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Loire Valley Wine Guide: Central Vineyard Geology: Flint

Flint

Having thoroughly explored the different limestones and marls of the Central Vineyards, which are divided between the Oxfordian, Kimmeridgian and Portlandian (or Tithonian) stages, we come now to flint. This is the final instalment of my attempt to describe the geology of this region, and how it pertains to viticulture and wine, before I move on to look at its climate and its various appellations.

Flint is a sedimentary crystalline form of quartz which is laid down in association with limestone, and like limestone it too has a biochemical origin. The purported progenitor material is biogenic silica. Organisms which utilise biogenic silica include sponges, where it is shaped into an internal skeleton and superficial spicules, and a variety of micro-organisms. I have already explored the process of silica deposition and flint formation in detail in On Limestone and Flint, but to very briefly recap, when these organisms die they sink to the sea floor, and subsequently degrade. The biogenic silica then enters the sediment, gathering in layers or nodules, and eventually the combination of high pressure and time will see it crystallise into flint.

Flint is therefore found within (or lying between) a wide variety of limestones, including that laid down during the Jurassic Oxfordian, Kimmeridgian and Jurassic-Cretaceous Portlandian stages which I have already discussed, but also in much more recent Cretaceous formations. Despite its apparent ubiquity, however, its relevance to the wines of the Central Vineyards is relatively restricted, as it crops up in significant amounts only in selected areas of the Sancerre and Pouilly-Fumé vineyards. To understand why this is, it is necessary to understand not only how limestone and flint is created at the bottom of ancient lakes and tropical seas, but also how they are weathered and eroded once it emerges from the water.

Weathering & Erosion

Limestone is a remarkably soft stone; the vignerons of the Loire Valley, who historically excavated their cellars in the limestone bedrock using mere hand tools, know this only too well. Calcite, one of the two main mineral constituents of limestone, has a hardness of 3 on the Mohs scale, the internationally accepted measure of hardness. Diamond, for comparison, has a hardness of 10. As a consequence, when the ancient seas and lakes that spawned these deep beds of rock finally receded, the process of weathering and erosion began immediately. With the passing of time, thick layers of limestone which had taken millions of years to lay down, and which in some parts were hundreds of metres deep, were worn away, leaving behind only harder materials more resistant to this action. This of course means flint which, having a hardness of 7 on the Mohs scale, is remarkably resistant to erosion.

Central Vineyard Geology: Flint

Thus the flint beds of the Loire Valley originate not from those layers of Oxfordian, Kimmeridgian and Portlandian limestone which still exist beneath the soil, but from long-lost layers of more superficial limestones and marls, which have over millions of years been worn away until nothing of the limestone remained and the flint was released. Flint thus has two ‘ages’. First, there is the age that relates to the deposition of the original material. Here in the Central Vineyards, the long-lost layers of limestone which gave rise to the flint were younger than the Oxfordian, Kimmeridgian and Portlandian layers which otherwise define the region’s terroir. The origin is usually definable, depending on the association of specific fossils within the flint, although this is not always possible. The second ‘age’ relates to the time at which the processes of weathering and erosion finally released the flint from its chalky incarceration; this is often much more difficult to determine.

The beds of flint which result from this long period of weathering and erosion feature nodules and stones, sometimes in isolation, sometimes mixed with silt, sand, clay or other weathered and organic materials (like that pictured above). While flint is scattered far and wide across the Central Vineyards, the richest beds tend to be gathered around a handful of specific hills or slopes, on both banks of the Loire.

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