HALF a million years ago, Britain was connected to mainland Europe by a broad chalk ridge across what is now the Dover strait. Somehow that ridge was then destroyed, creating the English Channel. Now we know what destroyed it: a megaflood.
Sanjeev Gupta and Jenny Collier at Imperial College London came to this conclusion after studying images from an ultra-high-resolution sonar survey of the channel鈥檚 bedrock by the UK Hydrographic Office. This showed deep valleys that could only have been gouged out by vast volumes of water. 鈥淎ll the bedrock landforms we see are characteristic of a megaflood,鈥� says Gupta.
He and Collier calculated that 1 million cubic metres of water per second must have flowed for several months sometime between 200,000 and 450,000 years ago. The flood carved the sea-floor valleys, some of which are up to 10 kilometres wide and 50 metres deep, with a flow rate 100 times the average flow rate of the Mississippi river today.
Advertisement
The megaflood鈥檚 source was a vast glacial lake that took up most of what is now the southern North Sea. Corralled by glaciers to the north and to the south by the Dover strait ridge 鈥� also known as the Weald-Artois ridge 鈥� the lake was fed by the Rhine and the Thames as well as glaciers. 鈥淭here was so much water coming in from the Rhine, the Thames and the glacial meltwater that it had no place to go,鈥� says Gupta. 鈥淪o at some point it breached a hole in the ridge.鈥� The water gushing through the hole tore down the ridge and scoured the sea floor to form the English Channel ().
Philip Gibbard, a geologist at the University of Cambridge, thinks the Imperial team are spot on in their analysis. 鈥淭hat lake was like a bath with the taps running,鈥� he says. 鈥淭he package of bedrock features they describe is very convincing.鈥�
Gupta and Collier are not the first to propose that a megaflood dug the English Channel. In 1985, marine geologist Alec Smith of Bedford College in London used a lower-resolution sonar survey to draw similar conclusions 鈥� but because that survey was only accurate to 10 metres, geologists were sceptical of his claims ().
Smith was right all along, says Collier. And Gibbard admits he was one of those who doubted Smith. 鈥淚 have already ordered my sackcloth and ashes,鈥� he says.
