Graph of modelled relative sea level change against time over the last 16, 000 years at John o' Groats, (After Lambeck, 1993, Hansom, 2001)
Sea level around 12000 BP. Green areas are land
Shelly sand and gravel in the raised beach at Keiss
Sea level change
World coasts have seen sea-level variation of approximately 100 metres within the past 11,500 years through melting of the ice caps. When the ice caps melt the sea level rises globally (eustatically). The relative sea-level at any location is measured proportionate to the nearby land, which is itself subject to tectonic movement both up and down.
During the Ice Age, there was over 1 km of ice over central Scotland. On melting the downward pressure was released and the land surface moved up (isostatic rebound) quicker than the eustatic sea level rose. This gives the appearance of a relative sea-level drop over much of Scotland during the Holocene. The sea level history of Scotland is described here.
In Caithness, where the ice thickness was greater than 500 m, eustatic sea-level rise was a little less than isostatic rebound giving evidence of a modest relative rise in sea level. The Main Postglacial Raised Shoreline lies at or a few metres above present sea level, with a submerged peat below the beach at Keiss. Buried tree stumps at or below sea level are also reported from the bays at Sinclairs, Lybster, Wick and Thurso. The MPRS rises to the south and west.
Flinn (1981) recognises a raised shore platform at around 3 m OD between Dounreay and Dunbeath. On these stretches of coast the raised platform often protects the base of the cliff from erosion so that the cliffs are weathered and largely inactive. Flinn suggests correlation with a low raised platform in Orkney just above sea level and with the main Lateglacial shoreline of Scotland.
There are intriguing reports of Lateglacial raised beaches at much higher elevations of 19-26 m OD at Dunbeath.A raised beach deposit at about 7m in the cliff face near Muckle Head, north Hoy, is covered by glacial till. This is evidence of higher relative sea level during one of the interstadial episodes of the last Ice Age and not part of the current phase. It may relate to raised marine features which lie beneath till on the Caithness coast.