The geology of the vineyards of Alsace

A geologists dream

Although the viticultural area of Alsace is small and confined, the multitude of the wines is striking. Partly, the variability can be explained by grape variety, partly by the preferences, style and skill of the producer. However some characteristics and properties of an Alsatian wine can only be explained by introducing the concept of terroir.

You will experience the first proof of the influence of terroir, and the necessity of the concept to understand Alsace wines, as a grower pours you two wines from the same grape variety, from the same vintage but different Grands Crus. You will find them different, and the more you sample, taste and think, the clearer and more obvious will the differences seem.

Your host will provide an explanation as you ask him or her to tell the story of the particular vineyard, which will be a unique spot on Earth. You will hear a story about the soil, the bedrock, heating and cooling properties and how the sun encourages the grapes to obtain full ripeness.

The concept of terroir can be used to summarize all the natural characteristics that give a vineyard specific qualities. Terroir is composed of geological factors, such as the chemistry of the rock and the soil, physical properties such as the exposition and slope as well as climatic factors such as solar irradiation, temperature and precipitation. Together, these factor decide each the supply of water, nutrients and light as well as the microclimate experienced by th evines.

Besides care and love, the growing vine needs carbon dioxide, water and nutrients to grow and survive. The carbon dioxide is assimilated from the air and reacts with water in the photosynthetic process to yield biomass and oxygen. The oxygen is partly released back to the atmosphere. The water is extracted by the roots from the soil, together with macro nutrients such as nitrogen (N), phosphorous (P), potassium (K), calcium (Ca) and magnesium (Mg). In additions, the vine needs trace elements such as iron (Fe) and boron (B) for its metabolism.

I plant is not very picky when it comes to nutrient uptake. If a nutrient element is abundant, it will grab it as a luxury. Together with factors such as water and temperature, this will affect the plants metabolism and hence the composition of the grape must. In short – the quality of the wine!

I define a "vin du terroir" as a wine with vineyard specific, inimitable characteristics, the nature of which can not be explained by an array of unambiguous measures, and where the winemaker is subordinate relative to the terroir.

Alsace terroir

The roots of the plants are, obviously, partly located to the soil and partly in the bedrock itself.

In Alsace, the scale of variability in soil composition and conditions is in the range of 100 meters. Hence, if you walk 100 meters in any direction, you will normally find soil conditions that are significantly different in at least one important aspect. It is therefore not surprising that each commune and each hillside is divided into hundreds of named parts. Obvious from the Blue Map series covering Alsace, even the Grands Crus include several sub-areas with names from centuries ago. It is clear that most named growths, or Lieux-dits, have been treated separately primarily because they yield wine with diferent aroma and flavour.

In terms of the bedrock, the variations show in similar scale, but not necessarily overlapping the soil variations. Hence, two grids of variations overlap each other, literally adding an extra dimension to the geological complexity. This feature is quite unique for Alsace. In comparison, the geology of Champagne or Beaujolais is simply boring.

Riesling is the variety that, by far, is most transparent to the influence of terroir with respect to aroma and taste. Pinot Gris and the Muscat varieties have such distinct varietal characteristics that the contribution by the terroir is rather difficult to identify. However, when it comes to growing conditions the Riesling (and Gewurztraminer) is arguably less picky than the other noble grapes of Alsace. This is because Riesling has a longer maturation period, high natural acidity and a relatively skin that can "survive" quite bad weather during several weeks of the critical autumn months.

It may be argued that the concept of terroir is of prime interest for wine lovers who find an intrinsic value in variation, while the concept is less valuable for consumers who have made up their minds about how the perfect wine should taste.

Formation of the vineyards of Alsace

The vineyards of Alsace are located on the foothills of the mountain range Les Voges. The Voges is a part of the elevated ridge that stretches from Massif central to Bayerischer wald.

With a few exceptions, the best terroirs of Alsace are situated along a geological fault zone, stretching from south to north along the Voges. This zone borders in the west to the Voges granitic mountains and in the east to the flat Plaine d´Alsace. In the south the zone starts in Thann and ends in the north at Marlenheim. The zone is almost 120 km long but only a few kilometres wide.

