GEOLOGY — Tour de France avec une différence
| Author | Dan Bosence |
| Published | February, 2004 |
The opportunity to act as a geological consultant to a wine importer on his annual tasting trip around France does not come around many times in a geologist’s lifetime. But the dark, cold and wet days of February were thankfully broken by this Dionysian opportunity.
The Tour de France team comprised Nick Brooks of Vinetrail, Bristol, Eric Zwiebel, Cellar Master of The Summer Lodge, Evershot, Dorset and this geologist with a fairly rudimentary, but rapidly improving, knowledge of French geology and with a capacity for, but very little knowledge of, wine tasting. Through dedication to my teaching of carbonate reservoirs for the MSc Basins course at Royal Holloway University of London, I missed the first leg of the journey. This was a shame as it included tastings in Champagne, Burgundy, Beaujolais and northern and southern Rhone. In fact I was met at Marseille airport (courtesy of Easyjet from Gatwick for £2.99) by two light headed and black mouthed friends who had spent the afternoon in Chateauneuf-du-Pape. Apart from missing the tasting of some of France’s best produce I also missed the opportunity of experiencing the relationships between wine and geology (or terroir) in an area where this is most clearly expressed, and has been closely studied. Indeed the underlying geology (metamorphic gneisses and schists or Mesozoic limestones) may form part of the definition of an appellation d’origine contrôlée ( or AOC), such as in Beaujolais. This gives me a good opportunity to cover this leg of the trip next year.
My trip started in the far south-southeastern corner of France where Palaeozoic metamorphic rocks from the axial zone of the Pyrenees outcrop along the coast that leads to the Spanish border. These steep terraced slopes have thin soils developed from underlying phyllites and some schists. Here, syrah and grenache grapes are grown to produce the heady wines of Collioure and Banyuls-sur-Mer.
At these caves we were able to undertake what is known as “vertical tasting”. Now this has nothing whatsoever to do with whether one is still vertical after the tasting but is in effect a form of wine stratigraphy; tasting the same wine from the same grapes from the same vineyard going back through time (2003, 2002 etc..). In this case it was possible to experience the development of rich aromatic, spicy flavours as the wine matured in its stacked oak barrels. However the difficult, and highly commercial bit is judging how the immature 2003 “wine” would taste in 10 years time. Well, we could have spent all evening in there, but duty called, and we needed to move on and find a good seafood restaurant to line our stomachs with something other than wine.
From Banyuls we travelled north to a grower northeast of Béziers (Saint-Chinian) where vineyards occur on Upper Cretaceous sandstones and Jurassic limestones that are faulted against Palaeozoic schists of the Montagne Noire. The grower had a rather good display of the local geology in his cave complete with a nest of dinosaur eggs found when excavating terraces in the Upper Cretaceous sands. Each rock type was said to produce wine of particular flavour; the Palaeozoic schists and shales “subtle flavours”, the Jurassic limestone “acidic and strong tannins”, and the sands “rich and silky smooth”. So could these flavours be picked up in the wine? The short answer is not easily. Because the skill of the vigneron is not only in finding which grape varieties grow best on which rock type, soil and climatic setting but also in the fermentation of these different juices (by several different methods), their assembly (different proportions of juices from the different parcels of the vineyards), and then maturation in oak barrels (of different types of oak fired to different degrees) to produce his different wines for marketing. So after a promising start with three distinct geological settings we were left drinking a beautifully strong, fruity flavoured red wine with a rich and smooth taste resulting from a combination of geological, climatic, botanical and oenological factors. A similar situation was found near Cahors on the river Lot where we tasted wines from Jurassic and Cretaceous limestones and also from a thick iron-rich soil (with siderite and haematite) developed as a Tertiary terra rossa soil on top of the Mesozoic limestones. The growers here were adamant that the different rock types produced very different wines but with different grape varieties in the different areas and the subsequent assembly of the wine produced a delicious aromatic, spicy and fruity rich red wine. There was no clear signature to me of the different rock types in the final product, even after a number of repeated tastings. At this point we had a long discussion about what was really needed here was to sample and analyse, rocks, groundwater and wines produced from the same grape variety, growing on a range of different lithologies and fermented and stored in exactly the same way. Sounds like an interesting MSci project to me, any geochemists interested?
