Part 2: Vineyard Geology

The first part of this article outlined some of the possible interactions between vines and vineyard soils. It indicated that the latter consist of a physical framework (with more or less pore space and organic matter) that is of geological origin. It also illustrated how geology figures prominently in the wine world. Whether or not it is justified, it is almost as though writers like to mention mineral and rock names, geological time periods, and the like. So this second part of the article explains some of these geological matters that frequently appear in the world of wine. 

Minerals

All matter consists of chemical elements and in minerals they are systematically bonded together to make rigid particles. By far the two most abundant elements in the Earth’s crust are oxygen (46%) and silicon (28%). Consequently the most common minerals are various silicates, in which these two elements are linked together, with nothing else involved in the case of the mineral quartz (SiO₂) but usually with a number of other elements also. 

However, some minerals relevant to vineyard soils are non-silicates. A few are composed of a single element. Graphite, for example, is composed solely of carbon. It helps make the soils at Priorat in Spain and Styria in Austria and is largely responsible for the dark color of rocks such as shales. Although sulfur is widespread in vineyard soils through fungicide applications it occurs naturally in many young volcanic soils, such as at Vulture, Vesuvius and Etna in S Italy.  In fact, around Tufo in Campania (home of Greco di Tufo white wine) sulfur was commercially mined until recently.

Non-silicate compounds include the oxides (e.g.: manganese oxide, the mineral pyrolusite, at Moulin-a-Vent, Beaujolais, France; aluminum oxide (bauxite) at Coteaux de Baux-de-Provence, France, and Pemberton, W Australia; hematite, the iron oxide giving the distinctive red color of  terra rossa, found in areas such as Istria, Croatia, and Coonawarra, Australia); and sulfates (such as gypsum (calcium sulfate) found in the soils at Ribera del Duero, Spain, and in W Colorado). But especially important are the carbonates, and above all calcium carbonate, the mineral calcite. Materials that are composed of this mineral are referred to as being calcareous, and this applies to a host of vineyard rocks and soils, including marble, travertine, tufa, marl, and the various limestones (see below). For historical rather than any particular scientific reasons, vineyards sited on calcareous rocks and soils have acquired a particular cachet and so such materials appear frequently in the wine literature.

Iron-rich clay loam derived from limestone: Terra rossa. S Istria, Croatia.

The silicate minerals are complex and numerous. A few examples follow. Olivine is a dark-colored iron-magnesium silicate common in the rock basalt. It is why, for instance, the vineyard soils next to Lake Balaton in Hungary are almost black. The family of pyroxene minerals includes augite and the amphibole family includes hornblende, both of which contain various permutations of iron, magnesium, calcium, etc. All these mineral names appear on wine labels and in vineyard designations, and help form a variety of igneous and metamorphic rocks (see later). Examples are found at Heathcote, Victoria, and Barbera di Colli Tortonesi in the Piemonte region of Italy.

The family of mica minerals include silvery muscovite, giving the sparkling effects seen at, say, Poncie in Fleurie, France, and shiny black biotite, found in some Alsace vineyards, such as the Grand Cru Brand, just outside Turckheim, Alsace. One Vinho Verde (Portugal) is actually labelled “Biotite”, after the vineyard soil.

Serpentine is a magnesium silicate and is unusual in that it can be inimical to grapevines. Its preponderance of magnesium can curb potassium uptake, leading to stunted growth. Moreover, serpentine is commonly associated with minerals rich in nickel and chromium which can exacerbate these effects. Consequently, in parts of Sonoma and Lake counties, California, the planting of vines is avoided.

As we have seen, the word clay is often used to denote a particularly fine grain size in soils. But it is also the name of a group of flaky silicate minerals that are especially varied and complex, and crucial in vineyard soils. In line with the grain-size usage, individual flakes of clay minerals are very tiny, .002 millimetre (two thousandths of a millimetre) at most, which means we cannot discern them by eye. And it is why clays can choke the pores in the soil and curb drainage. Moreover, some clay minerals expand when wet, adding to the clogging effect but also offering water storage capabilities. In other words, clays are fundamental to the drainage behaviour of vineyard soils.

