The path that leads to this quarry reach the centre of the hillock through the moor and groves. After having used a steep path, remnants of the quarry's exploitation path, nowadays the kingdom of enormous ferns, we suddenly come accross some water at the bottom of a phtanite (jasp) cathedral. This siliceous rock emerges at the top of the hillock, at about 80m. It is one of the rare place of Brittany where it can be observed at the surface thanks to the surrounding gneiss' erosion. Very hard, it was exploited up to the 60's to road-bed the paths.
The lake therefore formed by the meteoritic water, and as such according to seasons, becomes nowadays a favourable sport for observing Odonates, from the collared grass-snake to the aquatic insects. 14 species of odonates are identified there, 38% of the species commonly admited in Côtes d'Armor.