For millions of years, Britain was part of Europe. A high ridge of limestone—now famously exposed as the chalky cliffs of Dover—extended all the way to what is now France, allowing mammoths, hippos, and eventually humans to freely pass back and forth. But about 450,000 years ago, this rocky road was cut off by a flood of unimaginable proportions. Now, a team of researchers has detailed evidence of how this torrent of destruction was unleashed.
"They've managed to identify … how the ridge was breached," says geologist Philip Gibbard of the University of Cambridge in the United Kingdom, who was not involved in the current study. "I'm quite enthusiastic."
The idea that enormous floods quickly carved parts of the English Channel was first proposed in 1985. At the time, geologist Alec Smith of Bedford College in London noticed large, interweaving scour marks on maps of the English Channel sea floor. The patterns of erosion resembled those of the "channeled scablands," a starkly eroded landscape in eastern Washington state that was formed by floods from glacial lakes that burst their icy dams between 15,000 and 18,000 years ago.
Similar glaciers have existed in Europe. Some 450,000 years ago, a massive ice sheet spanned Scandinavia, reaching down into northern Europe and much of what are today the British Isles. Hemmed in by the ice was a large glacial lake. No one knows exactly how big it was, but it may have been several hundred kilometres wide, and it likely filled the southern part of the North Sea. Because so much water was stored on land as ice sheets, sea levels were likely 120 meters lower than today, exposing the bottom of what is now the English Channel.
With the chalk ridge forming a dam to the south, and the Rhine and other rivers flowing into the glacial lake, the water would have kept rising. Smith proposed that the lake would have eventually cascaded over the chalk cliffs. As evidence, he pointed to large, deep pits at the base of the former ridge—“plunge pools” that were formed by the force of the waterfalls and later filled in with sediment. Eventually, he said, a major flood would have created the scour marks on the channel bottom. Still unknown was what unleashed the deluge.
The dramatic idea was then forgotten for many years. "I don't think many people believed it," says Sanjeev Gupta, a geologist at Imperial College London. (Smith, who retired a few years after floating his idea, died in 2015.)
Now, Gupta and his colleagues have brought the cataclysmic breakup back to life. With data from U.K., Belgian, and French research vessels, the team got a clear picture of seven plunge pools on the channel floor (see map below), each nearly a kilometer wide and up to 140 meters deep—even larger than Smith had estimated. "It's clear that the scale is really a different kettle of fish," Gibbard says. "They're enormous."
The waterfalls pouring from the lake might have been 50 meters high or taller, Gupta says, although it's difficult to know. The relentless pounding of the cascades may have spun large blocks of stone inside the pools at the bottom, drilling into the bedrock. All this digging might have fatally weakened the ridge, eventually causing it to fail, Gupta and colleagues propose today in Nature Communications.
"I think they have a good rationale for this theory," says Kim Cohen, a geologist at Utrecht University in the Netherlands, who was not involved in the work.
It's not yet possible to know how long it took the waterfalls to erode the chalk ridge, Gupta says. The only way to tell would be to collect samples by drilling into the sediments that later filled in the pools. But that’s a tricky proposition, given the busy traffic of the shipping channel and its strong currents. "It's a dangerous area to do geological research," says Gibbard, noting that just getting the new submarine images was an accomplishment.
The images also clarify the extent of erosion from another round of catastrophic flooding during a subsequent ice age. Again, the evidence is submerged, because sea level is much higher today. But perhaps 160,000 years ago—the timing is uncertain—huge floods dug out what's known as the Lobourg Channel (see map), which the images revealed to be about 10 kilometers wide and 20 meters deep. This was probably the final breach of the remnants of chalk ridge, Gupta says.
These twin disasters meant trouble for ancient commuters. That's because they put travel between Britain and Europe at the mercy of sea level. Previously, when the seas rose and covered the floor of the channel, the chalk ridge would still have been passable. But when the first catastrophic flood tore through, Britain was cut off for the first time. (Of course, when sea level fell enough, the English Channel would have been dry again. Despite a major river flowing to the southwest, animals and people could pass back and forth easily.)
Britain most recently became an island about 9000 years ago. And—now that the United Kingdom has officially announced its intention to sever ties with the European Union—it will become even more of one.