But also, each tiny clay flake presents an enormous surface area for its size. And because of the way the mineral is constructed the clay surfaces are commonly very reactive. This is the basis (together with humus) of the much-mentioned cation exchange capacity (CEC), in other words, the fertility of a soil (or lack of it, if clays are sparse). Many vineyards soils are famously barren-looking and do not have much in the way of humus, so their fertility - and all that means for vine growth and grape production - is very largely governed by the CEC of the sparse clay minerals in the soil.

The clay mineral with the highest CEC and greatest propensity to affect soil drainage is montmorillonite, named for the town in western France. When swollen with water it makes soils “thicker” or “heavier”.  Some writers maintain that such soils lead to fuller, heavier wines but this is largely anecdotal. Most montmorillonite originates from the weathering of igneous rocks such as basalt, and related volcanic deposits. For example, the young volcanic rocks of the Tokaj-Hegyalja region, Hungary, are currently weathering into montmorillonite minerals, which, in addition to helping from some of the best vineyard sites, are being quarried for their industrial value.

Volcanically derived material can be reworked and re-deposited to form a variety of other rocks, including sedimentary rocks. Also, some limestone areas are covered with montmorillonite-bearing soils. Because limestone is notorious for poor water-holding these soils can come to the rescue in vineyards. In the area around Cognac, France, for example, the bedrock is a very dry limestone but sufficient water for the vines is usually held by the overlying montmorillonite-rich soils. Some assert that the special-ness of the Romanée-Conti vineyard in Burgundy owes a great deal to just the right proportion of montmorillonite in its soils

Other clay minerals include vermiculite, also with a high CEC and important in the glacial soils of the Finger Lakes vineyards of New York, and kaolinite, with a low CEC but abundant  in soils forming from the weathering of granite, such as at Darling, north of Cape Town, South Africa. In parts of Sonoma County, California, kaolinite soils weathering from volcanic rocks are preferred because of their good drainage and, because of the low CEC, restricted vigor. The most widespread clay mineral is probably illite, but it is of relatively little importance in vineyards because of its restricted swelling capacity and low CEC.

The iron-magnesium mineral chlorite is similar to the clay minerals in some ways, although restricted in CEC and water-swelling. However, it is an important constituent of rocks like slate and schist, and so occurs in vineyards wherever these form the bedrock.  Examples would be NE Corsica, in the vineyard soils of the Rheingau and Mosel, Germany, in the Banyuls and Durban, Corbieres (France) areas, and at Kastelberg, on the outskirts of Andlau, Alsace.  

Grey slate. Cederberg Vineyards, Cederberg Mountains, Coastal Region, South Africa.

Feldspar is the name of a large group of very important rock-forming minerals. It is especially important for igneous rocks - especially granite – and hence the kaolinite-rich soils derived from them. Thus vineyard soils in areas such as Dão, Portugal, or Temecula, California, are pale pink due to the feldspar weathering from the granite bedrock below.

Perhaps the best known mineral of all is quartz. The name pops up in a surprising array of contexts including the names of vineyards and wines, and in tasting notes. It is a common mineral, being composed of the two most common chemical elements in the Earth’s crust, and it is a tough, durable, and eye-catching and so is often noticed. Also, being simply silicon dioxide, SiO₂, often referred to as silica, it can assume a wide variety of forms (e.g. chert, agate and jasper) and just tiny amounts of impurities are enough to produce strikingly coloured variants (e.g. amethyst, rose quartz, carnelian). All are varieties of silica.

