Ancient Megaflood Made Britain an Island, Study Says

Kate Ravilious
for National Geographic News
July 18, 2007

A flood of biblical proportions cut the British Isles off from mainland Europe sometime between 450,000 and 200,000 years ago, according to a new study.

The research, based on three-dimensional sonar mapping of the English Channel, provides the strongest evidence yet that a catastrophic megaflood broke a land bridge that once connected what is now Britain and France.

"It is probably one of the largest floods ever identified," said Phillip Gibbard, a geographer at the University of Cambridge who wasn't involved in the study.

At its peak, the flood would have discharged water at a rate of about 264 million gallons (a million cubic meters) a second, gushing at speeds of up to 62 miles (100 kilometers) an hour, the researchers say. This is roughly equivalent to ten times the combined flow rate of all the rivers in the world.

In addition to making Britain an island, the authors add, the huge flood had wide-ranging environmental consequences.

For example, the gigantic pulse of freshwater entering the Atlantic Ocean likely caused a period of climate cooling in the Northern Hemisphere, Gibbard said.

"The introduction of ice and freshwater into an ocean drives climate oscillations and causes marked cooling events," he explained.

The flood also marooned many animals and plants, so those species gradually evolved into different forms than their mainland cousins.

And humans appear to have avoided the newly made island altogether, leaving it unoccupied for over a hundred thousand years.

Crumbled Chalk

Researchers have long known that a narrow ridge of chalk once connected Dover in southeast England to Calais in northwest France (see a map of France showing Calais' proximity to Britain).

During the ice ages, when sea levels were low, the ridge held back a glacial lake from inundating a large valley between the two regions.

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