edge of the great
Scottish submarine plateau. Here the sudden deepening of the Atlantic
would arrest its progress and cause it to break up into icebergs. In
those old times, therefore, a steep wall of ice would extend all along
the line of what is now the edge of the 100-fathoms plateau. From this
wall large tabular masses would ever and anon break away and float off
into the Atlantic--a condition of things which is closely paralleled at
present along the borders of the ice-drowned Antarctic continent.
By-and-by, however, a great change took place, and the big ice-sheet
melted off the Long Island and vanished from the Minch. We read the
evidence for this change of climate in certain interesting deposits
which occur in considerable bulk at the northern extremity of Lewis,
and in smaller patches in the Eye peninsula of the same island. In
those districts the old sub-glacial débris or till is covered with
beds of clay and sand in which many marine exuviæ are found--shells of
molluscs, entomostraca, foraminifera, etc. They clearly prove, then,
that after the ice-sheet had vanished Lewis was submerged in the sea
to a depth of not less than 200 feet, and they also prove that the
temperature of the sea was much the same then as now, for the shells
all belong to species that are still living in these northern waters.
It is very remarkable that the marine deposits in question seem to
occur nowhere else in any part of the Long Island. We cannot believe
that the submergence was restricted to the very limited areas where
the shell-beds are met with: it must, on the contrary, have affected
a very large portion, if not the whole, of the Outer Hebrides. Why,
then, do not we meet with shelly sands and clays, with raised beaches
and other relics of the former occupation of these islands by the sea,
covering wide areas in the low-grounds? How can we explain the absence
of such relics from all those districts which, being much under the
level of 200 feet, must necessarily have at one time formed part of the
sea-floor? The explanation is not difficult to discover.
Resting upon the surface of the shell-beds at Ness and Garabost we
find an upper or overlying accumulation of sub-glacial débris or till.
At Ness this upper till closely resembles, in general appearance, the
lower deposit that rests directly upon the rocks. It is a pell-mell
accumulation of silty clay, crammed with glaciated stones, amongst
which are many fragments of red sandstone and some extra-Hebridean
rocks, and interspersed through it occur also broken fragments of
sea-shells. The marine deposits lying below are usually much confused
and contorted, and here and there they are even violently commingled
with the upper till. They show, generally, a most irregular surface
under that accumulation, and are evidently only the wreck of what they
must at one time have been. Now the presence of this upper till proves
beyond doubt that the intense arctic conditions of climate once more
supervened. A big ice-sheet again filled up the basin of the Minch and
flowed over the Long Island--its under-tow creeping along the inner
margin of the lofty rock-barrier as before, and eventually stealing
over the low-ground at the Butt, where its bottom-moraine or till was
dragged over the marine deposits, and confusedly commingled with them.
The upper strata of the ice that streamed across the islands renewed
the work of abrasion, and succeeded in scraping away all traces of the
late occupation by the sea. If any such now exist they must lie buried
under the till that cloaks the low-ground on the western margins of
the islands. Hence it is that we find not a vestige of shelly beds in
any part of the Long Island which was exposed to the full brunt of
the ice-flow. At Garabost they have been ploughed through in the most
wonderful manner, and only little patches remain. At Ness, however,
they are more continuous. This is owing to the circumstance that the
ground in that neighbourhood is low-lying and offered no obstacle
to the passage of the ice out to sea. Hence the shell-beds were not
subjected to such excessive erosion as overtook them along the whole
eastern border of the Long Island.
Eventually, however, this later advance of the ice-sheet ceased. The
climate grew less arctic, and the great glacier began to melt away,
until the time came that its upper strata ceased to overflow the
islands. They then passed away to north and south, along the hollow
now occupied by the Minch, following the same path as the bottom-ice.
