QR Les Martres-de-Veyre 4
The Blue Rock Cliff
Here, the Allier has carved out its bed from an ancient alluvial terrace, creating a cliff some 8 metres high. The upper part of the cliff has a layer of cemented pebbles (puddingstone) two metres thick. Among the various types of pebbles, those made of basalt have a bluish colouring that has given the site its name.
Modest salt springs
This stratum of puddingstone overlies a lighter, harder sandy layer several metres high, at the base of which several small springs emerge. The largest has a modest flow rate of one litre per minute, and its water temperature is 16°C. Salty, with a composition similar to that of its predecessors on the terrace, it creates an outpouring of alluvium measuring several square metres, distinguishable by carbonate deposits and an environment of halophilic plants, in particular a lawn of lysimachia maritima.
Gaseous emissions
The other springs are temporary and, in dry periods, one of them only emits carbon dioxide, which makes a disturbing noise reminiscent of a drum roll. It is perhaps for this reason that the locals also refer to this area as the « rochers de l’Enfer » (Rocks of Hell).
In this part of the valley, carbon dioxide emissions are very high, including in the river, and they form distinctive little funnels in the sand. In the absence of wind, they can cause the asphyxiation of birds living on the banks.
Image captions
- View of the cliff crowned by travertine veneers. Below, basalt pebbles enclosed by the thick layer of puddingstone. Their bluish reflections have given the site its name.
- One of the salt springs flowing at the foot of the Blue Rocks. Note the air sacs in the mud created by carbon dioxide emissions.