About 8.2 ka ago sediments on the steep continental edge of the North and Norwegian Seas slid onto the abyssal plain of the North Atlantic. This huge mass displacement triggered a tsunami whose effects manifest themselves in sand inundations at the heads of inlets and fjords along the Norwegian and eastern Scottish coasts that reach up to 10 m above current sea level. At that time actual sea level was probably 10 m lower than at present as active melting of the last glacial ice sheets was still underway: the waves may have reached 20-30 m above the 8.2 ka sea level. So powerful were the tsunami waves in the constricted North Sea that they may have separated the British Isles from the European mainland by inundating Doggerland, the low-lying riverine plain that joined them before global sea level rose above their elevation at around the same time. Fishing vessels plying the sandbanks of the southern North Sea often trawl-up well preserved remains of land mammals and even human tools: almost certainly Doggerland was prime hunting territory during the Mesolithic, as well as an easily traversed link to the then British Peninsula. Mesolithic settlements close by tsunami deposits are known from Inverness in Scotland and Dysvikja north of Bergen in Norway and individual Mesolithic dwellings occur on the Northumberland coast. The tsunami must have had some effect on Mesolithic hunter gatherers who had migrated into a game-rich habitat. The question is: How devastating was it.
Hunter gatherers move seasonally with favoured game species, often returning to semi-permanent settlements for the least fruitful late-autumn to early spring season. The dominant prey animals, red deer and reindeer also tend to migrate to the hills in summer, partly to escape blood-feeding insects, returning to warmer, lower elevations for the winter. If that movement pattern dominated Mesolithic populations then the effects of the tsunami would have been most destructive in late-autumn to early spring. During warmer seasons, people may not even have noticed its effects although coastal habitations and boats may have been destroyed.
Norwegian scientists Knut Rydgren and Stein Bondevik from Sogn og Fjordane University College, Sognda devised a clever means of working out the tsunami’s timing from mosses preserved in the sand inundations that added to near-shore marine sediments. (Rydgren, K. & Bondevik, S. 2015. Most growth patterns and timing of human exposure to a Mesolithic tsunami in the North Atlantic. Geology, v. 43, p. 111-114). Well-preserved stems of stair-step moss Hylocomium splendens still containing green chlorophyll occur, along with ripped up fragments of peat and soil, near the top of the tsunami deposit which has been uplifted by post-glacial isostatic uplift to form a bog. This moss grows shoots annually, the main growth spurt being at the end of the summer-early autumn growing season. Nineteen preserved samples preserved such new shoots that were as long as or longer than the preceding year’s shoots. This suggests that they were torn up by the tsunami while still alive towards the end of the growing season, around late-October. All around the North Sea Mesolithic people could have been returning from warm season hunting trips to sea-shore winter camps, only to have their dwellings, boats and food stores devastated, if indeed they survived such a terrifying event.