Les Houches - Les Contamines-Montjoie : discovering alpine geology

Have you noticed the rocks surrounding the doors, niches and windows of this chapel? These are cargneules (dolomite cavities with lime cement), easily recognisable by their orange colour and porous appearance. These rocks are often mineralised by pyrite, an iron sulphide. When pyrite decomposes, it forms deposits of sulphur and iron oxides, giving the cargneules its characteristic yellow-orange colour. In this region, cargneules are linked to the Triassic geological period, which also includes gypsum (white rocks formed in an area affected by water evaporation). Gypsum, which plays a key role in tectonic movements, contains water that can be released under pressure, facilitating the formation of cargneules near rifts.

You are now standing on Col de Voza. Look up at Mont Blanc and you'll see the Bionnassay glacier overlooking the valley below. This glacier obscures another, much smaller one: the Tête-Rousse glacier. Located on the right-hand side of the Bionnassay, it cannot be seen from the valley. It was first discovered on the night of 11 to 12 July 1892, when a pocket of water ruptured at the front of the glacier. Several million cubic metres of water and ice tumbled down the mountain, devastating the villages below and the thermal baths of Saint-Gervais. 175 people died, swept away by torrential waters. In 2010, a new pocket was discovered and emptied between September and October of the same year.

As you go up the Nant Borrant, you enter the Contamines-Montjoie Nature Reserve. Created in 1979, it stretches over 5,500 hectares from the village to the Aiguille de Tré-la-Tête (3,892 m). It stretches from the mountainous zones to snowy lines, and is home to a rich biodiversity. To enjoy and admire this wild and pristine nature for many years to come, we need to follow a few simple rules: - Picking a flower means destroying a plant. - The presence of dogs in the reserve disrupts the ecosystem: dogs can destroy broods and eggs, they transmit pathogens and cause stress to the wildlife. This nature reserve is one of the few where dogs are allowed on a lead, because this nature reserve features a hiking path. - This area can only be explored on foot; motor vehicles, and overflying such as paragliding, hang-gliding, helicopters, drones, etc. is prohibited at a height of less than 300 m from the ground. - let's collect our rubbish and take it back with us. - Bivouacking is forbidden at the Plan Jovet and the Lacs Jovets, but is permitted in the rest of the reserve. Camping is prohibited throughout the nature reserve. - Swimming is prohibited at the Lacs Jovets and the Plan Jovet - Stay on the main path to limit the risk of destroying plants that are as scarce as they are inconspicuous, and to make sure that the future generations of hikers will still be able to admire this beautiful natural site. Let's share our nature and behave responsibly!

A nant is a term used to describe a mountain stream. The Bon Nant springs from the Contamines-Montjoie nature reserve at the Col du Bonhomme and flows into the Arve in Le Fayet some fifteen kilometres further down; it is one of the Arve’s largest tributaries. It forms the boundary between the Mont-Blanc and Beaufortain massifs and is used for hydroelectric power generation and fishing.