From here we moved to southwest France where the geology is dominated by the nearby Pyrenees. Since Oligocene times fluvial drainage systems developing from the uplifting Pyrenean mountain belt have transported sediment to the north into the Aquitaine Basin. The resulting clays, silts, sands and pebbly conglomerates (les poudingues) have been eroded into steep-sided valleys in the Jurançon area to the south of Pau. Here the climate is dominated by moist winds coming in from the Atlantic and contrasts significantly with the arid, Mesozoic limestone dominated areas to the east. Vines, which prefer to grow on well-drained soils, grow here on the tops of the hills and north-south oriented ridges in between the valleys and river systems that still drain the Pyrenees today. A speciality of this region is a rich sweet and spicy white wine produced from the late harvesting of sun dried, petit manseng grapes (or rather raisins). The locals tend to drink this as an aperitif along with lashings of pâté de foie gras; but that takes a stronger stomach than mine.
Onwards and northwards took us to the first classic area to be visited on this trip of French (well English really) wines of the Bordeaux area. Why English? Because the red wines (or clarets as we call them) in this area were developed by us over 300 years up to 1453 when we gave the area back to the French. The area has gone on to become arguably the best wine producing area in the world; but some might disagree with that! The geology of this region is relatively straightforward as it has been affected since at least Eocene times by river systems draining northwestwards off the Pyrenees. Today the Dordogne river and its tributaries wind through broad valleys into the Bay of Biscay. Overlying the Eocene alluvial sediments are more alluvial clays and sands of Oligocene age. Oligocene seas then transgressed, over the region and shelly limestones (Calcare d’asteries) the dominant building stone of the affluent towns and chateaux of St Emilion, Lussac and St Michel de Fronsac. The room and pillar excavation of this limestone for building stone provides excellent caves for wine storage with their stable year-round temperatures. Following limestone deposition the Oligocene seas subsequently retreated to the west and the ancestral Dordogne and its tributaries eroded into these limestones, clays and sands to deposit a series of Quaternary river terraces which produce some of the classic wines of the area (Chateaux Figeac and Petrus). So why are these wines of such high quality? The pundits discuss the importance of the well-drained soils of the river terraces and the underlying clays that restrict vine growth. This generally leads to a concentration of flavour in the grapes. The rest you have to put down to the combination of the cabernet and merlot grapes and the 2000 years of French (and British) expertise in winemaking and storage in this region.
By way of contrast the remainder of the trip took us north to the Loire valley and Bourgogne to taste the wines of Saumur, Sancerre, Pouilly Fumé and Chablis. This is a major white wine producing area of France dominated by the chinon blanc, sauvignon blanc and chardonnay grapes growing largely on thin soil developed on the Mesozoic limestones and marls of this region. Here the weather turned significantly northern and tasting young 2003 fermented grape juice at 9.30 in the morning in a freezing barn with snow blowing under the barn door has to be considered more a duty than a pleasure. The Mesozoic carbonate rocks are thought to impart the dry freshness (and “minerality” sic.) to these wines. Interestingly, at Saumur and at Pouilly we were able to compare wines produced in the same vineyard (same fermentation method) from Cretaceous chalks (light, fruity, dry and slightly acid) with those from the overlying Eocene siliciclastic sands and conglomerates (sweeter and more rounded flavours).
To conclude, you might well ask: how does the geology of an area affect the wines that are produced? Well the answer is complicated because the geology not only determines the chemistry (and possibly the flavour) of the groundwater drawn up by the very deep-rooted vines, but also the geomorphology and drainage of the area and its aspect; dip slopes and scarps are often determining factors for vineyards and AOCs. On top of that there are the many different grape varieties, blending of juices from different vineyards or parts of vineyards, methods of fermentation and different types of maturation and storage. There is still a lot to learn and the first leg of the trip has yet to be completed to give you a second instalment next year, with, hopefully some more conclusions on terroir.