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The submerged landforms of the St. Kilda archipelago, western Scotland
Marine Geology, 58 (1984)435--442
435
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands Letter Section T H E S U B M E R G E D L A N D F O R M S O F T H E ST. K I L D A A R C H I P E L A G O , WESTERN SCOTLAND
DONALD G. SUTHERLAND
Department of Geography, University of Edinburgh, Drummond Street, Edinburgh EH8 9XP (Scotland) (Received November 7, 1983;revised and accepted January 18, 1984)
ABSTRACT Sutherland, D.G., 1984. The submerged landforms of the St. Kilda archipelago, western Scotland. Mar. Geol., 58: 435--442. A detailed bathymetric map of the area around the St. Kilda archipelago has been prepared with a 10 m isobath vertical interval. The map reveals two major submerged erosional surfaces that culminate in clifflines. The lower surface at ca. - 1 2 0 m covers at least 40 km ~ and is thought to have been produced during low sea levels coincident with periods of maximum northern hemisphere glaciation. The upper surface is complex in origin, varying between - 8 0 and - 4 0 m. It is thought to have been produced during periods of partial northern hemisphere glaciation, the last such period, the Loch Lomond (Younger Dryas) Stadial, being responsible for the production of the ca. - 4 0 m cliff-platform junction. The last 40 m relative rise of sea level has been accompanied by very little marine erosion despite the very exposed situation of the islands and this sea-level rise is thought to have taken place during the Flandrian.
INTRODUCTION
Location, regional bathymetry, geology T h e St. K i l d a a r c h i p e l a g o ( 5 7 ° 4 9 ' N , 08°35'W) lies ca. 60 k m t o t h e w e s t o f t h e O u t e r H e b r i d e a n island c h a i n t o w a r d s t h e edge o f w e s t e r n S c o t t i s h c o n t i n e n t a l s h e l f (Fig. 1). T h e shelf, d o m i n a n t l y c o m p o s e d o f P r e c a m b r i a n Lewisian gneiss, slopes g e n t l y w e s t w a r d f r o m t h e O u t e r H e b r i d e s t o a d e p t h o f ca. 1 2 0 - - 1 4 0 m in t h e n e i g h b o u r h o o d o f St. Kilda. T h e gentle decline is o n l y i n t e r r u p t e d b y a N N E - - S S W t r e n d i n g d e e p t h a t is t h e s u r f a c e e x p r e s s i o n o f a n a r r o w fault~controlled M e s o z o i c basin t e r m e d t h e ' F l a n n a n T r o u g h ' b y J o n e s ( 1 9 7 8 ) a n d t h e ' O u t e r H e b r i d e s Basin' b y N a y l o r a n d S h a n n o n ( 1 9 8 2 ) . T h e St. K i l d a a r c h i p e l a g o , h o w e v e r , is t h e r e m a i n s o f a T e r t i a r y igneous c o m p l e x ( C o c k b u r n , 1 9 3 5 ; H a r d i n g , 1 9 6 7 ) , t h e igneous a c t i v i t y having b e e n d a t e d t o b e t w e e n 35 a n d 60 m . y . B.P. (Miller a n d M o h r , 1 9 6 5 ) . T h e r e is little s e d i m e n t a r y c o v e r o n t h e
0025-3227/84/$03.00
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Fig. 1. Location of St. Kilda to the west of the Outer Hebrides. Bathymetry in metres. Stippled area is land. The continental slope extends below the 180 m isobath. shelf around and to the east of St. Kilda and the landforms to be described in this paper have been produced by erosion of the Precambrian gneisses and the Tertiary igneous rocks. Data
Bathymetric, seismic and side-scan sonar data have been made available from a number of sources. The Marine Geology Unit of the Institute of Geological Sciences has sailed several cruise lines in and around the archipelago and high precision echo sounder, sparker, pinger and sidescan sonar continuous profiles are available from this source. Detailed surveys by both continuous b o t t o m profiling and closely spaced nearshore soundings have been carried out by the Hydrographic Department of the Admiralty and three cruises using high precision echo sounder and sidescan sonar continuous profiles by the Institute of O c e e a o ~ a p h i c Sciences have been used. Additional less precise information has been derived from the standard Admiralty chart soundings, from a private survey using a fishing b o a t b y the P e t r ~ y Department of the Institute of Geological Sciences and by the records of divers at the base of the cliffs (Rictley,
