MORAINIC RIDGES ON THE FLOOR OF THE
IRISH SEA
G. F. MITCHELL
Department of Geology, Trinity College, Dublin
In 1945 FARRINGTON published a short paper on ' The
Xevel of the Ocean in Glacial and Late-glacial Times,' because
as he said " The question of Quaternary sea-level is an important one, not only for its intrinsic interest but also for dis. eussions of the geomorphology of the continental shelf and
•slope and of the migrations of flora and fauna and of man."
The origin of the Irish flora and fauna has been discussed
many times. Evidence about the flora of Ireland in inter.glacial times is now accumulating (see WATTS, 1959), and it
is clear that the interglacial flora was very similar to that of
the present day, and included American and Lusitanian plants
that are still present in the Irish flora. Periods of cold broke
up and dispersed the interglacial floras, and during the
latest phase of cold (8,800-8,300 B.C., GODWIN, 1960) much
of north-west Europe, including Ireland, had a type of tundra
vegetation rich in arctic and sub-arctic elements. The bulk
of the modern Irish flora and fauna must have entered the
country overland from Britain immediately after the final
phase of cold. From the opening of the postglacial (Zone IV,
8,300-7,600 B.C.) we have many records of warm-loving plants,
and at this time temperature was no longer a bar to migration.
In a recent paper (MITCHELL, 1960) I pointed out that
a number of ridges, probably moraines of the last glaciation, run across the floor of the Irish Sea. Before
discussing these ridges as potential migration routes, I wish to
describe them a little more fully. My data came from Admiralty Chart 1824 A of the East Coast of Ireland with the Irish
Channel (1885). Needless to say such description is highly
subjective, and other workers may not agree with the picture.
A deep channel runs between Britain and Ireland. If a
contour is drawn on the walls of this channel at —325'
(—100 m.), the floor of the channel intersects this contour
in only four areas (i) between Carnsore Point and St. David's
Head, (ii) between Arklow and the Lleyn Peninsula, (iii)
between the north Leinster coast and the Isle of Man, and
(iv) between Inishowen and Islay, If a contour is drawn
.at —400' (—120 m.), there is no ground south of the line
335
joining Arklow and the Lleyn Peninsula that lies below
this level. The —500' (—150 m.) contour runs north from a.
point west of the Isle of Man, and the —600' (—180 m.)
contour marks a narrow deep (greatest depth 900' (275 m.)
west of Galloway, and another deep north of Rathlin Island
(greatest depth 700' ; 215 m.). This suggests that if this deep
channel was cut by running water, it was cut by water that
was moving to the north, though the fact that material of
glacial origin is probably thicker in the southern half of the
Irish Sea cannot be overlooked. Such a concept of the
" Irish Sea River " of course reverses that set out by
SCHARFF (1907, Fig. 13).
I picture the ice advancing down the dry floor of the Irish
Sea against the slope of the channel, and then spanning the
floor with moraines as it withdrew. When first created these
moraines linked up with the moraines that are still to be seen
on land above modern sea-level. But modern wave-erosion
and coastal drift have destroyed the parts of the moraines
that lay immediately outside the present coast-line. Thus
while we can see the ridges on the floor of the deeper parts
of the Irish Sea (Fig. 1), we cannot trace them through the
shallow coastal waters.
(i) Between Carnsore Point and St. David's Head.
This
ridge, with its lowest point at —320' (—98 m.), is not so clearly
defined as, and has a smoother topography than, the ridges
farther north. It may be an older deposit, or it may link with
the kettlemoraine that rises to 330' (100 m.) above modern
sea-level north of Curraghcloe in Wexford.
(ii) Between Arklow and the north side of the Lleyn Peninsula.
Here there is a complex of steep-sided ridges and basins trending towards the end-moraine on the north side of the Lleyn
Peninsula to the east and to the morainic gravels and sands
of north Wicklow on the west. The low point is at —275'
(—84 m.).
(iii) Between the north Dublin coast and the Isle of Man.
Steep slopes, enclosed basins, steep-sided valleys, all suggest
morainic topography. The south ridge (low-point —295' ;
—90 m.) is the most prominent. There are two less prominent
ridges farther north, each with low-points at —320' ; —98 m.
