Between about 99 and 65 million years ago, a shelf sea up to 300 m deep covered much of northern Europe. Almost nothing washed into it from the land. What settled instead was algae — the calcite plates of coccolithophores, microscopic and beyond counting, sinking for tens of millions of years. Compacted, that sediment is the Chalk Group. Chalk is not a pale rock, it is a deposit of bodies.

Nowhere is that deposit put to better-known use than 150 km east of Paris. The Comité Champagne dates its own subsoil to 90 million years and describes it as "fragments of marine micro-organisms"; three quarters of the rock at the surface is limestone, of which chalk is the part the vines prefer. The houses built into it as well as on it — Taittinger is one of seven still holding crayères, the chalk quarries under the Saint-Nicaise hill in Reims, cut for building stone since Gallo-Roman times and now a UNESCO site.

What makes growers care is water. Chalk is porous enough to work as a reservoir — the Comité Champagne puts it at 300 to 400 litres per cubic metre — and it pulls that water back up through its characteristics, "naturally regulating the vine's water consumption". Taittinger calls it a highly efficient sponge, draining a downpour and then drawing the reserve back up in a heatwave, "often making vines the only spots of green in the Champagne landscape in July and August".

Both claim this changes the tasting profile. The Comité Champagne puts it plainly: the same grape at the same latitude in a different subsoil "will not produce the same Champagne — even if it is made in exactly the same way". The mechanism they give is water stress, which "achieves that delicate balance of ripeness, acidity and berry aroma potential". Taittinger names the result — chalk, it says, gives champagne "its finesse, lightness and minerality".

The same rock, 300 km north

This chalk runs through Picardy and into the Champagne region vineyards east of Paris, under the Strait of Dover, and surfaces in England across Kent, Sussex and Dorset, stretching as far as Yorkshire.

Vines have been growing on this ground for nine centuries before anyone reasoned about what was underneath them: 45 vineyards were recorded in the Domesday Book of 1086, held by Normans and the great abbeys, in a south-east that was then warm enough to support them. The Chalk Group had been sitting there the whole time, unnamed and unconsidered.

In the modern era, the first commercial act to capitalise on this opportunity came in the summer of 1951. Major-General Sir Guy Salisbury-Jones, founder of Hambledon, was looking out of the dining-room window of Mill Down House with his stepson John, wondering what to do with the field below. He researched the feasibility of vines on the south-facing chalky slopes, took advice from friends at Pol Roger, the Champagne house favoured by Winston Churchill, and planted in 1952. It became England's first commercial vineyard.

Thirty-six years later, Nyetimber put in Chardonnay, Pinot Noir and Pinot Meunier "in the lee of the South Downs" — the varieties and the method rather than the rock. Its vines are spread across several sites, on greensand and chalk, and its head winemaker Cherie Spriggs puts the split at "about two thirds greensand and one third chalk". The estate that began modern English sparkling wine did not begin it on chalk.

Greyfriars went in the following year, on the chalk slopes of the Hog's Back outside Guildford, and says it was one of the first in England to plant the Champagne varieties. It gives the reason in the Comité Champagne’s own terms: the vineyard is "connected to Champagne by the same chalky bedrock that goes underneath the channel". Two plantings a year apart, after the same wine, on different rock.

In 2015 the argument was settled commercially rather than scientifically, when Champagne Taittinger and Hatch Mansfield launched Domaine Evremond at Westminster Abbey — the first time, the partnership says, that a Champagne house had invested in the United Kingdom with the ambition of producing a top-quality sparkling wine. Its Kent address is Chalk Hill.

But does it matter?

Most English sparkling wine is not grown on chalk. Margaret Rand reported it plainly in 2015: the chalk downland is there, "but most sparkling vineyards aren't on it". Spriggs adds that both drain well, and that drainage "is the important factor".

Some of that is arithmetic rather than preference. Kellett's own survey of the South Downs found that the best slopes are mostly north-facing, leaving "only about 2-3 percent of the landmass in the South Downs National Park" suitable at all. Chalk is common in southern England; chalk facing the right way, at a height that is not punished by wind, is not.

And some of it is genuine disagreement about whether the rock matters. Stephen Skelton MW, who planted the vineyard that became Chapel Down, argues from fruit rather than geology: apples and hops have grown in southern England for centuries, and "they didn't plant apples in windy sites", while "a lot of chalk downland is 150m up and very exposed". Bob Lindo of Camel Valley, on Cornish slate, is blunter — "soil is trumped by climate every time". Even Mark Driver, who planted Rathfinny on chalk, told Rand he held no firm view that chalk was necessary.

Spriggs is the one who describes a difference she can taste rather than measure. Acidity on chalk is not lower, she says, but it moves differently: "more even from the beginning to the back of the palate — which feels like less acidity". That is unfalsifiable in the technical sense and honest in the practical one, which is roughly where the chalk question sits.

Where the model points

The most systematic answer came from the University of East Anglia. Modelling terrain, soil and climate together, Nesbitt, Dorling and Lovett identified 33,700 hectares of prime viticultural land in England and Wales — "a scale just larger than the Champagne region of France" — and found many existing vineyards of a hectare or more "are sub-optimally located". The ground the model likes best is not all on the Downs. Large areas of Essex, which holds mainland Britain's warmest 30-year growing-season average at 13.9°C, and Suffolk came out as especially suitable.

Which returns the question to where it started, in 1086. Vines grew in the south-east then because the product was enjoyable, not because anyone had read the rock.