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Bordeaux Geology

Before homing in on Bordeaux, I thought it would be useful to first look at a few geological concepts. Fear not; I have no intention of presenting you with a geological timetable for you to learn. But it makes sense to at least look at the basic rock types, so that we all understand their origins, followed by a very broad overview of the geology of all France. The emphasis here is on the word broad, because in no way will this be a detailed examination. Nevertheless, taking an overview in this fashion not only allows us to understand why Bordeaux's geology is how it is, it is also useful for understanding other regions of France.

Keeping it simple, then, rock generally has one of these three basic origins:

Igneous: rocks of igneous origin form from lava, either following surface eruptions (gentle and ongoing, or sudden and catastrophic), or from crystallisation and solidification of deep lava which then rises up to the surface as the land rises. Examples include granite and gabbro.

Sedimentary: these rocks originated at the floor of ancient seas, and are formed from the accumulated sediments on the sea floor. These sediments might be mineral in origin, formed from sandy particles washed into the sea by rivers, or they may be organic, formed from the remains of sea animals that have died and drifted down to the sea bed. Examples of the former include sandstone, the latter limestone.

Metamorphic: these rocks are the result of changes in rock structure brought about by extremes of pressure and heat. The original rock may be sedimentary or volcanic. For example, limestone subjected to extreme pressure may turn to marble, sandstone may turn to quartzite, shale may turn to slate, schist, gneiss and ultimately orthogneiss, whereas gabbro might give us amphibolite or serpentine.

With these terms under our belt, let's take a brief overview of France's geology.

The Geology of France

From this basic understanding of rocks, it is sensible (and indeed correct) to assume that igneous rocks are, in general, older than sedimentary rocks. After all, such rocks were formed on the earth's surface as it cooled, and persisted for many millions of years before the seas and life forms necessary for the formation of the various sedimentary rocks appeared. Nevertheless, there are of course exceptions to this rule. In the Massif Central, for example, there are volcanic flows from more recent geological eras. Nevertheless, the rule holds true in general; sedimentary rocks are younger than igneous rocks, and for this reason - in France at least - sedimentary rocks tend to lie over more deeply buried igneous rocks.

Bordeaux Geology

After their age, another important feature of igneous rocks that differentiates them from sedimentary rocks is their durability. Igneous (and metamorphic) rocks are much, much harder than the softer sedimentary rocks; for this reason they resist the eroding forces of ice ages, glaciers and weather very well. As a consequence igneous rocks are often responsible for large land promontories; with reference to the grossly simplified map above (it seems to me that geology can be as complicated as you want to make it; I have chosen to keep it simple), in the case of France such igneous and metamorphic land masses include the mountains of the Massif Central, the Massif Armoricain, which is the large finger of land in the northwest that juts out into the Atlantic Ocean, and the Vosges mountains overlooking Alsace. These land masses are visible in the map above.

Sedimentary rocks, by contrast, are easily eroded leaving the harder igneous rocks standing proud. Nevertheless, they still persist in two large basins, the Bassin Parisien and the Bassin Aquitain, each of which represents the existence of ancient seas or lakes, and both lie over more deeply buried igneous rocks. These regions are coloured yellow in the map above. Such regions are not necessarily flat and featureless though; although by their nature the rocks here began life as a flat sea bed, through plate tectonics they may develop rifts or rise to become mountain ranges, as in the case of the Pyrenees and the Alps, and the rift that runs through Burgundy and the Rhône Valley.

The Geology of Bordeaux

It is clear from the map above that the region of Bordeaux features only sedimentary rocks. Nevertheless, this is naturally a very superficial view, and there is much complexity to the region than this sweeping statement suggests. Nevertheless, I will try to remain very broad in my approach here. The time for a detailed discussion of the merits of calcaire à astéries over other types of limestone is best limited to my guide to St Emilion, and an exploration of the benefits of planting on gravel as opposed to clay is best reserved for my account of St Julien or Pauillac. Or Graves, perhaps.

Bordeaux Geology

The region is broadly divided into two by the line of the Garonne and the Gironde. To the east of this axis is limestone, some laid down during the Cretaceous (the final era of the Mesozoic, and the last time dinosaurs walked the earth) and Tertiary (a subdivision of the more recent Cenozoic era). To the west we have less structured rocks, as although there is also some limestone there are also gravel beds and the sands of Les Landes, all of which originated in the Quaternary era (more recent than the Tertiary - within the last few million years).

To the West: Médoc and Graves

These vineyards are most marked by gravel, which extends northwards from near Langon, in Sauternes territory, in a not-quite-unbroken run northwards to the far reaches of the Haut-Médoc appellation, beyond St Estèphe and into the Médoc. The gravel mounds originated in the Pyrenees, washed down here by the action of rivers over many millennia during the Quaternary (during the last few million years). They are often referred to as Günzian; this term refers to the glacial phase during which the gravel was exposed along the banks of the Gironde.