On land we can note the Gormanstown moraine, the morainic
complex at the north of the Isle of Man, and the moraines
at St. Bees.
(iv) Between Inishowen and I slay. The deep below —600'
(—180 m.) off Galloway disappears as we pass the mouth of
the Firth of Clyde, but reappears off Rathlin Island ; has it
336
[ABERDEEN
so
100
, • SMESTOW
Fig. 1. Morainic ridges on the floor of the Irish Sea. Stages""of the
last glaciation are shown as in MITCHELL, 1960.
337
been filled up by outwash from the Clyde glacier ? Deep
water here is nearer to the Irish than the Scottish coast.
North of Rathlin Island we are in the area of the ice that
came down the west side of the Mull of Kintyre, reached
Ireland at Armoy and Inishowen and fanned out to the
north-west, building up a morainic ridge, now 180' (55 m.)
below modern sea-level, which curves back towards Islay.
We may sum up by saying that if modern sea-level were
to fall by 180' (55 m.) dry land would link Inishowen to Islay,
if it were to fall by 275' (84 m.) Wicklow would be joined to
Wales. While at first sight the Inishowen/Islay link would
seem to have been the one most readily available for postglacial migration of flora and fauna, this part of the British
Isles was affected by isostatic movement, and for reasons
which will be developed later in this paper, I do not think that
this route was open. For the route from Wales to Wicklow
to have been open, sea-level in the Irish Sea must have been
at least 275' (84 m.) below its present level in 8,300 B.C.
Can this have been the case ?
The consensus of opinion (as summarised by FLINT, 1957,
p. 270) is that at the maximum of the last glaciation sea-level,
due to water locked up in the ice-sheet, was at least 295'
(90m.) below its present level, and was very likely 320'
(100 m.) below its present level. A recent paper (DONN,
etc., 1962), based on revised estimates, suggests that sealevel was at least 345' (106 m.) below its present level, and
may have been 405' (122 m.) below its present level, at the
last glacial maximum in North America, dated to 18,000 years
ago. The maximum glaciation appears to have been rather
earlier in Scandinavia and in the British Isles, but as the
American ice-mass was probably more than four times greater
than that of Europe, it will have had a proportionately larger
effect on sea-level. Nonetheless between the time of the
glacial maximum in America 18,000 years ago, and the time
of concern to us, the opening of the postglacial period in
Europe at 8,300 B.C., a lot of water must have returned to
"the oceans and have raised their level.
In his 1945 paper FARRINGTON sought to show that
" a minimum estimate of the amount of the lowering of the
level of the ocean at the maximum should be much more
than 400' (120 m.)." At the time we are discussing in regions
which had formerly been covered by ice the land was rising
isostatically, and the sea was rising eustatically, and as
FARRINGTON pointed out " when the surface of the sea
and the land are rising at varying and unknown rates the
338
only definite proof of the return of'water to the sea is the record
of increasing depth." Endeavouring to estimate the amount
by which sea-level rose, he quoted BROGGER'S work on the
lateglacial deposits of the Oslo fiord. BROGGER (1900/1)
claimed that a steady increase in depth was recorded in the
fiord, but this work, which depended largely on the interpretation of shelly faunas, has since been severely criticised, chiefly
by HESSLAND (1943). But if this proof of FARRINGTON'S
views must be discarded, another can be introduced in its
place.
Tree-stumps and peat are recorded below sea-level at many
places in the North Sea and at the south end of the Baltic
Sea. In this latter area one locality is south of Karlskrona.
Here the water is at present 140' (43 m.) deep, and a'pinestump from the bottom has been given a C-14 date of 7,150
B.C. (St-120, OSTLUND, 1957). This locality lies within
the area of isostatic uplift, as is shown by beaches of the
Littorina Sea lying 33' (10 m.) above sea-level on the adjoining
coast. Therefore at the time of the Littorina-maximum the
tree-stump was 173' (53 m.) below sea-level. At a second
locality south of Kaseberg, water-depth 118' (36 m.), age of
pine-stump 7,400 B.C. (St-179, OSTLUND, 1957), Littorina
beach 16' (5 m.) above sea-level, the stump was 134' (41 m.)
below sea-level at the maximum Littorina transgression.
