Harrow and Hillingdon Geological Society
Ochres of Rousillon
Roussillon and its ochres and the Grand Canyon of Verdon.
This presentation described two areas visited in Provence, south-east France, in 1984.
Roussillon and its ochres.
Between the plateau de Vaucluse and the Montagne de Luberon, lies the Vallée de Coulon, which was the centre of ochre extraction in France in the area from Roussillon, through Gargas to Colorado de Rustrel.
Ochres are naturally occurring earth pigments comprising the minerals Goethite (nFeO.OH – yellow-brown), Limonite (FeO.OH.nH2O – Yellow-orange-brown) and haematite (Fe2O3 – deep cherry red). They occur as sedimentary accumulations of secondary minerals (laterites) formed by weathering under extreme conditions on the edge of the savannah, where temperature, organic material and rainfall were just right during the Pleistocene. The area is not far from Les Baux de Provence, after which the mineral Bauxite was named.
They have been used by artists from Cro-Magnon times and as a body decorative (they are antiseptic and non-toxic). Early settlers in North America referred to red Indians, not because of their skin colour but because of the use of ochres in war paint etc. They are also used in the formation of more massive paints. Mixed with oils, they dry very quickly, cover thoroughly and are weatherproof. Ochres are also found at Brixham in Devon, where they were used to weatherproof sails (Hence “Red sails in the sunset”).
The original centre of extraction was around Roussillon, where they were worked from Roman times, with the peak of activity between 1850 and 1940. 17 different colours were available. The last ochre workshops at Roussillon closed in 1950 and the area has been a protected site since 1943. Roussillon itself sits on top of Mont Rouge and is surrounded and undermined by ochre workings.
Gargas is the centre of modern extraction (in 1984) and the material extracted is fed into a flotation tower to wash the oxides off the sands, with the oxides run into settling pits to dry in the sun. Once dried, blocks are cut and sent for further use.
The Grand Canyon of Verdon.
On the edge of the Alpes Maritimes, the River Verdon flows through the Grand Canyon de Verdon into the artificial lake of Lac du St Croix on the Plateau de Valencole. This lake was created by Électricité de France for power generation and is to be used as cooling water for the nuclear fusion project based here.
The area above the lake is a Jurassic limestone escarpment cut by deep narrow canyons. The Grand Canyon of Verdon is sometimes no wider than 20 feet (6.1m) at the bottom and never wider than 320 feet (97.5m) and varies from 650 to 2,000 feet (198-610m) wide at the top. It was only really explored systematically in 1905 and in 1928 a path was opened from the south side to the most spectacular features (the Sentier Martel). Visitors are warned that appropriate clothing and food is required since it can be a 12 hour round trip. The path has 6 stairways with 252 steps. [Assuming 20cm risers, this would make it at least 50m deep.]