Considerable snow-fields, however, still covered the Outer Hebrides,
and large local glaciers occupied all the mountain-valleys, and,
descending to low levels, piled up their terminal moraines. Some of
these local glaciers appear to have gone right out into the Minch,
as in South Uist, and may have coalesced with the great glacier that
still filled that basin. It was during this condition of things that
most of the great perched blocks that are scattered so profusely over
the islands began to be dropt into their present positions. During
the climax of glacial cold, when the upper strata of the ice-sheet
streamed across the Hebrides, large fragments of rock would certainly
be wrenched off and carried on underneath the ice; but as only a few of
the Hebridean mountain-tops were then exposed, there would be a general
absence of such enormous erratics as are detached by frost and rolled
down upon the surface of a glacier, and any such superficially-borne
erratics would be transported, of course, far beyond the Long Island
into the Atlantic. When the ice had ceased to overflow the islands,
boulders derived from Skye and the mainland would no longer be carried
so directly out to the Atlantic, but would travel thither by the more
circuitous route, which the now diminished ice-sheet was compelled to
follow.
As the snow and ice melted off the Hebrides, the rocks would begin
to be exposed to the action of intense frost, and many fragments,
becoming dislodged and falling upon _névé_, small local ice-sheets,
and glaciers, would be stranded on hill-slopes and sprinkled over the
low-grounds, along with much broken débris and rock-rubbish. Eventually
all the lower-grounds would be deserted by the ice, glaciers would die
out of the less elevated valleys, and linger in only a few of the glens
that drain the higher mountain-masses. Such local glaciers have flowed
often at right angles to the direction followed by the great ice-sheet.
Thus, the ice-markings in the glens that come down from the Forest
of Harris to West Loch Tarbert, run from north to south, while the
trend of the older glaciation on the intervening high-grounds is from
south-east to north-west.
The morainic rubbish and erratics of this latest phase in the glacial
history of the Long Island may be traced down almost to the water's
edge, showing plainly that there has been no great submergence of that
region since the disappearance of glacial conditions. This is somewhat
remarkable, because along the shores of central and southern Scotland
we have indisputable evidence to show that the land was drowned to
the depth of at least fifty feet in post-glacial times. In the Outer
Hebrides, however, there are no traces of any post-glacial submergence
exceeding a dozen feet or so; that is to say, there is no proof that
the Outer Hebrides have been of much less extent than they are now. On
the contrary, we have many reasons for believing that they were within
comparatively recent times of considerably larger size, and were even
in all probability united to the mainland. The abundance of large
trees in the peat-mosses, and the fact that these ancient peat-covered
forests extend out to sea, are alone sufficient to convince one that
the Outer Hebrides have been much reduced in area since the close of
the glacial period. These now bleak islands at one time supported
extensive forests, although nowadays a tree will hardly grow unless
it be carefully looked after. That old forest period coincided in
all probability with the latest continental condition of the British
Islands--when the broad plains which are now drowned under the German
Ocean formed part of a great forest-land, that included all the
British Islands, and extended west for some distance into tracts over
which now roll the waves of the Atlantic. The palmy days of the great
British forests, however, passed away when the German Ocean came into
existence. The climatic conditions were then not so favourable for the
growth of large trees; and in the uplands of our country, and what are
now our maritime districts, the forests decayed, and were gradually
overgrown by and buried under peat-mosses. The submergence of the land
continued after that, until central and southern Scotland were reduced
to a considerably smaller size than now, and then by-and-by the process
was reversed, and the sea once more retreated, leaving behind it a
number of old raised beaches to mark the levels at which it formerly
stood.
The greatest submergence that overtook central and southern Scotland
in times posterior to the latest continental condition of Britain did
not exceed fifty feet, or thereabout; and the extreme limits reached by
the sea in the period that supervened between the close of the glacial
epoch and the "age of forests" was not more than one hundred feet. The
Outer Hebrides, however, were certainly not smaller in post-glacial
times than they are now, and we have no evidence to show that after the
"age of forests" had passed away the sea rose higher than a dozen feet
or so above its present level. Now there are only two ways in which all
this can be accounted for. Either the Hebrides remained stationary, or
stood at a level higher than now, while the central and southern parts
of Scotland were being submerged; or else there has been a very recent
depression within the Hebridean area, which has carried down below the
sea all traces of late glacial and post-glacial raised beaches. All we
know for certain is, that the only raised beaches in the Long Island
are met with in low maritime regions at only a few feet above the
present high-water mark. My own impression is that the whole district
has been submerged within comparatively recent times; for if the
present coast-line had endured since the close of the glacial period,
or even since the last continental condition of Britain, I should have
expected the sea to have done more than it has in the way of excavation
and erosion.
