Taste the Limestone, Smell the Slate is based on 12 feature articles published over the years in the magazine The World of Fine Wine. With the feedback I received on those essays, I judged that they were providing insight into the topics they covered, and readers found their content useful. It struck me, however, that in a magazine they were somewhat ephemeral, and that collecting them in book form could be a useful contribution to the wider wine literature. This book is the resulting anthology, together with six new essays, including the one that follows.
It’s the subtitle, A Geologist Wanders through the World of Wine, that really indicates the book’s nature. The content deals with an eclectic range of topics—the “wandering”—but all seen through the eye of a geologist. I like to think that, unlike my earlier wine book, which was more of a reference manual, this text allows readers to dip in and read, as each chapter is self-contained.
The first chapter outlines some basic ideas of how vines interact with vineyard soils and what this may mean for the wine in our glass. The second chapter recalls the long and little-told story of how we reached our present scientific understanding of photosynthesis. Several chapters discuss the cultural and wine significance of geological materials, such as schist, tuff, granite, flint, and limestone, while some are more concerned with places, such as the Loire Valley. Some approach broader issues, including the matter of metaphors, as in the excerpt provided here. And to underline the wide-ranging nature of the essays in the book, the final chapter examines how geology is handled in the worlds of whiskey and beer. The contrast may be surprising.
–Alex Maltman
We’re all aware of the difficulties of putting our taste sensations into words and that we often have to resort to using metaphors. And without even thinking about it, we know that just as “a heart of gold” and “a couch potato” aren’t literal, neither are, for instance, plummy- or leathery-tasting wines. The wines don’t actually involve plums or leather, obviously. Except, it seems, where tasters use “earthy” terms, such as mineral. It’s then so tempting to cling on to that implied connection with the soil.
It occurs to me that we use wine-taste metaphors in three different ways. In the first, we make a comparison between the wine and something else, where, although we may not know it, science has found that there happen to be flavor compounds in common. I don’t imagine that most tasters sensing, say, blackcurrant notes in a Cabernet Sauvignon are aware that exactly the same compounds producing that taste in blackcurrants can be in wine. It’s still a metaphor, but it has a tangible basis; there is a demonstrable commonality. The black pepper taste of Shiraz and the compound rotundone, found in black pepper, is another example.
Second are the more generally used metaphors where, as far as we know at present, there are no such direct links but useful recollections of other flavor experiences. Tasting a Barolo wine may remind us of a forest floor, a Marlborough Sauvignon Blanc of grass clippings.
Earthy-sounding examples of this include referring to the smell of warm or wet stones. We are all familiar with the odor of a shower of rain falling after a dry spell, and it’s a useful flavor comparator—but it has nothing to do with geology. It makes no difference whether the rain is falling on this or that rock, concrete, tarmac, or whatever, because the smell is due to organic compounds.
Any stonelike surface exposed to the air soon becomes veneered with all manner of bacteria, algae, molds, lipids, and the like, which are all around us. Some settle in the tiny depressions of the porous surface, and quite literally—it has been filmed—a falling raindrop can displace the compounds into the air as an aerosol, producing the well-known odor. It has been labeled petrichor.
Similarly, tilled earth and damp cellars produce familiar smells—known as geosmin—also of organic substances that emanate from certain bacteria and molds. (Some of these have astonishingly low sensory thresholds, down to a few parts per trillion.) We may smell them when soil is being tilled, and we recall this when describing a taste sensation in wine. They are metaphors.
The third way is a variation that’s intriguing to me, because it’s where most of the “earthy” metaphors fall. Here, we make the same kinds of comparisons, but with something that itself has no flavor—no taste or smell—or with something that we have no experience of. How can this work? I can only think that some constructive imagination must be involved—what the taste of a rock or mineral would be like if it had one. Here are some illustrations of what I mean by this third group.
Above my head as I write this, here in my Welsh farmhouse, is a roof covered with slate. It was put there nearly 200 years ago and is still almost as good as new—slate is such a good roofing material. Why so? One reason is its characteristic property of splitting cleanly into thin but robust sheets. Slate is a rock, a collection of several different minerals bonded together, and here they are arranged in such a way as to allow this useful splitting. The second reason is its durability and, in particular, its being waterproof. It doesn’t react with water; it doesn’t dissolve. That’s a pretty important property given the damp Welsh climate, but it’s also a critical matter for us here talking about wine.
It means that, no matter how much slate there is in a vineyard soil, it simply cannot get into the vine. Roots can absorb only dissolved things (apart sometimes from elements in active mycorrhizal fungi), and slate is wholly insoluble. So the idea that vines take up slate from slaty vineyard soil, and then transmit it to the grape juice to end up in the wine, patently cannot be true.
