may ask, is the cause of that general monotony of outline to which
reference has so frequently been made? At first we seem to get an
answer to our question when we are told that the islands of the Outer
Hebrides are composed chiefly of one and the same kind of rock.
Everyone nowadays has some knowledge of the fact that the peculiar
features of any given district are greatly due to the character and
arrangement of the rock-masses. For example, who is not familiar with
the outline of a chalk country, as distinguished from the contour of a
region the rocks of which are composed, let us say, of alternating beds
of limestone and sandstone and masses of old volcanic material? The
chalk country, owing to the homogeneousness of its component strata,
has been moulded by the action of weather and running water into an
undulating region with a softly-flowing outline, while the district
of composite formation has yielded unequally to the action of Time's
workers--rains, and frosts, and rivers--and so is diversified with
ridge, and escarpment, and knolls, and crags. When, therefore, we learn
that the Outer Hebrides are composed for the most part of the rock
called _gneiss_ and its varieties, we seem to have at once found the
meaning of the uniformity and monotony. It is true that although pink
and grey gneiss and schistose rocks prevail from the Butt of Lewis
to Barra Head, yet there are some other varieties occasionally met
with--thus soft red sandstone and conglomerate rest upon the gneissic
rocks near Stornoway, but they occur nowhere else throughout the Long
Island. Now and again, however, the gneiss gives place to granite, as
on the west coast of Lewis near Carloway; and here and there the strata
are pierced by vertical dykes and curious twisted and reticulated veins
of basalt-rock. All these, however, hold but a minor and unimportant
place as constituents of the islands. Gneiss is beyond question the
most prevalent rock, and we seem justified in assigning the peculiar
monotony of the Outer Hebridean scenery to that fact.
But when we come to examine the matter more attentively, we find that
there is still some important factor wanting. We have not got quite to
the solution of the question. When we study the manner in which the
gneiss and gneissic rocks disintegrate and break up at the sea-coast
or along the flanks of some rugged mountain-glen, we see they give
rise to an irregular uneven surface. They do not naturally decompose
and exfoliate into rounded dome-shaped masses, such as are so commonly
met with all through the islands, but rather tend to assume the aspect
of rugged tors, and peaks, and ridges. The reason for this will be
more readily understood when it is learned that the gneissic rocks of
the Outer Hebrides are for the most part arranged in strata, which,
notwithstanding their immense antiquity--(they are the oldest rocks
in Europe)--and the many changes they have undergone, are yet, as a
rule, quite distinguishable. The strata are seldom or never horizontal,
but are usually inclined at a high angle, either to north-east or
south-west, although sometimes, as in the vicinity of Stornoway, the
"dip" or inclination of the beds is to south-east. Throughout the
major portion of the Long Island, however, the outcrop of the strata
runs transversely across the land from south-east to north-west. Now
we know that when this is the case strata of variable composition and
character give rise to long escarpments and intervening hollows--the
escarpments marking the outcrops of the harder and more durable beds,
and the hollows those strata that are softer and more easily eroded by
the action of the denuding forces, water and frost. When the dip of the
strata is north-east we expect the escarpments to face the south-west,
and the reverse will be the case when the strata incline in the
opposite direction.
Seeing then that the Outer Hebrides are composed chiefly of gneissic
rocks and schists which yield unequally to the weather, and which, in
the course of time, would naturally give rise to lines of sharp-edged
escarpments or ridges and intervening hollows, with now and again
massive hills and mountains showing great cliffs and a generally
broken and irregular outline, why is it that such rugged features are
so seldom present at low levels, and are only conspicuous at the very
highest elevations? The rocks of the Outer Hebrides are of immense
antiquity, and there has therefore been time enough for them to assume
the irregular contour which we might have expected. But in place of
sharp-rimmed escarpments, and tors, and broken shattered ridges, we see
everywhere a rounded and smoothly-flowing configuration which prevails
up to a height of 1600 feet or thereabout, above which the rocks take
on the rugged appearance which is natural to them. By what magic have
the strata at the lower levels escaped in such large measure from the
action of rain and frost, which have furrowed and shattered the higher
mountain-tops?
I have said that long lines of escarpment and ridges, corresponding to
the outcrops of the harder and more durable strata, are not apparent in
these islands. A trained eye, however, is not long in discovering that
such features, although masked and obscured, are yet really present.