In a former article I have spoken of the sand-dunes and sandy flats
of the west coast of the Long Island. These receive their greatest
development in North Uist, Benbecula, and South Uist. Along the whole
western margin of these islands stretch wide shoals and banks of
yellow sand and silt, and similar shoals and banks cover the bed of
the shallow sounds or channels. In the middle of the Sound of Harris
one may often touch the bottom with an oar, and even run one's boat
aground. It is the same in the Sound of Barra, while, as I have already
mentioned, one may walk at low-water from Benbecula into the adjacent
islands of North and South Uist. Where does all this sand come from?
Certainly not from the degradation of the islands by the sea, for the
sounds appear to be silting up, and the general appearance of the sandy
flats along the west coast indicates that the land is upon the whole
gaining rather than losing. I have no doubt at all that this sand and
silt are merely the old sub-glacial débris which the ice-sheet spread
over the low shelving plateau that extends west under the Atlantic to
the 100-fathoms line. That plateau must have been thickly covered with
till, and with heaps and sheets of gravel and sand and silt, and it is
these deposits, sifted and winnowed by the sea, which the tides and
waves sweep up along the Atlantic margin of the islands.
There are many other points of interest to that I might touch upon,
but I have said enough perhaps to indicate to any intelligent observer
the kind of country he may be led to expect in the Long Island. Of
course the history of the glacial period is very well illustrated
in many parts of the mainland, which are much easier of access than
the Outer Hebrides. But these islands contain, at least, one bit of
evidence which does not occur anywhere else in Britain. In them we
obtain, for the first time, data for measuring the actual slope of the
ice-sheet. It does not follow, however, that the inclination of the
surface towards the Atlantic was the same all over the area covered
by the ice-sheet. The slope of the sheet that flowed east into the
basin of the German Ocean, for example, may have been, and probably
was, less than that of the Hebridean ice-flow. But apart altogether
from this particular point, I think there is no part of the British
Islands where the evidence for the former action of a great ice-sheet
is more abundant and more easily read, or where one may realise with
such vividness the conditions that obtained during that period of
extraordinary climatic vicissitudes, which geologists call the Glacial
Epoch.
Leaving these old arctic scenes, and coming down to the actual present,
no one, I think, can wander much about the Outer Hebrides without
pondering over the fate of the islanders themselves. Many writers
have asserted that the Celt of these rather out-of-the-way places
is a lazy, worthless creature, whom we Saxons should do our best to
weed out. One cannot help feeling that this assertion is unfair and
cruel. The fact is, we judge him by a wrong standard. He is by nature
and long-inherited habits a fisherman, and has been wont to cultivate
only so much land as should suffice for the sustenance of himself and
those immediately dependent upon him. In old times he was often enough
called upon to fight, wrongly or rightly, and thus acquired that proud
bearing which it has taken so many long years of misery to crush out.
He is, as a rule, totally unfit for the close confinement and hard work
which are the lot of the great mass of our mechanics--does not see the
beauty of that, and has rather a kind of contempt for the monotonous
drudgery of large manufacturing towns. One of the few situations in
town that he cares to fill is that of police-constable. Give him a
life in the open air, however trying it may be, and he will be quite
content if he can make enough to feed himself and family. If the
fishing chance to be very profitable he does not, as a rule, think of
saving the surplus he has made, but looks forward rather to a spell of
idleness, when he can smoke his pipe and talk interminable long talks
with his neighbours. No doubt this, judged by our own standard, is all
very shocking. Why doesn't he put his money in the savings-bank, and
by-and-by die and leave it to those who come after him? Simply because
he is a Celt, and not a Saxon.