The Cirque de Navacelles

Natural Heritage
Principal
Source : Extract of larzacinfo website
GPS coordinate : 43.892821 3.50996

The Cirque de Navacelles

As well as many other famous sites of the Grands Causses, the Cirque de Navacelles is visited by hundreds of thousands of tourists every year, who come to admire the cut out of the Vis canyon, across the limestone and dolomite of the Causse de Blandas.

At the bottom of the canyon, 300 meters lower than the Causse, the Vis descends in waterfalls to Navacelles. The village attached to rocks thus respecting a flat loop of cultivated fields at the bottom which circles a small limestone landform. The whole dominated by slopes covered by sharp scree, which forms a large amphitheatre, of which the bars of limestone make up the stands. This “cirque” is a place where the river Vis used to flow and, therefore, is very different from the other “cirques” of the region, like the one at Moureze and at the “Bout du Monde” (End of the world) near Lodeve, which are simply places where the limestone and dolomite ledge at the limit of the Causse du Larzac has retracted due to the effect of erosion.

This change in the path of the Vis is completely typical of rivers whose winding paths carve out loops or meanders. This is the case for the Vis from Vissec and especially from the source of the Foux, karstic resurgence which insures its flow the whole year, whereas its upper bed can be dry. The volume of water flowing from the spring of the Foux is never less than 1 cubic meter per second.

The speed of the water flow in the meanders of the Vis is unequal and the river hollows out its banks in some places, then it deposits silt on its convex banks where the water flow is slower. The meander becomes a tighter and tighter loop which is cut out by the river at its base. This phenomena is frequent : upstream from the Cirque de Navacelles, there are three more abandoned meanders, the one at Vissec, for example, which visible for a few kilometres.

Age of the carving of the meander of the Cirque de Navacelles

The Age of the carving of the meander of the Cirque de Navacelles is pretty well known. The tufas of vegetation where the water of the Vis falls down to Navacelles, also fill the meander. They exist as “terraces” above the bed of the river up to the village of Madières.

Study of prints of leaves which they enclose, and the carbon 14 dates of some samples taken from Madières, place the tufas at around 6000 years old. That is in the Holocene period after the Wurm, last cold period of the quartenaire era. It has been, therefore, only a few thousand years that the Vis has finished its work of undermining to find a more direct flow. The Vis has been raised up by the filling of its bed by the tufas, allowing it to overflow the base of its meander. Since then it has claimed a few meters across the loop.

We could also ask questions about the time necessary to carve out the Canyon of the Vis. This problem is more delicate and in fact, concerns the evolution of the whole region of the Grandes Causses. Without going into the whole complex history of these landscapes, which begins a few million years ago, it’s first necessary to realise that the region only achieved its altitude at the end of the tertiaire era and/or at the beginning of the quaternary era.

Before then, the rivers, different from the present water courses, ran across the surface of the causses and left their sediments made out of stones and schist, taken from the Cévennes and other ancient rocky regions. Through weathering the sediments have become red earth (“terres rouges”), now trapped and cultivated in the closed hollows (dolines) that are scattered around the Causses. Their mineral content (zircon, tourmaline, rutile, garnet, etc. … )which resists change, indicates their origin : metamorphic or granite rocks.

Near Saint-Maurice-Navacelles, schist and granite stones are still recognisable in spite of their alteration. The uplifting of the south of the Massif central and the region of the Causses has set off a change in the direction of the flow of the rivers. At the same time, the more important ones, like the Vis, the Dourbie, the Jonte, the Tarn and the Lot have sunk little by little. Their narrow valleys or canyons therefore could have been dug out in the time space of 2 to 3 million years.

In taking the last figures for the Vis, this excavation would have occurred at the rate of 1/10 of a millimetre per year. This figure should be considered to be an average, the climatic conditions which govern erosion having changed a few times during the quaternary era. The very recent sinking across the tufas could have been much faster. These vacuolar rocks being easier to erode than the limestone of the secondary era.