Wednesday, June 5, 2013

Loire Valley wine region soils: Origins and characteristics

Winegeeks has identified Kimmeridgian as the top vineyard soil in the world but has erroneously attributed that soil type to the totality of vineyards in Burgundy, Champagne, and the Loire Valley. I have set the record straight by, first, identifying the locations where Kimmeridgian soils can actually be found and, second, clarifying what soil types are to be found in Champagne, Burgundy, and the Loire. In this, my final post in the series, I discuss the origin and identity of the soils of the Loire Valley.

The Loire Valley wine region is an agglomeration of appellations located on the slopes and plateaus that line the river's 620-mile course and associated areas of its many tributaries. In the course of its passage, the Loire River transits varied climates and soil types and, as a result, the region is characterized by a diversity of wine styles. While not as famous as other French wine regions (Bordeaux, Burgundy, Rhone), the Loire is understood to produce the definitive expressions of Sauvignon Blanc, Chenin Blanc, and Cabernet Franc.

The broader Loire Valley wine region is further divided into five sub-regions which are, from east to west: Central Vineyards; Touraine; Saumur; Anjou; and Pays Nantais.

The figure immediately following is a high-level geological map of France. The second figure below shows a more detailed geology of the Paris Basin but also shows the course of the Loire River from its origin deep in the "old rocks" of the Massif Central, through the Jurassic and Cretaceous strata surrounding the Paris Basin, through the Tertiary strata of the Paris Basin, then again into the outlying strata of the Upper Cretaceous, and, finally, into the old rocks of the Massif Armoricain at Anjou. I will follow this geological course in describing the soils associated with the region.

Central Vineyards

I have previously discussed the formation of the Kimmeridgian chain and identified Pouilly, Sancerre, and Menetou-Salon of the Central Vineyards sub-region of the Loire Valley as components of that chain.

Lower Loire

The overarching geology of the Lower Loire is as follows: Cretaceous of Paris Basin overlaying Jurassic strata for Touraine; Cretaceous of Paris Basin over Jurassic for Anjou-Saumur until southeast of Angier and "old rocks" of the Massif Armoricain over Jurassic thereafter; and, old rocks of Massif Armoricain over Jurassic for Pays Nantais.

Touraine to Anjou

Soil formation in this zone has benefited from a number of contributory events. (i)The receding seas of the Late Cretaceous left deposits of sand and flinty clays in their wake (Wilson). These deposits have proven to be less-than-perfect as vineyard soils but they help in the aeration of the underlying chalk soils. (ii) During the Eocene period (Early Tertiary), rivers deposited sandy gravel which cemented into "flights of steps."

The Loire transits the Paris Basin through the soft rocks of the Cretaceous and the surroundings present as rounded hills -- with varying degrees of steepness -- topped by cap rocks formed from cemented Tertiary gravel. The composition of the slopes are shown in the figure below and are expanded upon in the table following.

Anjou and Pays Nantais

Southeast of Angiers, we see a clear soil delineation which the locals refer to as "Black Anjou" and "White Anjou." White Anjou refers to the white, chalky limestone of the Paris Basin while Black Anjou refers to the dark slate and stones of the Massif Armoricain -- the "old rocks."

The old rocks of the Massif Armoricain are the surviving elements of a once-mighty mountain range called the Hercynian whose origins extend back into the Cambrian Age of the Paleozoic Period. The material of this mountain range included schists, graywackes, limestone, lavas, tuff, granite, and puddingstones, among others, formed during the Paleozoic. These materials had, in turn, been deposited on remnants of the eroded Caledonian mountain range which had itself been formed in the Proterozoic Period.The Massif Armoricain was subjected to erosion and weathering during the Mesozoic such that its southern portion became a plain into which the seas of the Paris Basin advanced during the Upper Cretaceous.

In addition to the indignity of erosion to a plain in the south, the old rocks of the Massif Armoricain underwent a number of physical changes (Wilson):
  • The original thick shale deposits metamorphosed into schists and slates
  • Sandstones became hard quartzites
  • Limestones became compacted or marbelized
  • Granites and old volcanic rocks also metamorphosed.
The hardest of the old rocks survived the weathering and became ridges while others became excellent soils. These rocks contain biotite mica, serpentine, and other dark minerals which release valuable mineral nutrients when weathered to clay (Wilson).

In addition to the soils created by weathering/erosion and the intrusion of the Cretaceous Seas, the Pays Nantais region was affected by Mid-Miocene Atlantic Ocean intrusions which deposited fossiliferrous sands and pebbles in the region.

The geological map of the Massif Armoricain is shown below.

A summary of the soils distribution in the Loire Valley region resulting from the activities described in the foregoing is provided in the following three figures.

Soils of Central Vineyards, Loire Valley (Sources: Data -- Wilson (Terroir);
base map --  Guildsomm)

Soils of Touraine, Loire Valley (Sources: Data -- Wilson (Terroir);
base map -- Guildsomm)

Soils of Anjou and Pays Nantais, Loire Valley (Sources: Data -- Wilson (Terroir);
base map -- Guildsomm)

A discussion of the soils of the Loire Valley wine region would be incomplete without mention of tuffeau, the porous, chalky limestone which is held in high regard from Touraine to Saumur. This soil-type is generally found in tabular deposits containing fragments of mica and sand and adds valuable chemical and physical properties to the soil profile (Wilson). It is noted for its ability to retain heat (thus helping in ripening) and is well-drained. It is widely used as a building material in the region and winemakers generally carve their underground cellars out of tuffeau deposits.

This then concludes the series on defining in full the soils of Champagne, Burgundy, and The Loire Valley, soils which had been mis-characterized as Kimmeridgian. I have shown that Kimmeridgian represents only a small portion of the soils of each of these regions and that a wide diversity of soils exists within and between said regions.

©Wine -- Mise en abyme

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