VuedessucscreditlucaspaulinHere on the Plateau, Fred explains that the last eruptions were only 40,000 years ago! and that there are almost 180 volcanoes in the Haute-Loire.
Off we go to Suc d’Achon near St Jeures. This is a so-called “phonolithic” volcano, with a sugar-loaf dome and no crater. The lava was very viscous and solidified on site. Suc des Ollières emerges.
It’s a rather local word for a rounded summit, so it has a topographical meaning.
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LizieuxrandoIn the St Jeures region, these are often phonolithic domes. The view from the summit is very wide. A steep, narrow path leads to the summit. This beautiful, sonorous phonolith has a fine laminated structure that enables it to be cut into lauze used for roofing, paving courtyards or farm kitchens.
But it’s the Pic du Lizieux that really dominates here! 1388 m. This handsome mountain can be recognized from afar: the phonolithic peak dominates a tabular surface that emerges from the forest and thickens towards the north into a short, powerful flow.
After climbing Testavoyre (photo), a magnificent dome-coulée at 1,436 m, we stop off at Lac Bleu. Turquoise blue in all seasons, it is carved out of a phonolite flow from the Meygal. It was created only recently, when phonolite extraction from this quarry ceased, which meant that the pumps that dried out the quarry had to be shut down. The lauzière was renowned for the quality of the lauzes extracted.
Lac Bleu in ChampclauseAt its own pace, the Tortue reveals itself… it’s one of the three peaks of the Meygal range. It’s made up of a protusion, the tortoise’s head at 1265 m, and the endogenous dome, the tortoise’s shell at 1307 m. Let’s not forget Queyrières and its basaltic rock: the whole village seems protected around a beautiful sheaf of prisms.
Queyrières in the Meygal
Basaltic organqueyrierescreditjeremiemazet
MezenclucolivierThe Mézenc, the highest point in Haute-Loire at 1749 m and in Ardèche at 1753 m, is a magnificent phonolithic dome-castle, home to a rich and original flora. The three devil’s teeth or dents du Mézenc emerge in this landscape of moors and fairies. The Roche Pointue is a rhyolite dyke and a great climbing spot before the Croix de Peccata, while the Grosse Roche is trachytic. This is also the land of peat bogs formed in high-altitude depressions some 10,000 years ago. They sometimes correspond to the craters of volcanoes or topographic hollows in the granite bedrock. The peat, which is only a few meters thick, was used as fuel for the high-altitude farms at Chaudeyrolles and Champclause, and also as animal bedding, which was then “enriched” and used as fertilizer.
The swerve takes us to St Front, where the lake occupies an explosion crater, a maar volcano. The lake is no more than 6 metres deep. Erosion is filling it in, and it should turn into a peat bog within a few centuries.
Gr 430 20 Lac De Saint FrontOur geological journey ends near Tence and Le Chambon-sur-Lignon, where the Velay granite forms a plateau. More difficult to carve, it is used to build houses and farmhouse lintels, and sometimes as tombstones. The veins of light-colored granite are luminous and precious.
Source: La Haute-Loire, Richesses géologiques et Histoires des Paysages de -530 millions d’années à nos jours. Groupe géologique de la Haute-Loire. 17€
This “singing stone” has a flaky structure that enables it to be cut into sonorous lauzes that are smooth to the touch. The color varies from gray to dark green.
Whitish, grey rock, sometimes porous, often with feldspar crystals.
Granite” should not be confused with “granite”: in geology, the term “granite” refers to a plutonic magmatic rock with a specific mineralogical and chemical composition.
Dark in color, it’s a volcanic magmatic rock derived from rapidly cooled magma; stimulates microbial life, improves water retention in sandy soils and aerates heavy soils. Ideal for use in the vegetable garden or in compost!
In French “filon” means a vein of lava released by erosion.
French “cou”, more or less cylindrical relief, corresponding to the filling of a volcanic chimney.