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438 detailed interpolation. In general the density of data lines and points is sufficient to justify drawing isobaths at a 10 m vertical interval. THE SUBMERGED LANDFORMS Figure 3 clearly reveals the roughly circular area of the igneous complex standing proud of the surrounding low gradient sea bed. With the exception of the northeast corner, the igneous complex is separated from the surrounding sea bed by a marked break of slope that is particularly pronounced along the western margin. In profile, the marginal slope is frequently near-vertical and cliff-like and to the west it is fronted by a
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439
virtually level sea floor at 120--125 m depth. Seismic profiles and sidescan sonar traces confirm that this extensive surface is cut across bedrock with only a very thin, patchy sediment cover. This surface has been traced over an area of at least 40 km 2 to the west of the igneous complex. The combination of a wide erosional level surface culminating in a cliffline up to 40 m high facing towards the direction of maximum exposure suggests strongly that it is a marine erosional feature produced by a relative sea level of ca. - 1 2 0 m. Such an origin would appear to be confirmed by the 200--300 m wide benches that terminate in sharp breaks of slope at 110--120 m depth in the southeast part of the igneous complex. The difference in degree of development between the western and the southeastern features can be explained by the fact that with a sea level of - 1 2 0 m the southeast of the igneous complex would be exposed to waves generated over a very limited fetch whilst the west faced across the Atlantic. The islands of the archipelago occur towards the edges of the igneous complex and they are laterally continuous underwater with a series of sharp ridges such that the tops of the bounding slopes and cliffs of the igneous complex are frequently higher than the inner plateau-like surface. Cockburn (1935) noted that the structures in the ultrabasic (eucritic) rocks in both Hirta and Boreray dipped inwards towards the centre of the archipelago and, although no definitive ring structures were observed on St. Kilda, he hypothesised that this was indeed the nature of the igneous complex. The ridges around the edges of the complex are likely to result from geological control and may reflect the outcrop of a particular resistant rock around the periphery of the complex. By analogy with the rocks exposed in the islands, this rock would be the ultrabasic t y p e of Harding (1967) and the eucrite of Cockburn (1935). The greater part of the area of the igneous complex is c o m p o s e d of a very gently sloping surface that rises from just below - 8 0 m'in the northwest to approximately - 6 0 m on the central rise between Hirta and Boreray to decline again to slightly above - 8 0 m in the southeast. This surface also rises m o r e steeply (though still at a very low gradient) towards the islands and sea stacks which it meets at ca. - 4 0 m. On the northwestern side of the gentle central rise that joins Hirta and Boreray the surface is almost entirely bare rock whilst to the southeast there is a more general thin cover of shell sand. The area of this erosional surface is ca. 110 km 2 . Two sets of relationships seem significant in explaining the origin of this major feature. (a) Although it is generally of a very low gradient, there are steps in the surface that occur at c o m m o n altitudes and can occasionally be traced from profile to profile. Thus there is a step at ca. - 7 0 m to the north of Hirta that can be located on four separate profiles whilst a step at about the same depth occurs to the southeast of Boreray. Where best and most continuously developed these steps face outwards from the igneous complex. They indicate that the surface is complex in origin.
