To learn the mysteries of wine, we need to know geology. This simple observation can sustain doctoral dissertations, but was driven home to me recently in a 10-minute conversation with Roy Cloud, managing partner of the Washington importer Vintage 59.

Cloud was hosting a seasonal portfolio tasting at Johnny’s Half Shell near the Capitol; it was at the end of a week-long trip in which he chaperoned 16 winemakers through San Francisco and Chicago. He gave me a quick lesson in Sancerre, an appellation in the eastern Loire Valley that produces the world’s best sauvignon blanc.

Recommended Sancerre wines

Sancerre wines (Deb Lindsey/FOR THE WASHINGTON POST)

“The region is part of the Paris basin, an ancient sea bed that stretches from southwest of Paris through Chablis and Champagne and into southern England,” Cloud explained as he poured me a taste of Merlin-Cherrier 2010. It was delicious and round on the palate with a mineral note that echoed his geology discourse. “The cliffs of Dover are the same chalky limestone that gives such structure to these wines,” he said.

Within the Sancerre district, the soils break down into three distinct subtypes: Silex, or flinty soil, runs in a narrow band along a fault line that passes under the town of Sancerre; Kimmeridgian marl, also known as terres blanches because the chalky clay soil turns white in dry periods; and caillottes, or Oxfordian limestone, which has larger pieces of gravel due to erosion over the centuries. (The Oxfordian soil tends to be downslope from Kimmeridgian vineyards.)

Each of these soils imprints its own character on a wine’s flavor, so it is not enough to say that Sancerre’s white wines are “sauvignon blanc.” (Pinot noir grows in the region as well.) “Oxfordian soils give wines that are very broad, but they are delicate and perfumed,” Cloud said. “The Kimmeridgian soil yields fruitier, much more direct, pointed wines, aggressive in acidity and extremely age-worthy. Some of the top wines from a great year like 2010 can be laid down for 30 years.” Silex soils, he explained, yield brooding, flinty tasting wines that almost challenge us to tease out the inner core of fruit.

To illustrate his point, Cloud poured me a taste of Merlin-Cherrier’s 2008 Le Chene Marchand, from a single vineyard with gravelly Oxfordian soil. It was simultaneously intense and delicate, and impressively complex and long lasting. But I really flipped for the Domain Roblin Les Ammonites 2009, grown on the Kimmeridgian soil type. (Ammonites refers to the ancient seashells that can still be found in the soil.)

“There’s more clay in this soil, and it translates immediately into acidity and power,” Cloud said. I was taken by the wine’s laserlike focus and felt energized by its clarity of fruit.

Most Sancerre is a blend of grapes grown on the various soil types, and their terroirs are not typically listed on the label. Top single-vineyard cuvees can be harder to find and pricier, but even at about $37 the Chene Marchand and Les Ammonites represent good value given their quality.

There is a long tradition of Sancerre winemaking reflected in several names that keep appearing in various forms on labels, through generational shifts and marriages among families, such as Reverdy, Cherrier and Thomas. The wines set a global benchmark for sauvignon blanc, and yet they cannot truly be imitated, for they speak of their terroir in both a regional and vineyard-specific sense. They illustrate the connection between the wine in the glass and the land the vines spring from, clear and simple, down and dirty.

Recommended Sancerre wines

McIntyre blogs at Follow him on Twitter: @dmwine.