postglacial landscapes

coastal deposition

sea level and coastal hazards

submerged forest Bressay Sound

Graphs of modelled relative sea level change against time over the last 16, 000 years, along a south-north transect from Shetland to the Firth of Forth (After Lambeck, 1993, Hansom, 2001). A. Muckle Flugga  B. Sumburgh Head  C. Braeswick, Orkney D. John o'Groats

 

 

Sea level change

Shetland is the only part of Scotland where no evidence is known of sea levels higher than present. Raised marine sediments and shorelines seem to be absent from the islands. Instead the presence of many drowned valleys or voes points to long term marine submergence. Records of submerged peat exist for many sheltered bays and sounds. Marine submergence in historic times is indicated by archaeological remains that lie at or below present sea level.

Peat at sea level, Burra Voe, North Roe

On the Viking Bank, east of Shetland, sea level was more than 100 m below its present level between 15.5 and 13.6 ka BP. Parts of the Bank were dry land and others were covered by a shallow arctic sea. Between 13 and 11 ka low arctic waters covered the area, with a sea level 80-100 m below present. Thereafter water depths shallowed rapidly, perhaps partly in response to the isostatic collapse of the fore bulge of the Scandinavian ice sheet as it retreated (Peacock, 1995).

At Symbister Harbour, Unst, Hoppe sampled peat at between 8.6 and 8.9 metres below high-water mark which gave radiocarbon dates between 5455 BP and 6970 BP. This indicates relative sea level must have been at least nine metres lower than present around 6000-7000 years ago.

From about 5000 - 5500 radiocarbon years BP, Smith suggests that the sea surface around Shetland fluctuated in a range of about +/-2m , with a peak around 4000 radiocarbon BP and a consistent rise after about 2000 radiocarbon BP. There was a strong rise during the Mesolithic, from -15m HWMOST (-13m OD) at circa 7000 radiocarbon BP. Recent work by Bondevik and others at Bridge of Walls place sea level there at 3500 BP at 2 m below present.

Geophysical analysis by Lambeck and Peltier of relative sea level changes suggests that initially Shetland experienced isostatic uplift associated with the decay of the last ice sheet. Relative sea level fell until 13,000 yr BP. The model predicts that subsequently there was a continuous rise in sea level, standing at about -65 m at 10,000 BP, -10 metres at 5000 BP and at present at 3000 BP. It is noteworthy that the model predicts considerable variation in sea level across the islands during the Late Devensian, with relative sea level standing some 30 metres lower at Muckle Flugga (-90 m) than at Sumburgh Head (-60 m) at 13,000 BP. Moving closer in time, both the Peltier and Lambeck models predict sea levels which appear to be 2-3 m too low over the past 4000 years (Bondevik et al, 2005). The discrepancy may reflect underestimates of ice thickness over Shetland during the Late Devensian.