Left Bank Gravel, Right Bank Clay: Reading Bordeaux's Soils

One estuary, two banks: a shorthand, not a rule

Talk about Bordeaux wine often splits the region in two. The Left Bank means the Médoc and Graves, west of the Garonne and the Gironde estuary; the Right Bank means the Libournais around Saint-Émilion and Pomerol, by the Dordogne. The shorthand pairs the first with gravel and Cabernet Sauvignon, and the second with clay, limestone and Merlot.

It is a tendency, not a law. The appellation rules for Pauillac, Saint-Émilion and Pomerol all allow both Cabernet Sauvignon and Merlot, so the split comes from growers' choices on different soils, not from a ban. This article describes those soils, and touches on how the grapes behave.

Left Bank: gravel croupes beside the estuary

In the commune of Pauillac, gravel terraces lie parallel to the Gironde estuary, at altitudes of 3 to 30 metres. These Quaternary terraces average about 10 metres in thickness and almost entirely cover older marls and limestones of the Eocene and Oligocene.

The terraces are dissected into rounded mounds, the croupes, and their exposure, their nearness to the estuary and the free-draining soils favour very high-quality vineyards. Gravelly soils, more or less sandy, dominate on the croupes.

Basis of comparison

The comparison of the three appellations rests on soil and relief.

Right Bank: Saint-Émilion's limestone plateau and its slopes

At the centre of the Saint-Émilion vineyard lies a plateau of calcaire à astéries, "starfish limestone", dating from the Tertiary. Several types of soil occur on the plateau itself.

This rock can be dated more precisely: it was laid down during a marine transgression in the Stampian, a stage of the Oligocene (explanatory notice of the French geological survey, BRGM, Libourne map sheet). It is a limestone, not a marl or a slate.

Around the plateau, the molasse du Fronsadais, an Oligocene formation, outcrops on the slopes, which are generally steeper to the south and west than to the north. It mostly carries calcareous soils, often with a clay-silt texture. So in Saint-Émilion the "clay" of the shorthand is not one uniform clay layer but the texture of these slope soils.

Pomerol: gravel on top, clay below, iron in the subsoil

Pomerol's soil has three layers that work together. At the surface lie gravels, more or less compact or sandy, which drain and reflect light towards the foliage. Below them is a Tertiary clay substrate, a cool horizon that slows the ripening of the grapes and keeps the vine's water supply well regulated.

The subsoil also contains iron oxides, known locally as crasse de fer. They supply specific mineral elements and are a separate feature from the clay substrate, not another name for it.

Because Pomerol's soil is tied so directly to the speed of ripening and to the vine's water supply, the schematic below uses Pomerol as the example.

Height and slope

The Pauillac croupes are low: the gravel terraces lie between 3 and 30 metres above sea level, and the exposure of the croupes is one of their strengths.

In Saint-Émilion the slopes around the plateau are generally steeper on the south and west sides than on the north. No altitude is given here for Saint-Émilion or Pomerol, and no aspect for Pomerol; they are not estimated here.

Why Cabernet Sauvignon on gravel and Merlot on clay

Cabernet Sauvignon ripens later than Merlot and gives structure to blends; Merlot ripens earlier, with supple fruit (CIVB, Bordeaux's wine council).

Read together with the soil descriptions, the usual explanation follows: free-draining gravel suits the later-ripening Cabernet Sauvignon, while a cool clay horizon that slows ripening and regulates water suits Merlot. Treat this as a tendency. Both varieties are authorised in all three appellations, many of these wines are blends, and vintage and winemaking also shape the result.

The same caution applies in the glass. The contrast often drawn between a firmer Left Bank structure and rounder Right Bank fruit is a hypothesis to test against the wine in front of you, not a rule; in a blend you cannot assign each aroma, or all of the tannin, to one variety or one soil.

Practice: one question, a model answer and a self-check

Question: Using the Pomerol appellation rules, explain how each of the three layers of the soil can affect the vine and the ripening of its grapes.

Model answer: The surface gravels, compact or sandy, drain water and reflect light towards the foliage. The Tertiary clay substrate beneath forms a cool horizon that slows ripening and keeps the vine's water supply well regulated. The subsoil also contains iron oxides, the crasse de fer, which supply specific mineral elements. Together with the climate, these features form the basis of the character of Pomerol's wines.

Self-check (Maison's own guide, not an official exam scheme): one point for each layer described correctly with its effect (3 points), and one point for keeping the crasse de fer separate from the clay (1 point).