This insolubility also means that slate, like other rocks, has no taste. It’s easily tested if you have access to a rock saw and some different kinds of rock. Smooth, freshly sawn surfaces will give a cool, tactile sensation on your tongue, but they will have no aroma or taste. If you lick and smell them blindfolded, you won’t be able to tell the rocks apart.
In slate-dominated areas, such as Germany’s Mosel Valley, it’s easy to believe that all that slate must influence the wine, and almost certainly it does—indirectly. Things like the free drainage, light reflectance, heat retention, and nutrient availability may all play a part in the grape formation and possibly a wine’s character, but I am talking here about the flavor of wine in the glass and the notion that the slate itself can be detected in the wine.
Our tastebuds can deal only with liquids. Certain solids may be sufficiently soluble for our saliva to almost instantaneously begin dissolving them and transporting them to our taste receptors—sugar and salt are familiar examples. But slate is insoluble. Even if somehow slate in a vineyard soil did magic itself into a wine glass, then we would see it, and the idea of little specks floating in the wine is manifestly absurd.
So, our reporting a taste perception as slate must therefore involve some mental inventiveness. We may think of slate as a cold material, and we may associate a sharpness with its splitting into sheets, such that a slaty taste is almost always described from cool-climate, usually white, wines. And if it’s a Mosel wine, and we know of the importance of slate in the region, it becomes easy to conjure up the slate descriptor. Could we be inventing a sense of what slate ought to taste like?
These days, it’s fashionable in wine-tasting notes to specify an actual mineral. Let’s take as an example the mineral talc, as in “The wine shows strong talc notes.” But just as with the minerals that come together to form the rock slate, almost all individual minerals, including talc, are insoluble and hence have no taste. (The only significant exception is salt—sodium chloride—the mineral halite.) So, again, using these mineral names as descriptors must involve mental creativity, but it can get quite confusing. For one thing, the abstract picture in one taster’s mind might not correspond with that of another.
Talc is a mineral found in some highly magnesium-rich rocks. Its characteristic feature is a unique softness: it’s the softest mineral known. Thus, mentioning talc as a mouthfeel is understandable, but what about the more common use of talc in tasting notes, as an aroma?
Just as things have to dissolve to have taste, so they have to vaporize to have a smell. Aromatic solids or liquids turn to vapor easily, but geological minerals like talc simply do not. They have no smell (odor or aroma). However, there’s a complication with talc. Its softness is, of course, utilized to produce the well-known toiletry talcum powder, and this usually has had perfume of one sort or another added to it. So, as I see it, phrases like “The wine opens with aromas of talc” are alluding to the toiletry and its perfume, and not to the mineral at all.
It’s a rather parallel situation with graphite, the crystalline form of carbon found in some rocks. It also is soft and has a greasy feel, such that it’s used as a mechanical lubricant. So we might expect that mentions of graphite in tasting notes would be referring to mouthfeel, but they usually allude to taste or aroma, as in “The soils provide a graphite taste,” even though graphite doesn’t dissolve or vaporize.
The complication here, of course, is that graphite is also used for pencil lead. The soft, smudgy graphite is enclosed in wood, and this usually happens to be strongly aromatic. Cedarwood is traditional, and its distinctive sweet notes can often be apparent in wines such as oak-aged Cabernets. But surely this should be noted as a cedar aroma, or perhaps pencil shavings, rather than graphite? It seems to me that a tasting note would be more precise, and therefore more helpful in painting a picture of a wine, if it said “perfume of talcum powder” or “aromas of cedarwood” rather than talc or graphite. But then that charismatic allusion to a mineral, and beyond that the vineyard soil, would be lost.
For some commentators, chalk in a wine is an odor (“the chalk aroma of a Ribolla Gialla”), for others a taste (“the chalklike taste of fine Chablis”). Both, again, must be creative metaphors, seeing as chalk is a rock, wholly tasteless and odorless. Geologically, it’s a particular kind of limestone—and hence made of the mineral calcite, calcium carbonate—that’s dominated by the fossilized remains of a certain, very tiny, marine organism.
Most often, talking of chalk in a wine is referring to a grainy texture, a mouthfeel, perhaps in line with a perception of chalk as a dusty powder. Chalk, however, is not necessarily dusty. Some of the best-known natural occurrences of chalk are distinctly hard. Think of England’s White Cliffs of Dover or Beachy Head. For that matter, neither is it necessarily calcium carbonate.
