What do the wines of the Finger Lakes have in common with the wines of Pomerol in Bordeaux, Swartland in South Africa, Luján de Cuyo in Argentina, the Clare Valley of Australia, the Mosel of Germany, the Douro of Portugal and Savennières in the Loire? The vineyard soils of all these regions are derived from mud. Yes, that sticky substance that clings to our boots when it rains provides the underpinning of all these storied vineyard areas and many more as well.
Mud is an accumulation of fine-grained particles of clay and silt that will aggregate when moistened. We have probably all individually experienced stepping into a deeper-than-expected mud accumulation and sinking in up to our shoe tops. It’s hard to imagine such a sloppy mixture as the source of so many elegant wines. So, how does this metamorphosis from mundane to marvelous occur?
It is axiomatic that the best vineyard soils are noted for having a fine balance between water-draining and water-retaining properties. Vitis vinifera vines tend not to produce great grapes when they have their “feet wet.” Excessive water can lead to excessive leaf growth and large quantities of not particularly flavorful grapes. Many of the best vineyard sites allow most water to drain away but retain just enough so that the vine roots have access to water during a dry summer. The entity for water retention is, generally speaking, mud. More specifically, it is clay minerals like illite, kaolinite, smectite and chlorite that create these essential mini-reservoirs. Clay minerals fall within a category called phyllosilicates – minerals with a structural form of parallel sheets. They exist because of weathering – the slow disintegration of rocks as they are exposed to the physical and chemical forces on the earth’s surface.
Clay minerals have a microcrystalline form that resembles a stack of platelets. The platelet surfaces attract water molecules, which can double the size of the clay molecule or more. That makes these expansive clay minerals great as a provider of water for vine roots. They are also a nightmare for builders of houses, highways and other rigid objects, since the clays swell and crack foundations and roadbeds.
Our mud metamorphosis is happening right now. Rocks are forming in a slow but inexorable process. Clay washed from the mountains of western Montana is carried to Three Forks where the Madison, Gallatin and Jefferson rivers join to form the Missouri River which then joins the Mississippi near St. Louis. Waterborne for more than two and a half thousand miles those clay particles may now be settling to the bottom of the Gulf of Mexico at the end of the Mississippi Delta. Then, every hour of every day as long as the mighty river flows, those clay particles are then buried under more of the same.
That is how the rocks we know as shale, slate and schist begin their formation. It’s a slow process, but it’s the relentless nature of the action that is most impressive. If only one millimeter of clay per day is deposited, that will accumulate to over a foot of mud in a year. Over a one thousand year time span – a blink of an eye in geologic time – twelve hundred feet of clay will be deposited. That’s about the height of the Empire State building in New York. One can understand the compressive forces that such a massive pile of mud will exert on sediments below.
Over millions of years, then, the overburden pressures squeeze out the water and compress the mud into rocks, which we call mudstones or shales depending on their physical properties. The pressures are enormous, and the clay and silt particles tend to arrange themselves into layers or laminations which causes the resultant rock to split into layers, a property known to geologists as fissility. Shales comprise about half of all the sedimentary rocks exposed on the earth’s surface.
Sometimes our now transformed mud gets buried even more deeply and subjected to ever greater pressures and temperatures. When the original shales are subjected to these forces, they slowly begin to recrystallize and are metamorphosed into slate. Slate has the property of slaty cleavage – it splits easily into large plates. That makes it an excellent source for chalkboards, roofing materials, and decorative stones for flooring, sidewalks and the like. Although usually gray in color, slate can have blue, green, black or red hues depending on trace minerals within the original mud.
If slate undergoes further increases in pressure and temperature, the recrystallization continues. The crystals form a visible sheen on the rock, which is now called phyllite. Phyllite is a foliated rock. The term comes from the Latin folia, meaning “leaf” and refers to the sheetlike character of the rock. When we increase the pressure and temperature even more, the recrystallization continues and we form a rock with larger crystals that retains the foliated form. Our rock has now metamorphosed into schist. Further increases in heat and pressure result in coarser crystals and more visible bands in the rock which is now called gneiss. Ultimately, with enough heat and pressure, it may melt entirely and form a crystalline igneous rock known as plagiogranite with no trace of its foliated sedimentary origins.
When the metamorphosed slates and schists are pushed to the surface of the earth, they are unstable because their crystals were formed under completely different conditions than those on the earth’s surface. Thus, they are subject to chemical weathering that releases the clay minerals once more and starts the multi-million year process all over again. This is the rock cycle.
The transition of rocks from shale to slate to schist and beyond is a continuous spectrum. Thus, there are different definitions of the rocks even among geologists. A lightly metamorphosed slate for one observer might be called a highly metamorphosed shale by another. For vineyard owners, the terminology varies not just with the observer, but also with the traditional descriptions used in the region.
What does all this mean to a wine enthusiast? Many vineyard areas are dependent on clay minerals not only for their existence but also for their distinctive style. The soils the Haut-Médoc in Bordeaux become more clay-rich as one moves downstream. St. Estèphe vineyard soils contain more clay than those of Pauillac, St. Julien or Margaux. The wines of St. Estèphe reflect the cooler temperature profile of clay as well as the greater water retention with the powerful and ageable wines of the commune like Châteaux Montrose and Calon-Ségur. As we move north from St. Estèphe, the clay component becomes too great to yield wines of Cru Classé quality, and we see a considerable production of less exalted wines under the Médoc appellation. On the right bank of Bordeaux, Pomerol is especially noted for its clay-rich soils. The Merlot grape thrives in clay soils and the wines of Pomerol are evidence of this happy marriage.
In the Devonian Period, some 380 million years ago, thousands of feet of mud were eroded from an ancient North American mountain range and deposited in the nearby sea. Today, they are the thick sequences of shale beds that form the soils of the Finger Lakes AVA. Elsewhere in the US, clays derived from volcanic rocks form the distinctive red soils of the Dundee Hills and Chehalem Mountains of Oregon. Clay soils occur frequently in the Napa Valley, especially in Carneros. Further south, shales of the Monterey Formation have weathered to form clay-rich soils in Paso Robles.
There are many areas of the world where shale, slate, and schist soils are exalted as the reason for superior vineyard sites. In South Africa, most of the broad-ranging vineyards of Swartland rest upon the ancient Malmesbury Shale. Slate soils are famously lauded in the Mosel, Rheingau and Nahe of Germany. Additionally, Austria’s Wachau and Kamptal are noted for pockets of slate-based soil. Spain’s Priorat is also famous for its terraced vineyards in slate soils.
The Douro Valley of Portugal, the home of Port is famous for its hard schist soil and terraced vineyards. In France, schist-bases soils provide a home for Syrah grapes at Côte Rôtie in the Rhône and Chenin Blanc grapes at Savennières in the Loire. In Languedoc-Roussillon, growers have established a group called the “Terroirs de Schiste” for estates in Faugères, St. Chinian, Collioure and surrounds with schist soils.
Our pedestrian mud and its metamorphosed forms clearly support fine wines of all sorts and styles. The many vineyard sites share the life-giving, water retention properties of the clay minerals that compose their respective soils. So, the next time you are savoring a fine Bordeaux or Mosel Riesling, raise your glass to mud as well.