Britain was formed by the collision of three ancient continents, new research suggests.
England, Wales and Scotland were previously 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 called Armorica was also involved.
The research, published in journal Nature Communications, offers a “completely new way of thinking about how Britain was formed”, the authors said.
The team of researchers studied mineral properties in exposed rocks at 22 sites in Devon and Cornwall.
Their analysis suggests a boundary across the two counties, running from the Exe estuary in the east to Camelford in the west.
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 and mainland Europe, the researchers said.
Lead author Dr Arjan Dijkstra, a lecturer at the University of Plymouth, said: “This is a completely new way of thinking about how Britain was formed.
“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.
“But our findings suggest that 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.”
The authors said the discovery could explain why tin and tungsten are found in south-west England – as well as Brittany in France – but not other parts of England.
Dr Dijkstra said: “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.
“It explains the immense mineral wealth of south-west England, which had previously been something of a mystery, and provides a fascinating new insight into the geological history of the UK.”