Megaflood Of 400,000 Years bp Made Britain An Island

Whilst the Mesolithic people living in the huntin’, shootin’ and fishin’ paradise that was Doggerland at the end of the last Ice Age, had little idea of the watery world it would one day become as Britain became cut off from mainland Europe, it’s improbable they realised that similar events had happened in their own distant past. This from BBC News, who describe one event at around 400,000 – 425,000 bp…

It is believed that hundreds of thousands of years ago, when ice sheets had pushed down from Scotland and Scandinavia, there existed a narrow isthmus linking Britain to continental Europe.This gently upfolding chalk ridge was perhaps some 30m higher than the current sea level in the English Channel.

Palaeo-researchers think it bounded a large lake to the northeast that was filled by glacial meltwaters fed by ancient versions of the rivers Thames and Rhine. Then – and they are not sure of the precise date – something happened to break the isthmus known as the Weald-Artois ridge.

“Possibly this was just the build-up of water behind. Possibly something triggered it; it’s well known today that there are small earthquakes in the Kent area,” explained Imperial’s Dr Jenny Collier. Either way, once the ridge was broken, the discharge would have been spectacular.

One can imagine the effect such a deluge would have had on anything living in its path – maybe there were a few archaic humans dotted here and there across the landscape, along with a suite of Pleistocene wildlife to accompany them – which if caught in the sudden release of countless millions of tons of water from the breached ridge would have been swept up and destroyed in the cataclysmic torrent.

At its peak, it is believed that the megaflood could have lasted several months, discharging an estimated one million cubic metres of water per second. And from the way some features have been cut, it is likely there were at least two distinct phases to the flooding.

“I was frankly astonished,” said Dr Collier. “I’ve worked in many exotic places around the world, including mid-ocean ridges where you see very spectacular features; and it was an enormous surprise to me that we should find something with a worldwide-scale implication offshore of the Isle of Wight. It was completely unexpected.”

The image at top is what’s exciting the researchers, as it appears that what we now know as the Dover Strait was effectively created by the huge flood gouging out the English Channel, and despite sea levels later lowering enough for land-bridges once more to connect Britain with continental Europe, the die was cast, and following another catastrophic event at 200,000 bp, described in Nature

A second spill is likely to have occurred some 200,000 years later, during the most recent ice age, when again ice would have blocked up the north and created a lake where today’s North Sea lies. This flood was probably even more powerful than the first one, enlarging the Dover Strait to almost its present size. From then on, Britain would have been an island, only intermittently connected to the continent during times of extremely low sea level.

Dangerous floods are commonly associated with glaciation. But the surge of the English Channel was massively larger in size and impact than floods that occur fairly frequently in places such as the Himalayas today. “This was perhaps the biggest flood on Earth we have evidence for,” says (Philip) Gibbard, (a quaternary geologist at Cambridge University, UK)

And this isolation process looks only set to continue – if global climatic instability continues to raise sea-levels, more of Britain will disappear beneath the warmer waves of a wetter tomorrow, leaving perhaps only the highest hills and a few mountain-tops to give any indication that wide areas of dry land once abounded in the expanded North Sea of the future.

Update 20/07/07 – Cosmos Magazine have this – ‘Catastrophic Flood May Have Split Britain From Europe‘, in which they propose a date of 450,000 bp for the flood. Also worth reading for the perceived implications of the faunal and hominid populations that periodically entered into Britain over the course of hundreds of thousands of years.

see also: Nature – “The Megaflood That Made Britain An Island

NOVA- Mystery of the Megaflood

Britain’s Drowned World

BBC Radio 4 -“Doggerland” (‘Real Audio Player’ req’d)

Prehistoric Deluges

2 thoughts on “Megaflood Of 400,000 Years bp Made Britain An Island

  1. Fascinating and inspiring science.

    This idea was also proposed by Alec Smith (Nature, 1985) but although the sea-floor topography as shown on hydrographic maps was tantalisingly consistent, it seems that Gupta et al. have been able to use much higher resolution modern sidescan sonar data to confirm the hypothesis.

    Another interesting, and even larger, megaflood occurred through the Straits of Gibraltar at the end of the Messinian Salinity Event. Around 5.96 million years ago, uplift across the Pillars of Hercules and/or a fall in global sea-level led to the isolation of the Mediterranean basin from the world ocean.

    Over a surprisingly short time (perhaps less than 1,000 years) the high temperatures in the region led to a progressive desiccation of the basin and the widespread deposition of ‘Messinian’ evaporite deposits across the basin. The distribution of these across the area led Hsu (1983) to proclaim ‘The Mediterranean was a desert’.

    The presence on the basin floor of several cycles of evaporites record that this process was repeated multiple times, and at its lowest level, the Mediterranean salt lake surface may have fallen to at least 1 000 m below that of the North Atlantic, and, possibly to as much as 3 and 5 km below that of the North Atlantic.

    We are all familiar with the climatic extremes of Death Valley, which lies only 86 m below modern sea level – in comparison calculations suggest that surface temperatures at the Messinian basin floor may have been as much as 30 to 50 degrees C higher than on the surrounding land, which was already warm and certainly would have become hotter and drier still following desiccation of the Mediterranean Sea.

    Such a pronounced drop in base level resulted in massive erosion in all of the river systems flowing into the basin – seismic studies show the presence of a 2 400 m deep canyon beneath modern Cairo, for example.

    Eventually, at around 5.33 million years ago, full connection with the North Atlantic was restored and the Mediterranean was filled again, probably by means of a single catastrophic flood through the Straits of Gibraltar. This was achieved via a huge waterfall up to 1 000 m high, which would have refilled the basin in as little as 3 months (see this whimsical picture to gain an impression of how such a waterfall might have looked, had humans been around to marvel at it.

    An introductory review of the Messinian Salinity Crisis can be found on Wikipedia and more information is available by searching the many abstracts of scientific papers and geological lecture summaries to be found on Google.

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