The round-backed rocks are drawn out, as it were, in one persistent
direction, which always agrees with the _strike_ or outcrop of the
strata; and in many districts one notices also that long hollows
traverse the land from south-east to north-west in the same way.
Such alternating hollows and rounded ridges are very conspicuous in
Barra and the smaller islands to the south, and they may likewise be
noted in most of the larger islands also. Looking at these and other
features, the geologist has no hesitation in concluding that the whole
of the islands have been subjected to some powerful abrading force,
which has succeeded to a large extent in obliterating the primary
configuration of the land. The rough ridges have been rounded off,
the sharp escarpments have been bevelled, the abrupt tors and peaks
have been smoothed down. Here and there, it is true, the dome-shaped
rock-masses are beginning again to break up under the action of the
weather so as to resume their original irregular configuration. And,
doubtless, after the lapse of many ages, rain and frost will gradually
succeed in destroying the present characteristic flowing outlines, and
the islands will then revert to their former condition, and rugged
escarpments, sharp peaks, and rough broken hummocks and tors will again
become the rule. But for a long time to come these grey Western Islands
will continue to present us with some of the most instructive examples
of rounded and mammillated rock-masses to be met with in Europe.
From Barra Head in Bearnarey to the Butt of Lewis we are constantly
confronted by proofs of the former presence of that mysterious abrading
power, which has accommodated itself to all the sinuosities of the
ground, so that from the sea-level up to a height of 1600 feet at
least, the eye rests almost everywhere upon bare round-backed rocks and
smoothed surfaces.
II.
In the preceding article I have described the peculiar configuration
of the Long Island--rounded and flowing for the most part--and have
pointed out how that softened outline is not such as the rocks would
naturally assume under the influence of the ordinary agents of erosion
with which we are familiar in this country. The present contour has
superseded an older set of features, which, although highly modified
or disguised, and often well-nigh obliterated, are yet capable of
being traced, and are, no doubt, the conformation assumed by the rocks
under the long-continued action of rain and frost and running water.
We have now to inquire what it was that removed or softened down the
primal configuration I refer to, and gave to the islands their present
monotonous, undulating contour.
Any one fresh from the glacier-valleys of Switzerland or Norway could
have little doubt as to the cause of the transformation. The smoothed
and rounded masses of the Outer Hebrides are so exactly paralleled by
the ice-worn, dome-shaped rocks over which a glacier has flowed, that
our visitor would have small hesitation in ascribing to them a similar
origin; and the presence of the countless perched blocks and boulders
which are scattered broadcast over the islands would tend to confirm
him in his belief. A closer inspection of the phenomena would soon
banish all doubt from his mind; for, on the less-weathered surfaces,
he would detect those long parallel scratches and furrows which are
the sure signs of glacial action, while, in the hollows and over the
low-grounds, he would be confronted with that peculiar deposit of
clay and sand and glaciated stones and boulders which are dragged on
underneath flowing ice.
Having satisfied ourselves that the rounded outline of the ground is
the result of former glacial action, our next step is to discover, if
we can, in what direction the abrading agent moved. Did the ice, as
we might have supposed, come out of the mountain-valleys and overflow
the low country? If that had been the case, then we should expect to
find the glacial markings radiating outwards in all directions from
the higher elevations. Thus the low-grounds of Uig, in Lewis, should
give evidence of having been overflowed by ice coming from the Forest
of Harris; the undulating, rocky, and lake-dappled region that extends
between Loch Roag and Loch Erisort should be abraded and striated
from south-west to north-east. Instead of this, however, the movement
has clearly been from south-east to north-west. All the prominent
rock-faces that look towards the Minch have been smoothed off and
rounded, while in their rear the marks of rubbing and abrading are much
less conspicuous. It is evident that the south-east exposure has borne
the full brunt of the ice-grinding--the surfaces that are turned in the
opposite direction, or towards the Atlantic, having been in a measure
protected or sheltered by their position. The striations or scratches
that are seen upon the less-weathered surfaces point invariably towards
the north-west, and from their character and the mode in which they
have been graved upon the rock, we are left in no doubt as to the trend
of the old ice-plough--which was clearly from south-east to north-west.
Nor is it only the low-grounds that are marked in this direction.
