Wine Geology 101: A Book That Needs to Be Written

Piles of blue slate in the Graacher Himmelreich vineyard. Mosel Valley, Germany.
Piles of blue slate in the Graacher Himmelreich vineyard. Mosel Valley, Germany.

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 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.


Image courtesy
Image courtesy

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.

simplified rock cycle
Modified image courtesy of

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.

Most beach sand is just quartz, slowly breaking down.
Most beach sand is just quartz, slowly breaking down.

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).

Image courtesy
Image courtesy

Did you realize that limestone will eventually become marble under metamorphic pressure?!

Image courtesy
Image courtesy

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.

Marble quarry in the Apuan Alps, over the town of Carrara (Tuscany, Italy).
Marble quarry in the Apuan Alps, over the town of Carrara (Tuscany, Italy).

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:

Image courtesy Wikipedia.
Image courtesy Wikipedia.

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:

Image courtesy Wikipedia.
CaCO3, or calcium carbonate, means lime. Argile is from ancient Greek (argillus) for clay. Image courtesy Wikipedia.

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 1; 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 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 2


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.

Image courtesy
Image courtesy

Volcanic basalt is the dominant rock found under ocean basins and exposed in places like the lava flows of Hawaii. 3 , 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.


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:

Flint, often used in arrowheads.
Flint, aka. silex, often used in arrowheads.

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.


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.


  1. Dean Alexander at Diary of a Winebuyer ( has recently written a series of magnificent articles on the Terroir of Burgundy, in which he discusses the formation of soils from a limestone base, the development of marls, schists, and slate. Highly recommended, not only for his discussion of slope placement and soil effects upon the flavors of Burgundy, but also the development of Burgundy’s terroir over geologic time periods.

    • Gargantua says:

      I came across his site in researching this article and was impressed with his commitment. He goes really in depth with some pretty dense material.

  2. I really like this post. It is a good overview of concepts that are easy to miss or completely misunderstand.
    Also, thank you both for the kind words regarding my articles on Burgundy Terroir. This terroir thing has become something of a pleasurable obsession. Part of my early morning routine. There are so many tangents and details to explore! You have uncovered some terrific information, which I somehow missed in my research. Metamorphism of shale to gneiss, I missed. But this is probably because so little other than limestone is reported in Burgundy. Mud and shale are not sexy! Only limestone is sexy, so that is what is in the Burgundy literature (which I have become increasingly critical of for its lack of academic diligence and rehashing and propagating wine myths).
    My conclusion so far about terroir is wine quality and complexity comes down to enough water (more is probably better than less) and enough root space. I don’t think soil type affects the flavor or quality of wine, but it completely affects how much water is retained in the soil, and how much root space there is.
    However, vines really don’t seem to produce the very finest wine if they are unbridled in their growth, with deep loamy soils, and as much water as they can get. They can produce good wine, but not great wine. There has to be a Goldilocks element to its upbringing. Things have to be (more or less) just right.

    • Gargantua says:

      I’m fascinated by ‘the missing carrier’ for terroir; just why Chablis tastes as it does and why Riesling growing in blue vs gray vs red slate displays typicity. Clearly the fallacious notion absorbed by the unquestioning masses–that minerals are absorbed through roots–is implausible. It’s surprising how many professionals still believe that. But that leaves us with the question of why….I’ve read a few thought-provoking articles bringing attention to yeast, mycorrhizae, or the effects of mineral terroir on water retention; but nothing seems satisfactory yet. I hope to pen an article on this lofty topic someday, meanwhile it’s right in front of my mind, and I’m collecting ideas. Who knows if we’ll ever really know. Thanks for reading!

  3. Gargantua says:

    ERRATUM: As this is quickly becoming a reference for MW students, I have expunged an error I recently discovered. The paragraph on tufa and tuffeaux previously read:

    “You may have heard of tufa, or in French, tuffeaux. It’s a different type of limestone, that forms in a freshwater environment instead of the deep sea. Think of Yellowstone’s hot springs. Tufa are found near dry lake regions. 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 tufa.”

    That is incorrect, and this has been revised to reflect the ‘faux-ami’ nature of tufa/tuffeaux. Note also that the word tufa may ALSO refer to a completely different volcanic rock — tuff, a porous volcanic rock with parallel etymological origins that is sometimes called “tufa”.

    I will continue to update this article if I discover any further errors.

