Britain split from mainland Europe to become an island thanks to catastrophe — that might sound political, but in fact it’s geographic. Thousands of years before the UK opted to leave the European Union, a process called Brexit, a different separation occurred. Unlike the political one, it was relatively simple, and probably composed of just two stages.
First, a review of geography: England is separated from the rest of Europe by a body of water called the English Channel; the bit of water where England is closest to France is called the Dover Strait. But the strait wasn’t always there — it was likely created by two major erosion events, according to a study published today in Nature Communications. The first one likely happened around 450,000 years ago, around the same time Neanderthals first appeared in Europe. That’s when huge amounts of water spilled over from a large lake sitting at the edge of a massive ice sheet that stretched from Britain to Scandinavia. The second one may have occurred 160,000 years ago, when catastrophic flooding opened the Dover Strait. When the ice age ended and sea levels rose, water flowed into that gap. Just like that, Britain became an island.
It wasn’t always this way — Britain was once connected to the European continent through a chalk ridge that extended from Dover, in southeast England, to Calais, in northwest France. The chalk ridge collapsed to create the Dover Strait. That’s not in debate. But what caused the collapse has been debated for a long time. That’s why researchers analyzed seafloor data of the English Channel — in the hopes of resolving the debate.
To set the scene, imagine ice. Around 450,000 years ago, the world was in the midst of an ice age, which is why there was a massive ice sheet hanging out over Northern Europe. At the southern edge of this ice sheet, rivers and meltwater pooled to form a large lake. The chalk ridge functioned as a sort of massive dam. Scientists have long thought that spillover from the lake eroded the chalk ridge, eventually creating the Dover Strait. Today’s study provides “the first evidence” of this lake, says study author Sanjeev Gupta, a professor of Earth science at Imperial College, London. “Now we can really show how that process happened.”
Gupta and his colleagues analyzed new seafloor data from the English Channel, showing holes in the bedrock that are filled with sediments. Though these holes were originally found in the 1960s and 1970s, the new data were higher resolution, which let the scientists do more detailed analyses. Gupta and his team believe these holes — which are several kilometers in diameter and around 330 feet deep — were created when water spilled from the ancient lake, creating a depression in the rock below, a phenomenon known as a plunge pool. (You can see them at the foot of modern waterfalls as well.) The researchers also found a huge valley cutting through the center of the strait, carved into bedrock. This valley was created by massive floods, the study says.
The data suggest that at some point, possibly around 160,000 years ago, the chalk ridge damming the large lake collapsed. It could have happened because of an earthquake, Gupta says, or because thousands of years of waterfall erosion finally broke through the ridge. In any case, the megaflood carved the land bridge between Dover and Calais, creating the Dover Strait. Later, when the ice age ended and sea levels rose, water filled the gap — and England became an island, Gupta says.
The timing of the final ridge collapse is not completely clear, Gupta says. To get more definitive answers, scientists would need to drill through the sediments filling those huge plunge holes at the bottom of the sea. The rock samples could then be analyzed to get a clearer date. But that might be easier said than done. The English Channel is one of the busiest patches of water on the planet, and extreme tidal currents would make the job hard, Gupta says.
Still, today’s study shows just how a set of chance geological circumstances shaped Europe’s history forever: maybe, if it wasn’t for Brexit number one, Brexit number two would have never happened. “Britain would be part of the continent and the identity of the country would be very different,” Gupta says. “This nation arises from that island geography. History would have been completely different.”