As it crosses the canton of l’Isle Jourdain, the river Vienne exposes cristalline rocks from the Paleozoic era, of which there are many outcrops in the “Vienne Limousine” and in the Massif Central further to the south.
In these ten or so kilometres, there is a clear transition from the granitic basement to sedimentary rock.
To the south, the river Vienne has cut a valley through the granite and then the quartz diorite massifs. It is fast-flowing and littered with boulders that have been rounded by erosion. Further to the north, the valley widens as it comes into contact with sedimentary terrain.
In L’Isle Jourdain and in Millac, the river Vienne’s steep sided aspect made it suitable for the installation of three hydroelectric dams in succession, between 1918 and 1928; downstream from the Chardes Dam, the river meanders and its sides become asymmetrical.
On its right bank, a boulder field can be seen, the result of the erosion of a quartz diorite massif. Between Chardes and the Thierzat stream, a rocky mass towers above the narrow path. This is fractured in certain places; running water and the growth of vegetation are probably the cause of the successive landslides. Above the boulder field, there is a smooth, rounded boulder. Its unstable appearance is no doubt responsible for the site having been christened “La Pierre Folle” by our ancestors.
This circuit is one of 5 circuits known collectively as "La Pierre Folle", located in the Adriers, L'Isle Jourdain, Millac, Moussac and Nérignac area.
- Difference in height
- 242.61 m
- 243 meters of difference in height
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