A CLASS IN A GLASS – MAPPING THE GEOLOGIC ORIGIN OF SOILS IN THE WILLAMETTE VALLEY AMERICAN VITICULTURAL AREA (AVA), OREGON

Oregon's Willamette Valley AVA is a world-renowned producer of fine wines, with more than 700 wineries and a dramatic geological history. Visitors can taste wine from a variety of geologic terranes which reflect a 50-milion-year history, ranging from oceanic volcanism, accretion, sedimentation, and Columbia River flood basalt, to uplift, Cascade arc volcanism, Missoula glacial outburst floods, and windblown loess. This varied landscape also includes 11 nested AVAs, and their deep history provides a framework for tasting the potential contributions of terroir to the character of the wines.

As part of a task force convened by the Willamette Valley Wineries Association, we were charged with illustrating the distribution of Willamette Valley soils and telling the story of their origins. We started with the USDA Natural Resources Conservation Service (NRCS) soils maps and classified them according to their parent material. Soil Series descriptions of parent material were aggregated from the map unit level, as described in the Gridded National Soil Survey Geographic Database (gNATSGO) for Oregon. Generalized parent material descriptions (basalt, sedimentary rock, glacio-lacustrine, etc.) were further classified using geologic attributes from United States Geological Survey (USGS) and Oregon Department of Geology and Mineral Industries (DOGAMI) databases. A new (2020) geologic map of the greater Portland metropolitan area provided detailed information for 6 of the nested AVAs in the northern Willamette Valley.

In our origins map, the story of the soil extends into deep time, revealing the tectonic evolution of the Cascadia convergent margin. A thumbnail sketch and timeline of the geologic history is color-keyed to map units, described as soil “parents”. Although it looks a bit like a geologic map, it lacks the typical technical rigor and is more approachable for lay audiences. Vintners can better understand and tell their vineyards’ stories, and wine consumers and those in the wine trade can now appreciate the contribution of 50 million years of earth history to their glass of wine.