polygons in north-east Scotland, Gemmell and Ralston (1984) concluded that ... ice-front, have been recognised in the area west of Perth and also (Browne et al.
Letters to the Editors SOME RECENT DISCOVERIES OF ICE-WEDGE CAST NETWORKS IN NORTH-EAST SCOTLAND
A COMMENT
SIRS In their recent description of newly discovered occurrences of ice-wedge polygons in north-east Scotland, Gemmell and Ralston (1984) concluded that such permafrost features, except perhaps for those in 'Moraineless Buchan', were most probably formed during the cold period of the Loch Lomond Stadial. This conclusion appears to rest on an assumption that the earlier period of recession of the main Devensian ice was in general characterised by a 'rapid and substantial warming', thus precluding permafrost activity at that time. The most significant amelioration associated with the period of recession of the main Devensian ice took place at about 13000-13500 years B.P. at the beginning of the Windermere Interstadial (Coope and Pennington 1977). It coincides with the warming-up recorded at this time in Atlantic cores by Ruddiman and Maclntyre (1973) and probably also with the change in the Scottish late-Glacial marine sequence from the arctic faunas of the Errol Beds to the high boreal faunas of the Clyde Beds (Peacock 1975) and the Powgavie Clay (Paterson et al. 1981). The Errol Beds, which were laid down in the wake of the retreating main Devensian ice-front, have been recognised in the area west of Perth and also (Browne et al. 1984) in the vicinity of Inverkeithing. It is apparent, therefore, that, by the beginning of the Windermere Interstadial, a major part of Scotland had already been deglaciated, and on the evidence of the faunas, that this deglaciation had taken place in a cold climate. No confirmatory radiocarbon dates are yet available, but the pre-Windermere Interstadial deglaciation of the Scottish landmass must have been protracted, as the sequence of shorelines formed in east-central Scotland before the time of the Main Perth Shoreline, which probably relates to the period of amelioration (Paterson 1974), exhibits a wide range of gradients (Cullingford and Smith 1966). Andrews and Dugdale (1970) estimated from its gradient that the oldest shoreline in eastern Fife has an age in excess of 18000 years B.P., a figure which may be somewhat high (Sissons 1974). However, even the cautious suggestion of Cullingford and Smith (1980) that the features of pre-Main Perth Shoreline age date from the later part of the period 17000-13000 B.P. implies that large areas of Scotland were deglaciated in a cold climate for about 2000 years before the onset of the Windermere Interstadial. On this basis we suggest that permafrost-related features, including those reported by Gemmell and Ralston (1984), may well have formed during the Scott. J. Geol. 21, (1), 107-108, 1985
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recession of the main Devensian ice. W e do n o t thereby suggest that a p r e Windermere Interstadial age is proven for any of the features, and it is n o t the purpose of this note to deny that permafrost was active during the Loch Lomond Stadial, b u t to indicate that this is n o t the only possible age for the ice-wedge networks. It is n o t e w o r t h y that a very large proportion of k n o w n fossil frost wedges (Sissons 1974, fig. 2; Gemmell and Ralston 1984, fig. 1) appear to lie outside the ice-limit at the time of formation o f the Main Perth Shoreline, a line coinciding very roughly w i t h the limit d r a w n (Sissons 1967, fig. 59) for the n o w discredited Perth Readvance. A possible interpretation of this is that, after the onset of the Windermere Interstadial, permafrost was never again so active, even during the Loch L o m o n d Stadial. REFERENCES
ANDREWS, J. T. and DUGDALE, R. E. 1970. Age prediction of glacio-isostatic strandlines based on their gradients. Bull. Geol. Soc. Am. 81, 3769-71. BROWNE, M. A. E., GRAHAM, D. K. and GREGORY, D. M. 1984. Quaternary estuarine deposits in the Grangemouth area, Scotland. Rep. Br. Geol. Surv., 16, No. 3. COOPE, G. R. and P E N N I N G T O N , W. 1977. The Windermere Interstadial of the Late Devensian. In Coope, G. R. (ed). Fossil coleopteran assemblages as sensitive indicators of climatic changes during the Devensian (Last) cold stage. Philos. Trans. R. Soc. Lond. B280, 337-9. CULLINGFORD, R. A. and SMITH, D. E. 1966. Late-glacial shorelines in eastern Fife. Trans. Inst. Br. Geogr. 39, 31-51. and 1980. Late-Devensian raised shorelines in Angus and Kincardineshire, Scotland. Boreas 9, 21-38. GEMMELL, A. M. D. and RALSTON, I. B. M. 1984. Some recent discoveries of ice-wedge cast networks in north-east Scotland. Scott. J. Geol. 20, 115-8. PATERSON, I. B. 1974. The supposed Perth Readvance in the Perth district. Scott. J. Geol. 10, 53-66. , ARMSTRONG, M. and BROWNE, M. A. E. 1981. Quaternary estuarine deposits in the Tay-Earn area, Scotland. Rep. Inst. Geol. Sci. No. 81/7. PEACOCK, J. D. 1975. Scottish late- and post-glacial marine deposits. In Gemmell, A. M. D. (ed.). Quaternary studies in north east Scotland. Aberdeen Dep. Geogr., Univ Aberdeen. RUDDIMAN, W. F. and MCINTYRE, A. 1973. Time-transgressive deglacial retreat of polar waters from the North Atlantic. Quaternary Res. 3, 117-30. SISSONS, J. B. 1967. The evolution of Scotland's scenery. Edinburgh, Oliver and Boyd. 1974. The Quaternary in Scotland: a review. Scott. J. Geol. 10, 311-37. British Geological Survey,
