Les Gorges du Fier

Visit Date: Tuesday, 15th October, 2013

Cutting through the Savoie's front range, the Gorges du Fier lies between the Alps, the Jura, the Lac du Bourget and the Leman Bassin. The area was marked by the last great Ice Age, The Wurm, a period during which glaciers slid as low as Lyon. Born at the foot of the Mont Charvin (2,400m), the fier slides past Thones, then throws itself into the Rhone north of Annecy, near Seyssel. It measures 71.9km (44.68 miles). 

It's main affluents are the Nom, the Fillière, the Thiou, the Viéran, and the Chéran. The lake of Annecy also feeds the Fier, via the Thiou, thus ensuring a relatively constant source of water. 

The Fier's module is of 44.6m3/s, for a specific flow of 33l/km2/s (characteristic of mountain watercourses). 

The annual variations are significant, with an averace of 68.7m3/s in April, and a mere 23.6m3/s in August.

The Tuf, also called Travertine, is a sedimentary limestone rock varying from grey to yellowish in colour and presenting a caked and roughly layered texture. It is formed by contact with mosses and other organic particles. 

When water appears in long horizontal bands, these are known as Strata Joints. They allow people to note the angular value of the Urgonian Age's geological strata. 

In the very small cave next to the high water marks, you can see the "poudingue" in the Urgonian design and red clays at the cave's deepest recess. 

Alternating freezing and thawing weakens the rock, finally leading to rock slides.

The Fier dug its gorge through the Urgonian limestone in the middle Cretaceous Period (between -114 and -107 million years ago). Between 80 and 100 meters thick, this important mass is composed of compact, erosion-resistant limestone, itself the result of a coral reef which, widely disseminated, gave birth to many great Alpine rock faces. 

The visible strata are all composed of Urgonian limestone which varies widely in appearance and composition. Differing heights of ancient sea-levels left behind marine sediments (shells, etc.) of various textures and aspects.

Urgonian limestone is 80% composed of calcium carbonate. This comes from the accumulations of sea shells and coral. One can sometimes note variations of colour on the same rock wall, and this seems to be without any organisational logic. This is due to different oxidation properties of the bare white rock as it meets and then ages in the atmosphere when a block of limestone detaches itself and falls from the face.

Near the end of the tour, the "Mer des Rochers" (Sea of Rocks) is a karstic lapiaz, which developped under organic cover by the water's chemical action. Today's Fier runs through the lapiaz' canals. 

"Footprints" can be seen that are particular to lapiaz; they are dissolution holes, or orifices on the surface of the rock.

Blocks of rock fall down when balancing forces break down as blocks come to overhang a face eroded from below.

Erosion in the Giants' Kettles occurs due to water circulation/rotation and water-borne materials. The power of the water coupled with the centrifugal force (strongest near the edges), keeps the lightest materials near the centre.

The Kamenitza are produced by natural chemical erosion. They form on the surface when water accumulates in a small natural cavity and slowly dissolves the limestone, thus widening and deepening the cavity. A small hole thus forms on the surface of the lapiaz.

(All information taken from the panels placed within the Gorges du Fier)