Natural Brexit: Britain’s original separation from Europe left scars on English Channel

As it turns out, Brexit was not the first time Britain has separated from the European mainland. Scientists say that England and France were once connected by a ridge of land, until powerful waterfalls from an overfull lake demolished their connection.

The findings, published in the journal Nature Communications, help shed light on the emergence of Britain as an island, and on the changes this separation wrought in the greater region’s climate, ecology and human history.

“The opening of the Strait has significance for the biogeography and archaeology of NW Europe, with particular attention on the pattern of early human colonization of Britain,” the study authors wrote.

Some 450,000 years ago, Europe was a very different place. Glaciers covering the North Sea locked up much of the world’s water, leaving sea levels much lower than they are today. The English Channel was not a wide strip of water separating present-day England and France, but instead a frozen, river-ribboned tundra connecting the two lands.

The debate over how the dry tundra turned into a wide waterway has dogged scientists for decades. Was it a sudden change or a gradual process?

“The mechanism and history of the breaching of the Dover Strait is a question of importance to not only understanding the geographic isolation of Britain from continental Europe, but also the large-scale rerouting of northwest European drainage and meltwater to the North Atlantic via the Channel,” the study authors wrote.

In their paper, a team of European scientists led by Sanjeev Gupta of Imperial College London says they’ve found new evidence backing up an old but until now unproven idea: that Britain was cut off from France thanks to some devastating waterfalls.

The evidence for that lay in a series of strange “plunge pools” at the sea floor of the Dover Strait. Discovered in the 1960s while engineers were surveying the sea floor, these depressions could stretch roughly seven kilometers wide and hundreds of meters deep. These pits had been filled with looser sediment, forcing officials to reroute construction of the Channel Tunnel. In the 1980s, Bedford College marine geologist Alec Smith suggested that powerful, prehistoric waterfalls dug those enormous holes, but at the time, scientists lacked the data to determine whether this idea was true.

But now, using bathymetric maps to study the sea floor, the scientists found that Smith’s hypothesis was largely correct. Their analysis shows that Britain was once connected to the mainland thanks to a chalk ridge that extended from Dover (home of the famous white-chalk cliffs) in England to Calais in France, right across the Dover Strait. This ridge kept a proglacial lake — a lake formed in front of a glacier — at bay, until some unknown event caused it to spill over the natural dam, plunging into the valley below. This must have occurred at several spots along the ridge, leaving the telltale string of seven or so oversized plunge pools stretching from Dover to Calais.

The scientists can’t say for sure exactly what caused the lake to overflow and break the chalk dam. Perhaps a chunk of ice broke off the glacier and plunged into the lake, causing it to slosh over the ridge like sugar cubes dropped into a generous cup of tea. It’s also possible earthquakes helped to weaken the dam. In any case, the results certainly left telltale scars on the sea floor.

It was a second major event, however, that finished the job, separating Britain from the mainland for good. The Lobourg Channel, a valley at the bottom of the channel that stretches 80 kilometers long and 10 kilometers wide, was likely carved after a series of smaller lakes brimmed over. Though the scientists are unclear on precisely how far apart in time these two massive events took place, they think the second episode may have occurred around 160,000 years ago.

The researchers say more study is needed (though it may be easier said than done in the well-trafficked strait).

“The Fosses Dangeard sediment infills are an outstanding target for future drilling in order to precisely constrain the chronology of events shaping the breaching history of the Strait, and its palaeogeographic consequences,” the study authors wrote.

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The findings, however, could refine our understanding of when various species — humans included — arrived in Britain. How different would ancient and modern history have been, for example, if Britain had not become an island but had instead remained a peninsula, rather like Denmark today?

“Such a chronological framework is necessary to better understand the timing of when Britain first became isolated from mainland Europe during interglacial high sea-level phases,” the study authors wrote. “This has profound significance to understanding the ability and timing of biota, including humans, to colonize the British Isles.”

Understanding how Britain became an island nation will also help scientists understand how such a dramatic rerouting of a massive body of water would have affected the climate, they added.

“The rerouting of meltwater from the British-Scandinavian Ice Sheet and its injection into the North Atlantic has implications for inter-hemispheric climate variability,” the scientists wrote.

amina.khan@latimes.com

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