M. ARMSTRONG
Murchison House,
I. B. PATERSON
West Mains Road, Edinburgh EH9 3LA MS. accepted for publication 1st October 1984
Letters to the Editor ICE WEDGE POLYGONS IN NORTH EAST SCOTLAND: A REPLY
SIRS T h e suggestion by A r m s t r o n g and Paterson (this volume) that the ice-wedge polygons described by Gemmell and Ralston (1984) might have formed during the Late-Devensian recession of the ice is welcomed. We w o u l d still contend however that the balance of the evidence favours the formation during the Loch L o m o n d Stadial of those features recorded outside the 'moraineless' area of Buchan. N e t w o r k s within that portion of Buchan must still be considered undated. A r m s t r o n g and Paterson correctly indicate that a major amelioration of the lateglacial climate occurred at 13 500-13 000 years B P , and use this to suggest that ice-wedge development could therefore have occurred in association with the wasting of the Late-Devensian ice mass during the period 18 000-13 500 years B P . This is a realistic contention only if a) the climate during that period was sufficiently cold for ice-wedge formation, i.e. winter temperatures below — 15°C to —20°C (French 1976); and if b) land had been exposed at that time upon which the polygons might form. We consider that there are elements of doubt about whether these conditions obtained during the period in question (18 000-13 500 years BP), at least in that part of Scotland with which w e are concerned. Direct evidence for the lateglacial climate in Scotland is singularly lacking, save by extrapolation from records from other areas, and from the marine faunas of the Errol and Clyde beds and similar deposits around the coast. It is widely recognised that the surface waters of the Atlantic remained cold during the period 20 000-13 000 years B P (Ruddiman and Mclntyre 1981), particularly in response to a major influx of cold meltwater and icebergs resulting from the initial decay of the Late-Devensian ice-sheets in N o r t h America and Scandinavia. Certainly even during the milder conditions of the Lateglacial (Windermere) Interstadial there was a dearth of temperate-water fauna in the nearshore environment in Scotland (Peacock 1981), possibly due to the effects of glacial meltwaters. This being the case, it is hardly surprising that the faunas associated with the earlier Errol and Clyde beds, deposited at a time of m u c h greater melt activity, are characteristic of cold waters. This does not necessarily mean that the climate at the time of deposition of these beds was equally severe. As Gray and Lowe (1977) have indicated, the use of marine molluscan assemblages to determine palaeotemperatures must be open to a measure of doubt. Although the climate of the areas to the south of the major Late-Devensian ice-caps was both cold and dry (Ruddiman and Mclntyre 1981), the indications are that as early as 17 000 years B P levels of s u m m e r insolation were higher than Scott. J. Geol. 21, (1), 109-111, 1985
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those of the present day (Berger 1978). Such evidence raises doubts that significant periglacial activity accompanied the decay of the Late-Devensian ice-mass in Scotland (Gray and Lowe 1977; Price 1983). The case for the existence of conditions suitable for the formation of ice-wedge networks during the period in question must be considered 'not proven'. The extent of ice-free land available for polygon formation during the period 18 000-13 000 years BP is also open to doubt. Although Armstrong and Paterson propose that large tracts of Scotland had become ice-free before the onset of the lateglacial Interstadial, Price (1983) suggested that the final dissolution of the ice around 13 000 years BP was rapid, the total deglaciation possibly lasting as little as 3-4000 years. Gray and Lowe (1977) postulated an even more rapid decay lasting as little as 1000 years. Ruddiman and Mclntyre (1981) noted that although ice masses had lost as much as 50% of their volume by 13 000 years BP, mainly as a result of calving and melt, they still occupied 75-80% of the area they covered at the Devensian maximum. This discrepancy may be reconciled by considering the loss of volume to have taken place by thinning rather than retreat. We now have a mechanism for ice-mass decay without exposing large