London
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 30 kilometres wide, that ran from Dover to Calais and protruded several hundred kilometres 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 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 30 to 90 metres 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 140 metres deep and several kilometres 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 30,000 cubic metres 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, Britain 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.
Gupta and Collier have now teamed up with Belgian and French seismologists to analyse 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.
Gupta and 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,” Gupta said.
— New York Times News Service