Quartz is a common vein-forming mineral in rocks, so strikingly irregular blebs and zones of milky-white material crossing bedrock may well be made of quartz. Because of its resistance to weathering, quartz is also a common constituent of sediments, such as screes, river gravel and beach sand.  A dense, gray, rather opaque form of silica is known as flint. It is well known in some of the Loire vineyards, in France. Although the term flint frequently appears in tasting notes, like all forms of silica the material is actually odorless and tasteless – the very reason that silica is used to make wine glasses and bottles. The word silex also appears in wine writings in English although it is simply the French word for flint.

Rocks

Though minerals are fundamental to vineyard soils and possibly visible as tiny fragments, more noticeable in the soil will be larger earthy pieces – stones, cobbles even – and these almost certainly will consist of several minerals tightly bonded together. Such a coherent, solid aggregate of minerals is termed a rock. Bedrock, therefore, is a unified, bonded assemblage of minerals. Rocks are classified into three groups according to their origin.

Igneous rocks

Igneous rocks form by solidifying from a melt. The process is easy to visualise with molten lava pouring out of a volcano, cooling as it flows and gradually becoming more viscous until it finally solidifies – to give an igneous rock. Together with the various fragmented material that may billow from a volcano, these rocks are also known as volcanic igneous rocks. The most common rock formed in this way is basalt. It weathers readily to the high-CEC clay mineral montmorillonite and so gives rise to fertile soils. Numerous vineyards around the world are located on basalt: Somlo Hill in Hungary, Madeira, and the Canary Islands are just a few examples. The galette-like (see below) pebbles at Cayuse in Walla Walla Valley are composed of basalt.

Similar kinds of processes also happen underground, out of sight. Parts of the Earth’s crust are hot enough for the rocks to exist in a molten state. With time such melts try to move upwards, intruding the progressively cooler rocks above until the mass finally solidifies before reaching the surface. These are known as intrusive igneous rocks. With further passing of time, the solidified material may be forced further upwards at the same time as erosion is removing the rocks at the surface above. So eventually the mass may itself become exposed at the Earth’s surface. The intrusive rock, although formed deep below ground, is now visible at the Earth’s surface and is available for weathering and soil formation. Granite is the most important intrusive igneous rock. It makes the soils at, for example, Cornas, and in areas around Stellenbosch, South Africa.

Sedimentary rocks

As soon as igneous rock are exposed at the Earth’s surface, be they formed from a lava or deep underground, they are subjected to weathering and erosion. Their surface parts become progressively broken up into loose particles. Some will be fragments of the igneous rock, some may be individual mineral grains that were helping form the parent rock, some will be beginning their transformation into weathering products such as clays. In vineyards we would call such material soil but in the wider geological context it is called sediment.

Through the actions of gravity and running water, ice, wind etc., the sediment becomes moved across the land surface, much of it eventually finding its way out to sea. At any stage, and especially if the material gets as far as the sea- bottom, the sediment may reside long enough for the particles to become bonded together into a coherent mass. This solid aggregate of deposited particles is termed a sedimentary rock.

The constituent grains may be individual minerals or they may be fragments of pre-existing rocks. Those parents could have been any kind of rock, not just igneous. Consequently, sedimentary rocks are varied in appearance. Through time additional sediment, probably differing slightly in nature, is swept in and so the bedding or stratification characteristic of sedimentary rocks is built up (figure 3). As with igneous rocks, the Earth’s internal forces may in time uplift this submarine material to form dry land, whereupon weathering and erosion again commences and the whole process starts over again.

The names for these varied sedimentary rocks depends on the size of the constituent fragments, exactly in line with the grain-size terms for soils. Thus we have claystone or mudstone where the particles are the tiny size of clay minerals, then siltstone and sandstone as the grain size increases. Above this, we talk of conglomerates if the fragments are pebble size and reasonably smooth; breccia where they are angular. Where mudstone tends to splinter along uneven weaknesses it is the soft, easily eroded rock called shale. From the latin for clay (argilla) comes the prefix argillaceous, in French argilo, added to rock names where there is a significant clay content. In particular circumstances sediments can be deposited that have mixed grain sizes, say, sandy particles mixed up with clays. The resulting rock is called greywacke. All these rocks are widespread in the world’s vineyards and some have their names on wine labels.

