You’ve probably heard of vineyards covered in slate or schist. But did you know that slate can turn into schist? Or that shale can transform into slate, then schist, and later into gneiss?
I didn’t. And I wish I’d known this years ago.
Wine lovers like myself are in DIRE need of a bare bones guide to geology for wine. Someone needs to do for geology and terroir what Karen MacNeil did for wine with her Wine Bible — make it user-friendly by extricating needless jargon. And that’s a tall order, because unfortunately, geology starts off intuitive, then grows hopelessly complicated with an unending barrage of esoteric terms.
I’ve alerted Kevin Pogue, a wine geology expert specialized in Washington terroir. Unfortunately, he has a book on the Columbia Basin to write first, and admits that the project would prove daunting.
Meanwhile, I’ve found an excellent resource: an online geology course crammed with explanations and visuals at GeologyCafe.com. Creator Phil Stoffer is an ex-librarian and geology professor in MiraCosta College in California, and is committed to open-source science education … what a beautiful humanist.
I’ve culled the essentials from his site that seem useful to wine lovers.
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So, yes: shale, when under pressure (via both heat and friction), will transform into slate, and with greater pressure, into schist, and finally gneiss. Gneiss (along with granite) is one of the hardest rocks out there.
Now. Time to channel your inner 3 year old, and ask: what is shale?
Shale is a composite of mud and clay, that’s been compacted into a solid.
But what is clay?
Clay is a sediment, a mixture of sand and the tiny decay of other silica-based and aluminum-rich minerals that break down in water.
This leads us to the rock cycle (sort of like the rain cycle for rocks). Once you can visualize and understand this, everything starts to make sense, and you feel a bit like a bad-assed wizard.
The only way for a rock to go through a Caitlyn Jenner-like transformation is through energy: via erosion, heat, pressure, or explosion.
There are only three types of rocks: igneous rocks, which are cooled off magma; sedimentary rocks, which are compressed sediment; and metamorphic rocks, which are other rocks that are pressed and heated into a new form.
The forces of erosion — shorelines, wind, water and sun — are startlingly powerful, and are key to rock transformation. Shifting shorelines leave layers of sediment, which will later turn into slate and all sorts of things. The next time you are standing on a beach, meditate on the rising and sinking tide; then the sun and wind drying those exposed sediment elements out; and finally on the strange fact that the ‘sand’ beneath your feet is actually just partially decomposed quartz. Because quartz is so very hard, it decomposes into tiny ‘sand’ fragments, and can be transported long distances as sediment; it comprises the vast majority of beach sand.
Imagine this process over millions of years, and then imagine it working in tandem with shifting tectonic plates. Consider tectonic plates as dinner plates. Imagine these dinner plates are bedrock — very hard things, like granite, and that they still have thin layers of deposited food ‘sediment’ on them from some sloppy bachelor that ate off the same plate for weeks without washing them. Usually, if they’re pushed together via uneven surfaces, one will slide on top of another (subduction and induction, two words that feel very cool to say).
Metamorphic pressure comes into play when part of these dinner plates gets pushed deep into the hot, magma-laden earth. The heat and pressure fuse things (like sedimentary layers created by the sea, aka. the dried remnants of the sloppy bachelor’s meals) into new rocks by joining them with other nearby things. Here’s a neat graphic below that shows what some common things started off as (on the very top layer, near water) and what they can morph into with metamorphic pressure (nearest the magma).
Did you realize that limestone will eventually become marble under metamorphic pressure?!
Suddenly the hiccup in the Burgundy vineyards caused by the marble industry on the north and south ends of the Côte de Nuits Villages appellation makes sense! Solid pink and beige marble are carved from the earth. Those marble quarries in Comblanchien and Corgoloin divide the Côte de Nuits from the Côte de Beaune.
Very often, aside great limestone wine terroirs, you’ll find a marble industry. Consider Tuscany, and its famous Carrera marble used in the greatest sculptures and monuments of the Western world.
But what is limestone?
Basically: very, very old seafood. Limestone is a solidified accumulation of lime, that is to say calcareous skeletal remains (shells, coral, plankton, or old algae). In the case of Kimmeridgian limestone — that great sculptor of Chablis, the finest white wine on earth — it is encrusted with fossils from sea creatures such as Exogyra virgula and Ammonite.
Chalk is limestone, but it’s special: it’s a white or gray colored surprisingly pure variant of limestone that’s up to 99% pure lime. It’s made of microscopic nanofossils, so tiny they can condense into such a dense lime form. The next time you taste a great Chardonnay-based champagne from the Côte des Blancs, imagine these guys:
Or maybe the next time you see a chalkboard, imagine the millions of tiny sea creatures that donated their bodies to your project of scrawling something completely inane.
Now that we understand lime, we can tackle marl. This evil, bastard word has perplexed me for years, and I despise how people throw it out there as though it’s as perfectly stable and as simple as, say, slate. Wrong.
Marl is an unstable concept. It’s a variable composite of lime or clay. You can’t have all of both: it is either nearly all lime, or nearly all clay, with a bit of the other. It may be solid or pliable. (No wonder no one understands this). Here’s how you can visualize it:
If it’s hardened, you can call marl a marlstone. If mud is hardened, you can call it mudstone.