440 (b) The islands and sea stacks t h a t rise from this surface do so from a c o m m o n depth of ca. - 4 0 m. Similari3; the two valleys on the largest island of Hirta can be traced down to the level of this surface. This indicates that the surface is a marine erosional one. The difference in outline of the northwest.facing submerged valley (Glen Bay) and the southeastfacing submerged valley (Village Bay) on Hirta is thought to be due to the latter containing quantities of sediment. This is suggested, first, because the Village Bay area on land is characterized by a complex sequence of glacial and periglacial deposits that are largely absent from the land neighbouring Glen Bay (Sutherland et al., in press), and second, because Glen Bay is very m u c h more exposed than Village Bay and any sediment that m a y once have floored it would probably been reworked offshore. In summary, the upper erosional surface of the St. Kilda archipelago culminates in clear marine erosional features at ca. -40 m. The surface is complex and its formation has probably been the result of marine erosion at a variety of levels between -80 and -40 m. O n the available profile evidence a step in the profile seems identifiable at ca. -70 m and this m a y represent a particularsea level in the relativelyrecent past. At present the sea appears to be unable to effect more than minor erosion of weathered bedrock and drift and apart from a series of bedrock ramps around the heads of the bays and some minor caves there are no major marine erosional features such as rock platforms. The relict nature of the cliffs around Hirta is confirmed by fossil cemented block deposits that were formed during a period of periglacial climate and t h a t are found on the face of cliffs on the southwest coast of the island. This lack of significant marine erosion above - 4 0 m despite the extremely exposed position of the islands suggests t h a t the extensive submerged marine erosional surfaces were formed during a time when marine erosional processes were of much greater efficiency and/or that those sea levels were occupied for a much greater period of time. DISCUSSION The low sea-level altitudes identified must be regarded as relative. The St. Kilda igneous complex intrudes the Precambrian Outer Isles shield which may be expected to be 'stable' (in contrast to the North Sea sedimentary basin, for example) but two local effects of opposite sense may have affected the absolute altitude of these features: hydro-isostasy resuiting from the rises in sea level si.-~ce the features were formed, and decay of a possible 'fozebulge' r e ~ t o the last Scottish ice sheet t h a t terminated some distance to the east. Neither of these factors, nor their interaction at different times, can be accurately quantified and hence no absolute figures for sea-level change can be deduced. It is notable, in such an exposed situation, that there is comparatively little marine erosion at presen~ sea level and that the final 40 m of relative
441
sea-level rise was also accomplished without effecting any significant marine erosion. This suggests that the planation of the bedrock surfaces and formation of the cliffs was achieved during periods of relatively stable sea level when climatic conditions were perhaps different from the present. The magnitude of the sea-level changes is also significant in this context for given that glacio-eustasy was the major control on sea-level change in the late Quaternary the lower sea levels indicated by the planation surfaces must have coincided with periods of northern hemispheric glaciation and hence of severe oceanic climate in the North Atlantic. It is therefore suggested that the -120 m surface, which is the lowest evidence in this area for sea-level lowering, has been formed during periods of maximum glaciation, the last of which occurred during the Late Devensian (i.e. centred on ca. 18,000 yrs B.P.). On the same argument the complex surface between -80 and -40 m is thought to have been formed during periods of partial northern hemispheric glaciation, the last such period being the Loch Lomond Stadial between 11,000 and 10,000 yrs B.P. when local relative sea level may have been at the -40 m depth as this is the clearest (and hence most recently formed) cliff-platform junction. It would follow that the subsequent 40 m rise in sea level occurred during the mild conditions of the Flandrian when, as today, marine erosion has been markedly less effective. The features described around St. Kilda are in part similar to those described by Flinn (1964, 1969, 1977) around the Orkney and Shetland islands. Flinn noted that the major sea cliffs of these islands descended well below present sea level to what he termed 'the final break of slope'. The final break of slope he identified at a depth o f - 8 2 m (45 fthm) around the Shetland isles and at -64 m (35 fthm) around Orkney and he suggested (1969) that the final break of slope occurred at shallower depths farther south around the British coast although it is not always apparent that it is the same features that are everywhere being compared. If the break of slope at -40 m at the base of the St. Kflda cliffs is the same feature as that described by Flinn then this implies that the break of slope rises in a southwesterly direction. It should be noted, however, that all of the depths around the north of Scotland that Flinn records fall within the depth range (i.e. ca. - 8 0 to - 4 0 m), of the upper St. Kllda erosion surface and without the very detailed nearshore information available for the St. Kilda archipelago the depth of the final break of slope could be in error by 10--20 m. If the explanation offered here for the upper erosional surface at St. Kilda is correct and if it is indeed the same feature as described by Flinn, then the final break of slope and associated planated surfaces around the north of Scotland would have been formed during periods of partial northern hemispheric glaciation.