areas of the ground surface to whatever climatic regime was present at the time. The loss of volume of the ice would also allow a degree of isostatic uplift to take place whereby shoreline development could be initiated without widespread deglaciation. It is therefore unsafe to claim that the existence of suites of raised shorelines in many parts of eastern Scotland is a good indication of widespread deglaciation significantly earlier than the onset of the lateglacial Interstadial. If the raised shorelines of eastern Scotland could be dated accurately, then the above arguments might be capable of resolution. The complexity of factors affecting isostatic rebound, coupled with the warping of shorelines as a consequence of tectonic subsidence of the North Sea basin, make it unsafe to infer that any part of eastern Scotland was deglaciated by 18 000 years BP on the basis of altitude and tilt of raised shorelines (Sissons 1981). Even the suggestion by Cullingford and Smith (1980) that the raised shorelines of much of eastern Scotland were formed during the later part of the interval 17 000-13 000 years BP need imply little more than indicate that a narrow strip of ice-free ground may have formed prior to the lateglacial Interstadial. How then can this be reconciled with the presence of Errol beds, indicative of ice-free conditions, west of Perth and near Inverkeithing (Browne et al. 1984)? We suggest that a likely interpretation is that the retreat of the Late-Devensian ice front in the North Sea basin was largely achieved by calving. This process would promote rapid retreat in deep water, but would produce a virtual standstill of the ice edge once this became land based. As the ice thinned and retreated towards the present east coast of Scotland, deep inlets like the Firths of Forth and Tay would have acted as calving bays. In this way open water sediments could have been deposited well to the west of the general line of the Scottish coast without necessarily implying that much of the landmass was ice-free.
LETTERS TO THE EDITOR
111
T h e c o m m e n t by A r m s t r o n g and Paterson regarding the distribution of fossil ice-wedges relative to the ice-limit at the time of formation of the Main Perth Shoreline must be regarded with caution. N o t only is the age of that shoreline uncertain, if indeed its formation was synchronous t h r o u g h o u t Scotland, but within north-east Scotland there is n o secure evidence to indicate the position of any contemporaneous ice-limit. Such a limit therefore cannot be used as evidence to suggest the time of formation of ice-wedge polygons in the region. T h e polygons at Pugeston, developed on the D S - 7 shoreline of Cullingford and Smith (not D - 7 , as inadvertently stated by Gemmell and Ralston), cannot be older than the later stages of the 17 000-13 000 years B P interval of time assigned by t h e m for the development of the shoreline sequence. This being one of the lower shorelines in the region, the development of frost wedges on its surface is likely to postdate the deglaciation of the site by a considerable period of time. If Cullingford and Smith are correct in their dating, then the Loch L o m o n d Stadial, k n o w n to have been cold enough for ice-wedge development, seems a m o r e likely time for the formation of the n e t w o r k s than the last stages of the decay of the Late-Devensian ice mass.
REFERENCES
BERGER, A. 1978. Long term variations of calorific insolation resulting from the earth's orbital elements. Quaternary Res. 9, 139-67. BROWNE, M. A. E., GRAHAM, D. K. and GREGORY, D. M. 1984. Quaternary estuarine deposits in the Grangemouth area, Scotland. Rep. Br. Geol. Surv., 16, No. 3. CULLINGFORD, R. A. and SMITH, D. E. 1980. Late Devensian raised shorelines in Angus and Kincardineshire, Scotland. Boreas 9, 21-38. FRENCH, H. M. 1976. The Periglacial Environment. London, Longmans. GEMMELL, A. M. D. and RALSTON, I. B. M. 1984. Some recent discoveries of ice-wedge cast networks in north-east Scotland. Scott. J. Geol. 20, 115-8. GRAY, J. M. and LOWE, J. J. 1977. The Scottish Lateglacial environment: a synthesis. In Gray, J. M. and Lowe, J. J. (eds), Studies in the Scottish Lateglacial Environment, 163-81, Oxford, Pergamon Press. PEACOCK, J. D. 1981. Scottish Late-glacial marine deposits and their environmental significance. In Neale, J. and Flenley, J. (eds). The Quaternary in Britain, 222-36, Oxford, Pergamon Press. PRICE, R. J. 1983. Scotland's Environment during the last 30,000 years., Edinburgh, Scottish Academic Press. RUDDIMAN, W. F. and MCINTYRE, A. 1981. The mode and mechanism of the last deglaciation: oceanic evidence. Quaternary Res. 16, 125-34. SISSONS, J. B. 1981. The last Scottish ice sheet: facts and speculative discussion. Boreas 10, 1-17. Department of Geography,
University of Aberdeen, Aberdeen MS. accepted for publication 10th December 1984
A. M. D. GEMMELL I. B. M. RALSTON
