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Wednesday, July 18, 2007

How Britain became an island

A new high-resolution sonar map of the floor of the English Channel shows that the process which turned Britain into an island was a catastrophic flood and not gradual erosion.

It has long been known that when the level of the oceans lowered during various ice age maxima, Britain became attached to the European mainland. The North Sea and Channel were both above sea level then. The Somme and Seine rivers joined to form a single large river flowing west down the Channel valley while the Thames and Rhine joined to flow across the dry North Sea into a melt-water lake against the front of the ice sheet which formed a wall from Holland to the middle of England.

At the furthest advance of the ice sheet, most of the island would have been covered by glaciers. Only the southern part of England remained bare. As the ice retreated from England and Scotland, animals were able to walk over from the mainland to repopulate the newly temperate island. Even when the seas returned to the present level, geologists believe a narrow isthmus connected England and France during most interglacial periods. This allowed animals like hippos, which could not have lived through the ice ages, to colonize the island during the warmest periods.

This didn't happen after the last ice age or the one before it. Many animals than moved into northern Europe didn't make it into Britain. This has led geologists to believe that the isthmus disappeared rather quickly. While the fossil story is compelling, there has been very little direct geological evidence of the event that removed the isthmus until now.

Scientists at the Imperial College and UK Hydrographic Office used high-resolution sonar to produce a new map of the Channel bottom and have discovered deep grooves and streamlined features spreading out from the Calais-Dover ridge. These features are similar to the ones made in Eastern Washington by the Lake Missoula floods. That makes them believe the ridge was breached in a single catastrophic event.

In one way, this flood would have been different than the Lake Missoula floods. The latter floods, caused by the almost instant collapse of an ice dam, took place in a matter hours. The Channel Floods would have taken a few weeks to carve the ridge down to it's present level.

The scenario they paint for the Channel goes something like this: Near the peak of an ice age, when the ice sheet in the North Sea was furthest south, the melt-water lake filled by the Thames and Rhine would have been filled to near the present day coast line, while the Channel would have been several hundred feet lower. Once the water topped the narrow ridge between the lake and the Channel valley, perhaps assisted by a small earthquake, it would have rushed down the west side causing massive erosion and lowering the ridge. The event would have spiraled out of control. As the breach got larger, more water would get through tearing it larger still and allowing sill more water. The entire North Sea lake would have drained in a few weeks carving the most of the present Channel, which is thirty meters deep and thirty kilometers wide. For the rest of the ice age and future ice ages, the path for migrating wildlife would be blocked by the Rhine River flowing through this canyon.

This type of flood also appears in the geologic record. Sixteen thousand years ago and inland sea called Lake Bonneville breached a ridge between Utah and Idaho and flooded down the Snake River. Ten years ago, Walter Pitman and William Ryan made news suggesting that the Black Sea was catastrophically filled from the Mediterranean about 7,600 years ago. Ryan has a history of this sort of announcement. In the sixties, he and Kenneth Hsu announced the biggest flood of them all, the filling of the Mediterranean itself when the Gibraltar dam was breached five million years ago.

More recently, we have the example of the collapse of Teton Dam on June, 5 1976 (film and pictures here). This was a 305 foot tall earthen dam in Idaho that failed as it was being filled for the first time. Film and pictures taken on the day it failed show a small leak developing into major erosion and the collapse of the dam, with forty percent of its mass being washed away, over about four hours time. Other famous dam ruptures, like the Johnstown Flood, were the result of the same type of catastrophic erosion.

There are still many pieces of the English Channel story to be discovered. No one quite sure when the breach took place. They only have it narrowed down to 200-400,000 years ago. That spread includes at least two ice ages. I'm looking forward to more information on this one.

Labels: science

posted by John at 8:29 PM |