When Britain Split From Europe, in a Big Way

http://www.nytimes.com/2017/04/04/science/britain-europe-split-land-bridge.html

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The separation of Britain from Europe, set in motion by Prime Minister Theresa May last week, began a historic process but one not as abrupt as the first Brexit. That was the catastrophic destruction of the land bridge that for 10 million years had joined Britain physically to the Continent.

The bridge was a rock formation, about 20 miles wide, that ran from Dover to Calais and protruded several hundred miles into France and Britain. It was made of chalk, as can be seen in the cross-section where it has been ripped away at the white cliffs of Dover.

After many years of work, starting with the underwater surveys made in preparation for digging the Channel Tunnel, geologists have at last assembled a picture of the mighty forces that tore the bridge away and gave Britain its identity as an island, rather than a mere peninsula of Europe like Denmark and Scandinavia. Their account appears in Wednesday’s issue of Nature Communications.

In the last ice age, sea levels rose and fell as water was locked up in ice sheets during cold periods and released to the oceans in warm ones. At high sea levels, water would nearly encircle Britain but never surmounted the land bridge, which stood 100 to 300 feet above the waves.

That was until a cold period that began 450,000 years ago. A vast glacier that covered all but the southern parts of Britain edged out across the North Sea and joined up with the glacier covering Norway. With the North Sea dammed, the rivers that then drained into it, including the Rhine and the Thames, started to form a large lake, also swollen with meltwaters from the glacier.

As the level of the glacial lake rose, its waters started to cascade over the Dover-Calais land bridge that formed its southwestern wall. Laden with abrasive pieces of flint dissolved from the chalk, the waterfalls scoured out vast holes in the bedrock beneath, some 450 feet deep and several miles in length.

The western side of the land bridge retreated as the waterfalls eroded it, and finally a section gave way. In a cataclysmic flood, up to a million cubic feet of water per second roared through the breach, scouring deep valleys as the vast glacial lake emptied itself into the English Channel.

This event took place 430,000 years ago, to judge by a thick layer of sediment this old that has been found on the floor of the Atlantic Ocean next to the English Channel mouth.

The first breach in the land bridge may have been relatively small. The sediment record on the ocean floor indicates that a second megaflood occurred 160,000 years ago. It seems a second lake built up in the North Sea, its southern boundary being a wall of sediment left after the sudden exit of the first lake. When this wall collapsed, perhaps because of an earthquake, the lake rushed out, sweeping away the rest of the bridge and ensuring that at high sea levels, as at present, England would be an island.

Several aspects of this series of events have been proposed before but not proved. In 2007, a team led by Sanjeev Gupta and Jenny S. Collier of Imperial College London obtained previously unavailable records of the detailed underwater topography of the English Channel. These showed that west of the Strait of Dover, a network of deep valleys had been cut through the channel’s bedrock.

The streamlined nature of the walls and submarine islands suggested they had been shaped by a flood of enormous force. This evidence, the researchers said, supported the idea that a catastrophic breach of the Dover-Calais land bridge had unleashed a megaflood into the English Channel.

Dr. Gupta and Dr. Collier have now teamed up with Belgian and French seismologists to analyze the channel’s bedrock more closely. In particular they have looked at a series of deep pits in the bedrock between Dover and Calais. The sediment-filled pits were discovered in preparing the route for the Channel Tunnel and named the Fosses Dangeard (fosse is French for pit) after a French geologist.

The tunnel had to be rerouted to avoid the dangerous pits, which were assumed to have been gouged out by glaciers. But it’s now known that the ice never reached that far south.

Dr. Gupta and Dr. Collier interpret the pits as giant plunge pools created by cataracts cascading down from the land bridge. The depth of the pits suggests the cataracts must have fallen from a considerable height. The new seismic data show that some of the pits are elongated as if the land bridge was progressively shrinking until a breach unleashed the glacial lake behind it.

“Our paper shows for the first time that a lake existed and that there were waterfalls coming over the land bridge,” Dr. Gupta said. A deep bedrock valley that passes through the Strait of Dover from the east, the Lobourg Channel, would have been carved by the megaflood from the second lake, in his view.

Philip Gibbard, a geologist at the University of Cambridge, said Dr. Gupta and Dr. Collier’s reconstruction of events was “exciting and deeply plausible.” Victor Baker, an expert on very large floods at the University of Arizona, said the new data definitely showed that an ancient megaflood had occurred in the English Channel, and that everything so far known was consistent with the idea that the Dover-Calais land bridge had been destroyed by two catastrophic floods.

Dr. Gupta said he hoped to add more detail by drilling into the sediments that now fill the Fosses Dangeard. But the channel is the world’s busiest shipping route, and studying the Fosses requires crossing two shipping lanes.

“It’s quite hard to persuade ships’ captains to do it,” he said. “I’d say we know more about Mars than the submarine geology of these continental shelves around the world.”

Geologists may now understand how England became an island, but in their time scale few things are forever. “Brexit” or no, England will again become part of Europe when the oceans’ waters are locked back into the glaciers of the next ice age.

“If and when sea level goes down again, which it probably will, we’ll again be able to walk from here to the Netherlands,” Dr. Gibbard said from his home in Cambridge, England.