442
ACKNOWLEDGEMENTS Thanks partment partment for access
are due to the Marine Geology Unit and the Petrography Deof the Institute of Geological Sciences, the Hydrographic Deof the Admiralty and the Institute of Oceanographic Sciences to unpublished data.
REFERENCES Cockburn, A.M., 1935. The geology of St. Kilda. Trans. R. Soc. Edinb., 58: 511--547. Flinn, D., 1964. Coastal and submarine features around the Shetland Islands. Proc. Geol. Assoc., 75: 321--340. Flinn, D., 1969. On the development of coastal profiles in the north of Scotland, Orkney and Shetland. Scott. J. Geol., 5: 393--399. Flinn, D., 1977. The erosion history of Shetland: a review. Proc. Geol. Assoc., 88: 129--146.
Harding, R.R., 1967. The major ultrabasic and basic intrusions of St. Kilda, Outer Hebrides. Trans. R. Soc. Edinb,, 66: 419--444. Jones, E.J.W., 1978. Seismic evidence for sedimentary troughs of Mesozoic age on the Hebridean continental margin. Nature, 272: 789--792. Miller, J,A. and Mohr, P.A., 1965. Pota~ium--argon age determinations on rocks from St. Kilda and Rockall. Scott. J. Geol., 1 : 93--99. Naylor, D. and Shannon, P.M., 1982. The Geology of Offshore Ireland and West Britain. Graham and Trotman, London, 161 pp. Ridley, G., 1980. The British Sub-Aqua Club St. Kilda survey expedition 1979. Underwater World, 3 : 12--15.
435
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands Letter Section T H E S U B M E R G E D L A N D F O R M S O F T H E ST. K I L D A A R C H I P E L A G O , WESTERN SCOTLAND
DONALD G. SUTHERLAND
Department of Geography, University of Edinburgh, Drummond Street, Edinburgh EH8 9XP (Scotland) (Received November 7, 1983;revised and accepted January 18, 1984)
ABSTRACT Sutherland, D.G., 1984. The submerged landforms of the St. Kilda archipelago, western Scotland. Mar. Geol., 58: 435--442. A detailed bathymetric map of the area around the St. Kilda archipelago has been prepared with a 10 m isobath vertical interval. The map reveals two major submerged erosional surfaces that culminate in clifflines. The lower surface at ca. - 1 2 0 m covers at least 40 km ~ and is thought to have been produced during low sea levels coincident with periods of maximum northern hemisphere glaciation. The upper surface is complex in origin, varying between - 8 0 and - 4 0 m. It is thought to have been produced during periods of partial northern hemisphere glaciation, the last such period, the Loch Lomond (Younger Dryas) Stadial, being responsible for the production of the ca. - 4 0 m cliff-platform junction. The last 40 m relative rise of sea level has been accompanied by very little marine erosion despite the very exposed situation of the islands and this sea-level rise is thought to have taken place during the Flandrian.
INTRODUCTION
Location, regional bathymetry, geology T h e St. K i l d a a r c h i p e l a g o ( 5 7 ° 4 9 ' N , 08°35'W) lies ca. 60 k m t o t h e w e s t o f t h e O u t e r H e b r i d e a n island c h a i n t o w a r d s t h e edge o f w e s t e r n S c o t t i s h c o n t i n e n t a l s h e l f (Fig. 1). T h e shelf, d o m i n a n t l y c o m p o s e d o f P r e c a m b r i a n Lewisian gneiss, slopes g e n t l y w e s t w a r d f r o m t h e O u t e r H e b r i d e s t o a d e p t h o f ca. 1 2 0 - - 1 4 0 m in t h e n e i g h b o u r h o o d o f St. Kilda. T h e gentle decline is o n l y i n t e r r u p t e d b y a N N E - - S S W t r e n d i n g d e e p t h a t is t h e s u r f a c e e x p r e s s i o n o f a n a r r o w fault~controlled M e s o z o i c basin t e r m e d t h e ' F l a n n a n T r o u g h ' b y J o n e s ( 1 9 7 8 ) a n d t h e ' O u t e r H e b r i d e s Basin' b y N a y l o r a n d S h a n n o n ( 1 9 8 2 ) . T h e St. K i l d a a r c h i p e l a g o , h o w e v e r , is t h e r e m a i n s o f a T e r t i a r y igneous c o m p l e x ( C o c k b u r n , 1 9 3 5 ; H a r d i n g , 1 9 6 7 ) , t h e igneous a c t i v i t y having b e e n d a t e d t o b e t w e e n 35 a n d 60 m . y . B.P. (Miller a n d M o h r , 1 9 6 5 ) . T h e r e is little s e d i m e n t a r y c o v e r o n t h e
0025-3227/84/$03.00
© 1984 Elsevier Science Publishers B.V.
