Old supercontinent mystery unlocked by Canada chunk in Australia
Deposits left by underground volcanic eruptions in the region suggest that the landmass of Armorica, which includes modern France, also played a role in creating Great Britain.
Dr. Arjan Dijkstra, who led the study, said that this was a "completely new way of thinking about how Great Britain was formed".
"It was always assumed that the boundary between Avalonia and Armorica was below the natural border of the English Channel," he said.
"But our findings suggest that although there is no physical line on the surface, there is a clear geological boundary that separates Cornwall and South Devon from the rest of the UK."
The researchers examined 22 locations in Devon and Cornwall and found a border that runs roughly from the mouth of the Exe in the east to the city of Camelford in the west.
Rock samples from these ancient sites were returned to the laboratory and subjected to intense chemical analysis to understand their past. The scientists published their findings in the journal Nature Communications.
In more recent history, Great Britain was connected by a land bridge to mainland Europe that allowed humans and animals to migrate between the regions, but the new discovery shows an even deeper connection.
"We always knew that you could walk from England to France about 10,000 years ago", added Dr Dijkstra.
"But our findings show that millions of years earlier the links between the two countries would have been even stronger.
"It explains the immense mineral wealth of South West England, which was previously a mystery and offers a fascinating new insight into the geological history of the United Kingdom."
This area contains an abundance of valuable metals such as tin and tungsten. These metals also occur in Brittany and other parts of the European mainland, but are not common in the rest of the UK.