In warm, shallow seawater, organisms can grow which extract calcium carbonate from the water to make skeletons and shells. In such situations, the calcium carbonate may even be precipitated directly from the water to make an ooze of the mineral calcite. Where such materials are transformed into solid rock we have the rock limestone. It is rarely composed of pure calcite but usually has some clay content, hence an argillaceous limestone. A substantial clay content makes the rock a marl. In turn this can grade into a calcareous clay. Around 70-90 million years ago, during the time known to geologists as the Cretaceous, in northern Europe the seas were exceptionally rich in a particular kind of microscopic algae with calcite skeletons. Their remains accumulated on the seafloor in untold quantities to produce an unusual limestone called chalk, celebrated in some vineyards in Champagne and southern England.

Limestone is revered by some wine enthusiasts and some even speak of the “magic” of limestone. Apart from usually offering good drainage, however, it is not clear what this might be. Limestone brings no special ingredients to vineyards and can, through nutrient deficiencies, be problematic. Much of the apparent mythology may come from various classic French vineyards just happening to be on calcareous soils, such as parts of Champagne, Chablis and the Côte de Beaune, Burgundy. Note that these are all white wine areas. Limestone is generally deficient in iron, which is needed in the pigmentation of red grapes.

Rust en Vrede vineyard, Heldeberg Mountain, Stellenbosch, South Africa. The vineyard soils are derived partly from unseen underlying granite and partly from the sandstones that make the towering cliffs in the background. Note the distinct stratification of these sedimentary rocks, here very slightly inclined to the left (north).

Metamorphic rocks

In some situations, underground igneous or sedimentary rocks are not uplifted but buried further, say beneath yet more sediment accumulating above them. As they find themselves deeper underground they will be progressively warmed, as with the igneous rocks because of the Earth’s internal heat. As the temperature slowly grows and the rock mass experiences more and more pressure because of the increasing load of material above, it is prompted to undergo a variety of chemical and physical changes. Internal rearrangements of the chemistry and minerals occur, leading to metamorphic rocks.  At any stage the progressive burial may cease and the metamorphic changes arrested. Then, a combination of forces uplifting the mass and erosion at surface can cause the material to be exhumed.

The internal changes at depth commonly imbue a planar aspect to the rock. It can be in any orientation. Where it is not possible to discern the individual minerals doing this, but where there is a distinct tendency to split along these secondary planes, the rock is known as a slate. If the minerals are visible – though typically the clean-splitting tendency is less marked – it is called a schist. If the secondary planes consist of bands of different minerals it is termed a gneiss.  

Other metamophic rocks have no such secondary planes, and here the rock name depends on the dominant constituent mineral. Amphibolite, important in parts of the Muscadet region in France, is composed of the mineral amphibole. Marble is made of calcite and so is the metamorphic equivalent of limestone. Quartzite is made of quartz, commonly developing from precursor quartz-rich sandstones. It is a tough rock; erosion of it has made the famous rounded pebbles – in French, the galettes  – of Chateauneuf-du Pape, and other places such as near Boutenac, in Corbieres, France.

Quartzite cobbles or galettes, derived from conglomerate bedrock. Villemajou, Corbieres, France.

A word on bedrock, soils, stones, etc.

Before we leave this technical terminology, let’s be clear about some basic words. All vineyards are founded on bedrock, overlain to various degrees by loose soil, perhaps containing fragments of rock, potentially in all sorts of shapes and sizes. In hilly areas the bedrock itself may be visible, in hillsides or crags protruding up through the overlying loose material, in what geologists call outcrops of bedrock. This bedrock is actually the very outermost part of the solid Earth itself.

