3.4 Relative sea level changes during the last 15,000 years


Location of the Early Holocene iintertidal site at Clachan Old Harbour, Raasay © Sue Dawson

The relative sea level history of Scotland during the Lateglacial and Holocene (last 15,000 years) is complex. This is due to the overburden of ice when the last Scottish ice sheet covered the region. Ice cover was concentrated in the western Highlands with thinner areas of cover towards the peripheries of Scotland. Thus, the area around Oban in the west had greater thickness of ice than areas of the Outer Hebrides, the North coast and the Northern Isles. This leads to varying amounts of isostatic rebound and therefore, the position in the landscape where relict Shorelines can be seen today. For the Oban area, this translates to visible shorelines dated to c. 10ka BP (c.8000 cal BC) yrs up to 10 metres OD. However, the same Lateglacial Shoreline is well below present sea level in the areas of Coll and Tiree, Islay and the Solway coastline. Predicted shorelines for the Outer Hebrides and the Orkney Isles suggest they are located between 20-30 metres below present.

The net result of relative sea level change for Scotland is thus that there are areas where the sea bed has been dry land within the last 15,000 years. As this is the period within which Scotland has an increasing record of human presence, it is likely that these areas were inhabited.  They offer the possibility that submerged archaeological sites may be preserved.  Perhaps the best known area is that around the Orcadian archipelago where the sea did not reach present levels until about 4000 years ago (Strategic Environmental Assessments (SEA) of North Sea Areas:http://www.offshoresea.org.uk/site/scripts/sea_archive.php) 4), but another area lies to the west of the Western Isles (SEA7; Jordan et al. 2010), and there are small localized areas elsewhere, e.g.: around Coll, Tiree, and Islay.  Although there are no specific data on relative sea level rise for these areas, it is assumed that sea level reached roughly its present level between 5000–4000 years ago meaning that any submerged archaeological sites are likely to relate to Mesolithic or early Neolithic settlement.  Interestingly, both Orkney and the Western Isles stand out from the rest of Scotland in that they have relatively little evidence for Mesolithic settlement.  Mesolithic sites are few and far between in Orkney and even more so in the Western Isles.  Given the importance of coastal resources in the Mesolithic and the apparent concentration of sites around Scotland's coastlands this may be significant as an indication that evidence for the first 5000 years of human settlement in these areas is lying off shore.  Recent fieldwork in Orkney suggests the probable preservation of stone structures relating to the Neolithic on the seabed (Wickham-Jones et al. 2009).

It is also worth remembering that large-scale areas of the Scottish shelf have been dry land for considerable periods over the past 700,000 years  as a whole - a period during which there were episodes of human (Palaeolithic) habitation elsewhere in Britain.  England and Wales have a good record of early sites (Stringer 2006), particularly in the south, but there are so far no Lower or Middle Palaeolithic sites in Scotland (Saville 1997).  Environmental and osteological evidence suggests that this submerged landscape has, at times, been suitable for human occupation (Stringer 2006) and it is possible that surviving Palaeolithic sites from the 'Scottish sector' of the sea bed still survive - comparable sites on land having been destroyed or buried by the actions of the last Ice Age which blanketed mainland Scotland.

Submerged archaeology is likely to comprise a considerable resource for Scotland, a resource that, unlike other parts of Britain, has been largely neglected to date.  Increased pressures on the submarine landscape make the need to deal with this resource ever more urgent.

Work on submarine archaeology and landscapes around Britain to date, includes:

  1. http://www.st-andrews.ac.uk/~crb/website/website13_4_10/
  2. http://www.arch.soton.ac.uk/Research/Aggregates/shelve-intro.htm
  3. http://www.hwtma.org.uk/intro-bouldnor-cliff
  4. http://www.iaa.bham.ac.uk/research/fieldwork_research_themes/projects/North_Sea_Palaeolandscapes/index.htm
  5. http://www.wessexarch.co.uk/projects/marine/alsf/seabed_prehistory/round2.html
  6. http://www.bmapa.org
  7. http://www.science.ulster.ac.uk/cma/slan/
  8. Wessex 4min video: www.youtube.com/watch?v=TcGBnVI0gM0