THE SUBSIDENCES AND DISLOCATIONS OF THE PLATEAUX
There can be no doubt that considerable alterations of level have
taken place over the volcanic areas of North-Western Europe since
the eruptions that produced the basalt-plateaux, These alterations
embrace general and local subsidences, and also dislocations by which
considerable displacements of the crust either in a downward or upward
direction have been effected.
i. SUBSIDENCES
The mere fact that in many places the lower members of the series of
terrestrial lavas have been submerged under the sea may be taken to
prove a subsidence since older Tertiary time. Along the west coast of
Skye this depression is well shown by the almost entire concealment
of the bottom of the plateau under the Atlantic. In the Faroe Isles
the subsidence has advanced still further, for not a trace of the
underlying platform on which the basalts rest remains above water. In
Iceland, too, the complete submergence of the base of the Tertiary
volcanic sheets points to a widespread subsidence of that region.
Another strong argument in favour of considerable depression may be
derived from a comparison of the submarine topography with that of the
tracts above sea-level. It is obvious that the same forms of contour
which are conspicuous on the land are prolonged under the Atlantic.
If we are correct in regarding the valleys as great lines of subærial
erosion, their prolongations as fjords and submarine troughs must be
considered as having had a similar origin. We can thus carry down the
surface of erosion several hundred feet lower than the line along which
it disappears under the waves.
I know no locality where this kind of reasoning is so impressively
enforced upon the mind as the west end of the Scuir of Eigg. The
old river-bed and its pitchstone terminate abruptly at the top of
a great precipice. Assuredly they must once have continued much
further westward, as well as the sheets of basalt that form the main
part of the cliff. Yet the sea in front of this truncated face of
rock rapidly deepens to fully 500 feet in some places. Had any such
hollow existed in the volcanic period it would have been filled up
by the long-continued outflowings of basalt. Making every allowance
for concealed faults and local subsidences, we can only account for
this submarine topography by regarding it as having been carved out,
together with the topography of the land, at a time when the level of
the latter was at least 500 feet higher than it is now.
The subsidence which is thus indicated along the whole of the
North-West of Europe probably varied in amount from one region to
another. We seem to have traces of such inequalities in the varying
inclinations of different segments of the basalt-plateaux. The angles
of inclination are almost always gentle, but they differ so much in
direction from island to island, and even among the several districts
of the same island, as to indicate that certain portions of the
volcanic plain have sunk rather more than other portions.
Thus in the Faroe Islands, where the bare cliffs allow the varying
angles of inclination to be easily determined, a general gentle dip
of the basalts in a south-easterly direction has been noted among the
central and northern islands by previous observers. This inclination,
however, is replaced among the southern islands by an equally gentle
dip towards the north-east. The centre of depression would thus seem to
lie somewhere about Sandö and Skuö. The highest angle of inclination
which I noticed anywhere was at Myggenaes, where the basalts dip E.S.E.
at about 15°.
Among the Western Isles, also, where similar variations in the
inclination of the basalt-sheets are observable, it might be possible
by careful survey to ascertain the probable position of the areas of
maximum depression, and to show to what extent differential movements
have affected the originally nearly level volcanic floor. It would
doubtless be found that everywhere the dominant movement has been one
of subsidence. The vast outpourings of lava would tend to leave the
overlying crust unsupported, and to cause it to sink into the cavities
thus produced.
Perhaps the most extensive subsidence of this kind, at least that which
admits of most satisfactory investigation, because it still remains
above sea-level, is displayed by the vast hollow in the Antrim plateau,
which embraces the basin of Lough Neagh and the valley of the Lower
Bann. This depression measures about 60 miles in length by about 20
in breadth. Its axis follows the N.N.W. trend so characteristic of
the volcanic features of Tertiary time. The depression may be said to
involve the entire basaltic plateau of Antrim, for with the exception
of a few insignificant areas along the borders, especially on the east
side between Larne and Cushendall, the whole region slopes inward from
its marginal line of escarpments, which reach heights of 1800 feet and
upwards, towards the great hollow in its centre (see Map VII.).