When was the Littorina maximum in Sweden ? In Sodermanland where the Littorina limit is at 177' (54 m.), a mud
at 171' (52 m.) has a C-14 date of 5,000 B.C. (U-42, OLSSON,
1959) ; near Oslo where the Littorina beach is at 165' (50 m.),
shells from the beach have a C-14 date of 4,900 B.C. (T-123,,
NYDAL, 1960) ; in Smaland the Littorina beach has a C-14
date of 5,100 B.C. (St-191, OSTLUND, 1957). I do not think
that the maximum in the south of Sweden was greatly different
in age.
Therefore we can say that between 7,150 B.C. and 5,000
B.C. the water in the south of the Baltic deepened by at least
173' (53 m.), giving an average rate of rise of 8' (2.5 m.) per
century. We have already seen that the final episode of cold
in Europe came to an end about 8,300 B.C., and that warmloving plants came crowding back to the north-west. During
the period from 8,300—7,150 B.C. ice must have been melting,
and there is no reason to think that sea-level rose more slowly
than in the following period. A rise at the average rate of
8' (2.5 m.) per century from 8,300—7,150 B.C. would have
raised sea-level by 92' (28 m.).
What happened after 5,000 B.C. ? Until that time sealevel was rising rapidly, and was able to gain on the coasts
339
of Scandinavia even though that area was rising isostatically.
The rise in sea-level must have fallen off, because even though
the rate of land-rise was falling off also, sea-level could not
keep pace, and the Littorina beaches began to rise above
the level of the sea. Where is this further but slower eustatic
rise in sea-level recorded ? No post-glacial raised beaches
are known in south-west England or south-west Ireland which
appear to have been areas of crustal stability. A layer of
peat at Burnham-on-Sea, Somerset at a depth of 15' (4.5 m.)
below sea-level, pollen zone Vila, has a C-14 date of 4,300
B.C., Q-134, GODWIN & WILLIS,(1959). A peat from Foynes
in the Shannon Estuary 55' (17 m.) below sea-level, pollen-zone
VIb, points in the same direction, though no C-14 date is
available.
,
We thus arrive at the following total rise in sea-level since
8,300 B.C.
From 8,300-7, 150B.C.
From 7,150-5,000 B.C.
92'/28 m.
173753 m.
From 4,300 B.C. to to-day
TOTAL
1574.5 m.
28O'/85.5 m.
Such a total rise is certainly of the order required if morainic
land-bridges across the Irish Sea were to have been available
for plant and animal migration at the beginning of the postglacial period. It is fair to point out that at every step the
most disadvantageous figures have been taken. It has been
assumed that in Sweden there was no interval in time between
the growth of the trees and their submergence below the waters
of the Baltic ; there may have been a considerable interval.
The recorded rise from 7,150—5,000 B.C. is only the amount
by which the eustatic rise of the sea exceeded the isostatic
rise of the land, not the full amount by which sea-level rose
eustatically. No rise is allowed for the period from 5,000—
4,300 B.C., though the rise certainly continued during this
period.
The figures given for the morainic ridges are the lowest
points on the ridges to-day ; these low-points may have been
higher when the ridges were first built up. Basins between
the ridges may have held lakes and outflow from the lakes
may have lowered the exit channel. It is obvious that the
rise in sea-level must have been rapid, or else the morainic
features would have been destroyed by wave attack on their
unconsolidated materials (just as has subsequently happened
in shallow coastal waters). But when the ridges were first
breached by the rising sea, tidal scour would have cut a channel
340
even more rapidly than the sea could rise and so protect the
moraine from wave attack and tidal scour ; such a channel
•could have been of considerable depth (see CHARLESWORTH, 1930, p. 387).
Therefore it seems not unreasonable to me to consider
that for a short time after the beginning of the postglacial
period sea-level was more than 280' (85m.) below its present
level, that there were at this time morainic bridges across
the Irish Sea, and that the bulk of the modern Irish flora
and fauna entered the country across these bridges. Such a
low sea-level at the opening of the post-glacial period implies a
still lower level at the maximum of the last glaciation, and
•considerable areas of dry land may have existed to the west
and south of the present Irish coast line. But as the typical
•cold vegetation of Zone III (8,800—8,300 B.C.) has been
recorded from Wexford, Kerry and Galway, it seems likely
that a similar vegetation with tundra affinities will also have
covered the land so exposed.
