 The Walla Walla AVA contains at least four distinct soil profiles scattered across the valley – slackwater terrace, loess, river gravel and flood plain silt.The majority of Walla Walla's vineyards are located on a combination of slackwater terrace and loess.The silt and volcanic ash that make up the region's loess soils contain remnants from the eruption of Mount Mazama (which also formed the Crater Lake nearly away in Oregon).