436
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Fig. 1. Location of St. Kilda to the west of the Outer Hebrides. Bathymetry in metres. Stippled area is land. The continental slope extends below the 180 m isobath. shelf around and to the east of St. Kilda and the landforms to be described in this paper have been produced by erosion of the Precambrian gneisses and the Tertiary igneous rocks. Data
Bathymetric, seismic and side-scan sonar data have been made available from a number of sources. The Marine Geology Unit of the Institute of Geological Sciences has sailed several cruise lines in and around the archipelago and high precision echo sounder, sparker, pinger and sidescan sonar continuous profiles are available from this source. Detailed surveys by both continuous b o t t o m profiling and closely spaced nearshore soundings have been carried out by the Hydrographic Department of the Admiralty and three cruises using high precision echo sounder and sidescan sonar continuous profiles by the Institute of O c e e a o ~ a p h i c Sciences have been used. Additional less precise information has been derived from the standard Admiralty chart soundings, from a private survey using a fishing b o a t b y the P e t r ~ y Department of the Institute of Geological Sciences and by the records of divers at the base of the cliffs (Rictley,
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Fig. 2. Data used in compilation of detailed bathymetric map. 1 = IGS cruise lines; 2 = IOS cruise lines; 3 = Hydrographic Department continuous profiles; 4 = AdmirMity chart soundings. 1980). Figure 2 shows the distribution of the various cruises and sounding lines used in the construction of the bathymetric map. In constructing the bathymetric map, most weight was placed on those data sources for which position was by Decca Navigator and the technique was o f a continuous profiling nature. Much o f the sea floor around St. Kilda is o f low-gradient surfaces separated by distinct, sharp breaks o f slope and this facilitated interpolation b e t w e e n cruise lines. To the southwest o f Hirta, however, the sea bed is particularly irregular with local relief o f up to 40 m and this, c o m b i n e d with a paucity o f data, has inhibited
438 detailed interpolation. In general the density of data lines and points is sufficient to justify drawing isobaths at a 10 m vertical interval. THE SUBMERGED LANDFORMS Figure 3 clearly reveals the roughly circular area of the igneous complex standing proud of the surrounding low gradient sea bed. With the exception of the northeast corner, the igneous complex is separated from the surrounding sea bed by a marked break of slope that is particularly pronounced along the western margin. In profile, the marginal slope is frequently near-vertical and cliff-like and to the west it is fronted by a
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439
virtually level sea floor at 120--125 m depth. Seismic profiles and sidescan sonar traces confirm that this extensive surface is cut across bedrock with only a very thin, patchy sediment cover. This surface has been traced over an area of at least 40 km 2 to the west of the igneous complex. The combination of a wide erosional level surface culminating in a cliffline up to 40 m high facing towards the direction of maximum exposure suggests strongly that it is a marine erosional feature produced by a relative sea level of ca. - 1 2 0 m. Such an origin would appear to be confirmed by the 200--300 m wide benches that terminate in sharp breaks of slope at 110--120 m depth in the southeast part of the igneous complex. The difference in degree of development between the western and the southeastern features can be explained by the fact that with a sea level of - 1 2 0 m the southeast of the igneous complex would be exposed to waves generated over a very limited fetch whilst the west faced across the Atlantic. The islands of the archipelago occur towards the edges of the igneous complex and they are laterally continuous underwater with a series of sharp ridges such that the tops of the bounding slopes and cliffs of the igneous complex are frequently higher than the inner plateau-like surface. Cockburn (1935) noted that the structures in the ultrabasic (eucritic) rocks in both Hirta and Boreray dipped inwards towards the centre of the archipelago and, although no definitive ring structures were observed on St. Kilda, he hypothesised that this was indeed the nature of the