Recent research supports the view that a tsunami occurred widely in the northern North Sea and adjacent areas of the Atlantic Ocean at around 6,000 BC. The event is known to have altered coastlines and probably affected coastal populations, but its greatest significance today appears to be in the sediments it deposited, which provide a stratigraphic marker horizon of value to geologists and archaeologists alike.
‘Tsunami’ is a word adopted by geologists to describe a sea surface disturbance seen in a series of waves. A tsunami may originate on the sea floor, such as in land movement along a fault or in a landslide, or in a volcanic explosion. It may also be caused by the impact of a meteorite on the sea surface. The tsunami waves spread away from the area of the event. Places in their path often experience an initial drawdown of water before the first wave arrives. The size and number of the waves depend upon the nature and scale of the cause, together with the depth of the sea and the shape (bathymetry) and composition of the sea floor. Out at sea, tsunami waves may be no more than a metre or two high, but they grow as they reach shallow water and have been known to reach heights of over 30 metres. Although not much higher than the greatest storm waves, they are of a much longer period, travel faster and involve much greater energy. They can occur in all oceans and seas of the world. Some indication of the devastation big tsunamis achieve is shown in the photograph of a demolished church (below).
Evidence for a North Sea and Atlantic tsunami consists of deposits lying within Holocene (post-glacial) sediments landward of the present day coastline. Such sediments are found where land has risen after glacial ice sheet decay (isostatic uplift) more than the sea level. The map (below) depicts locations where the deposits have been found, together with the area of the likely cause of the tsunami, the second Storegga slide.
In northern England, eastern and northern Scotland, there is a thin (circa 2-100 centimetres) but persistent layer of sand occurring within former estuarine deposits (see section photo page 470. When first found in the Forth valley, the layer was thought to have been deposited by a river flood. It was later believed to have been the product of a North Sea storm surge. However, the singularity of the layer in over 2000 years of estuarine sedimentation at widespread locations indicates that a more unusual origin is likely.
The culprit seems to be have been the second Storegga slide offshore south-west Norway, when masses of material moved down the continental slope and across the abyssal plain over an area of at least 20,000 square kilometres. Radiocarbon dates for sediments resting on slide deposits are similar to dates for peat resting on the sand layer. This tsunami deposit has now been found at 23 locations in eastern Scotland and northern England and dated at 16 of them. A typical drawn section showing the stratigraphic context is illustrated on page 470. The likely age is around 6000 BC, taking the mean of dates for the base of the peat conformably resting upon the sand layer; there may have been erosion below the sand, so dates for peat beneath could be too old.
Evidence for the tsunami in Norway has been identified from rocky hollows, known as isolation basins, in the south-west and far north. Here a layer of sand and gravel containing fragments of plants and wood is seen within marine sediments at low levels, and within brackish-marine or lacustrine sediments at higher elevations. Radiocarbon dates have been obtained from biogenic sediments beneath, within and above the tsunami layer, but the best age estimate is from the base of conformable sediments resting on the layer: the Norwegian dates are slightly older than the Scottish ones, although both sets are statistically comparable.
To the west, in Iceland, evidence more akin to the Scottish record has been found, with a prominent layer of sand occurring within peat, and a similar date obtained.
How big was this tsunami? The widespread evidence indicates a major event, but its magnitude is difficult to estimate. In Scotland, the difference in altitude of the sand between where it crosses the intertidal mudflat beneath, and its limit on the rising slope behind is no more than about 5 metres. In Norway a somewhat less accurate comparison between the altitude of the highest isolation basin threshold and an estimated contemporary shoreline gives values of up to 10 metres. However, these estimates are likely to be on the low rather than high side. Studies of present day tsunamis indicate that deposits can sometimes be many metres below the wave height. Model simulations support the implications that the event was of considerable scale, but at present do not throw much more light on the highest levels reached. It thus seems possible that the tsunami caused by the second Storegga slide was a spectacular event along the coast, which would have left its mark on the minds of those who witnessed it. Flooding may have extended kilometres inland in low-lying estuarine areas, and the erosion of coastal spits and bars would have been notable.
The activities of Mesolithic hunter-gatherer communities are likely to have been disrupted by the tsunami. At Inverness, excavations have disclosed tsunami sands resting upon scattered artefacts of a hunting camp: we believe there may well be a direct link, but we cannot prove it. Near St Andrews, a similar camp appears to have been relocated about this time. Migration across the then dry land of the southern North Sea could easily have been affected by the tsunami reinforcing the narrow channel which was opening up about that time and which would isolate Britain from the European mainland [some time between circa 6000 and 4000 BC].
The event may prove of greater interest in geology and archaeology as a marker horizon. The thin yet widespread layer of sand which accumulated in the space of a few hours may ultimately enable correlation between occupation layers at sites up to hundreds of kilometres apart. Major events in environmental change can be compared from location to location. Recently a preliminary estimate of the pattern of land uplift in Scotland has been made using the altitude of the intertidal mudflats when the event occurred. With more data, such an estimate may prove of greater accuracy than those obtained by other methods, since the ‘marker’ occurred over such a brief period of time. The Storegga tsunami may thus prove a valuable benchmark in understanding Mesolithic populations and the landscapes they inhabited.
