The Level of the Ocean in Glacial and Late-Glacial Times

The Level of the Ocean in Glacial and Late-Glacial TimesAuthor(s): A. FarringtonSource: Proceedings of the Royal Irish Academy. Section B: Biological, Geological, andChemical Science, Vol. 50 (1944/1945), pp. 237-243Published by: Royal Irish AcademyStable URL: http://www.jstor.org/stable/20490836 .

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[237] 50 B 12

XIL.

THE LEVEL OF THE OCEAN IN GLACIAL AND LATE-GLACIAL TIMES.

By A. FARRINGTON.

[ReaA 12 FaaniArY. Published 21 A\k&cu, 19454

IN 1928 Antevs (1) published an estimate of the fall in level of the surface

of the ocean due to the locking up of water in the ice-sheets of the last

glaciation. He arrived at the conclusion that the fall in level lay btween

the limits of 290 and 305 feet. This estimate, like most of those that went

before it, was ba ed on a calculation of the cubie content of the ice-sheets. In 1927, shortly before hi death, Ramsay was occupied in a similar

study, more elaborate than that made- by Antevs, in which he attempted to allow for the elevation and depression of the ocean floor as the resalt of the withdrawal and return of water dunrng the growth and decay of

thle ie-heets. IRamsay's work was published posthumously in 1930 (2). He dealt with the matximum of the Quaternary glaciation, and estimated

that the lowering of the surface of the ocean lay between 183 metres

(600 feet) and 275 metres (900 feet). An this allows for a recovery of the

ocean floor of about one-third of the lower.ing of the surface level of the

ocean 1amsay's estimate of the amount of water held in the ice-sheets is

between three and four times that of Antevs -

It is perhaps surprising that R]tamsay undertook this -survey at all as

he says aptly (2, p. 33): "The &lculation of the Quaternary ice masses is based on statements of the areas formerly glaciated and on estimates

of the thickness of the vanished glaciations, the latter factor Can vary

according to ;the author's view, the former will be, free from uncertainty

when all parts oI the globe are geologically surveyed, especially with

regard to the traces of' Quaternary glaciations." How great the

uincertainty is may be seen from the fact that Antevs estimates the volume

of the ice masses in Eurasiaduring the last glaciatio'n at 5,350,006 cubic

kilometres (1-, p 81), whereas Ramay's estimate for the same continental

area during the maximum glaciation is 43,250,000 cubic kilonietres Whatever may have been the difference between the iee mawses of these

two phases it eannot have been as great as this. Incidentally, Antevs and

Ramsay use the same authorities for obtaining their widely differing estimates.

P3ROC. RLIA., you.U, SECT.* B. [2BJ



2W38 Proceedings of the Royal Irish Academy.

It seems clear that estimates of the Quatereary lowering of the level of the ocean surface based on calculations of the cubic content of the

ice masses will be subject to considerable doubt until there is more detailed knowledge available about conditions of world glaciation, e-specially with

respect to northern Asia.

The question of Quaternary sea-level is an important one, not only for

its intrinsic interest but also for diseussions of the geomorphology of the continental shelf and slope and of the migrations of flora and fauna and

of man. It mav, therefore, be worth while to approach the problem from

a new direction. The glacial anrd post-glacial movements of land and sea have been noist

elosely studied in the Baltic region and in Scandinavia. Most of the

studies have. been made around the shores of the land-locked sea, and are,

therefore, of little value for our purpose. The 0slo fiord, however, has been in connexion with the ocean by means of the Norwegian deep from the time the Ice left it daring the last glacial retreat up to the present

day. It is the only place so far studied where one may find a direct and

continuous contact between the ocean and the land-surface since glacial

times, for it is probable that the greater part of the continental shelf was,

dry until comparatively recently. It is assumed in this discussion that after the withdrawal of the ice-sheet

in noth-wvestern Europe the movement of the land and sea have been upward in the main. There may have been minor oscillations of the sea but the rise of the land was continuous, even though it varied in the

rate of uplift. The small negative movements of the peripheral zonie are ignored.

