Day 6: Friday 25 July 2003

Protrusions and dykes: the plumbing system

On this day we commenced at the Vallèe de Chaudefour Réserve Naturelle.

Author providing scale for the Dent de la Rancune (tooth of rancour?)

We were looking for other evidence of volcanism in the form of dykes eroded in relief, glaciation of deposits, and hot springs, all to be expected in proximity to volcanic activity.

We walked through the Fôret Danuale de Chaudefour and were treated to a splendid view of the eroded end face of Dent de la Rancune, showing in relief.

We then came to a waterfall, Cascade de la Perouse, which cascaded about 10m down the hillside.

La source St Anne: A natural spring, working on the principle of an artesian well. The water percolates to depth and in doing so dissolves minerals into solution. These minerals are the expected ones in such a location: CO₂, H₂S, SO₂. Dissolving of minerals into solution is much facilitated if the temperature of the circulating waters is elevated.

The spring water was not warm at the outlet but tasting the water revealed iron (Fe) and sulphur. H₂S is after all hydrogen sulphide, which smells like rotten eggs! There was plentiful evidence of the deposition of iron and sulphur from the waters. Any residual heating due to the volcanism was not evident.

Brenda abseils into the abyss

This was the end of geological field trips and after lunch at Lac Chambon, at which some of the party had an enjoyable swim, the party then headed over to Lac Pavin (occupying the crater of an extinct volcano) where some members of the party were instructed in the arts of abseiling down a small cliff by the lakeside under the expert tuition of Nicolas, our guide and mentor, who provided the equipment.

By Frank Bailey