Many researchers have worked on the Storegga tsunami: see for example Marine Geology 82, 1988, 271-276 and Boreas 26, 1997, 29-53. Model simulations are provided in Marine Geology 105, 1-21. Details of the Storegga slides appear in Philosophical Transactions of the Royal Society, Series A, 1988, 325-388. The excavations at Inverness are discussed in Journal of Archaeological Science 17, 1990, 509-512.
David E. Smith, Director Centre for Quaternary Science, Coventry University, Coventry, CV1 5FB
What was the impact of the Storegga tsunami on the inhabitants of the Scottish coast?
Caroline Wickham-Jones sifts the evidence…
The tsunami that hit Scotland some 8000 years ago is certainly a major factor in studies of the Mesolithic settlement of the time, as David Smith has described. Unfortunately, however, recent Scottish fieldwork in the Mesolithic has mainly taken place in the west, so that it has not been possible to examine a site where evidence of the tsunami overlies archaeological remains. Tsunami sediments have been identified on Mesolithic sites in the east, including Broughty Ferry, Morton and Inverness, but these sites were excavated well before the significance of the white tsunami sands was recognised in the 1980s. At these sites the tsunami deposit sits directly over the occupation deposits, but it is impossible to be sure of the relationship between the two. The force of the tsunami event means that it may well have eroded the ground surface prior to the deposition of sands, thus exposing earlier settlement remains. Though the significance of this deposit has only recently been recognised it is interesting to note that as long ago as 1886 Hutcheson commented on the force of the exceptional tide that had apparently disturbed the remains of occupation at Broughty Ferry and covered them with sand.
Recently attention has begun to shift back to the potential of the Mesolithic remains of the east coast areas of Scotland, but tsunami deposits in direct relationship with archaeological remains are proving elusive. In 1996 a small site was excavated at Fife Ness, to the south of St Andrews. Though the remains here pre-dated the tsunami, they were overlain by ploughsoil and there was no sign of tsunami material. Graeme Warren (Edinburgh University) has been conducting excavations at the Sands of Forvie, north of Aberdeen, for Historic Scotland. The material here is probably more recent, however, as is that at Low Hauxley in Northumberland, excavated by Clive Bonsall (also from Edinburgh University). Meanwhile a team from Newcastle University have been working on the Northumberland coast at Howick. Here the site is probably older, but as yet no clear tsunami deposit has been found in the vicinity.
It would be nice to find the remains of squashed Mesolithic people under the tsunami deposit, but I fear that the force of this event is such that this is an unlikely scenario. Perhaps some Current Archaeology reader might know whether any research has been done on the disturbance caused by recent tsunami events and the archaeological traces left in relation to known pre-existing settlement.
From a human point of view it is likely that the sea would first of all have gone out, all but disappeared. This must have been a source of wonder and no doubt a great harvest of fish and other sea creatures would have been laid bare for the taking. My guess is that people would have been divided. Some, I am sure, would have ventured out to investigate and collect the riches exposed on the sea bed. Others (and I hope I might have been one of them – perhaps I must have been if my ancestors were on the east coast at the time) instinctively made off in the opposite direction. Whatever the nature of their Gods, this was something out of the ordinary. If they were lucky, they would have made it to higher ground, a little way inland, in time to survive the cataclysm. Traces of the tsunami deposits have, however, been found well up some of the east coast river valleys. The disruption to life as people knew it would have been complete: many settlements disappeared without trace; members of individual families were never seen again; the shape of the land itself, on which people relied, was altered. People certainly had the resources to pick up the pieces and build new communities, but the mental images and the anguish must have left a lasting mark.
The tsunami reminds us that natural disasters are not a recent phenomenon. In the long time-scale of human history even apparently secure places like Britain can be vulnerable.
I would like to thank Graeme Warren (the Sands of Forvie) and Clive Waddington and Robert Shiel (Howick).
For a general discussion of the tsunami in relation to the Mesolithic of Scotland see Wickham-Jones Scotland’s First Settlers (Batsford 1994). For excavation reports see: Broughty Ferry: Hutcheson Proceedings of the Society of Antiquaries of Scotland XX, 1886, 166-9. Fife Ness: Wickham-Jones & Dalland Internet Archaeology, 5, 1998 http://intarch.ac.uk/journal/issue5/index.html. Inverness: Wordsworth Proceedings of the Society of Antiquaries of Scotland 115, 1985, 89-103. Low Hauxley: Bonsall Proceedings of the Prehistoric Society, 50, 1984, 398. Morton: Coles Proceedings of the Prehistoric Society 37, 1971, 284-366. Sands of Forvie: Warren Discovery & Excavation in Scotland 1999, 10-11.