Why do I reject the route between Donegal and Scotland ?
I reject it because here we are in a region of isostatic disturbance, and I believe that the amount of vertical movement
involved has been very much underestimated. DALY (1940,
p. 318) gives figures suggesting that while the uplift in
Fennoscandia exceeded 275 m., uplift in Scotland was only
30 m. I consider that the scale of movement for Scotland was
of the same scale as that for Scandinavia. Because they rise
from the ocean floor, the part of the Hawaiian Islands that
projects above modern sea-level is not specially high ; if the
islands rose from sea-level they would be one of the most
impressive mountain ranges on earth. Because the isostatic
depression of Scotland carried it far below modern sea-level,
only a fraction of the recovery is recorded above modern
sea-level by the ' 100-ft. beach ' (which of course gave DALY
his figure of 30 m.). When we have records of glacial marine
clays in Scotland (see for example SYNGE, 1956, p. 140),
these are more likely to indicate not that sea-level in the middle
of a glacial period was higher than it is to-day, but that the
land was isostatically depressed to such an extent that the
sea, even at its low glacial level, could invade the area within
the present coast-line whenever sea-level was appearing to
rise relative to the nett effects of isostatic change in sealevel and eustatic change in land-level. In many of the
developments of modern civilisation it has been taken for
granted that sea-level remains constant, and many millions
of pounds have been invested in harbour development on this
assumption. When considering isostatic and eustatic move341
ments of glacial times we cannot let ourselves be mesmerised
by the convenience of modern sea-level as a datum-line for
modern human measurements. We should recall that if the
ice of the Antarctic continent were to melt, sea-level would
rise by about 150' ; 45 m.
In approaching the Islay/Inishowen and the Firth of Clyde
areas, where we find the moraines of a re-advance from Scotland whose age cannot be more than 40,000 years nor less,
than 12,000 years, I make the assumption that the maximum
phase of the last glaciation was over as far as the British.
Isles were concerned, and that the isostatic movement was
upward. The growth of later smaller masses of ice might
retard the movement, but could not bring it to a halt or
reverse it. The lateglacial invasion of Scotland by sea-water,
and the formation of beaches and marine clays to-day found.
100' (30 m.) above modern sea-level, probably took place
about 12,000 years ago, for at Garscadden, near Glasgow,
the freshwater mud of the warmer Aller0d period (Zone II)
which began at 10,000 B.C. rests directly on the marine clay.
At Garscadden the marine clay is at 82' (25 m.) : discussing
the fact that the clay is not at 100' (30 m.), DONNER
(1959) pictures that the Garscadden deposits record a fall
in lateglacial sea-level. But the warm Aller0d period was
just setting in, and it seems most unlikely that the rate of
return of water to the ocean fell off at this point. I prefer to
think that at this time the isostatic uplift of Scotland was.
outstripping the rate at which the sea, still far below its
present level, was rising.
In the first part of this paper I have argued that at 8,300
B.C. sea-level in the Irish Sea was more than 280' (85 m.)
below its present level. When the marine lateglacial clays
were forming about 10,000 B.C., there was more ice on the
earth's surface than at 8,300 B.C., and therefore sea-level
must have been as low as —280' (—85 m.), if not substantially
lower, when the clays were forming. The so-called ' 100-ft.
beach ' must have formed at the same level as the sea of its.
day, and therefore since it was formed this beach has been,
isostatically uplifted not 100'; (30 m.) but at least 380' (116 m.).