When the surfaces of both the sea and the land are rising at varying and unknown rates the only definite proof of the retarn of water to the sea is the recording of mncremiag dep-th. If there is no increase in depth

it may be that both are stationary or both ae rising at the same rate. If

there is shallowing, it may mean either that the land is rising with the sea stationary or that both are rising with the sea moving more slowly than the land. If there is an inerease in depth with the land stationary, then the inerease gives the actual amount of the rise of sea-level: if the land is also rising, then the inerease in depth gives the amount by which

the rising sea-level has- outstripped the recovery of the land. Any record of increasing depth in the conditions under consideration is li%ely to give too small an estimate for the rise of sea-level.

The late-glacial deposits of the Oslo fiord have been tudid in considerable detail by Brbgger (4), and hisJ)observations are the basis of the present study. In his paper evidence is available for a steady increase in depth. He shows that at the time when the ice stood at the outer Ra



FARRINGTON-The Ocean in Glacial dud Late-Glacial Times. 239

the older yoldia clay was being deposited in water of a depth of from,

10-30 metres. The older yoldia clay is overlaid by the younger yoldia clay, the fauna of which indicates deposition in water of a depth of from

40 to 60 metres. Upon the younger yoldia clay lies the oldest area clay, which was laid down at a depth of from 80 to 100 metres. There is no

reason to suppose that the land was sinking at this period, as it had only

recently been relieved of a great load of ice. If the land was moving

upwards, as was most likely, the eustatic rise of sea-level would have been

greater by an unknown amount. On this evidence a rise in the level of

the ocean of 70 metres is a conservative estimate.

The Ras were deposited at the en;d of the Baltic Ice Lake (5; 6), and

on the withdrawal of the ice from these moraines the exposed area was

covered by the sea initiating the Yoldia Sea of the Baltic aa. The

evidence of eustatic rise in sea-level is no longer clear in Scandinavia, for

the land movement is so great that the criterion of increasing depth canhot

be applied. It is, however, known that at the period of the Ancylus Lake the bed

of the North Sea- was dry land down to what is now 50 metres below

sea-level (7; 8). A gap, including the Yoldia Sea period, intervenes between the two known rises in sea-level, so that the total of these two

amounts, 70 and 50 metres, is less than the total rise of the sea since the

beginning of the stand of the sea at the outer Ra.

The rise of 50 metres is an approximate figure, for the floor of the

North Sea may well have still be-en rising as a result of the withdrawal

of the earlier Scandinavian ice, and there may have been sagging beneath

the load of returned water. The 70 metres is a minimum figure, but it

is dependent on the accuracy of the depth determinations made from the

faunas of the various clays. These determinations are checked to some

extent by the fact that the older yoldia clay is found over a vertical range

of 120 metres. Brsgger, working with the heights of the deposits above

the present sea-level, estimates a total submergence of up to 240 metres

for the period from the beginning of the outer Ra to the time of the

epi-glacial moraines of the Oslo valley. This estimate is more subject to

overstatement than computations made from increasing depth of water.

For one thing, there is an unknown amount of submergence due to the

inflow, without any rise of surface, of the sea over the depressed land.

The more conservative estimate of 70 metres is preferred for the present

discussion giving a total rise in ocean level of at least 120 metres, say,

400 feet, since the beginning of the stand at the outer Ra.

By the time the last European icesheet had retreated to the outer Ra

its bulk was already greatly wasted. Also, the Ra is evidence of a halt

in its retreat. Accordingly, the bulk of the water which was responsible



'4240 Proceedings yf the Royal Irish Academy.

for the eustatic rise of 70 metres at the Ra must have come from extra

European sources, as must also the greater part of the rise of 50 metres,

as the melting of the small amount of Scandinavian ice remaining at the

beginning of Ancylus time could have had only small effect. Such a rise

of- 120 metres is more than enough to account for the melting of the whole

of the American ice-sheet as estimated by Antevs (1, p. 81), and Antevs

calculates that the North American ice-sheet accounted for 75 per cent.

of the water returned to the ocean. Does this mean that the whole of

the North American ice-sheet melted after the European ice-sheet was

virtually gone? This would be contrary to the abundant evidence that

there was general contemporaneity of the Quaternary ice-sheets over the

whole world. The alternative is that the melting of the extra-European

sheets took place contemporaneously with that of the European sheet. In

this case the 400 feet of recorded rise can only represent a part of the

total rise; from which it follows that Antevs' estimate of the amount of

ice is much too small.

