Shades of Schistosity

Shale

As a truly committed student of wine, you probably know that shale is a type of soft, foliated sedimentary rock composed (at least in part) of clay minerals and (sometimes) volcanic ash. Shale has visible stratification and a tendency to break or split along “layers” (known as “planes of weakness” or “rock cleavage” in geo-speak). This tendency to split along planes is known as fissility (which is just such a fun word).

You might also know that shale is well-represented in the vineyards of the world, including the Finger Lakes AVA (in New York State) and the Santa Cruz Mountains AVA (in California). Other wine areas known for shale-influenced soils include the western side of Paso Robles, the Mayacamas Mountains (between Napa and Sonoma), Heiligenstein (Alsace), and Austria’s Wachau region.

Shale is fascinating on its own but there’s more to the story, as shale can be transformed into slate, schist, or gneiss. These three types of rock are produced via varying degrees of metamorphism—changes resulting from heat, pressure, and deformation—and they all have different appearances and characteristics. Some of these differences are discussed below:

Slate

Slate: Slate, formed from shale, is a finely grained rock that may be formed under relatively low temperature and pressure conditions (low-grade metamorphism). Slate tends to be one solid color in addition to being very hard and brittle; when broken, it will form flat, smooth surfaces. Germany has several vineyards areas celebrated for their slate soils; these include the Mosel and the Rheingau—both of which also have significant outcroppings of shale (now we know why). Other wine areas rich in slate include the Clare Valley, the Cebreros VCIG (in Castilla y León), and Chile’s Aconcagua Valley.

Schist: Schist is formed (from slate or mudstone) under moderate levels of heat and pressure (metamorphic forces). Schist is identifiable by its visible “grains” (in layered formation), dull luster, and schistosity—the layer-like foliation that is found in certain coarse-grained metamorphic rocks. Despite the fact that it reminds me of a wine-geeky, made-up word (like matchsticky or porch-pounder), schistosity is a real thing.

Schist

Several vineyard regions are regarded as rich in both slate and schist; these include Priorat (Spain) and the Douro Valley (of Portugal). Parts of Tuscany are known for galestro—a soil rich in both clay and schist. Other areas known to be rich in schist include Corbières, Côte-Rôtie, Kastelberg (Alsace), the Valais (Switzerland), Ribeira Sacra, and Savennières. These areas are often described as having schistous soils—although schistous is definitely a made-up/wine geek word and does not appear in the geological lexicon (Maltman: Vineyards, Rocks, and Soils, p. 103). Schistes, however, is a real word (in French)—there is even a wine association to prove it: L’Association des Terroirs de Schistes.

Gneiss

Gneiss: Given the right combination of intense heat, pressure, and (perhaps) chemical activity, schist can transform into gneiss. Gneiss has visible “bands” of various colors composed of various minerals (gneissose banding). Having been formed under intense metamorphic pressures, gneiss is much heavier and harder than slate and schist and does not typically break along its foliation planes.

Vineyard regions known for gneiss include the Pays Nantais, Margaret River, Wachau, Kamptal, and the Middleburg AVA in Virginia (USA).

• References/for more information:

• Feiring, Alice (2017). The Dirty Guide to Wine. New York: The Countryman Press.
• Franzmeier, Donald, William McFee, John Graveel, and Helmut Kohnke (2016). Soil Science Simplified, 5th edition. Long Grove, Illinois: Waveland Press.
• Maltman, Alex (2018). Vineyards, Rocks, & Soils: The Wine Lover’s Guide to Geology. New York, New York: Oxford University Press.
• Robinson, Jancis and Julia Harding: The Oxford Companion to Wine, 4th Edition. Oxford, 2015: The Oxford University Press.
• White, Robert (2009). Understanding Vineyard Soils. Oxford University Press.
• http://www.terroirsdeschistes.com/
• https://geology.com/rocks

The Bubbly Professor is “Miss Jane” Nickles of Austin, Texas… missjane@prodigy.net