I’m guessing that the dusty image comes, at least for those of a certain age, from old classrooms and their chalkboards, but blackboard chalk is calcium sulfate, the mineral gypsum. The famous chalk dust of disputed line-calls in tennis is titanium dioxide, the mineral rutile. In other words, a chalkiness in wine is an imagined mental association, with no obvious relationship with vineyard geology.
Now, Chablis and iodine. Most wine enthusiasts know that the bedrocks at Chablis formed on an ancient seafloor and contain striking fossilized seashells. It all puts you in mind of the sea! Consequently, things maritime often appear in tasting notes on Chablis, such as an iodine note. For example, “that exclusively Chablis note of iodine.” And, naturally, it’s usually related to the soils: “Fossilized oyster shells give Chablis its unique iodine flavor.” So is this metaphorical, or perhaps here something more literal?
One complication is that few of us have actually smelled, let alone tasted, iodine. It is an element that can be made to vaporize so it does have a smell, which the books say is repellent and toxic. What most of us think of as iodine is actually a medicinal tincture, with a strong smell of some organic solvent, such as alcohol. It isn’t the aroma of iodine.
Iodine is a well-characterized chemical element that can be measured, so some years back a student (Li-Ming Tan) and I analyzed iodine in soil and vine-leaf samples from grand cru sites, together with finished Chablis wines. Then, for comparison, the same down the road, with soils and Chardonnay vines growing on Burgundy’s hill of Corton. We also, just for fun, analyzed some supermarket Chardonnay wines from Argentina’s Mendoza and Australia’s Barossa. All the analyzed values for iodine were absolutely minuscule. But what was striking—okay, surprising—was that the very lowest values came from the Chablis samples and the highest from the Barossa. Go figure!
And then there’s the big one. Apparently the most commonly used wine descriptor these days: the conundrum that is minerality. What is it about this topic that continues to fascinate? Maybe it’s to do with a melding of the pragmatic usefulness of the word, evocative of the much-loved link between wine and the land, with the continuing lack of consensus on what the term actually means. Minerality can mean what you want it to mean!
For some, “minerality” may be unknowingly triggered by something in the wine, sulfur compounds or acidity, perhaps, and hence an example of my second type of metaphor. For others, it’s in my third group, an allusion to actual geological minerals even though they have no taste or smell.
The word appeared just a few decades ago, suddenly, like a new meteor in the night sky, and the wine world noticed. Wines were soon no longer described as steely, lean, and austere, but mineral. At first, it was restricted to cool-climate white wines, like Chablis and German Rieslings, but soon, anything.
But given the blur surrounding the term, it’s hardly surprising that science has found it challenging to identify what in the wine triggers the sensation.
I’m not aware of anyone today disputing that minerality must be metaphorical, but, even so, it seems that, because of the very word itself, making some connection with the ground is irresistible. Thus, mention of minerality is often accompanied by something about the soils—the presence of stones, for example, or deep roots. Stones exist because they have resisted weathering—they are inert. They affect the physical properties of the soil, but they can’t give anything to the vine roots. Deep roots are seeking water; there are few available nutrients down in the subsoil (by definition, little weathered), and there’s certainly nothing magical down there.
Comparisons are sometimes made between wine minerality and the taste of bottled water. Most such water, however, has been drawn directly from the ground, where normally it will have been held for long periods (the average residence time in the UK is over a century) and in immediate contact with the host aquifer. So anything that’s remotely soluble is taken in by the water, unlike the selective, regulated uptake of nutrient elements by vine roots.
Consequently, the mineral concentrations in bottled water, which obviously lacks the powerful aromatic compounds that give wine its flavor, are typically greater than those found in wine. And crucially, unlike in wine, they can include plentiful chloride, sulfate, carbonate, and bicarbonate, all of which demonstrably contribute to taste and mouthfeel.
In the wine world, however, minerality and similar “geological” words used in tasting remain nebulous. It’s almost as though they have taken on a life of their own! None of this is to suggest it is mistaken to use geological words in flavor descriptions, or that it is unscientific. Indeed, science likes to employ metaphors: the tree of life, soil health, the greenhouse effect, genetic fingerprinting, et cetera.
But we have to be mindful that, with minerality and the like, we are not just making comparisons; we are imagining things, mentally conjuring what rocks and minerals ought to taste like. It’s not wrong. After all, we need all the words we can get in our tasting lexicon. Putting our taste perceptions into words: that was never simple.
This excerpt first appeared in Taste the Limestone, Smell the Slate, written by Alex Maltman and published by Académie du Vin Library in April 2025. It has been minimally edited for style, length, and audience. Used with permission.
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Awesome read!