Ascend Suaina (1300 feet), and you shall find it showing evident signs
of having been abraded all over, from base to summit. The same, indeed,
is the case with all the hills that stretch from sea to sea between
Uig and Loch Seaforth. Beinn Mheadonach, Ceann Resort, Griosamul, and
Liuthaid, are all strongly glaciated from south-east to north-west.
North and South Harris yield unequivocal evidence of having been
overflowed by ice which did not stream out of the mountain-valleys,
but crossed the island from the Minch to the Atlantic. A number of
mountain-glens, coming down from the Forest of Harris, open out
upon West Loch Tarbert, and these we see have been crossed at right
angles by the ice--the mountains between them being strongly abraded
from south-east to north-west. It is the same all over South Harris,
which affords the geologist every evidence of having been literally
smothered in ice, which has moved in the same persistent direction. The
rock-faces that look towards the Minch are all excessively naked; they
have been terribly ground down and scraped, and the same holds good
with every part of the island exposed to the south-east.
Now, the mode in which the rocks have been so ground, scraped, rounded,
and smoothed betokens very clearly the action of land-ice, and not of
floating-ice or icebergs. The abrading agent has accommodated itself
to all the sinuosities of the ground, sliding into hollows and creeping
out of them, moulding itself over projecting rocks, so as eventually to
grind away all their asperities, and convert rugged tors and peaks into
round-backed, dome-shaped masses. It has carried away the sharp edges
of escarpments and ridges, and has deepened the intervening hollows in
a somewhat irregular way, so that now these catch the drainage of the
land and form lakes. Steep rocks facing the Minch have been bevelled
off and rounded atop, while in their rear the ice-plough, not being
able to act with effect, has not succeeded in removing the primeval
ruggedness of the weathered strata.
I have said that the movement of the ice was from south-east to
north-west. But a close examination of the ice-markings will show that
the flow was very frequently influenced by the form of the ground.
Minor features it was able to disregard, but some prominent projecting
rock-masses succeeded in deflecting the ice that flowed against them.
For example, if we study the rocks in North Harris, we shall find that
the Langa and the Clisham have served as a wedge to divide the ice,
part of which flowed away into Lewis, while the other current or stream
crept out to sea by West Loch Tarbert. The Langa and the Clisham,
indeed, raised their heads above the glacier mass--they were islets in
a sea of ice. It is for this reason that they and the Tarcull ridge in
South Harris have not been smoothed and abraded, but still preserve
their weathered outline. All surfaces below a height of 1600 feet which
are exposed to the south-east, and which have not been in recent times
broken up by the action of rain and frost, exhibit strongly-marked
glaciation. But above that level no signs of ancient ice-work can be
recognised.
We see now why it is that the hill-slopes opposite the Minch should,
as a rule, be so much more sterile than those which slope down to the
Atlantic. The full force of the ice was exerted upon the south-east
front, in the rear of which there would necessarily be comparatively
"quiet" ice. For the same reason we should expect to find much of the
rock débris which the ice swept off the south-east front sheltering on
the opposite side. Neither clay nor sand nor stones would gather under
the ice upon the steep rocks that face the Minch. The movement there
was too severe to permit of any such accumulation. But stones and clay
and sand were carried over and swept round the hills, and gradually
accumulated in the rear of the ice-worn rocks, just in the same way as
gravel and sand are heaped up behind projecting stones and boulders
in the bed of a stream. Hence it is that the western margin of Harris
is so much less bleak than the opposite side. Considerable taluses
of "till," as the sub-glacial débris is called, gather behind the
steeper crags, and ragged sheets of the same material extend over the
low-grounds. All the low-grounds of Lewis are in like manner sprinkled
with till. Over that region the ice met with but few obstacles to its
course, and consequently the débris it forced along underneath was
spread out somewhat equally. But wherever hills and peaks and hummocks
of rock broke the regularity of the surface, there great abrasion took
place and no till was accumulated.
Thus the position and distribution of this sub-glacial débris or
bottom-moraine tell the same tale as the abraded rocks and glacial
striæ, and clearly indicate an ice-flow from the south-east. This is
still further proved by the manner in which the upturned ends of the
strata are frequently bent over underneath the till in a north-westerly
direction, while the fragments dislodged from them and enclosed in the
sub-glacial débris stream away as it were to the same point of the
compass. Not only so, but in the west of Lewis, where no red sandstone
occurs, we find boulders of red sandstone enclosed in the till, which
could not have been derived from any place nearer than Stornoway. In
other words, these boulders have travelled across the island from the
shores of the Minch to the Atlantic sea-board.
