Birth of an island: megaflood severed Europe from Britain.

Hundreds of thousands of years ago, as an ice age was ending, the spillover from an immense glacial lake in northern Europe sliced through a broad ridge that for millions of years had connected what is now England to the continent. The flood that resulted, one of the largest that scientists have ever identified, quickly created a breach that makes Britain the island that it is today.

The narrowest part of the English Channel is the 33-kilometer-wide Strait of Dover. The cliffs on both sides of this waterway were once part of a broad chalk ridge that connected England to France, says Sanjeev Gupta, a geologist at Imperial College London. The lowest spot on this ridge probably sat about 30 meters above today's sea level, he notes.

About 450,000 years ago, the Northern Hemisphere was locked in an ice age. A kilometers-thick ice sheet smothered Scandinavia, most of Britain, and much of the North Sea, and water carried by Europe's north-flowing rivers collected in a large lake along the ice sheet's southern boundary, says Gupta. As that ice age waned, meltwater from the ice sheet boosted the lake's level.

Eventually, the lake began to spill over the chalk ridge, cutting rapidly into the soft material and, in a matter of weeks, turning into a chasm-carving torrent. Evidence for the resulting flood lies on the bottom of the English Channel, Gupta and his colleagues report in the July 19 Nature.

A sonar survey just south of England revealed a 100-km-long, submerged feature that scientists have dubbed the Northern Paleovalley. This valley, which contains little if any sediment, is as much as 50 m deep in spots, says Gupta. Large, flat-topped islands in the valley have streamlined shapes, suggesting that they were carved by massive amounts of water flowing over what had been dry land. Broad grooves carved into the bedrock, some of them at least 100 m wide and 15 km long, curve to follow the valleys terrain--a hint that the features were quickly created by a colossal deluge.

The size of the paleovalley's islands suggests that the floodwaters could have run as much as 20 m deep, says Gupta. Considering the width of the now-submerged valley, the scouring flow that created it could have carried about 1 million cubic meters every second and raged for months, he adds.

The purported flood probably rivaled the floods that scoured portions of the northwestern United States at the end of the most recent ice age, says Philip Gibbard, a geologist at the University of Cambridge in England. Those inundations, which occurred when a glacial lake burst through the edge of the ice sheet that constrained it, sculpted a chaotically eroded terrain in eastern Washington that geologists aptly call the Channeled Scablands.

Several of the features seen on the floor of the English Channel "certainly resemble those seen in the Channeled Scablands," says Timothy J. Walsh, a geologist with the Washington State Department of Natural Resources in Olympia. The new findings "are interesting and are bound to generate a lot of discussion," he notes.