Brexit might be causing political chaos but whatever Theresa May has up her sleeve it is unlikely to be as catastrophic as the first separation of Britain from the continent.

A new study has revealed how giant waterfalls and, later, a megaflood severed our connection to France, resulting in the creation of island Britain and the watery moat of the English Channel.

“A chance series of geological events set the stage for Britain becoming an island,” said Sanjeev Gupta, professor of earth science at Imperial College London and co-author of the research.

“If it weren’t for these events, in a sense the history of Britain would have been completely different,” he added, pointing out that if the ridge had never been breached, Britain would have remained attached to northern France with easy access to the rest of Europe.

The research, published in the journal Nature Communications by Gupta and colleagues in the UK, France and Belgium, draws together a number of long-held theories and previous studies of the Channel’s seafloor with new, high resolution mapping of the landscape under the sea of the strait of Dover to unravel how Britain became separated from France. “It really tells us about the details of how this breaching happened, which we’d only basically guessed at previously,” said Gupta.



Bathymetry map of the strait of Dover showing prominent valley eroded through the centre. Note the rock ridge made of chalk in southern Britain and northern France which would have connected across the strait prior to breaching. Photograph: Imperial College London/Professor Sanjeev Gupta and Dr Jenny Collier

The result is a dramatic tale. About 450,000 years ago Britain was connected to France by a long, rocky, chalk ridge, approximately 32km long, behind which was a great lake, likely dotted with icebergs, with ice stretching across what is now the North Sea. “It would have been a dramatic landscape,” said Gupta. The Channel itself would have been dry, except for small rivers, while the surrounding land would have been forbidding. “It would have been cold, grey, rocky, with very, very sparse vegetation, like Svalbard or Siberia,” he said.

But, whether as a result of melting of the ice sheet or some other reason, it seems this dam-like ridge began to overflow.

“We find these huge holes basically, depressions, eroded into the bedrock in the Dover strait, and we believe the best explanation for these is that these were giant plunge pools – basically formed by waterfalls from the lake water plunging over [the ridge],” said Gupta.

The sediment-filled holes are enormous, he added, reaching up to 140 metres in depth and up to 4km in diameter, and exist in a line running across the seafloor. But while they were first discovered decades ago, leading to a change in course for plans for the Channel tunnel, the authors say the new study is the first clear evidence that they are the remains of huge plunge pools.

Erosion of the ridge by the water, the team add, at least partially ruptured the dam-like structure, releasing pent-up lake water which possibly carved some of the valleys previously found in the centre of the Channel. But there was more to come.

Analysis of the new data has revealed details of a huge valley, etched into the seafloor, running from behind the location of the former rock ridge into the Channel, cutting across the fossil plunge pools and connecting with the valleys in the centre of the Channel. That, says Gupta, suggests it was formed by catastrophic flooding, possibly sometime about 160,000 years ago, during another glacial period. “We think this is actually coming from lakes further upstream,” he said, adding that the megaflood probably also gouged the centre of the Channel.

Together, said Gupta, the two stages completely opened up the Dover strait, although Britain probably only fully became an island about 125,000 years ago, after sea level rises linked to a warmer climate. “You could say it was a violent beginning to Brexit 1.0,” he said.