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Planet Earth

Megaflood in English Channel separated Britain from France

Not Exactly Rocket ScienceBy Ed YongJune 8, 2009 5:30 PM

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Hundreds of thousands of years ago, one of the largest floods in Earth's history turned us into an island and changed the course of our history. Britain was not always isolated from our continental neighbours. In the Pleistocene era, we were linked to France by a land ridge called the Weald-Artois anticline that extended from Dover, across what are now the Dover Straits.

This ridge of chalk separated the North Sea on one side from the English Channel on the other. For Britain to become an island, something had to have breached the ridge.

Now, Sanjeev Gupta and colleagues from Imperial College London have found firm evidence that a huge 'megaflood' was responsible. They analysed a hidden series of massive valleys on the floor of the English Channel - vast gouges of bedrock 50 metres deep and tens of kilometres wide.

These valleys were first noticed by geologists in the 1970s but until now, no one really knew what caused them. Gupta decided to find out with the help of some modern technology. He used high-resolution sonar to create a contour map of the Channel floor, and found that this hidden world was remarkably well preserved.

He saw a clear picture of the huge, linear valleys, branching out in a westerly direction. In and among the valleys lay long ridges and grooves running parallel to the channel, V-shaped scours that taper upstream, and streamlined underwater islands up to 10km long.

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All in all, these images show that the valleys are geological scars, formed by erosive torrents of water travelling west from the Dover straits. Their size and features are consistent with a massive flood, carving out the land in its wake.

During the Pleistocene, the North Sea was actually a giant lake, closed off at its northern edge by merged ice sheets from Britain and Scandinavia, and at its southern edge by the Weald-Artois ridge.

This lake was fed by both the Thames and the Rhine rivers. That, combined with the melting ice, eventually burst the Weald-Artois barrier, sending the lake's water surging into the Channel.

Gupta estimated that the flood would have lasted for several months and involved at least two episodes. At its peak, one million cubic metres of water flowed into the Channel every second, a thousand times more than the Victoria Falls.

The megaflood changed both the local geography and the course of British history. It reorganised the drainage of the Thames and Rhine rivers to the Channel rather than the North Sea. And most importantly, it permanently separated Britain from continental Europe.

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The flood made migration into the newborn island more difficult and aside from some early attempts at settlement, Britain was completely devoid of humans for about 100,000 years.

Once humans finally colonised this green and pleasant land, our island status has affected our entire history from our power as a naval empire, to our strategies during the Second World War to our national character.

Reference: Gupta, Collier, Palmer-Felgate & Potter. 2007. Catastrophic flooding origin of shelf valley systems in the English Channel. Nature 448: 342-346.

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