19 July 2007

Catastrophic flood split Britain from Europe

Cosmos Online
Monumental flooding isolated Britain from mainland Europe hundreds of thousands of years ago and halted early human migration there.
Catastrophic flood split Britain from Europe

Dam buster: Map showing ancient regional geography about 450,000 years ago. It depicts ice sheets, which expanded into northern Europe and Britain. The red line marks the damming of a massive lake by a rock ridge, which was later breached. Credit: S. Gupta & A. Whitchurch

SYDNEY: Catastrophic megafloods isolated Britain from mainland Europe hundreds of thousands of years ago, say geologists, halting early human migration there.

Using high resolution sonar to study the morphology of the English Channel seafloor, experts from the Imperial College in London have identified evidence of not one, but two, monumental prehistoric floods. These were possibly the biggest in history.

“We have found a large, deep valley carved into the floor of the English Channel, which we believe was caused by high water volumes released during a catastrophic flood event,” said study author Jenny Collier. “If this event had not happened, the U.K. would have remained a peninsular of Europe.”

“Catastrophic flood event”

In their paper published in the journal Nature today, Collier and her co-author Sanjiv Gupta, reveal that high interglacial sea levels led to marine flooding of the English Channel and the North Sea sometime before 200,000 years ago.

Using bathymetric (or sea depth) surveying, Collier’s team created a detailed three-dimensional map of the English Channel. They used this to identify a large valley – the Northern Palaeovalley – which hinted at large-scale erosion by a massive, catastrophic flooding event. The pair recorded detailed images of a gash torn into the chalky bedrock that is tens of kilometres wide and 50 m deep.

At around 450,000 years ago, this first flood may have been caused by a breach in a fragile strip of land the authors refer to as a “rock dam” at the site of today’s Dover Strait. This dam was the southern wall of an enormous glacial lake that spanned what is now the southern North Sea. The lake was then fed by Thames and Rhine rivers and was dammed on its northern side by glaciers.

The breach of the dam instigated huge drainage of the lake, at an estimated rate of a million cubic metres a second for several months, leading to the deepening of the valley that now lies on the seafloor.

At this time the world was firmly in the grip of an ice age, which locked up much water in vast ice caps and made sea levels much lower than they are today. However, as the world warmed several hundred thousand years later, sea levels rose once more, creating another massive flood that widened the channel and forever divided Britain from the mainland.

Migration stopper

These climate changes not only lead to the separation of the British Isles, but may have also halted the migration of ancient humans from Europe, potentially explaining puzzling episodes of population growth and decline.

“This prehistoric event rewrites the history of how the U.K. became an island and may explain why early human occupation of Britain came to an abrupt halt for almost 120 thousand years,” said Gupta.

Another implication is that flood was the cause of a well-known ancient reorganisation of northwestern European rivers, said the researchers.

“It is a very elegant study detecting evidence of catastrophic flood features on the seabed using state-of-the-art remote sensing technology,” commented Ted Bryant an expert on ancient climate and catastrophes at the University of Wollongong in New South Wales, Australia. “This can be done easily for surface features on the Earth and at expense using satellite images on Mars. To do so using sonar of the seabed is stunning.”

“Furthermore, there is no doubt that this flooding cut any land bridge between England and the continental. The demise of human occupation in Britain 100,000 years ago is nicely accounted for by this flood event and the subsequent diversion of northern European rivers through the English Channel at lower sea-levels,” said Bryant.


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