Geologists have assembled picture of how land bridge to Continent was destroyed
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 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 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 occured 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.
