IN 1607 a sudden flood around the Bristol Channel in south-west Britain killed at least 2,000 people. It was the worst natural disaster ever recorded in a country which, last week, devoted headlines to an earthquake that knocked over a few chimney pots. But on that sunny January 30th four centuries ago, a fully laden 60 tonne ship in Appledore harbour, north Devon, was picked up and dumped in nearby marshland. The waters took one Mistress Van before she could reach the higher rooms of her house. She was caught unawares because she lived more than four miles (around 6km) from the sea.
That this huge flood happened is undisputed. It is commemorated in church plaques in the coastal counties of Somerset and Monmouthshire. But why it happened is another matter. For the past five years Edward Bryant and Simon Haslett of the University of Wollongong in New South Wales, Australia, have propounded the idea that a storm surge may have been wrongly accused. They think the flood was caused by a large tsunami that began off Ireland. They outline their evidence in this month's Journal of Geology.
The Bristol Channel is shaped like a funnel. That means it experiences smaller storm waves in its inner regions, which are narrow and shallow, than in its wider, deeper parts. Conversely, a tsunami wave rolling up the channel would grow in height as it became increasingly constricted. Dr Bryant and Dr Haslett looked at large boulders that had been transported onto the land by the sea in this region. Had a storm surge moved these boulders, Dr Bryant and Dr Haslett calculate that its waves would have to have been seven times higher than those of the largest storm waves ever recorded in these parts. A tsunami is more plausible.
Next they looked at the arrangement of the boulders. Rather than being dribbled erratically over the land, many form overlapping “trains” like roof tiles, oriented along the direction from which a tsunami would have struck. This pattern is more pronounced as the channel becomes narrower and turns into the Severn estuary.
Finally Dr Bryant and Dr Haslett found four examples of coastal bedrock that has recently been sculpted by high-energy vortices. That is an after-effect associated with historical tsunamis.
What might have caused such a tsunami is unclear. One possibility is a submarine landslide. Another is an earthquake at sea. Such earthquakes are rare in the geologically placid British Isles. But, as the people of Kent have just discovered, they do occasionally happen.