Britain's split from Europe began more than 200,000 years ago during an ice age. At this time Britain was a peninsula of northwest Europe. Melting water from vast ice sheets filled a giant lake in the southern half of the North Sea. It was held back by a chalk ridge stretching from the southeast of England to the northwest of France.
Eventually this glacial lake filled up with so much water that the dam burst at the Strait of Dover, unleashing vast torrents of water in an enormous flood. The flood was so violent it ripped through the chalk ridge and gouged a deep valley from the Dover Straits to past the Isle of Wight. That valley became a new waterway, the Channel. It drained the rivers of northwest Europe into the Atlantic and severed Britain from the mainland.
The final division from Europe was settled about 10,000 years ago, at the end of the last ice age, when rising temperatures again melted ice sheets, sea levels rose and the plains (Doggerland) connecting Britain to Europe were flooded in what became the North Sea, while the Channel River became the Channel. Britain became an island separate from mainland Europe.