There have been four principal glacial phases during the last million years, named Günz (more than 600 000 years ago), Mindel (about 400 000 years ago) Riss (about 200 000 years ago) and Würm (during the last 100 000 years, ending about 10 000 years ago). At the beginning of the Günzian period, the sea level was high, and thus the level of the Gironde was also high. As the ice age took hold, huge ice caps and glaciers spread across the planet, and sea levels dropped as a result. As a consequence the Gironde began carving a deeper, lower-level course, and the banks of previously deposited gravel were now exposed. During subsequent glacial phases - Mindel, Riss and Würm - further banks of gravel were exposed, as the Gironde sank to its lowest level, which was 15 000 years ago. Then, as the climate warmed, sea levels rose, and the Gironde did likewise, submerging many of these previously exposed gravel banks. It is widely accepted that what gravel remains on the banks of the Gironde is largely Günzian and perhaps also Mindelian in origin. The gravel from the Riss and Würm era lies beneath the Gironde.

Bordeaux Geology

Thus, although it is tempting to imagine the gravel - like that above, pictured in a St Estèphe vineyard - as little more than a beach for the modern-day Gironde, its deposition and the course taken by the Gironde throughout history is a little more complex than that. Nevertheless, the process described above has left us with banks of gravel which are deposited just a few metres above and away from the course of the modern-day Garonne and Gironde. This has led to the saying that the best vineyards are always planted within sight of the Gironde, a belief that does not merely reflect superstition but the fact that such vineyards are, by definition, planted on the best gravelly terroirs. In addition, the existence of small streams, known locally as jalles, run through the marshy land into the Gironde at right angles to its flow. Today these streams have been incorporated into the Médoc's drainage system, but they predate its existence by many millennia, and are responsible for dividing up this gravelly sweep into distinct mounds around the modern-day towns of St Estèphe, Pauillac, St-Julien-Beychevelle and so on, and they have therefore defined the communal appellations we know today.

View any of Bordeaux's gravel vineyards and it is tempting to believe, especially when approaching the great gravel mounds from the north-west along the D204 when they rise up in the distance, that they are indeed nothing more than huge and featureless piles of stones. Of course this is not true; the vine does not require rich soils, but pure gravel might just be too far in the opposite direction. Investigations carried out by Professor Gérard Seguin of Bordeaux University in 1978 revealed that the deeper structure of the gravel mounds is more complex than we might have imagined; each one is rich in seams of moisture- and nutrient-retaining clay, sand and silt. It was, of course, not just gravel that the ancient Gironde deposited. As the ancient river flooded and receded year after year, it is only natural that there would be mixing of these layers of sand, gravel and clay. So although the gravel perfects the drainage, these deeper seams hold the water and nourishment, making these soils ideal for the vine.

To the East: St Emilion & Pomerol

As already indicated, the lands east of the Gironde-Garonne line are in many cases quite different to those to the east. Nevertheless there are also some similarities; beneath the gravels of the Médoc, Graves and Sauternes there is often limestone, just like that classically associated with St Emilion. And around Pomerol, there are banks of gravel which have the same Günzian origin as those of the Médoc. Indeed, around Pomerol in particular, there are also banks of gravel originating from the other glacial periods, Mindel, Riss and Würm, and so you could say the gravel here is more complex than that of the Médoc. Nevertheless, I will save these more appellation-specific analyses for my commune-by-commune guides, and try to maintain a broader overview here.

Bordeaux Geology

Keeping it simple, then, it is limestone that defines many vineyards of the right bank appellations, and these rocks are of much more ancient origin than the recently deposited gravels of the Médoc and Graves. Deposition began during the Triassic, several hundred million years ago, and continued as the waxing and waning of lakes and seas intermittently covered and then exposed the land here. Each time the land was submerged with water a new phase of limestone deposition occurred, so that a deep excavation through the bedrock might reveal many different layers of this rock. The characteristic calcaire de Blaye is one of the older deposits, having been laid down about 40 million years ago. Looking back across to the left bank for a moment, the calcaire de St Estèphe is somewhat younger, this layer arriving perhaps 37 million years ago. The classic right-bank limestone known as calcaire à astéries, however, is one of the youngest, having been laid down about 25 million years ago. This particularly hard limestone is named for the billions of fossilised starfish within it, and forms the cap rock of the famous limestone plateau that now defines St Emilion. It is only fair to note, however, that it can also be found under many left bank vineyards as well; it's just that you have to dig through a lot of gravel to get to it.

Over many millions of years the limestone has weathered, reducing its depth, introducing fractures and fissures. As a consequence today the right bank appellations are focused on a small number of limestone plateaus, and the degraded limestone and clay slopes around their edges are also important for viticulture. There is, however, much more to the geology of the right bank than this famous limestone. Much of the St Emilion appellation has alluvial and windblown sand (pictured above, close to the Dordogne) for its soil, and there is also gravel towards the west, gravel which also runs into and defines much of Pomerol. To the east, beneath Castillon, there is more limestone, the Calcaire lacustre de Castillon, its origin a freshwater lake rather than the astérie-rich sea. And the rocks and soils of Fronsac also have a freshwater origin, a mix of deep limestone and marly soil known as the Molasse du Fronsadais. Clearly, although the general rule here is limestone, the story for each appellation and commune is complex. As with the appellations of the left bank, I will deal with each in more detail in my commune-by-commune pages, later in this guide. (31/3/13)