It’s just as well political boundaries are not set by ancient geological events or Brexit might be even messier than it currently is. If they were, the European Union could put in a bid to retain the counties of Cornwall and Devon when the rest of the UK leaves. Research on the formation of Britain has found that, 400 million years ago, these areas were part of the landmass known as Armorica, which became what is now France and Spain. Great Britain is the product of three landmasses colliding, not two as previously thought.

Britain and Ireland contain parts of two ancient continents, named Laurentia, which bore what is now Scotland, and (rather charmingly) Avalonia, which England and Wales are formed from. It had been assumed the collision of these two continental landmasses created the British Isles, but by studying the geology of these south-western counties researchers revealed they actually come from a third, Armorica. 

“It has always been presumed that the border of Avalonia and Armorica was beneath what would seem to be the natural boundary of the English Channel,” said Dr Arjan Dijkstra of the University of Plymouth in a statement. However, Dijkstra has challenged this in Nature Communications.

According to Dijkstra, while the eastern part of the boundary between Armorica and Avalonia does indeed run under the Channel, further west it crosses land. “Although there is no physical line on the surface, there is a clear geological boundary which separates Cornwall and south Devon from the rest of the UK,” Dijkstra said. 

Around 285-295 million years ago volcanic eruptions brought thin tubes of magma from deep within the Earth to south-west Britain. The remnant rocks reveal different ratios of strontium and neodymium isotopes on either side of Dijkstra's line, with those to the south resembling those on the continent. Since these tubes are formed from material in the mantle, they are a window into where pieces of the Earth's crust lay when the volcanism happened.

The finding explains why Cornwall and Devon were once rich sources of tin, tungsten, and copper, with Cornish tin mines dating back more than 4,000 years. Similar deposits are found in France and northern Spain, but the rest of England has no such metallic wealth.

It is presumably coincidental that these ancient geologic boundaries resemble cultural and historic ones. The point where Avalonian rocks meet those from Laurentia almost perfectly matches the English/Scottish border. Meanwhile, the parts of Armorica now attached to Britain were a Celtic holdout against the Anglo-Saxon invasions. Maybe the EU should put a claim on Cornwall after all.