Geology of Grandfontaine

The Grandfontaine valley is made up of marine volcanic rocks, 370 million years old and covered by thick sedimentary layers that have given rise to red sandstone, among others. The formation was strongly disturbed by tectonic activity and then eroded. The rocks you are walking over today are the deep roots of the old Hercynian mountains. The Grandfontaine soil is rich in iron, which explains the numerous mines winding their way into the mountains, and which are now forgotten or closed off for obvious reasons of safety. Some mines were named after colours: red, yellow, grey, black... perhaps due to the different crystals that were found there.