Having said so much about the glaciation of Lewis and Harris, I need
not do more than indicate very briefly some of the more interesting
features of the islands further south.
I spent some time cruising up and down the Sound of Harris, and found
that all the islets there had been ground and scraped by ice flowing
in the normal north-west direction, and sub-glacial débris occurs on
at least one of the little islands--Harmetrey. But all the phenomena
of glaciation are met with in most abundance in the dreary island of
North Uist. The ridge of mountains that guards its east coast has been
battered, and ground down, and scraped bare in the most wonderful
manner, while the melancholy moorlands are everywhere sprinkled with
till, full of glaciated stones, many of which have travelled west
from the coast range. Benbecula shows in like manner a considerable
sprinkling of till, and the trend of the glacial striæ is the same
there as in North Uist, namely, a little north of west. There are no
hills of any consequence in Benbecula, but the highly-abraded and
barren-looking mountains that fringe the eastern margin of North Uist
are continued south in the islands of Roney and Fuiey, either of which
it would be hard to surpass as examples of the prodigious effect of
land-ice in scouring, scraping, and grinding the surface over which it
moves.
South Uist presents the same general configuration as North Uist,
its east coast being formed of a long range of intensely glaciated
mountains, in the rear of which ragged sheets and heaps of sub-glacial
débris are thrown and scattered over the low, undulating tract that
borders the Atlantic. No part of either Benbecula or North Uist has
escaped the action of ice, but in South Uist that knot of high-ground
which is dominated by the fine mountains of Beinn Mhor and Hecla
towered above the level of the glacier-mass, and have thus been the
cause of considerable deflection of the ice-flow. The ice-stream
divided, as it were, part flowing round the north flank of Hecla,
and part streaming past the southern slopes of Beinn Mhor. But the
ice-flow thus divided speedily reunited in the rear of the mountains,
the southern stream creeping in from the south-east, and the northern
stream stealing round Hecla towards the south-west. The track of this
remarkable deflection and reunion is clearly marked out by numerous
striæ all over the low-grounds that slope outwards to the Atlantic
coast. The till, it need hardly be added, affords the same kind of
evidence as the sub-glacial deposits of the other islands, and points
unmistakably to a general ice-movement across South Uist from the Minch
to the Atlantic.
The influence which an irregular surface has in causing local
deflections of an ice-flow is also well seen in Barra, where the striæ
sometimes point some 5° or 10°, and sometimes 25° and even 35° north of
west--these variations being entirely due to the configuration of the
ground. This island is extremely bare in many places, more especially
over all the region that slopes to the Minch. The Atlantic border is
somewhat better covered with soil, as is the case with South Uist and
the other islands already described.
Vatersey, Saundry, Papey, Miuley, and Bearnarey, are all equally well
glaciated; but as they show little or no low-ground with gentle slopes,
they have preserved few traces of sub-glacial débris. In this respect
they resemble the rockier and hillier parts of the large islands to
the north. Till, however, is occasionally met with, as for example on
the low shores of Vatersey Bay, and on the southern margin of Miuley.
Doubtless, if it were carefully looked for it would be found sheltering
in patches in many nooks and hollows, protected from the grind of the
ice that advanced from the south-east. I saw it in several such places
in the islet of Bearnarey, where the striæ indicated an ice-flow as
usual towards the north-west.
We have now seen that the whole of the Long Island has been ground,
and rubbed, and scraped by land- or glacier-ice which has traversed
the ground in a prevalent south-east and north-west direction. We have
seen also that this ice attained so great a thickness that it was able
to overflow all the hills up to a height of 1600 feet above the sea.
It is needless to say that such a mass could not have been nurtured on
the islands themselves. They have no gathering grounds of sufficient
extent, and if they had, the ice would not have taken the peculiar
direction it did. Instead of flowing across the islands it would have
radiated outwards from the mountain-valleys. Where, then, did the ice
come from?
