Several aspects of this series of events have been proposed before but not proved. In 2007, a team led by Sanjeev Gupta and Jenny S. Collier of Imperial College London obtained previously unavailable records of the detailed underwater topography of the English Channel. These showed that west of the Strait of Dover, a network of deep valleys had been cut through the channel's bedrock.
The streamlined nature of the walls and submarine islands suggested they had been shaped by a flood of enormous force. This evidence, the researchers said, supported the idea that a catastrophic breach of the Dover-Calais land bridge had unleashed a megaflood into the English Channel.
Dr. Gupta and Dr. Collier have now teamed up with Belgian and French seismologists to analyze the channel's bedrock more closely. In particular they have looked at a series of deep pits in the bedrock between Dover and Calais. The sediment-filled pits were discovered in preparing the route for the Channel Tunnel and the Fosses Dangeard (fosse is French for pit) after French geologist.
The tunnel had to be rerouted to avoid the dangerous pits, which were assumed to have been gouged out by glaciers.
But it's now known that the ice never reached that far south.
Dr. Gupta and Dr. Collier interpret the pits as giant plunge pools created by cataracts cascading down from the land bridge. The depth of the pits suggests the cataracts must have fallen from a considerable height. The new seismic data show that some of the pits are elongated as if the land bridge was progressively shrinking until a breach unleashed the glacial lake behind it.
“Our paper shows for the first time that a lake existed and that there were waterfalls coming over the land bridge,” Dr. Gupta said. A deep bedrock valley that passes through the Strait of Dover from the east, the Lobourg Channel, would have been carved by the megaflood from the second lake, in his view.
Philip Gibbard, a geologist at the University of Cambridge, said Dr. Gupta and Dr. Collier's reconstruction of events was “exciting and deeply plausible.”
Victor Baker, an expert on very large floods at the University of Arizona, said that the new data definitely showed that an ancient megaflood had occurred in the English Channel, and that everything so far known was consistent with the idea that the Dover-Calais land bridge had been destroyed by two catastrophic floods.
Dr. Gupta said he hoped to add more detail by drilling into sediments that now fill the Fosses Dangeard. But the channel is the world's busiest shipping route, and studying the Fosses requires crossing two shipping lanes.
“It's quite hard to persuade ships' captains to do it,” he said. “I'd say we know more about Mars than the submarine geology of these continental shelves around the world.”