Soil is derived through the fragmentation of bedrock. Where the rock fragments are easily visible we usually call them stones. Actually, the word is little used in Geology because it lacks precision. If we pick up a stone it’s more than likely to be a piece of rock, but it could be a chunk of a mineral, a piece of quartz, say. Or it could be two or three pieces of mineral joined together. (There have to be many mineral pieces before we can start calling it a rock.) And if the stone is reasonably smooth and a few centimetres across we may call it a pebble.

If it’s smaller than this then we’ll be talking about gravel, sand, etc. and if it’s bigger then we’re into cobbles and even boulders. Sometimes we see in mountainous vineyards, such as at Elqui, Chile, just the tops of enormous boulders. Despite their size they are nevertheless fragments; they are loose, detached from the bedrock.

Finally, let us consider the frequent claims in wine writing that a particular vineyard bedrock is “mineral-rich”. What does this statement mean? As we have seen, all rocks are made of minerals, not some more than others. So maybe it means rich in nutrient minerals? That’s the same as saying fertile, and rocks themselves cannot be fertile. Of course, some rocks may weather more easily, yielding clays with a high CEC, and hence giving nutrient-rich soil. However, the “mineral-rich” claim is usually made in the context of “minerality” in the wine, which is not normally associated with high-vigor wines growing on fertile soils. However it is looked at, the claim has no meaning.

Geological time and fossils

Wine writings often mention periods of geological time. A well known example is the controversy over whether or not true Chablis can be produced from Portlandian as well as Kimmeridgian soils. However, these two terms refer to geological time intervals and not to the actual nature of the material, what geologists call its lithology. During Kimmeridgian and Portlandian times (each lasting for five million years or so, either side of about 140 million years ago) the kinds of geological material being produced would vary through time and from place to place. So for the soil properties we have been discussing it is the lithology that is relevant, and not the time of their formation.

Yet it is common in wine writings to declare the geological age of the vineyard, despite it having little relevance to the vines. For example, the vineyards of the Wachau region of Austria are often vaunted because of their “Primary” age. This term, long obsolete in Geology anyway, means nothing unless the lithology is indicated and, better still, the relevant soil properties. Similarly, current efforts to sub-divide the Italian regions of Barolo and Barbaresco are distinguishing between vineyards sited on Langhian, Serravalian, Tortonian, and Messinian rocks. These are names for fine divisions of geological time, which convey little meaning for vines (and, it has to be said, are terms that many a geologist would struggle with!).

In addition, there is often confusion between the age of bedrock and the overlying soil. They are usually vastly different. The fragmentation and weathering processes that reduce bedrock to soil have been happening geologically very recently (and still are); in general the age of soils rarely exceeds a few thousands or, perhaps, tens of thousands of years. The Gimblett Gravels of New Zealand, for example, formed in a flood little more than a century ago!

In contrast, production of the actual bedrock, be it solidification from a rock-melt, sedimentation on the sea floor, etc., more than likely happened hundreds of millions of years or so ago. Many vineyards have bedrock a billion years or more in age, such as at Stellenbosch, South Africa, Western Australia, parts of the Middle Loire in France, and a number of U.S. states, including Virginia and the Texas Hill Country.

Sedimentary rocks formed in the last few hundred million years may contain fossils. In fact, as it happens, fossilised sea-shells catch the eye in a number of the world’s vineyards, both in the bedrock and as loose fragments in the soil debris. Chablis and Sancerre in France are well known examples, together with the Cederberg Mountains of South Africa and Central Hawkes Bay, New Zealand. However, although when we taste the wines resulting from such soils we may be reminded of things to do with the sea, for the vines fossils are indistinguishable from any other piece of rock.

Organisms soon vanish after death, apart sometimes from any hard parts such as shells or bones. Providing circumstances are right, these can become fossilised, either by internal rearrangements and replacements to give a durable mineral structure, or by dissolution to leave an imprint in the host rock. Either way, the fossil is a replica, normally with none of the original organism remaining. It is composed of exactly the same geological minerals that make rocks and stones (most commonly calcite and quartz). Hence fossils in a vineyard bring nothing different to the nutrition of the vines or the composition of the resulting wine.