Looking across the Minch we see Skye and the mountains of the
north-west Highlands, and those regions, as we know, have also been
subjected to extreme glaciation. From the appearances presented by
the mountains of Ross-shire we are compelled to believe that all that
region was buried in ice up to a height of not less than 3000 feet--the
ice-sheet was probably even as much as 3500 feet in thickness. The
evidence shows that the under portion of this vast ice-sheet flowed
slowly off the country into the Minch by way of the great sea-lochs.
Thus we know that an enormous mass crept down Loch Carron and united
with another great stream stealing out from the mountains of Skye, to
flow north through the hollows of Raasay Sound and the Inner Sound
into the Minch. So deep was the ice that it completely smothered the
island of Raasay (1272 feet high) and overflowed all the lofty trappean
table-lands of Skye. From the Coolins, as a centre-point, another
movement of the ice-sheet was towards the south-west, against the
islands of Rum, Cannay, and Eigg. Further north similar vast masses of
ice streamed out into the Minch, from Loch Torridon, Gairloch, Loch
Ewe, and Loch Broom. The direction of the glaciation in the north of
Skye, which is towards north-west, shows that the glacier-mass which
overflowed that area must eventually have reached the shores of the
Long Island. In short, there cannot be a reasonable doubt that the
immense sheet of ice that streamed off the north-west Highlands must
have filled up entirely the basin of the Minch, and thereafter streamed
across the Outer Hebrides. But it may be objected that if the Outer
Hebrides were overflowed by ice that streamed from the mainland across
the north end of Skye, we ought to get many fragments of Skye rocks
and Ross-shire rocks too in the sub-glacial débris or till of Lewis
and Harris, and the north end of North Uist. But all such fragments
are apparently wanting. True, there are bits of stone like the igneous
rocks of Skye often met with in the Hebridean till, but as veins or
dykes of precisely the same kind of rock occur in the Long Island
itself, we cannot say that the stones referred to are other than
native. A little reflection will show us, however, that it is extremely
improbable indeed that stones derived from Skye and the mainland should
ever have been dragged on under the ice, and deposited amongst the till
of the Long Island. There is only one part of the whole Outer Hebrides
where we might have anticipated that fragments from the mainland should
occur; and there, sure enough, they put in an appearance.
But before I attempt to explain the non-occurrence of Skye rocks in
the till of the Outer Hebrides, let me show in a few words what the
glaciation of the Long Island, Skye, and the north-west Highlands
teaches us as to the general aspect presented by the ice-sheet. The
height reached by the surface of the ice in Ross-shire and the Long
Island respectively indicates of course that the main movement was
from the mainland. We must conceive of an immense sheet of solid ice
filling up all the inequalities of the land, obliterating the glens,
and sweeping across the hill-tops; and not only so, but occupying the
wide basin of the Minch to the entire exclusion of the sea, the surface
of the ice rising so high that it overtopped the whole of the Outer
Hebrides, and left only the tips of a few of the higher mountains
uncovered. The slope of the surface was persistently outwards from the
mainland, and the striation of the Long Island indicates clearly that
the dip or inclination of that surface was towards the north-west. Nay,
more than this, we are now enabled for the first time to say with some
approach to certainty what was the precise angle of that inclination.
If we take the upper surface of the ice in Ross-shire to have been 3000
feet (and it was not less), then the slope between the mainland and
the Outer Hebrides was only 25 feet in the mile, or about 1 in 210. It
is quite possible, however, and even probable, that the actual height
attained by the ice-sheet in the north-west Highlands was more than
3000 feet. I think it may yet turn out to have been 3500 feet, and if
this were so it would give an inclination for the surface of the ice of
about 35 feet in the mile. In either case the slope was so very gentle
that to the eye it would have appeared like a level plain. Over the
surface of this plain would be scattered here and there a solitary big
erratic or two, while in other places long trains of large and small
angular boulders would stream outwards. All these would be derived from
such mountain in Skye and the mainland as were able to keep their heads
above the level of the ice-flow; while a few also might be dislodged by
the frost and rolled down upon the glacier from the tips of the Clisham
and the Langa in Harris, and Hecla and Beinn Mhor in South Uist. Every
such block, it is evident, would be carried across the buried Hebrides,
out into the Atlantic in the direction indicated by the glaciation of
the Long Island--that is, towards the north-west.