Lough Neagh, which occupies the deepest part of this hollow, and covers
about one-eighth of the whole area of subsidence, is the largest sheet
of fresh water in the British Isles, for it exceeds 150 square miles in
extent of surface. Yet, for its size, it is one of the shallowest of
our lakes, its average depth being less than 40 feet. Its shallowness,
compared with its wide area, marks it out in strong contrast to most
of the larger British lakes. Its surface is only 48 feet above the
level of the sea.
The origin of Lough Neagh, the theme of various legends, has been
seriously discussed by different writers, but most exhaustively by the
late E. T. Hardman of the Geological Survey.[434] This author connected
the formation of the lake-basin with a series of large faults which
are found intersecting the rocks around the basin, and passing under
the water in a general north-easterly direction. He showed that these
faults have produced serious displacements of the strata, amounting
sometimes to as much as 2000 feet, and he believed that it was by the
concurrent effect of such dislocations that the depression of Lough
Neagh had been caused.
[Footnote 434: "On the Age and Formation of Lough Neagh," _Journ. Roy.
Geol. Soc. Ireland_, vol. iv. (1875-76), p. 170; also Explanation of
Sheet 35 of the _Geol. Surv. Ireland_ (1877), p. 72.]
It is possible that these displacements may have contributed to at
least the earlier stages in the history of the Antrim subsidence. They
have undoubtedly taken place after the outpouring of the basalts, for
these rocks are involved in their effects. But in the hollow of the
Bann valley north of Lough Neagh the faults which have been detected
in the basaltic plateau are few and trifling. The bold and bare
escarpments, that so clearly display the relations of the rocks, reveal
few traces of any important transverse dislocations. Nor has any proof
of large longitudinal faults parallel with the axis of depression been
obtained within the area of the Bann valley.
The earliest evidence for the existence of a lake on the site of the
present Lough Neagh has been supposed to be furnished by certain fine
clays, sands, seams of lignite and clay-ironstone, which have been
referred to the Pliocene period. These deposits have been regarded as
indicating the accumulation of fine sediment with drift vegetation
brought down into a quiet lake by streams entering from the south.
Their fresh-water origin was believed to be further corroborated by the
occurrence of shells belonging to the lacustrine or fluviatile genus,
_Unio_.[435]
[Footnote 435: These shells were regarded as forms of _Unio_ by the late
W. H. Baily; but Dr. Henry Woodward assigned them to _Mytilus_. See
Prof. Hull's _Physical Geology and Geography of Ireland_, 2nd edit. p.
101. The shells have been more recently dug out by Mr. Clement Reid,
who has found them to be the common _Mytilus edulis_.]
The thickness of this series of strata, their position above sea-level,
and their distribution are important parts of the evidence for the
geological history of the locality. At one place the deposits are said
to have been bored through to a depth of 294 feet, and Mr. Hardman
believed them to be not less than 500 feet deep. The same observer
found that they certainly reach a height of 120 feet above the sea,
and he was of opinion that in some places their height was not less
than 140 feet. The deposition of strata to the depth of 300 feet below
a level of 120 feet above the sea would, of course, entirely fill up
Lough Neagh, and spread over a large tract of low ground around it.
The pottery-clays and lignites, however, appear to be confined to the
southern half of the lake, from which they rise gently into the low
country around.
The distribution of these deposits and their extraordinary variations
in altitude, as described by Mr. Hardman, present great difficulties
in the attempt to regard them as the sediments of a Pliocene lake.
A more recent examination of the ground by Mr. Clement Reid of the
Geological Survey has led that able observer to believe that two
totally different groups of strata at Lough Neagh have been confounded.