The ' 100-ft. beach ' is well seen round the shores of theFirth of Clyde, particularly in the Ardrossan area. Thereis on the floor of the Firth of Clyde, both to the north-west and
the south-east of Ailsa Craig, a well-defined moraine, which:
now lies 150' (46 m.) below modern sea-level. At the time of
the ' 100-ft. beach ' this moraine must have been 250' (76 m.)
below the sea-level of the day. TING (1937) has pointed out
that steep slopes and hollows without outlet occur on the;
342
surface of the moraine, and it follows that it must have been
deposited sub-aerially, not below water or from a floating
ice-sheet. If the glacier which built up the moraine advanced
on dry land down the floor of the Firth of Clyde, then sealevel must have been 250' (76 m.) below the sea-level at the
time the ' 100-ft. beach ' was built up, and this sea-level was
in turn at least 280' (85 m.) below modern sea-level. Therefore at the time the moraine in the Firth of Clyde was built
up, sea-level was at least 530' (150 m.) below its present level.
It is clear that the extent of isostatic depression and recovery in Scotland (and in the north of Ireland) during the
last glaciation is, compared with what is known of similar
events in Scandinavia, still largely unexplored, and cannot
be further pursued here. Doubtless the isostatic depression
of Inishowen, which was farther from the centre of glaciation
than the Firth of Clyde, was on a smaller scale, but nonetheless
we cannot asume that if at 8,300 B.C. sea-level was 180'
(55 m.), or even 250' (76 m.) below its present level there
would have been a morainic land-bridge between Islay and
Inishowen.
I believe that Dr. Farrington was right in 1945, that the
glacial fall in sea-level has been underestimated, and that in
very early postglacial time morainic land-bridges provided
migration-routes across the Irish Sea.
BIBLIOGRAPHY
BROGGER, W. C.
1900-01 On the Late-Glacial and Postglacial changes of level in the
Christiania Region.
Norges Geologiske Undersogelse,
No. 31.
CHARLESWORTH, J. K.
1930 Some Geological Observations
on the Origin of the Irish Fauna
and Flora.
Proc. Roy. Ir. Acad., 39, B,
pp. 358-390.
DALY, R. A.
1938 The Architecture of the Earth,
New York.
DONN, W. L., FARRAND,
1962 Pleistocene ice volumes and
W. R., & EWING, M.
sea-level lowering.
J. Geol., 70, pp. 206-214
DONNER, J.
1959 The late- and post-glacial raised
beaches in Scotland.
Suom. Tied. Toim., Ser. A, III,
Geol.-Geog., 53, pp. 5-25
FARRINGTON, A.
1945 The level of the ocean in glacial
and late-glacial times.
Proc. Roy. Ir. Acad., 50, B,
pp. 237-243.
343
FLINT, R. F.
GODWIN, H., WALKER,
D., & WILLIS, E. H.
GODWIN, H., &
WILLIS, E. H.
HESSLAND, I.
MITCHELL, G. F.
NYDAL, R.
OLSSON, I.
OSTLUND, H. G.
SCHARFF, R. F.
SYNGE, F. M.
TING, S.
WATTS, W. A.
1957
Glacial and Pleistocene Geology
New York
1957 Radiocarbon dating and postglacial vegetational history :
Scaleby Moss
Proc. Roy. Soc., B, 147, pp.
352-366
1959 Cambridge University Natural
Radiocarbon Measurements I
Amer. J. Sci., Radioc. Suppl.,
1, pp. 63-75
1946 Marine
Schalenablagerungen
Nord-Bolraslân (Sweden)—Marine shell deposits of northern
Bohuslan (Sweden)
Bull. Geol. Inst. Uppsala, 31,
pp. 1-348
1960 The Pleistocene history of the
Irish Sea
Advanc. Sci., 17, pp. 313-325
1960 Trondheim Natural Radiocarbon Measurements II
Amer. J. Sci., Radioc. Suppl.,
2, pp. 82-96.
1959 Uppsala Natural Radiocarbon
Measurements
Amer. J. Sci., Radioc. Suppl.
1, pp. 87-102
1957 Stockholm Natural Radiocarbon
Measurements I
Science, 126, pp. 493-7
1907 European Animals London
1956 The Glaciation of North-east
Scotland
Scott. Geog. Mag., 72, pp. 129143
1937 The coastal configuration of
western Scotland
Geog. Ann., 19, pp. 62-83
1959 Interglacial deposits at Kilbeg
and Newtown, Co. Waterford
Proc. Roy. Ir. Acad., 60, B,
pp. 79-134.
344