Kimmeridgian and the Seven

Chablis is in Burgundy the way Alaska is in the United States: administratively yes, and in every other respect somewhere else. It lies about a hundred kilometres north-west of Dijon, closer to Champagne than to Beaune, in a cold continental pocket on the river Serein. Nobody driving from the Côte d'Or to Chablis feels they are travelling within a region.

The distance matters because it changes what the same grape does. Chardonnay in the Mâconnais ripens easily and gives breadth. Chardonnay in Chablis ripens with difficulty and gives acid, tension and a quality people call flinty or saline for want of better words. The wine is recognisably a different proposition from white Burgundy further south, and that difference is climatic before it is anything else.

The rock gets more credit than it may deserve, but it is genuinely distinctive. The defining soil is Kimmeridgian: a Late Jurassic marl (clay mixed with limestone) packed with the small fossil oyster Exogyra virgula. It outcrops in a band that runs from the Dorset coast through Chablis and on towards Sancerre, and in Chablis the grands crus and the better premiers crus sit on it. Above it, on the plateau, lies younger Portlandian limestone, which is harder and poorer, and which carries the Petit Chablis appellation.

That is the one geographical fact worth memorising, because it clears up the region's most common misunderstanding. Petit Chablis is not Chablis from young vines, and it is not a declassified second wine. It is a separate appellation defined by which rock is under it. It tends to be leaner and simpler and is meant to be drunk quickly, and it is not a lesser version of the same thing so much as a different thing from up the hill.

The grand cru arrangement is also unlike the Côte d'Or's. There is a single appellation, Chablis Grand Cru, covering seven contiguous climats on one south-west facing slope above the town: Les Clos, Vaudésir, Valmur, Grenouilles, Blanchot, Preuses and Bougros. The climat is named on the label but the appellation is shared, which reflects the modest scale; the entire grand cru is barely a hundred hectares on one hillside. Les Clos is the largest and generally the longest-lived. The right-bank premiers crus that continue along the same slope, Montée de Tonnerre and Fourchaume above all, share its aspect and its Kimmeridgian and are the sensible place to look for weight without grand cru money. Left-bank climats such as Montmains and Vaillons are finer-boned and quicker.

Then there is frost, which is the fact that dominates Chablis growers' lives. Spring frost at budburst has destroyed large parts of the crop repeatedly and recently, which is why the vineyards are strung with sprinkler systems and dotted with paraffin candles in April. The sprinkler trick is elegant physics: water sprayed onto a bud freezes, and freezing releases latent heat, which holds the bud at exactly zero degrees rather than letting it drop to minus four. A shell of ice is a blanket. It looks like a catastrophe and is the opposite.

The consequence for a buyer is that Chablis vintages vary in volume far more than Côte d'Or vintages do, and therefore in price. It also means that the older stereotype of Chablis as austere and thin belongs to a colder era. Warming has made ripe Chablis normal, and the argument in the region has quietly shifted from whether the fruit will ripen to whether the wines are losing the nervous tension that made them worth having.