He noticed the _Mytilus_-clay to be a dark blue mass full of derived
boulder-clay stones, and yielding _Mytilus edulis_ and seeds of a
sedge. This deposit cannot be Pliocene, but must be of Glacial or
post-Glacial age, possibly contemporary with the Clyde beds. The
junction of this clay with the pipe-clays is not at present seen,
but the lithological contrast between the two groups of strata is so
strong as to indicate their independence of each other. Mr. Reid found
the white, red and mottled pipe-clays with their masses of lignite
to present a strong resemblance to the Bagshot group in the Tertiary
series. It is possible, as already suggested, that the pipe-clays and
lignites may belong to the sedimentary zone that separates the lower
and upper basalts of Antrim. At all events they furnish no proof of
any Pliocene lake, and may not indicate more than a deeper part of the
depression in which the tuffs, lignites and iron-ore were laid down.
The existence of the _Mytilus_-clay shows that in Glacial or
post-Glacial times the valley of the Bann was a strait or fjord into
which the sea entered. Thick masses of drift have been laid down all
round and over the depression now occupied by Lough Neagh, insomuch
that had any older lake existed here in Glacial times, it could hardly
have escaped being filled up.
The observer, who from one of the basalt-heights looks down upon the
expanse of Lough Neagh and the broad peat-covered plain that continues
the level platform of the lake-surface down the valley of the Bann,
cannot but be impressed with the size of this wide hollow in the heart
of the Antrim plateau, and with the evident continuity of the whole
depression from the lake to the sea. If he be a geologist, he will
be further struck by the fact that while the Chalk and other older
rocks appear from under the basalt-escarpments all round the plateau,
at heights of many hundred feet above the sea, the floor of this
wide hollow is entirely covered with basalt. Had the depression been
merely due to denudation, the rocks that underlie the volcanic series
would have been exposed to view. The base of the basalts which, on
either side of the depression, is often more than 1000 feet above the
sea-level, sinks below that level in the hollow of the Bann and Lough
Neagh.
This inequality of position may have been partially brought about by
faults like those around Lough Neagh, and may thus have been begun long
before the Glacial period. But it appears to me to be mainly due to a
wide subsidence, of which the axis ran in a N.N.W. and S.S.E. direction
from the present coast up the valley of the Bann and the basin of Lough
Neagh to beyond Portadown.
We may conceive that after the cessation of the outflows of basalt,
the territory overlying the lava-reservoir that had been emptied would
tend to subside, partly by ruptures of the crust producing faults
and partly by a downward movement of a more general kind. In course
of time, these disturbances turned the drainage into the hollow now
traversed by the Bann. Denudation would necessarily accompany them, and
the surface of the country would be continually eroded and lowered.
Lough Neagh has been carefully sounded by the Admiralty, and its
chart affords much suggestive material for the consideration of the
geologist.[436] From the soundings there given it has long been known
that the lake deepens towards its northern end, and attains a maximum
depth of 102 feet. But it is not until we trace on the chart a series
of contour-lines for successive depths, as shown by the soundings, that
we realize the remarkable form of the lake bottom. We then discover
that below a depth of 50 feet a well-defined channel extends for
rather more than half the length of the lake. This channel begins to
be distinctly perceptible between Kiltagh Point and Langford Lodge. It
first runs in a northerly course on the west side of the centre of the
Lough, but when it comes into a line with Saltera Castle on the western
shore, it wheels round so as to conform to the curve of the Antrim
coast-line, which it follows northward until, about two miles from the
exit of the lake, its outline ceases to be traceable on the gently
shelving bottom. Its total length is thus about 12 miles.
[Footnote 436: Lough Neagh surveyed and sounded by Lieut. Thomas Graves,
R.N.]
There can hardly be any doubt that this channel is a former bed of the
River Bann. It occupies exactly the position which that stream would
take if the lake were drained, and its depth and breadth correspond to
those of the valley-bottom of the present river. If this conclusion be
accepted, some important conclusions may be further deduced from it.
