Willamette Valley Soil Types
- By Morgan Harris MS
- 10 Nov 2020
- 5 MIN
- Level 301
The Willamette Valley possesses one of America's most established terroirs for Pinot Noir, but with only 55 years of winemaking history, the region only recently grasped a more complete picture of its geology. As can be gleaned from the long-established particularly of terroirs in Burgundy, soil type is a major component towards building particularity in wine. We can’t discount the vineyard’s aspect, micro-climate, and various other factors as contributing to a vineyard’s character, but soil is a major component contributing towards the final character of a finished wine.
As a newer wine growing region that is still developing vineyard sites, direct inferences as to how a soil type correlates aesthetically with a finished wine are tenuous (as in, “soil type Y will yield quality X in a wine”). We can, however, gain familiarity with the catalogue of soil types in the valley to see if any themes emerge when tasting wines from different sub-AVAs of the Willamette.
Broadly, soil types are placed into three major categories: sedimentary, igneous, and metamorphic. Sedimentary soils are formed by the movement of water: sands, gravels, clays, and their various derivatives. Igneous soils are formed by molten rock from deep inside the earth, either forming underneath the ground (intrusive) or coming up out of the surface of the earth (extrusive). Metamorphic soils are formed via pressure, usually around intrusive igneous events or tectonic movement.
Several major geological events formed the particular soils of the Willamette Valley. Beginning approximately 50 million years ago, all of western Oregon was on the bottom of what is now the Pacfic Ocean. Around 12 - 15 million years ago tectonic action began lifting that ancient marine sediment to the surface, forming large parts of what is now Western Oregon. The uplifting of these marine sedimentary sandstones corresponded with volcanic eruptions in the eastern part of what is now Oregon, with layers of igneous basalt forming overtop. Importantly for vineyard soil comparisons, the marine sedimentary soil is derivative from siltstone or sandstone, and is not the limestone and/or chalk soil that would be found in Burgundy.
In terms of other major soil types of the Willamette, windblown Loess (a fine, dust-like soil), derived from much older, weathered formations, has been deposited in a thin layer gradually over the last million years. Additionally, during the end of the last ice age around ten to twelve thousand years ago, the massive Missoula floods, caused by the rupture of a titanic glacial lake in what is modern day Idaho, carried a sizeable deposit of alluvial soils into the more low-lying parts of the valley.
As to where these individual soil types are most frequently found, it is difficult to make broad-based pronouncements on an AVA-by-AVA basis. However, generally, in the south-eastern side of the valley, vineyards have more volcanically-derived soils, and on the northwestern side they have more marine-sedimentary-influenced soils. Adding to the difficulty of generalization, vineyard soils are often layered, with particular soil depths composed of differing, striated layers; a vineyard may have a shallow top horizon of Loess or Missoula flood sediment, but then may be more decomposed basalt or marine sedimentary deposits as you move deeper.
As with many topics in wine, there is no simple answer to the question of terroir in the Willamette, but a strong baseline understanding of any region’s soil types will certainly increase any sommelier’s perspective on tasting the wines.