And I Thought it was just Chalk!
by Mike Gilmore, Dordogne
We live at Velines in north-east Aquitaine close to where the Dordogne flows onto the huge plain of 'Les Landes'. Initially we bought a cottage 7 km away by the river (the garden is on a Quaternary bed of the Dordogne, hence lovely alluvial soil, lush vegetation and a wealth of wildlife, such as swallowtail butterflies, 5 pairs of nightingales, night herons etc.). However we needed more space and bought a large 'maison de maitre' on the hill overlooking the cottage at Velines ( 'lieu vente' from 'Locus ventosus' during the Roman occupation; perhaps we shall rename the house Chicago when we change its name, as there are two houses called 'La Torre' here).
On the way up to the village, as on the opposite bank, there are many exposures of degraded rivercliff, which is a white limestone, and LOOKS like chalk!
I decided to buy a 1:50 000 BRGM geological map of the area. However the government owned company is in the process of making a new series and the map became available only in last September, thus I tried to guess the geological sequence. I assumed the white limestone was chalk, though I have to admit that Chalk was not obvious on the only map I had (1:500 000-La Carte de France). Thus I imagined the topography to be chalk dipping westwards, with Tertiary rocks largely eroded away, but exposed to the west, particularly in the lovely and famous village of Saint Emilion. In other words, I imagined that the Dordogne had 'cut down' through the chalk, much like the well known 'Mole Gap', by Box Hill, North of Dorking in Surrey.
Well, my new map arrived in September on the same day as the construction team for our new swimming pool. I had warned the chief engineer there was probably chalk under the soil, but no it was clay, as far as he was concerned and thus 'pas de probleme', so I watched and videoed as the 'Minipelle' took off the thin topsoil and then, lo and behold, hit the limestone and slowly toppled over like R2D2. Out cold, so they then had to use our tenant farmer's JCB.
Meanwhile I had been studying the map, and the limestone is lacustrine. Formed when the Oligocene seas invaded the subsiding Aquitaine basin, Velines would have been a few kilometres inland, in humid marshy conditions (probably a bit like the Georgia and South Carolina coastlines at present). This is a particularly tough limestone. It has many fossils, mostly lamellibranchs, echinoids and bryozoans. It is part of the Inferior Oligocene (g-C1) and is known as 'the clays and limestones of Castillon' (the latter is Castillon-la-Bataille), where John Talbot was defeated and the 100 Year's War ended; the locals still cheer the English in the annual re-enactment as taxes were imposed on wine by the conquering French, and as usual they have never been revoked. So, so much for my chalk!
The next surprise was that to the west end of the garden, there is a little layer of crumbly sandstone, c. 0.5m, but this deepens as the garden slopes upwards to the east entrance (c. 2.0m). It is very crumbly, and is full of broken echinoids, broken shells and colossal numbers of oyster shells (Ostrea longirostris). This is a marine transgression known as g2A: it is thought that the sea water was shallow and calm as a study in a quarry has shown the presence of Pararotalia lithothammica, Discorbis, Halkyardia minima, Stomatorbina concentrica, Valvulina, Triloculina, Quinqueloculina and Massilina. So now I guess that the sheer mass of pebbles one has to remove in order to plant a shrub were probably once a beach, as to the oyster shells I found lying loose and assumed to be part of an old 'le quatorze juillet' party, well shame on me. Anyway the village is built on an outlier of this transgression.
I was always taught never to make assumptions in Veterinary Medicine, and the same certainly applies to Geology.
I hope, this month to visit a very large working quarry nearby. It was apparently started by the English in the reign of Eleanor of Aquitaine (wife of Henry Plantagenet), and is still being worked. It was started close to the Dordogne in order that large blocks of sandstone could be cut and rolled down over logs to flat barges on the river. As the river is tidal to Castillon, these could be floated down on the ebb tide to where the rivers Garonne and Dordogne meet forming the wide Gironde: once they stopped at slack water they could be redirected upstream on the flood tide to Bordeaux, and unloaded as building blocks. Hence why the area between the rivers was called 'Intra Duo Maria' by the Romans, nowadays 'Entre-deux-Mers'. So, OUGSME members, next time you are speeding down the N10 through the great pine forest of Les Landes (planted by Napoleon to transform the watery area of the Roman Aquitania), know that there is a wealth of fascinating tertiary geology beneath your tyres. The only UK comparison is the Hampshire Basin, where another tyrant, too planted a forest.