You may have heard of tufa, and in French, tuffeaux. These are two different types of limestone. Tufa forms in a freshwater environment instead of the deep sea; think of Yellowstone’s hot springs. Tufa are found near dry lake regions. Tuffeaux, while seeming identical, is actually a yellowish, chalky marine limestone that has blended with sand and other fossil debris 1https://dominique.millet2.free.fr/histouraine5.html; see also Terroir: The Role of Geology, Climate and Culture in the Making of French Wines, By James E. Wilson (geologist.), p. 236, excerpt visible at https://books.google.com/books?id=gt517z302YcC&lpg=PA236&ots=LDlxofPSr9&dq=turonian%20yellow%20tuffeau&pg=PA236#v=onepage&q=turonian%20yellow%20tuffeau&f=false.. Some of my favorite Chenin grows in these limestone variants, notably François Chidaine’s Montlouis-sur-Loire ‘Les Tuffeaux’ bottling, a blend of parcels which delivers an insane value and a beautiful aging curve for a pittance. Chidaine is convinced the roots break through the tender tuffeaux 2https://www.francois-chidaine.com/accueil/les-vins/aoc-montlouis/les-tuffeaux/.
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One of the surprising things with metamorphic rocks is that there are many geological roads to schist. You can create schist not only from slate, but also from igneous basalt, a rock which rises via volcanic explosion from the depths of the earth.
Volcanic basalt is the dominant rock found under ocean basins and exposed in places like the lava flows of Hawaii. 3https://geologycafe.com/class/chapter2.html , cf. Figure 2-13. Black basalt characterizes one of my favorite vineyards in Germany: the Forst vineyard, in the Pfalz region of southern Germany (notably Forster Pechstein).
I am madly in love with wines which grow in granite, and am surprised how often I infer this given their unique noses, even in the face of changing varieties. Northern Rhône Syrah is the melted expression of granite. Alsace — that insane encyclopedia of terroirs — includes granite in its Brand vineyard and other steep slopes; this renders my favorite Alsatian Riesling. Cru Beaujolais thrives in granite. And my favorite Muscadet, which used to be called Granite de Clisson before the vineyard was branded as a new AOC Clisson, is grown in white granite.
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I want to clarify five last tricky terms I’ve grappled with as a wine lover.
Alluvial soils are loose, unconsolidated sediments, recently deposited by flowing water; think of stream channel beds, or flood plains. The sediment has not yet been solidified into a mass.
Silt is decomposed quartz and feldspar. If they make up granite, too, it is because they are the two most common minerals on earth. Silt feels like flour if dry, or slippery if wet. It’s often dredged up by construction near rivers.
Loess: Silt is easily transported in water or other liquid and is fine enough to be carried long distances by air in the form of dust. Thick deposits of silty material deposited by wind are called loess.
Loam is an elusive bastard in the same way marl is. Loam is a mix of sand, silt, and clay; no one element can be in majority. Typical breakdowns are 40% sand, 40% silt, and 20% clay. You thus have different types of loam soils: sandy loam, silty loam, clay loam, sandy clay loam, silty clay loam, and — infuriatingly — just ‘loam’. In the USDA textural classification triangle, the only soil that is not predominantly sand, silt, nor clay is called “loam”. Loam soils generally contain more nutrients than sandy soils. As we all know, wine tastes better when the grapes struggle in poor nutrient soils, so we shouldn’t hear about loam too often in wine terroirs.
How terrifying is this?
Silex is tricky, because it’s just Latin for ‘hard rock’. It usually refers to flint, a crystalline derivative of quartz, which is nearly pure silica. You may recall flint is used to light fires by striking steel or iron to produce sparks. Its tough, crystal nature made it ideal for shaping arrowheads as well. Flint occurs as masses in sedimentary rocks like chalk and limestone, so it’s inextricably linked to limestone. It’s many different colors, but it’s always smooth and opaque, and looks like this:
Does the above image remind you of Didier Dagueneau’s Silex label? Well, it makes sense: this is the prized Loire Valley terroir element for most of Sancerre and Pouilly Fumé, and while I suffer from a prejudice against Sauvignon Blanc and Sancerre, I fell madly in love with Gérard Boulay’s 2013 Sancerre à Chavignol, and 2010 Clos de Beaujeu. This actually instead speaks volumes to my love of Chablis, because Chavignol is more Chablis-like Kimmeridgian limestone than flint! But don’t cry for silex: Dagueneau’s Silex is amazing, and my beloved Chidaine Montlouis-sur-Loire Tuffeaux terroir is laden with silex, just like his amazing Montlouis ‘Les Choisilles’, and a great deal of other Chenin growing in Vouvray.
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Demystifying the geology of wine terroirs goes a long way in allowing wine lovers to decipher their preferences in wine.
At present, no one can fully explain how grapes’ flavors are inflected by their mineral terroir. Maybe someday we’ll better understand the connection between the rocks and the way wines taste: perhaps different rocks encourage certain species of yeast to grow, and that yeast ends up on the grapes’ skins before expressing itself during vinification. For now, it’s a mystery.
Meanwhile, if you’re ready to plunge even deeper into geology, I’d really recommend auditing Phil’s geology course at Geology Cafe. You’ll see the world in a completely different, more wizard-like way afterwards.