Researchers based in London have uncovered the event to which Britain owes its island existence.
Writing in this week's Nature Sanjeev Gupta and Jenny Collier, who are both based at Imperial College, used sonar to scan the floor of the English channel looking for clues to what cut the UK adrift from France. What they uncovered was evidence of a catastrophic flood that carved out the straits of Dover at some point between between two hundred and four hundred thousand years ago.
At this time England was linked to the continent by a rock ridge made of chalk that ran between Dover and France. This meant that the Thames and the Rhine both flowed northwards to empty into the North Sea. But then ice sheets triggered by the arrival of a glacial period formed an ice dam, which blocked the flow of water and triggered the formation of a lake the size of Wales. The water backed up all the way to the rock ridge. The team can't say for sure what triggered it, and it could have been a small earthquake, but something suddenly caused a breach in the ridge, which quite literally unleashed the floodgates. The accumulated water surged through, carving out deep chaotic gouges in the chalk, which were the giveaway signs picked up by the team's sonar.
"What's really exciting about this result is that it ties in with what the archaeologists are finding about early humans in Britain," says Sanjeev Gupta. "Between 200,000 and 60,000 years ago there is no evidence of human activity here; so it could be that because this flood triggered the Rhine to divert south through the newly formed Straits of Dover it would have been very difficult to cross, and as sea levels rose with the end of the glacial period (ice age) Britain would have been cut off."