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.