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May 2010 trip

Vine Trail Geological Tours 3 -The Loire Valley - May 2010

A week’s trip by Nick Brookes and Dan Bosence to growers in the Loire Valley in May 2010 allowed us to savour and try to understand the relationship of the Loire wines to the geology of the area. What has geology have to do with the wine? You might be asking.

On a general level geology controls the shape of the landscape and the aspect of the slopes. South facing slopes are important for ripening grapes this far north in France. On a more detailed level though geology determines soil type and therefore, with climate, what grape varieties will produce the best wine. Three main grapes are grown in this region; Muscadet on the ancient rocks of the west and Cabernet Franc and Chenin Blanc in the central and eastern areas and unusually they are used to make single grape wines. Blending of different grapes is the exception and not the rule in the Loire.

If single grape wines such as Muscadet and Chenin Blanc are produced on different soils and rock types then if geology does control the character of the wines then one might expect wines of different character from these different areas. This was one of the topics we hoped to follow up on our tasting trip as well as to see how different vignerons made different wines depending on the geological setting of their vineyards.

These notes start with an introduction to the geology of the Loire and its tributaries and then moves on to our itinerary.

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Geological evolution of the Val de Loire

The geological evolution of the Loire valley region stretches right back to some the oldest rocks in France formed in the Brioverian period of time from 2,000 to 500 million years ago. So old, that the area has been subjected to two phases of compression to form mountain chains and enough time to erode these mountains to sea level. The Brioverian rocks are known as metamorphic rocks and were formed many kilometres down into the Earth’s crust at high temperatures and pressures which led to their being recrystallised into rocks called phyllites, schists and gneisses. In addition these had molten rocks, or magmas, that were injected into them which then crystallised to form igneous rocks such as granite and gabbro.

All this took place in a time of compression of this part of the Earth’s crust to form an ancient mountain range, and it’s the core of these old mountains that have been uplifted and eroded to form the landscape in the lower, or western region of the Loire valley, the Pays Nantais. In the geological map of the region the Brioverian rocks are coloured distinctive reds, pinks and oranges.

 

 

Geological map and main wine regions of the western Loire region of France. A geological map is a normal topography and road map that is overprinted with colours that indicate different rock types. In this map the western areas Pays Nantais and western part of Anjou-Saumur have bands of ancient Brioverian and Palaeozoic metamorphic and igneous rocks (see text for explanation) aligned northwest-southeast and coloured pink red, green and turquoise. These are overlain by much younger Jurassic (blues), Cretaceous (light greens) and Cenozoic (yellows) limestones arranged in east-facing crescents in the eastern Anjou-Saumur and Touraine regions (source for geological map www.brgm.fr). The wines regions, written across the top of the map and with the north – south dividing lines, do not follow the geological boundaries and relate to socio-historical boundaries.

Alongside these ancient rocks are younger sedimentary rocks, just 500 to 400 million years old (part of the Palaeozoic time) which contain the earliest animal fossils; groups such as arthropods and molluscs that had no backbones. Clearly these must have accumulated in ancient seas that covered the eroded core of the earlier mountain chain. So, after the formation of the Briovarian mountain range it was uplifted and eroded down to its roots. This early landscape then subsided and was flooded by the Palaeozoic seas that accumulated muds and sands (the dark greens and blue on the map) on top of the metamorphic basement.

The map shows a strong northwest to southeast alignment of the Brioverian and the Palaeozoic rocks together with heavy black lines between the different rock types which are faults, or fractures. All this indicates that both groups of rocks were subjected to a later period of compression and mountain building known as the Variscan orogeny to form what is called the Armorican Massif with its distinctive NW-SE structural grain. Today, this structural grain is reflected in NW-SE oriented hills and steep wooded valleys such as that of the Sèvre Nantais and Layon rivers.

To the east of Angers and the Sarthe river there is clearly a different geology and these are much younger rocks. Light blue, pale green and yellow colours indicate Jurassic, Cretaceous and Cenozoic rocks respectively. These formed from 200 (the earliest Jurassic) to 10 (the late Cenozoic) million years (m.y.) ago and are the oldest in the west (north and south of Angers) and younger towards Orléans to the east. These are sedimentary rocks that accumulated in warm, shelf seas and are mainly limestones; calcium carbonate rocks formed from the accumulation of the remains of shelly fossils on the sea floor. The largest area (coloured pale green) is Cretaceous aged chalk or craie formed in deep shelf seas from the remains of planktonic organisms; essentially the same rock units that form the chalk downs of England.

These younger rocks occur as a series of east facing crescents that is quite different from the NW-SE orientation of the older Armorican rocks. The line of junction between the older and the younger rocks represents nearly 200 million years of erosion of an ancient landscape before flooding by the Jurassic seas about 200 m.y. ago. Geological surfaces such as this are known as unconformities and are clearly seen on the map with the change in orientation of the strata and change in rock types and their ages. This is also reflected in the building stones used in the chateaus built near the unconformity, as in Angers. Here, the dark coloured basement rocks are used decoratively alongside the cream limestones.

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Château d‚'Angers built in the 13th century, during reign of Louis IX, from dark coloured basement rocks and lighter coloured Mesozoic limestones. Both rock types occur nearby as Angers is built near the unconformity between these two major geological regions of the Loire Valley. (Picture from franceguide.com)

Therefore, the western and eastern parts of the Val de Loire are very different geologically and this means they have different soils and different landscapes. The older Brioverian and Palaeozoic rocks have been subjected to a long period of weathering and erosion to form deeply incised valleys and steep wooded slopes of the Pays Nantais. The climate here is influenced by the Atlantic so it is moist and temperate. The soils are rich in silica and silicate minerals, such as quartz, feldspar and amphibole, and this means that they develop acidic soils, which are often enriched in iron. However, the Mesozoic and Cenozoic rocks are dominated by limestones that are younger and still have an essentially horizontal orientation that results in plateaus and more undulating landscape that are incised by the Loire and its tributaries to form river cliffs. The rivers expose the limestones and many have been excavated to form dwellings or caves for wine storage. These calcium carbonate, or lime-rich rocks, weather to form thin alkaline soils and the climate in the east of the region, above Angers, is more continental with cooler winters and a drier climate.

The Loire and its many tributaries cut through all these rock types to form river cliffs and sloping valley sides. Despite it being the longest river in France it does not have large river terraces with deep alluvial deposits. On the geological map the alluvial deposits are shown as thin cream coloured strips on either side of the rivers. This is in contrast to the Dordogne and Garonne rivers to the south that have large river terraces forming the classic wine producing regions of Bordeaux and Médoc. The reason Wilson2 gives for this is that the rocks of the Loire area are more resistant to erosion than the soft Cenozoic deposits of the south so that the Loire kept to its long-lived channel rather than meandering to and fro and forming extensive river terraces.

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