During an ice age some 450,000 years ago, a land bridge linked Britain to the mainland. Frigid temperatures had trapped large amounts of water in polar ice, and ocean levels fell to levels hundreds of feet lower than they are currently. At the time, a giant ridge of chalk — made of layers of pulverized shells of long-dead algae — connected the southern corner of Britain to northwestern France.
“We had the rock ridge standing proud,” study author Jenny Collier, a marine geophysicist at Imperial College London, told The Washington Post. It towered hundreds of feet above the low ocean that curled around Britain.
The bridge also acted as a dam, holding back a lake of melted glacial water. But the dam didn't last. Today, only the very edges of the ridge are visible as majestic bluffs, including the White Cliffs of Dover. Geologists didn't know whether the dam dissolved slowly, as a result of gradual erosion, or all at once.
According to the new findings, two swift cataclysms did the dam in. First, several huge waterfalls cascaded over the bridge, weakening the rock structure. Then, about 200,000 years later, a megaflood blasted through the breach, completely severing Britain from the continent.
Scientists had hypothesized about the existence of ancient and destructive waterfalls, but evidence was limited. “It had been proposed in the mid-'80s,” Collier said. “But it dropped out of view and no one took it very seriously.”
The authors of the new study pointed to a key piece of evidence favoring the waterfalls: a row of massive holes in the bedrock below the English Channel, just to the side of where the dam once stood. Surveyors discovered the holes, some of which descended 450 feet into the rock, in the 1960s and 1970s. (This was during planning stages for the construction of the tunnel between England and France; tunnel engineers had to dig around the huge depressions.)
To get a better picture of the holes' characteristics, the scientists coupled old topographic data with new imaging techniques. The giant pockmarks seemed to be the product of geologic violence more severe than what would be expected in the Dover Strait. “We’re not in the Himalayas, we’re in the south of Britain, where everything is quite gentle,” Collier said. The scientists determined that the most likely sculptor behind each hole was a waterfall — a giant water jet, on the scale of Niagara, that plummeted over the ridge and scoured the bedrock.
These holes were “exactly in the right place” where water would have spilled over the top of the dam, Collier said. The waterfalls also cut into the dam itself. A section of it failed, releasing a huge volume of glacial lake water.
The new seafloor images also supported the theory that floodwaters broke through the ridge a second time. The scientists found a more recent system of valleys carved into the Dover Strait, coinciding with an event that occurred 200,000 years after the giant waterfalls. Though evidence for the later flood was “more conjectural,” Collier said, there is other evidence to support such an event. About 160,000 years ago, when Collier and her colleagues estimate this flood struck, the population of early humans who lived in the area crashed.
The flood would have been a knockout blow, wiping away the ridge. Such a cataclysmic uncoupling changed human history, the scientists noted. “This is Brexit 1.0 — the Brexit nobody voted for,” Sanjeev Gupta, a co-author of the study and an Earth scientist at Imperial College London, said in a news release. “Without this dramatic breaching Britain would still be a part of Europe.”