Britain was formed by the collision of three ancient continents, research suggests.
England, Wales and Scotland were thought to have been created by the merging of Avalonia and Laurentia more than 400 million years ago.
However, geologists at the University of Plymouth now believe a third mass of land, Armorica, was also involved.
After studying mineral properties in exposed rocks at 22 sites in Devon and Cornwall, they concluded there was a boundary across the counties.
The areas north of the border share geological roots with the rest of England and Wales, while those in the south are linked to France, they found.
Lead author Dr Arjan Dijkstra, said: “It has always been presumed that the border of Avalonia and Armorica was beneath the natural boundary of the English Channel. But our findings suggest a geological boundary which separates Cornwall and south Devon from the rest of the UK.”
The team said it could explain why tin and tungsten are found in south-west England and Brittany in France but not elsewhere in England.
Dr Dijkstra added: “We always knew that around 10,000 years ago, you would have been able to walk from England to France.
“But our findings show that millions of years before that, the bonds between the two countries would have been even stronger.”