So is there an ideal vineyard soil?

No. As we have seen, the basic properties of water behavior and nutrition have to be in place but soil properties interact with a whole matrix of other factors, especially those to do with climate. So a soil that seems superb in one place may well perform differently elsewhere. People make lists of the “the best wine soils” but they are based on particular, successful places and have no general application. Abundant claims are made for the importance of certain types of bedrock and soil for a particular grape varietal, for the character of an area’s wine, for wine quality, etc. etc., but they lack consistency. After all, rarely are the claimants subjected to the potentially humbling experience of a truly blind comparative tasting!

Pinot Noir, for example, obviously thrives in the thin, calcareous soils of the Côte d’Or, France, but it does also in the thick alluvium of Bio Bio, Chile, the schists of Otago, New Zealand, as well as both the basaltic and the sandy soils of the Willamette Valley, Oregon. Riesling probably conjures up to many the distinctly slaty soils of the Rhine and Mosel valleys in Germany. In fact some talk of the hallmark of Riesling wines being a taste of slate. But not far away, in Alsace, world-class Rieslings are produced from vineyards sited variously on sandstone, marly limestone, conglomerate, granite, volcanic rocks, and others. Even within France, the Gamay grape – so closely associated with granite of Beaujolais – also thrives on the volcanic soils of Chateaugay, Côtes d’Auvergne. Classic varietals like Cabernet Sauvignon from the gravels of Bordeaux and Chardonnay from the calcareous sedimentary rocks of Burgundy are finding successful new homes in the metamorphic soils of the Midi.

Regarding wine character, we all know what to expect from a bottle of, say, Muscadet or Beaujolais. But the geology of the Muscadet region includes a whole range of metamorphic and sedimentary rocks, as does Beaujolais, except that there is a lot of granite there as well. The Côte-Rôtie in France is classified and usually treated as an entity, producing wines that are consistently different from nearby districts, yet the hill has a diverse geology that includes alluvial gravels, schists, gneisses, and granite.

Conversely, the Kimmeridgian Formation is much lauded in wine writings because from it come the legendary French wines Pouilly-Fumé, Sancerre, Chablis, and some Champagne. However, the Kimmeridgian snakes all the way from the Mediterranean right across France to the English Channel (and on below it, to England and beyond, through the village of Kimmeridge). Yet for most of its course, the Kimmeridgian yields indifferent wines, or, most commonly, none at all. Clearly, other factors are dominating.

There are bold assertions on which soils give what qualities to wine, but they, too, are wildly inconsistent. Contrast “wines produced on limestone are delicate and elegant, with outstanding finesse” with “the region’s limestone soils give unmatched levels of luscious flavor that make this wine one of the world’s most opulent”. The granite at Dambach, Alsace, yields wines said to have “a beautiful elegance and very fine fruitiness” whereas wines produced on granite at Cornas attract descriptions such as “impenetrable”, “meaty”, “powerful, and “brutal”.

Such inconsistencies are encapsulated in the statements regarding which soils are supposed to give a mineral quality to wine: “the high granitic content of the soil in Elgin gives the wine its mineral finish”; “yellow ferricrete and white quartzite soils give a unique minerality”; “Kimmeridgian limestone is the source of the trademark minerality”; “gravel soils give our Rieslings great minerality”; “minerality in wine is most often associated with soils that were once upon a time immersed under saline seas”; “the Muscadet minerality originates from soils rich in amphibole”.

So, in conclusion, just how important is vineyard geology for wine? It is clear that vineyard soils play a major part in the performance of vines, especially through water and nutritional effects. But affecting character and flavour of the final wine? This is far less clear. There is certainly a plethora of subjective, anecdotal evidence. But as for a scientific basis for the continuing preoccupation of wine being impacted by vineyard geology, well, the jury is still out. 

-Alex Maltman, Professor, Institute of Geography and Earth Sciences, University of Wales at Aberystwyth, United Kingdom