1. The presence of a former course of the Bann on the bottom of Lough
Neagh proves the lake to be much younger than the Ice Age. The thick
boulder-clays and Glacial gravels which so encumber the country
around and descend under the lake, would assuredly have filled up the
river-channel had it existed at the time of their deposition. The
channel has obviously been cut out of these drifts since the Glacial
period. When the erosion took place, the present Lough Neagh could
not have existed, but the Bann followed a continuous course across
the plain which the lake now covers. The river probably maintained
its place for a long period, so as to be able to excavate so wide and
deep a bed in the drifts, if, indeed, it did not to some extent slowly
carve its bed out of the underlying basalts. It must be remembered that
sediment is being continually poured into Lough Neagh, and that some
of the silt must have accumulated in the submerged river-course, thus
lessening its depth and width. That the channel should still be so
marked may be used as an argument for the comparatively late date of
the subsidence.
2. The submerged river-course is a clear proof of subsidence. The
present Lough Neagh cannot be looked upon as a glacial lake formed by
rock-erosion or by irregular deposition of drift. Its floor must have
been a land surface when the Bann cut out its bed upon it. The whole
area has sunk down, the drainage has been arrested, and some 20 miles
of the course of the Bann are now under a sheet of shallow water.
This subsidence was not brought about by faults. It seems rather to
have resulted from a general sinking of the ground. The movement was
probably comparatively rapid, otherwise the river-course would hardly
have survived so well.
3. These inferences, based upon purely geological considerations, have
an interesting bearing upon the allusions to the origin of Lough Neagh
contained in some ancient historical documents. Various legends have
from an early period been handed down as to the first appearance of
this sheet of water. These myths, though differing in details, agree in
describing such a sudden or rapid accumulation of water as destroyed
human life, in a district which had previously been inhabited by man.
The earliest records indicate that the alleged catastrophe took place
in the first century of the Christian era.[437] It appears to me not
improbable that the tradition,thus preserved in these legends, may
have had its basis in the actual disturbance which, on geological
grounds, can be shown to have determined the existence of Lough Neagh.
Though the event may go back far beyond the first century, there can
be no doubt that, in a geological sense, it was one of the most recent
topographical changes which the British Isles have undergone.
[Footnote 437: For versions of the legends, see Dr. Todd's "Irish Version
of the Historia Britonum of Nennius," _Roy. Hist, and Archæol. Assoc.
Ireland_; Dr. Reeves' "Ecclesiastical Antiquities of Down," etc., p.
370; Mr. J. O'Beirne Crowe's "Ancient Lake Legends of Ireland," No. 1
in _Journ. Roy. Hist. and Archæol. Assoc. Ireland_, vol. i. (1870-71),
p. 94; _Giraldus Cambrensis_, vol. v. cap. ix. p. 91--"de lacu magno
miram originem habente." Moore's well-known lines embody the popular
belief that round towers and other buildings were submerged by the
inundation.]
Thus the Antrim basalt-plateau, in addition to the high interest of its
volcanic history, has the additional claim to our attention that it
has preserved, more fully and clearly than any other of the plateaux,
the evidence for the latest subterranean movements that followed the
long series of volcanic eruptions during Tertiary time. It contains the
record of a post-Glacial subsidence that gave birth to the largest lake
in Britain.
ii. DISLOCATIONS
Though I have not observed any features among the Tertiary
basalt-plateaux of the British Isles that can be compared to the
remarkable rifts and subsidences of Iceland, it can be shown that these
piles of volcanic material have undoubtedly been fractured, and that
portions of them have subsided along the lines of dislocation.
Careful examination of the basalt-escarpments of the Inner Hebrides
discloses the existence of numerous faults which, though generally
of small displacement, nevertheless completely break the continuity
of all the rocks in a precipice of 700 or 1000 feet in height. Not
infrequently such dislocations give rise to clefts in the cliffs. Some
good illustrations of this feature may be noticed on the north side
of the island of Canna, where the precipice has been fissured by a
series of dislocations, having a hade towards the west and a throw
which may in some cases amount to about 20 or 25 feet. The cumulative
effect of this system of faulting, combined with a gentle westerly
dip, is to bring down to the sea-level the upper band of conglomerate
which further to the east lies at the top of the cliff. Again, the
basalt-escarpment on the west side of Skye, from Dunvegan Head to Loch
Eynort, is traversed by a series of small faults. On the east side of
Skye and in Raasay, a number of faults, some of them having perhaps a
throw of several hundred feet, has been mapped by Mr. H. B. Woodward.
