8 December 2009
Britain cut off from Europe by ancient 'super-river'
Researchers have found sediment on the ocean floor off France which originated in the north of the channel which must have been transported by the river originally fed by the Thames and the Rhine. The samples, taken from the Atlantic sea bed, have provided scientists with the final piece in a geological jigsaw, enabling them to reconstruct the story of the 'Fleuve Manche' (Channel River) - a giant waterway that flowed through the area now occupied by the English Channel.
Earlier studies have already suggested that the river existed during a sequence of ice ages that began 450,000 years ago. It formed when a huge glacial lake in the North Sea overflowed, causing a prehistoric 'mega-flood', which sent water surging into the basin between Britain and France and gouging through the hills of chalky rock connecting them. To date, however, the timings and nature of the river have been based on a mixture of evidence from the English Channel and sedimentary deposits in coastal Europe, many of which are incomplete due to erosion. An Anglo-French team of researchers claim they found a more perfect record of the river's existence beneath the sea.
The study was carried out in the Bay of Biscay, where the Fleuve Manche met with the ocean and discharged layers of sediment which have gone undisturbed for thousands of years. "This is the first time we have looked at what flowed out of the Channel and into the Bay during these crucial periods," Professor Phil Gibbard, from the University of Cambridge's Department of Geography and one of the project leaders, said. "It provides the final piece in the puzzle, forming a complete record that reconstructs the dramatic events that cut Britain off from Europe and gave it its island status."
Half a million years ago, Britain was connected to Europe through a range of low hills. Many major European rivers flowed into the North Sea, unable to cross the natural barrier this stretch of hills created. During periods of glaciation, however, two huge ice sheets, the British and the Fennoscandian, trapped water between the glacier to the north and the land barrier to the south. As the rivers continued to pour into this huge glacial lake, it overflowed like a bath, breaching the land barrier between Britain and France and sending its contents crashing into the Channel, then a wide river valley. Two such flooding events carved through the bridge so significantly that when the ice eventually melted and the sea rose, water covered the area, cutting Britain off.
Like a huge conveyor belt, the Fleuve Manche carried sediment from northern Europe towards the sea. As it met the ocean itself, however, the conveyor stopped and its energy was lost, leaving the perfectly-preserved layers of sediment behind on the ocean floor. As a result, the team were able to test their samples for points at which they contained sediment from the super-river and for the intervening periods, when any deposits were left as a result of normal sedimentation, since the Channel was submerged.
The study revealed conclusively that the Fleuve Manche existed during three different ice ages, 450,000, 160,000 and 90 to 30,000 years ago. In each case, the volume of the sedimentary material increased significantly - the result of surges of debris pouring into the Bay of Biscay. The implications for British prehistory of knowing exactly when the ice and the river were at their height are profound.
During glacial periods, the water level fell significantly enough to allow plants, animals and humans to cross into Britain. In temperate times, however, Britain would have been cut off, as it is now.
Source: Telegraph.co.uk (30 November 2009)