Britain is an island thanks to a megaflood that dug out the English Channel, according to a study released yesterday by the journal Nature.

The catastrophic flood, one of the mightiest in recorded history, occurred between 450,000 and 200,000 years ago and created a barrier to human migration to Britain that probably lasted for tens of thousands of years.

Imperial College London researchers Sanjeev Gupta and Jennifer Collier used high-resolution sonar to build up a highly-detailed picture of the floor of the Channel.

They captured spectacular images of a vast submerged valley, tens of kilometres wide and up to 50 metres deep, that was carved into the Channel's chalky-bedrock.

They believe the valley is the flood-scoured remnant of a land bridge that was destroyed when a natural dam, running across what is now the Dover Straits, was breached.

This so-called Weald-Artois ridge was the southern wall of a gigantic lake which spanned what is now the southern North Sea.

Two rivers, the Rhine and the Thames, fed the great lake, and towering glaciers provided its northern wall.

The Weald-Artois ridge collapsed, presumably be-cause of a rise in the lake's level, unleashing a megaflood that lasted for several months. A million cubic metres of water per second poured southwards, Gupta and Collier calculate.

The torrent formed the 'Channel river,' digging out what is now a valley on the Channel floor.

At that time, Earth was in a glacial period. Sea levels were much lower than they are today, as so much water was locked up in icesheets.

When the ice age came to an end, sea levels rose once more and the valley was permanently drowned, leaving today the sister cliffs of Dover and Cap Gris-Nez as signatures that Britain and France were once joined.

In addition to changing the region's topography forever, the 'Channel river' must also have altered the course of human history, the paper suggests.

"This prehistoric event rewrites the history of how the UK became an island

and may explain why early human occupation of Britain came to an abrupt halt for almost 120,000 years," suggests Gupta.

PHOTO CAPTION

This image, released in 2006 by NASA. acquired by the MODIS on the Aqua satellite, shows parts of England (Upper-L) and France, separated from each other by the English Channel. (AFP)

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