Chaos du Moulin - Moulin du Diable - Roche Tremblante - Camp d’Artus

Useful Information

Location: Huelgoat, department Finistère. Armorique Natural Regional Park, Arrée Mountains.
(48.367433, -3.744573)
Open: no restrictions.
Fee: free.
Classification: SpeleologyTectonic cave
Light: bring torch
Guided tours: self guided
Photography: allowed
Accessibility: no
As far as we know this information was accurate when it was published (see years in brackets), but may have changed since then.
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Forêt de Huelgoat (Huelgoat Forest) is a forest with strange granite formations which form cliffs, gorges and small caves. The rocks are the reason why the area is a forest, because there is not enough soil, and it is not flat enough for farming. The state-managed forest has a size of 2,500 acres. You can walk through the forest on various public trails starting at the town. The mysterious forest was always a place of legends and fairy tales.

The Celtic giant Gargantua once came to Huelgoat. He was very hungry and asked the locals for something to eat. But the locals were quite poor and offered him only buckwheat porridge. Being quite hungry, he became furious and swore revenge for this insult. He went to the richer lands of Léon, where he satisfied his appetite. Then walked along the northern coast of Brittany and found some round boulders. With his huge hands he took the large boulders, rounded by the surf, and hurled them towards Huelgoat. They fell all along the river and throughout the forest.

Grotte d'Artus, Forêt de Huelgoat, Bretagne, France. Public Domain.

Grotte d'Artus (Arthur's Cave) is a small cave formed by huge boulders. A second cave is named Grotte du Diable (Devil's Cave). The Gouffre de Huelgoat (Huelgoat Abyss) is a section where the River d'Argent flows between huge boulders. Roche Tremblante (Trembling Rock) is a huge block which lies on a much thinner foot, looking like a huge mushroom. But there are also archaeological sights in the forest. Chaos du Moulin (Chaos Mill) and Moulin du Diable (Devil's Mill) are medieval water mills along the river. And finally there is Camp d’Artus (Artus' Camp), which is the ruin of a celtic fort. They used the abundance of rock to build the walls. The Bretagne always had a strong connection to the legends of King Arthur and the sorcerer Merlin. As a result many places in the forest were named after those legends.

The boulders are a result of the erosion of granite. The granite was formed by an intrusion of magma, which stuck deep underground und cooled very slowly. Nevertheless, the cooling of the rock cause a reduction in volume and thus the creation of cracks. The intrusion was formed like a huge bulb or mushroom, a structure called diapir by geologists. The heat difference from the surface to the center caused the formation of cracks parallel to the surface and perpendicular to the surface. So the massive rock is actually split into huge blocks with thin cracks. Afterwards the rocks above the granite were eroded and the diapir is now on the surface. The erosion of the granite worked along those cracks, separating the blocks and rounding them, a typical form of weathing granite. When those blocks became unstable the collapsed and formed the caves and crevices. So while the boulders are formed by erosion, the caves are actually tectonic caves, the result of movement.