Sea-level change, Quaternary to the present, Southern Uplands
|Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
Present sea level is relatively high compared to its position during most of the Quaternary, so much evidence of past fluctuations has been either destroyed by marine erosion or submerged. Relative sea levels around southern Scotland have been determined by both depression of the land under the ice load during glaciation and its subsequent recovery — isostatic effects — and by changes in global sea levels — eustatic effects. The latter are mainly determined by the amount of water contained within continental ice-sheets and global sea level has risen from a lowstand of about 120 m below current OD some 20 ka BP as those ice-sheets have melted. The interaction of isostatic and eustatic effects means that former sea levels, as portrayed in sea-level curves, vary considerably around the coast of Britain (P912374). The rate of isostatic uplift was greatest during and immediately after deglaciation and has since fallen exponentially. Two distinct sets of raised beach and estuarine deposits occur along the coast of the Solway Firth, where the amount of glacio-isostatic recovery was sufficiently great to bring about an early period of falling sea level. The older, Late Devensian (‘late glacial’) set was created during and shortly after retreat of MLD ice, whereas the younger, far more extensive set formed during the mid to late Holocene. Although some fragmentary raised beaches, rock platforms and marine planation surfaces in this area have been claimed as Late Devensian, the evidence is equivocal. Equally, claims that deglaciation of the Irish Sea Basin was accompanied by widespread deposition of glaciomarine sediments up to 150 m above present OD cannot be substantiated.
The sea-level curves for southern Scotland reveal lowstands in the early Holocene as glacio-isostatic rebound outstripped global sea-level rise. Sea level may have been as low as 60 m below OD within the northern Irish Sea Basin. The subsequent Main Postglacial Transgression followed a reduction in the rate of isostatic rebound and resulted in the formation of the set of Holocene raised beaches and associated estuarine silts, fine-grained sands and clays comprising the Carse Clay Formation (P912389). Peat beds and tree stumps (‘submerged forests’) interbedded with the estuarine deposits are exposed periodically on the foreshores of the Solway (e.g. at Redkirk Point (NY 302 650)) and near Girvan on the Firth of Clyde, where they provide clear evidence of Holocene sea-level rise.
Along the northern shores of the Solway sea-level rose towards a distinct mean tidal range highstand of about +3 to +4 m OD by 7.7 to 7.4 ka BP in the mid Holocene (P912374). There, and in other nearby coastal areas, this rise created the Main Postglacial Shoreline, which is represented by fragmentary raised beach deposits of well-rounded shingle backed by a degraded cliff line (P774197), with extensive raised tidal flats and saltmarsh deposits up to about 9 m OD. Since their formation, these features have been uplifted by continuing, slow isostatic rebound. Elsewhere around the coast, small accumulations of blown sand occur, the most extensive being the dunes at the head of Luce Bay.
On the rocky eastern seaboard of southern Scotland, south of Cockburnspath, there is limited raised beach evidence for sea-level change. In contrast there is a more or less continuous former coastline northwards towards Dunbar. There, the Holocene coastal sediments rest on a rock platform that may have a polyphase history of Quaternary planation.
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