As the time occupied by the recorded rise-that is, from the end of

the Baltic Ice Lake period to the maximum during the post-glacial

period-is roughly 5,000 years, or less than one-third of the estimate for

the retreat of the European ice-sheet from the maximum of the last

glaciation, a minimum estimate of the amount of the lowering of the level

of the ocea;n at the maximum should, therefore, be much more than 400 feet.

CORRELATIONS. The difficulty of correlating the Irish glacial episodes with those of

north-western Europe is considerable. Correlations have been made between the terminal moraines (9 and 10) and also between the stages of forest

development (10) in the two areas. These correlations are tentative, and perhaps the discussion may be reopened. If the conclusions suggested in

the earlier part of this note have weight they may be of use in correlation.

It has been shown above that the eustatic rise of sea-level in the

European area must have been caused by the melting of ice in other parts

of the world, so that it was general and contemporaneous as far as Europe

is concerned, with no material complication from the melting of local ice. Let us cosider one small problem.

The Highlad Glaciation of Scotland has beenl correlated with the

period of the Ra moraine and with the Athdown Mountain Glaciation of

Ireland', (3, 9 and 10). McCallien, however, suggests (11) that the "District Glaciation" of Scotland was "post-glacial," and equates the Lammermuir-Stranraer-Antrim coast advance with the Ha, but he gives no reason for the correlations.

1 For the nomenclature of the Irish glaciations, see

Earrington, Proc. R.I.A., vol. 50, B, 6.



FARRINGTON-The Ocean in Glacial and Late-Glacial Times. 241

It has been shown that at the beginning of the Ra period the level of

the ocean was at least 400 feet lower than at present. As the 100-foot

beach of Scotland is contemporaneous with the Highland Glaciation (3, p. 373, and 11, p. 189), if the latter is correlated with the Ra, then

Scotland must have been depressed by at least 500 feet at that period.

This is, of course, possible though hardly likely. But evidence has been given (3 and 11) that the relative sea-level had

fallen before the Highland glaciers had reached their maximum. This makes the suggested correlation improbable; for this was the period when,

judging from events in the Oslo area, the relative rise of sea-level was

most rapid. It is not likely that the depression of the land was as great

or the rate of rise as rapid in Scotland as in Scandinavia, and it would be

surprising to find emergence in the Scottish area at a time when the rise

of sea-level was most rapid.

It is more probable, therefore, that the Highland glaciers would have reached their maximum at a somewhat later period than that of the Ra.

The rate of eustatic rise had fallen off considerably by the beginning of

Ancylus' time and, therefore, an emergence of land in Scotland at that

period would be more likely. If the Highland Glaciation has this dating the necessaxy depression of the Scottish area for the formation of the

100-foot beach would be less than half the minimum necessary with the

earlier dating. In correlating the Highland Glaciation with the Athdown Mountain

Glaciation of Ireland there is also difficulty. It is known from studies

iii Scandinavia that there was little delay in the initiation of the recovery

of the land on the disappearance of the ice-load. To find the land in

Scotland emerging while a rise of sea-level was still going on implies that

a considerable mass of ice had not long disappeared from the area. The

Highland Glaciers were too small to have been responsible for the

depression, and in any ce the land was rising while they were at their

maximum.

In contrast, in Ireland the Athdown Mountain Glaciation is separated

by a considerable interval from the Midland General Glaciation (12 and

13). The Athdown Glaciation has been dated to Zone III (13), and as

this zone has been recognised right across Ireland, as has Zone II under

lying it, it may be taken as proved that the Midland General Glaciation

had disappeared from Ireland completely, to be followed by a milder period, before the oncoming of the Athdown Glaciation.

With what then is the Athdown Mountain Glaciation to be correlated? It has been shown (12 and 13) that it is not contemporaneous with the

Midland General Glaciation as originally suggested (14). Nor does it appear to have been contemporaneous with the Highland Glaciation,

It has been claimed that moraines of the Central Plain in Ireland mark

a definite halt in the retreat of the Midland General Glaciation, or even



242 Proceedings of the Royal Irish Academy.

a readvance (10 and 14). These moraines have beeen given a greater

importance than formerly by Movius (10), who correlates them with the beginning of the Gotiglacial period on the continent. Charlesworth (14) described these moraines lying across Ireland from Galway Bay to Trim,

but the line which he discusses does not agree with that later drawn by

Movius at the southern and eastern limits of his Irish Gotiglacial period

(10, fig. 8). Furthermore, there is no clear evidence that the features that

Movius has used to delimit the ice-sheet of this period are continuous, or

that they are of the same age. The late Dr. W. B. Wright drew (15)

quite a different line marking a glacial stage aross the Central Plain.