The largest dislocation observed by me among the basalt-plateaux of
the Inner Hebrides is that already referred to (p. 209), which runs at
the back of the Morven outlier, in the west of Argyllshire, from the
Sound of Mull by the head of Loch Aline to the mouth of Loch Sunart,
along the line of valley that contains the salt-water fjord Loch Teacus
and the fresh-water lakes Loch Durinemast and Loch Arienas. While the
Cretaceous deposits and the bottom of their overlying basalts rise but
little above the sea-level on the south-west side of this line, they
are perched as outliers on hill-tops on the north-east side, where they
rise to 1300 feet above the sea. The amount of vertical displacement
here probably exceeds 1000 feet. The fault runs in a north-westerly
direction, and has obviously been the guiding influence in the erosion
of the broad and deep valley which marks its course at the surface.
This dislocation is only the largest of a number by which the
basalt-plateau has been broken in the district of Morven. Their effects
are well shown in the outlier of basalt which caps Ben Iadain, where
two parallel faults bring down the lavas against the platform of
schists on which they lie (see Fig. 266).
Many faults have been traced in the Antrim plateau, and are represented
on the Geological Survey Maps. In general they are of comparatively
trifling displacement. Occasionally, however, they amount to several
hundred feet, as in those already referred to as occurring near
Ballycastle and around the southern part of the basin of Lough Neagh.
To what extent the dislocations that traverse the British Tertiary
basalts are to be regarded as comparable to those which in Iceland
have been referred to subsidence caused by the tapping and outflow of
the lower still liquid parts of lava-sheets must be matter for further
inquiry. So far as my own observations have yet gone, the faults do not
seem explicable by any mere superficial action of the kind supposed.
Where they descend through many hundreds of feet of successive sheets
of basalt, and dislocate the Secondary formations underneath, they
must obviously have been produced by much more general and deep-seated
causes.
It is conceivable that, if these dislocations took place during the
volcanic period, they broke up the lava-plains into sections, some
of which sank down so as to leave a vertical wall at the surface on
one side of the rent, or even to form open "gjás," like those of
Iceland. But it is noteworthy that the fissures, which have been filled
with basalt and now appear as dykes, comparatively seldom show any
displacement in the relative levels of their two sides. In Iceland,
also, the great lava-emitting fissures seem to be in general free from
marked displacements of that kind.
[Illustration: Fig. 382.--Reversed fault on the eastern side of Svinö,
Faroe Isles.]
The faults in the Inner Hebrides, so far as I have observed, are all
normal, and indicate nothing more than gentle subsidence. But among the
Faroe Islands I have come upon several instances of reversed faults,
which, in spite of the usually gentle inclinations of the basalts,
probably point to more vigorous displacement within the terrestrial
crust.
On the east side of Svinö a fault with a low hade runs from sea-level
up to the top of the cliff, a height of several hundred feet. It has
a down-throw of a few yards, but is a reversed fault, as will be
seen from Fig. 382. Another similar instance may be noticed on the
north-east headland of Sandö, where, however, on the upcast side, the
basalts appear as if they had been driven upward, a portion of them
having been pushed up into a low arch (Fig. 383).
[Illustration: Fig. 383.--Reversed fault on the north-east headland of
Sandö, Faroe Isle.]
When the Tertiary basalt-plateaux of the Hebrides and the Faroe
Isles come to be worked out in detail, many examples of dislocation
will doubtless be discovered. We shall then learn more of the amount
and effects of the terrestrial disturbances which have affected
North-Western Europe since older Tertiary time. In the meantime
evidence enough has been adduced to prepare us for proofs of very
considerable recent displacements even among regions of crystalline
schists, like that which has been disrupted by the Morven faults above
alluded to. While the study of the Tertiary volcanic rocks demonstrates
the vast general denudation of the country since older Tertiary time,
the proofs that these rocks have been faulted acquire a special
interest in relation to the origin and evolution of the topography of
the region.