igneous complex. The ridges around the edges of the complex are likely to result from geological control and may reflect the outcrop of a particular resistant rock around the periphery of the complex. By analogy with the rocks exposed in the islands, this rock would be the ultrabasic t y p e of Harding (1967) and the eucrite of Cockburn (1935). The greater part of the area of the igneous complex is c o m p o s e d of a very gently sloping surface that rises from just below - 8 0 m'in the northwest to approximately - 6 0 m on the central rise between Hirta and Boreray to decline again to slightly above - 8 0 m in the southeast. This surface also rises m o r e steeply (though still at a very low gradient) towards the islands and sea stacks which it meets at ca. - 4 0 m. On the northwestern side of the gentle central rise that joins Hirta and Boreray the surface is almost entirely bare rock whilst to the southeast there is a more general thin cover of shell sand. The area of this erosional surface is ca. 110 km 2 . Two sets of relationships seem significant in explaining the origin of this major feature. (a) Although it is generally of a very low gradient, there are steps in the surface that occur at c o m m o n altitudes and can occasionally be traced from profile to profile. Thus there is a step at ca. - 7 0 m to the north of Hirta that can be located on four separate profiles whilst a step at about the same depth occurs to the southeast of Boreray. Where best and most continuously developed these steps face outwards from the igneous complex. They indicate that the surface is complex in origin.
440 (b) The islands and sea stacks t h a t rise from this surface do so from a c o m m o n depth of ca. - 4 0 m. Similari3; the two valleys on the largest island of Hirta can be traced down to the level of this surface. This indicates that the surface is a marine erosional one. The difference in outline of the northwest.facing submerged valley (Glen Bay) and the southeastfacing submerged valley (Village Bay) on Hirta is thought to be due to the latter containing quantities of sediment. This is suggested, first, because the Village Bay area on land is characterized by a complex sequence of glacial and periglacial deposits that are largely absent from the land neighbouring Glen Bay (Sutherland et al., in press), and second, because Glen Bay is very m u c h more exposed than Village Bay and any sediment that m a y once have floored it would probably been reworked offshore. In summary, the upper erosional surface of the St. Kilda archipelago culminates in clear marine erosional features at ca. -40 m. The surface is complex and its formation has probably been the result of marine erosion at a variety of levels between -80 and -40 m. O n the available profile evidence a step in the profile seems identifiable at ca. -70 m and this m a y represent a particularsea level in the relativelyrecent past. At present the sea appears to be unable to effect more than minor erosion of weathered bedrock and drift and apart from a series of bedrock ramps around the heads of the bays and some minor caves there are no major marine erosional features such as rock platforms. The relict nature of the cliffs around Hirta is confirmed by fossil cemented block deposits that were formed during a period of periglacial climate and t h a t are found on the face of cliffs on the southwest coast of the island. This lack of significant marine erosion above - 4 0 m despite the extremely exposed position of the islands suggests t h a t the extensive submerged marine erosional surfaces were formed during a time when marine erosional processes were of much greater efficiency and/or that those sea levels were occupied for a much greater period of time. DISCUSSION The low sea-level altitudes identified must be regarded as relative. The St. Kilda igneous complex intrudes the Precambrian Outer Isles shield which may be expected to be 'stable' (in contrast to the North Sea sedimentary basin, for example) but two local effects of opposite sense may have affected the absolute altitude of these features: hydro-isostasy resuiting from the rises in sea level si.-~ce the features were formed, and decay of a possible 'fozebulge' r e ~ t o the last Scottish ice sheet t h a t terminated some distance to the east. Neither of these factors, nor their interaction at different times, can be accurately quantified and hence no absolute figures for sea-level change can be deduced. It is notable, in such an exposed situation, that there is comparatively little marine erosion at presen~ sea level and that the final 40 m of relative
