Erratics: evidence of an evolving landscape

Before the abundance of narcissus in Les Avants, the region underwent vast landscape changes, punctuated by glaciations. Around 20 000 years ago, a vast glacial landscape stretched from the Alps to the Jura, with the Rhône glacier covering the Cubly to a depth of 300 m.

This last ice age came to an end around 10 000 years ago. Today, many traces remain, such as the erratic boulders that were transported by the glaciers before being deposited when the ice melted.

The small erratic boulder visible on this site weighs around 4 tonnes and comes from the Dorénaz region in Valais, on the right bank of the Rhône valley. It has therefore travelled more than 30 km since it was torn off by the force of the glacier. Some come from even further afield, such as the granites of the Mont Blanc massif found on the Suchet pastures in the Vaud Jura, more than 100 km away. They have different geological characteristics to the local rock, which is why they can be defined as erratic boulders.

Intrigued, scientists in the early 19th century developed the theory of glaciation to explain the presence of such blocks far from their place of origin. They can be found in several places in the region, including Les Pléiades, Le Lautaret and La Lécherette at an altitude of 1550 m, the only evidence of the passage of the Rhône glacier over the Col des Mosses. The largest boulder in the region can be found near Sonzier, at the foot of Cubly. It also comes from the Dorénaz region and weighs almost 400 tonnes!