Areas of Brittany and Western France were previously mountainous and located close to the Equator, according to research led by the University of Plymouth.
Scientists have for the first time been able to reconstruct the isotopic composition of rainwater from 300 million-year-old minerals.
It has allowed them to unpick parts of the region’s history and explain how it looked in the late Carboniferous period, just before the time of the dinosaurs.
They analysed granite samples found today between Brest and Nantes, and also took samples from the Western part of the French Massif Central (Limousin).
During the late Carboniferous period, these areas were located near the Equator, and were part of the Variscan Belt mountain chain that extended into Spain, Germany and Southern England. The deformation processes associated with the building of the Variscan mountain range is also the reason why most of the rock layers on the coast of Devon and Cornwall are folded, faulted and mostly dipping towards the south.
Violent mountain building processes caused these areas to be deformed and cracked, forming major fault zones, and this allowed rainwater to sink to 15km below the surface.
By measuring the radioactive decay of Argon, they dated the samples to around 300 million years old. Comparing the relative ratios of oxygen and hydrogen isotopes also showed that the Variscan samples matched those which came from moderate to high altitude, indicating that the rain fell on a mountain range.