Notes on Trip Through France September 2011

During September 2011 I was the ‘back-up team’ for my son and three friends as they cycled from Narbonne in the south of France to Cherbourg in the north. It wasn’t a busy program and there was time to note the general geology along the route. The geological picture on this route is based on two ancient granitic crystalline blocs–the Massif Central in the Auvergne and the Massif Amoricain in Brittany, both of Hercynian origin. Between lies the Poitou gap fault structure now infilled with largely Jurassic marine deposits and later alluvium. Volcanic cones of the Auvergne were seen, fleetingly, on the route south to Narbonne, together with a sign for Lodeve a home for one of our members for many years.

The first four days from Narbonne involved climbs to some 1200m, initially crossing a very mixed deformed band of ridges and valleys with granites and schists dominating, until the large area of the “Causses” were reached. These comprise a widespread plateau of limestone karst with deep steep-sided gorges cut by rivers such as the Lot, Tarn, and Dordogne on the west. The cycle route crossed many of these gorges, which are very spectacular with caves, potholes, and springs feeding the wide rivers at the bottom of the gorges making for fairly strenuous but spectacular cycling country.

The Causses plateau is nearly level, dipping westward, but the eroded surface with deep-cut karst limits agriculture to sheep and some rough forestry, largely sweet chestnuts. The limestone is barely covered by soil and exposures of the strata beautifully visible. Within the gorges the limestone is massive, akin to our carboniferous Dales limestones, but significantly softer, a feature taken advantage of by the cave-dwellers of the past. Fossils are plentiful but well collected within reach, especially around Sarlat le Canedas and other tourist hotspots! The route took us NW into Perigord and Aquitaine to the coast at Rochefort sur Mer. The harder limestone dipped beneath chalk east of Limoges and the countryside took on a rolling undulating landform as chalk was succeeded by a stony alluvial soil around Cognac to the coast. At one of the overnight stops the hotel owner had a collection of chalk fossils of, in our terms large, bivalves, gastropods, sponges and corals–taken from a nearby quarry.

Geologically there was little of special interest across the easier lowlands and coastlands until north of Nantes on the southern edge of Brittany. Though the route skirted the highest ground examples of hard granite ridges were visible, and especially the plug of Mont-St Michel stood out. Uses of the granites could be seen in the houses and buildings, and a typical example of the many varieties was shown by gravestones in a single churchyard. North of Mont-St Michel the Cherbourg peninsular continued the granite/schist west coast cliffs, spectacular though difficult to view except as stone for rough walling as we have in Cornwall.

Barrie Heaton.


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copyright Hull Geological Society 2011