Raised beaches along the Firth of Forth

Significance: one of the most closely studied raised beach sequences in the world

 

Shorelines in SE Scotland from Scottish Sea Levels                                                                                                                                                                                                                  

The raised beaches of the Firth of Forth were first studied in detail in the 1960s by a group of geomorphologists from the University of Edinburgh led by Brian Sissons. Over a remarkable period of activity from 1965-1972, Sissons, Smith, Cullingford and others mapped, levelled and dated the main shoreline fragments from west of Stirling to Fife Ness. The shoreline diagram above was made possible by this epic effort and remains one of the clearest of its type in the world.

The diagram shows the lateral extent of the shorelines, together with elevation and gradient. These three elements relate directly to the formation of the shoreline:

  • the lateral extent of the higher shorelines reflects the former presence of glaciers in the Forth valley. These high shorelines pass westwards into glacial outwash deposits.

  • the elevation of the shorelines reflects post-glacial uplift in response to isostatic rebound during and after the retreat and melt of the last ice sheet

  • the gradient of the shorelines reflects tilting away from the main centre of ice loading and subsequent isostatic rebound over the south-west Grampians

The shoreline diagram points to a complex pattern of ice retreat, isostatic rebound and global sea level change in the post-glacial period extending from 15 thousand years ago to the present.

  • the highest beaches of East Fife and East Lothian developed rapidly as soon as the ice front retreated into the Firth of Firth. These beaches originally had storm ridges, formerly displayed north of Dunbar, but most of these delicate landforms have been lost to ploughing or building. The total uplift for the highest shorelines is the current height (up to 35 m asl) plus the fall of sea level from the time of formation to today (up to 60 m).

  • the ice front retreated west of Edinburgh

  • the ice front stabilised at or re-advanced to Perth. The extent of the Main Perth Shoreline suggest that sea level remained stable for several centuries

  • ice retreated west of Stirling - and glaciers probably largely disappeared from the Scottish Highlands. This is the period of the Windermere Interstadial, a warm period that marked the termination of the last ice sheet

  • during the following Loch Lomond Stadial (Younger Dryas) the temperature dropped sharply. Glaciers reformed on Rannoch Moor and flowed down the Loch Lomond basin to end moraines at Menteith. Global sea level fell and the crust of this part of Scotland was again depressed under the weight of ice.

  • at the start of the Holocene glacier ice has disappeared from Scotland. Global sea levels were rising rapidly, faster than crustal rebound

  • as global sea level rise slowed down as the last ice sheets melted in North America and Eurasia crustal rebound began to dominate, leading to the formation of a staircase of gently tilted raised beaches a few metres above present sea level

Recently, Smith and others have further refined the pattern of postglacial rebound in Scotland by using deposits from the Storegga Tsunami at ~7300 BP.