The Alsatian vineyard "benefits" from exceptional geodiversity

• The Rhine ditch that carries the Alsatian vineyard was mainly formed around 30 million years ago, according to our partner The Conversation.

• But geology alone does not explain the wine-growing richness (both the cultivation of the vine and the production of wine) of Alsace.

• This analysis was conducted by geologist Nicolas Charles.

On the hillsides, protected by the Vosges from precipitation and cool winds

(hot, dry wind on the downstream side of a relief blocking a mass of humid air upstream), it is a thin ribbon green a few kilometers wide and more than 100 kilometers long which extends between Mulhouse and Strasbourg.

But what a ribbon… It is a vineyard located on the borders of the geographical and geological borders of France, welcome to Alsace!

The Alsatian vineyard occupies a unique place on the edge of a major tectonic structure, the western European rift, which extends from the North Sea to the Mediterranean.

The Rhine ditch which carries the Alsatian vineyard on its western border, was mainly formed like these "cousins" the ditches of Bresse, Loire or Limagne, during the Oligocene, there about 30 million years old.

This topographic depression, oriented north-south, results from the stretching of the earth's crust.

As such, the East African rift is a current analogue of the Alsatian structure.

On either side of the Rhine ditch, or Rhine graben, the Vosges and the Black Forest constitute the shoulders of the rift, which is bordered by a network of faults which accommodates the deformation undergone by the earth's crust.

Two major faults delimit the Vosges from the plain of Alsace, the Vosges fault and the Rhine fault.

Between these two faults, the hills under the Vosges carry the immense part of the Alsatian vineyard.

This area is traversed by innumerable faults constituting the “field of fractures of the sub-Vosges hills”.

The faults here have shifted the subsoil into multiple blocks which today form a real geological mosaic, where the rocks exhibit a remarkable geodiversity.

Limestone, marl and sandstone from the Mesozoic (-252 to -66 million years) and conglomerates, limestone and marl from the Cenozoic (-66 to 2.6 million years) make up the hills under the Vosges.

On the edge of the Alsace plain, there are deposits at the bottom of the slope (colluvium and dejection cones) then alluvium (pebbles, gravel, sand and silt) from the Quaternary (-2.6 million years to today). hui) which carry the rows of vines.

A vineyard inherited from antiquity

On the edge of the Vosges, it is granite, gneiss, or even schist that serve as the foundation for the vines.

A long geological history is therefore at the origin of Alsatian geodiversity, from the Variscan chain nearly 350 million years ago to the alluvium that is still deposited in the rivers today.

Mountains, seas, lagoons, lakes, volcanoes, glaciers or even rivers have succeeded in the meanders of long time contributing to the identity of this Alsatian subsoil which today bears a singular vineyard inherited from antiquity.

Of course, geology alone does not explain the vitivinicultural richness (both the cultivation of the vine and the production of wine) of Alsace.

Terroirs are the result of a set of characteristics (topography, water, soil, climate, winemaker know-how, botany, biology, etc.) to which geology contributes, on its own scale, to this “ecosystem”.

The centuries-old experience of winegrowers has led to the production of single-varietal wines (with the exception of Edelzwicker and Gentil) which recognize the diversity of the 13 Alsatian terroirs to which each of the grape varieties adapts.

Moreover, it is a French exception, Alsatian wines take the name of their grape variety and not of the terroir or the geography.

It should be noted that the Alsace grands crus display the locality of the vines because they have been recognized for a long time (for some as early as the 9th century) to provide prestigious wines.

51 in number, these appellations account for 4% of the vineyard.

Alsace is the kingdom of white wines: Sylvaner, Riesling, Gewurztraminer, Muscat, Pinot blanc, Pinot gris “tokay”… Pinot noir is an exception giving red or rosé wines.

It must be remembered that the demarcation of land on the sole basis of geology is not sufficient.

Also to be considered are climatic (rainfall, temperature, airflow), topographic (relief, slope, exposure), soil (nature of the soil, stoniness, porosity, chemistry and mineralogy, etc.), biological (variety and port -graft, micro-organisms), and of course vitivinicultural (winemaker's know-how).

It is by integrating all these aspects that we can better understand the typicity of a wine.

Rather Sylvaner or Gewurztraminer

In Alsace, a balance must be found between ampelographic and geological considerations.

Within the many rock types, the main mineral constituents are quartz (siliceous sand), clay and limestone.

Claude Sittler, geologist at the University of Strasbourg, has thus proposed a correspondence between the taste sensations of Alsatian wines and the mineral elements resulting from the geodiversity of the vineyard.

Power and astringency for clay, vivacity and acidity for quartz, opulence and softness for limestone.

Everything is a question of balance between the siliceous sand which brings porosity to the soil, the clay which captures the water and returns it to the vine but not too much, and the limestone which delivers mineral elements to the vine in moderation. to avoid chlorosis.

Knowing these basics, the winegrowers adapt the grape varieties to the terroirs to give their wines a personality.

The Sylvaner will give light and discreetly fruity wines on a fairly deep substrate with a loamy texture.

Granite and sandstone from which light soils with a coarse sandy texture are formed, will be particularly suitable for Riesling which will give wines with beautiful mineral notes with fine acidity.

But the king of Alsatian grape varieties also adapts to marl-sandstone soil (wines with delicate aromas, with liveliness and finesse) or alluvial soil (early, light and fruity wines).

The marl-limestone soils will be adapted to Gewurztraminer with full-bodied and aromatic wines.

Our "ALSACE" file

Why not end on an epicurean note?

Alsace is also a land of gastronomy and wine plays a large part in this with its alliance with regional dishes.

And if we started with a Crémant d'Alsace, where fine bubbles and fruity freshness will serve as a start.

Riesling and Sylvaner go wonderfully with the famous sauerkraut.

If you don't fancy cabbage, opt for baeckeoffe, a traditional stew that will be delighted to be accompanied by a Pinot blanc or a Riesling.

Finally, if you are still hungry, the kouglof will marry wonderfully with a Gewurztraminer, unless you start with a Crémant.

And of course, all this in moderation!

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This analysis was written by Nicolas Charles, geologist at the Geological and Mining Research Bureau of the National Geological Service.

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