441
sea-level rise was also accomplished without effecting any significant marine erosion. This suggests that the planation of the bedrock surfaces and formation of the cliffs was achieved during periods of relatively stable sea level when climatic conditions were perhaps different from the present. The magnitude of the sea-level changes is also significant in this context for given that glacio-eustasy was the major control on sea-level change in the late Quaternary the lower sea levels indicated by the planation surfaces must have coincided with periods of northern hemispheric glaciation and hence of severe oceanic climate in the North Atlantic. It is therefore suggested that the -120 m surface, which is the lowest evidence in this area for sea-level lowering, has been formed during periods of maximum glaciation, the last of which occurred during the Late Devensian (i.e. centred on ca. 18,000 yrs B.P.). On the same argument the complex surface between -80 and -40 m is thought to have been formed during periods of partial northern hemispheric glaciation, the last such period being the Loch Lomond Stadial between 11,000 and 10,000 yrs B.P. when local relative sea level may have been at the -40 m depth as this is the clearest (and hence most recently formed) cliff-platform junction. It would follow that the subsequent 40 m rise in sea level occurred during the mild conditions of the Flandrian when, as today, marine erosion has been markedly less effective. The features described around St. Kilda are in part similar to those described by Flinn (1964, 1969, 1977) around the Orkney and Shetland islands. Flinn noted that the major sea cliffs of these islands descended well below present sea level to what he termed 'the final break of slope'. The final break of slope he identified at a depth o f - 8 2 m (45 fthm) around the Shetland isles and at -64 m (35 fthm) around Orkney and he suggested (1969) that the final break of slope occurred at shallower depths farther south around the British coast although it is not always apparent that it is the same features that are everywhere being compared. If the break of slope at -40 m at the base of the St. Kflda cliffs is the same feature as that described by Flinn then this implies that the break of slope rises in a southwesterly direction. It should be noted, however, that all of the depths around the north of Scotland that Flinn records fall within the depth range (i.e. ca. - 8 0 to - 4 0 m), of the upper St. Kllda erosion surface and without the very detailed nearshore information available for the St. Kilda archipelago the depth of the final break of slope could be in error by 10--20 m. If the explanation offered here for the upper erosional surface at St. Kilda is correct and if it is indeed the same feature as described by Flinn, then the final break of slope and associated planated surfaces around the north of Scotland would have been formed during periods of partial northern hemispheric glaciation.
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ACKNOWLEDGEMENTS Thanks partment partment for access
are due to the Marine Geology Unit and the Petrography Deof the Institute of Geological Sciences, the Hydrographic Deof the Admiralty and the Institute of Oceanographic Sciences to unpublished data.
REFERENCES Cockburn, A.M., 1935. The geology of St. Kilda. Trans. R. Soc. Edinb., 58: 511--547. Flinn, D., 1964. Coastal and submarine features around the Shetland Islands. Proc. Geol. Assoc., 75: 321--340. Flinn, D., 1969. On the development of coastal profiles in the north of Scotland, Orkney and Shetland. Scott. J. Geol., 5: 393--399. Flinn, D., 1977. The erosion history of Shetland: a review. Proc. Geol. Assoc., 88: 129--146.
Harding, R.R., 1967. The major ultrabasic and basic intrusions of St. Kilda, Outer Hebrides. Trans. R. Soc. Edinb,, 66: 419--444. Jones, E.J.W., 1978. Seismic evidence for sedimentary troughs of Mesozoic age on the Hebridean continental margin. Nature, 272: 789--792. Miller, J,A. and Mohr, P.A., 1965. Pota~ium--argon age determinations on rocks from St. Kilda and Rockall. Scott. J. Geol., 1 : 93--99. Naylor, D. and Shannon, P.M., 1982. The Geology of Offshore Ireland and West Britain. Graham and Trotman, London, 161 pp. Ridley, G., 1980. The British Sub-Aqua Club St. Kilda survey expedition 1979. Underwater World, 3 : 12--15.