Harrow and Hillingdon Geological Society

Catastrophic Flooding

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For a long time there has been talk about a possible flood causing the separation of England from France but only recently has there been proof that this actually happened.

A research team from Imperial College, in collaboration with the UK Hydrographic Office and the Maritime and Coastguard Agency, were collecting data on the sea floor (of the English Channel) for civil safety at sea when they made this discovery.

Swath bathymetry, using high-resolution sonar waves, which allows a whole strip or ‘swath’ of the seabed to be mapped from a single location was used; the sonar equipment being attached to a catamaran.

Spectacular images of a huge valley, 15 km wide and 50m deep were found: a valley this big could only have been created by an enormous volume of water.

The most striking evidence that the valley was created by a flood is the presence in the valley of small ‘islands’ with a distinct shape. The ‘islands’ are streamlined because the flood carved them into this shape to reduce the amount of friction between the land and the water – this would not have happened in an ordinary river and is similar to that shown in other places where megafloods have taken place. Scours would have been formed by huge eddies and by the action of boulders carried along in the flood.

At the time of the flood ice sheets would have covered most of Britain and the nomadic people would have migrated south as Britain was still part of mainland Europe.

To the north of the Channel basin was a large glacial lake in the area now occupied by the southern part of the North Sea. This was fed by both the Rhine and the Thames, stemmed to the north by glaciers and dammed to the south by the isthmus called the Weald-Artois chalk ridge which spanned the Dover Straits. It is believed that a rise in the lake level led to a breach in the ridge causing a catastrophic flood along the area of the English Channel (which was exposed at this time due to a drop in sea levels). The breach and subsequent flooding affected the pattern of human occupation in Britain as migration north (after periods of glaciation) could no longer take place overland. This explains the gap found in human remains and artefacts.

It is possible that two separate floods took place between 200,000 years and 450,000 years ago.

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