The Montagne de la Serre
– 3 million years
The Montagne de la Serre lava flow is 10 km long, and is bounded to the sides by the Veyre and Auzon valleys, and at the far end by the village of le Crest. It dominates the southern part of the Limagne Plain, and is flanked by other examples of inverted relief, such as the Plateau de Gergovie.
The lava flow originated from the Puy de la Vigeral, a volcano situated at 1,000 m, on the Plateau des Dômes, where it overshadows Lake Cassière.
Formation of the Montagne de la Serre
Around 5 My, before emplacement of the Montagne de la Serre lava, the sedimentary phase came to an end and the sedimentary infill of the graben reached the same level as the adjacent plateau.
Around 4 – 3 My an eruption resulted in a lava flow which spread across the plateau before being channelled into a valley in the sediments.
This flow lay partly on the Plateau des Dômes (shown in red) and partly on the sediments (shown in yellow).
Towards 3 – 1 My the basement and sediments were uplifted together, triggering intense erosion.
Towards 4 – Erosion led to the preferential removal of the softer sediments surrounding the flow. The more resistant lava formed a protective layer which preserved both the lava and the sediments lying directly underneath it.
The result is an upstanding area, forming what is now known as the Montagne de la Serre. The evolution from valley-infill to upstanding plateau results in inverted relief.
Did you know ? The sedimentary rocks at the contact between the Montagne de la Serre and the Limagne Plain, particularly at Chadrat, contain numerous fossil fish and sedimentary structures (stromatolites) which are indicative of a lacustrine environment in the Limagne around 30 My, similar to that of the current Lake Chad. Traces of these features can be seen in the stones used to construct the hamlet of Chadrat.