  4. Bob Henry says:

    Admirable that you go back and fact check/update posts over time.

    The Web would be a more trusted source of information if others adopted such practices.

    And on this subject . . .

    From the Los Angeles Times “Op-Ed” Section
    (February 10, 2012, Page A19):

    “Syntax? Logic? Why?”

    By Michael Kinsley
    Bloomberg View columnist

    It’s been going on for too long, right before our eyes. Inevitably, someone was going to blow the whistle, and wouldn’t you know it would be Felix Salmon, the famous financial blogger for Reuters?

    . . .

    Nothing, though, prepared me for the dazzling brilliance of Felix’s blog item this week [circa February 2012] about the quality of writing on the Internet. … his basic point is that on the Web, sheer quantity trumps quality. …

    … Felix’s blog post … argue[s] that all aspects of good writing — accuracy, logic, spelling, graceful turns of phrase, wisdom and insight, puns (only good ones), punctuation, proper grammar and syntax (and what’s the difference between those two again?) — are all overrated.

    . . .

    … Now one of our nation’s leading bloggers has confessed what we all suspected: that bad writing is inherent to the online world. …

  5. Steven Murphey says:

    so what is the correction for tufa/tuffeau ?

    • Gargantua says:

      The article as it reads now incorporates the correction. If you wish to read the previous version (incorrect text), it is visible for archive purposes in the comment above dated October 29, 2015 at 1:24 am.

    • Gargantua says:

      Wish I could view an excerpt! Thanks for sharing–looks promising.

  6. Varun says:

    This was a very well written article. Caitlyn Jenner reference was genius! Thanks for sharing the info.

    • Gargantua says:

      Thanks for reading, Varun. I’ll publish a new set of articles once I recover from this election outcome.

  7. I live in a Weingut on the Mosel and I am interested in the research into both the geology of wine growing areas and the microbiology of the vines themselves. I understand that recent research shows that the soils affect the microbiology of the vines and so the soil only indirectly affects the taste of the grapes and wine. So maybe someone should follow this up!

    • Gargantua says:

      I too have thought a great deal about this topic–the possible interaction between geology and yeast; perhaps certain soil types are capable of encouraging different types of yeast to populate and in turn grow in the soil, then later on the grapes and stems themselves. And perhaps this is the missing skeleton key for why terroir produces seemingly consistent results from say blue or red slate, or limestone or granite terroirs. I’m tempted to write this article but lack much of the requisite microbiological and chemical expertise. I’ve been compiling a sourcebook of evidence for around three years…

  8. How has no one mentioned Terroir by James E. Wilson? All about underlying geology, written by a geologist. Although it’s not very clear on what you call the “missing carrier”, which is the crux of the matter. But that is more the province of chemistry and plant biology.

    • Gargantua says:

      The missing carrier question is the one that fascinates me, and however that may interact with terroir. RE Wilson’s Terroir, it’s true, it’s an amazing book, and I need to come back to it. It’s also rather demanding of the reader; even though Wilson punctuates the text with interesting historic anecdotes, it’s an academic bound text, and not something you can carry about easily. It also goes into such lengthy detail that it made it a bit of a challenge to finish each chapter (Burgundy is 55 pages). Can you believe I had set this book aside after finishing Alsace, and arriving at Burgundy? How on Earth did I not get to starting Burgundy. Piles of books, I guess. Thanks for the reminder!

  9. The biggest problem with Wilson is that he didn’t explain things as he went and just threw out names as if any knowledgable wine geek would know what they were. Because of this unless you are a geology major (or graduate student) you’ll never have a hope of understanding what he’s writing about. In that sense, it’s a total failure – and that is why it is so hard to read. For the past 8 months, I’ve been working on a new treatise (a book this time) on the geology of Burgundy, starting with how the conditions under which the sedimentation was laid down and only now does Wilson make sense. I don’t know if anyone will read it but it needs to be done to advance our understanding of the wine. I have discovered that our perception of Burgundy as handed down from Coates and others is all wrong and even viewing the great work of Vannier-Petit only puts names to stones and conglomerates which we still have no reference by which to understand. I aim to change all of that.

    • Gargantua says:

      Well keep us posted!

    • Gargantua says:

      You’re welcome! Thanks for reading. Be sure to check the “Archives” link above for other equally thoughtful articles I’d poured my heart into. 🙂

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