Nicholas Wade, writing in the NYT, has the story of the big Brexit 430,000 years ago.
That was the catastrophic destruction of the land bridge that for 10 million years had joined Britain physically to the Continent.
The bridge was a rock formation, about 20 miles wide, that ran from Dover to Calais and protruded several hundred miles into France and Britain. It was made of chalk, as can be seen in the cross-section where it has been ripped away at the white cliffs of Dover.
In the last ice age, sea levels rose and fell as water was locked up in ice sheets during cold periods and released to the oceans in warm ones. At high sea levels, water would nearly encircle Britain but never surmounted the land bridge, which stood 100 to 300 feet above the waves.
That was until a cold period that began 450,000 years ago. A vast glacier that covered all but the southern parts of Britain edged out across the North Sea and joined up with the glacier covering Norway. With the North Sea dammed, the rivers that then drained into it, including the Rhine and the Thames, started to form a large lake, also swollen with meltwaters from the glacier.
As the level of the glacial lake rose, its waters started to cascade over the Dover-Calais land bridge that formed its southwestern wall. Laden with abrasive pieces of flint dissolved from the chalk, the waterfalls scoured out vast holes in the bedrock beneath, some 450 feet deep and several miles in length.
Eventually the land bridge collapsed though it didn't disappear until subsequent flooding episodes much later. When the sea level was low during ice ages, though, you could still walk across the channel.