The geological basis for today’s Alsace is a granitic mountain range, stretching from south to north, roughly from Basel to Frankfurt. It was formed 300 million years B.P. as a result of the Variscian folding. The granite rock itself is more than 570 million years old and has similar mineralogical composition as the Fenno-scandian range.

The ages of the dinosaurs

After the Variscian folding, the present Voges and Schwartzwald were the boundaries of one mountain range without the present (Rhine-)valley in between. During the course of millions of years, the range gradually lowered. As the temperature rose, the polar caps melted, and finally the former mountain range was below the sea level. We are now at Triassic which spans 245-208 million years B.P. During this period sediments of varying composition were deposited. Somewhat simplify, one can say that the chronological sequence of deposition, bottom and up, was sandstone (Bundsandstein), limestone (Muschelkalk) and marl (Keuper).

The first period was thus Bundsandstein (245-241 million years B.P.) During this period, erosion of the continents produced sandy particles that was transported and deposited in shallow seas. This layer was many tens of meters thick and covered the granite mountain under sea level. Bundsandstein is a German term which simply means bottom sandstone.

Thereafter followed Muschelkalk (241-235 million years B.P.) At this time the sub-continent was located close to the equator. In the warm oceans, there was an enormous production of shelled organisms, especially scallops (fam. Pectinidae). The muschelkalk sediments exerted a pressure, causing the sandy sediments below to form sandstone.

After Muschelkalk, we reach the period Keuper (235-208 million years B.P.) Marl was formed on the shores of the sea during this period. Marl is a rock formed by clay size particles of clay minerals, limestone and organic material. Marls are thus quite soft and it weathers easily to nutrient rich clay soils. In Alsace daily conversation keuper is associated with marly soils and bedrock in general and not specifically related to Triassic formations.

At the end of Triassic, three distinct layers covered the granite of Alsace.

During Jurassic, which followed Triassic, various marls and limestones where deposited. Often these sediments contained gypsum formed from evaporated sea water (as at Grand Cru Schoenenbourg), iron compounds formed under reducing conditions (as at Grand Cru Froehn), and fluorite (as at Grand Cru Kantzlerberg).

From Jurassic until 45 million years B.P. our continent was relatively stable. Thereafter the continental drift caused the folding process that created the basis for present landscape of Europe. The Alps were formed and the ancient north-south mountain range, hidden for hundreds of millions of years, was subject to an upward lift. During this uplift the layers of Triassic rock cracked. But the most dramatic event was that the whole central part of the mountain range collapsed, leaving the Voges and Schwarzwald on the edges of a giant depression.

This depression is known as the classic Rhine Graben formation. In French, the term is Fossé Rhenan.

The photograph, taken close to Voegtlinshoffen, shows the obvious signs of the broken granite by the foothills.

The lower parts of the fault zone Jurassic and Triassic rock was altered during Oligocene (35-23 million years B.P.) to marly conglemorates rich in limestone.

Recent times

During our own geological period, Quartenary, the climate has varied causing a series of glaciation periods in Europe. Even the Voges have been covered, but mainly to an altitude of 1000 meters. Hence, above 1000 meters the Triassic layers have been scraped of. At lower altitudes, such as in northern Alsace, the granite is still covered by Bundsandstein. This sandstone has been used for spectacular buildings all over France.

To find evidence for the recent glaciation, a glance at the naked Ballons d´Alsace should suffice.

The Fossé Rhenan has been filled with water during several periods, filling the Plaine d´Alsace with exceptionally fertile soil material. From a geological perspective, the present Rhine river as pathetic.

To sum up, the soil and bedrock of Alsace is thus a result of continental drift, physical and chemical weathering, biological processes in addition to the forces induced by gravity, snow, ice and water.

The result is a unique variations in growing conditions, creating a unique range of world-class wines.

The hamlet of Zellenberg, overlooking the Rhine Graben from an elevated slice of resistant Bundsandstein, surrounded by Oligocene material