Some preliminary work done in 1935 by the author of this note suggests

that this whole question should be reviewed in the light of recent research. There is much evidence suggesting that the glacial features north of Trim are not all of the same age as those running east and west from Galway

to Trim. But the evidence given by Jessen (13, pp. 245-246) is so

weighty that it is not possible in the present state of our knowledge to

go beyond it. This evidence is totally against the probability that the

moraines of the Central Plain represent any important stage in the retreat.

The same evidence precludes the correlation of these moraines with the

Athdown Mountain Glaciation. The only glacial stage in Ireland with which it seems possible to

correlate the Athdown Mountain Glaciation is that of the Scottish

Readvance on the Antrim coast. Charlesworth (16, p. 288) suggests that

the corries in the Mourne Mountains are of the same age as the readvance.

TABLE.

Britain Ireland Scandinavia Highland

Glaciation Lammermuir- Antrim Coast

Stranraer Readvance Outer Ra and Athdown

Mtn. Glaciation

This interpretation of late-glacial and post-glacial events would involve

a reconsideration of various other problems. For instance, a depression of

ocean level lasting up to so comparatively recent a time would give ample

ice-free land surface to the west and south of Ireland for the survival of

the Lusitanian elements of the fauna and flora, especially when it is

considered that the 400 feet is only a fraction of the total lowering of

sea-level. Again, the view that Ireland was almost completely ice-free and

was separated from Wales only by drainage from the ice-front while the

North Channel was still blocked by ice makes the northern route very

unlikely for the immigration of new species into Ireland in late-glacial

times.



FARRINGToN-The Ocean in Glacial and Late-Gldvial 7Times. 243

BIBLIOGRAPHY.

1. Antevs, Ernst. 1928 The Last Glaciation, American Geographical

Society, Research Series, No. 17.

2. Ramsay, Wilhelm. 1930 Fennia, 52; No. 5.

3. Wright, W. B. 1937 The Quaternary Ice Age, London.

4. Br^gger, W. C. 1900-1901 On the Late-glacial and Post-glacial

changes of Level in the Kristiania Region, Norges Geologiske

Unders^gelse, No. 31.

5 Sauramo, Matti. 1929 The Quaternary Geology of Finland, Bulletin

de la Commission G?ologique de Finland, No. 86, Helsingfors. 6. Nordhagen, R. 1933 De Senkvartaere Klimavekslinger i Nordeuropa

og Deres Betydning for Kulturfor&kningen, Oslo.

7. Jessen, K. 1935 The Composition of the Forests in northern Europe in Epipalaeolithic time. Det. Kgl. Danske Videnskabernes

Selskab, Biologiske Middelelser XII.

8. Godwin, H. 1940 Post-glacial changes of relative land and sea level

in the English Fenland, Phil. Trans. Roy. Soc, London, 230, B, 239-303.

9. Charlesworth, J. K. 1931 A tentative reconstruction of the

successive margins of the Quaternary Ice-sheets in the region of

the North Sea, Proc. Roy. Irish Acad., 40, B, 4.

10. Movius, Hallam L., Jr. 1942 The Irish Stone Age. Cambridge. 11. McCallien, W. J. Late-Glacial and Early Post-Glacial Scotland,

Proc. Soc. of Ant. of Scotland, 61, pp. 174-206.

12. Farr?ngton, A. 1934 The Glaciation of the Wicklow Mountains, Proc. Roy. Irish Acad., 42, B, 7.

13. Jessen, K., and A. Farr?ngton. 1938 The Bogs of Ballybetagh near

Dublin, with remarks on Late-Glacial conditions in Ireland, Proc. Roy. Irish Acad., 44, B, 10.

14. Charlesworth, J. K. 1928 The Glacial Retreat from Central and

Southern Ireland, Q. Journ. Geol. Soc, 84, pp. 293-242.

15. Wright, W. B. 1938 Tools and The Man, London.

16.* Charlesworth, J. K. 1939 Some Observations on the Glaciation of

North-east Ireland, Proc. Roy. Irish Acad., 45, B, 11,



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The Level of the Ocean in Glacial and Late-Glacial Times