The Walla Walla Valley lies within the Columbia Plateau. Fifteen million years ago, this area was inundated by giant volcanic eruptions, creating the largest lava flows on earth. The lava from this period hardened into the iron-rich basalt that forms the bedrock for the whole region. Throughout the Columbia Plateau, however, the top-soils are non-native, deposited across the region by ancient floods, volcanic eruptions and wind.
During the last ice age, a lobe of glacial ice from the Cordilleran ice sheet advanced south into the Idaho panhandle and blocked the Clark Fork River. This lead to a series of incredible glacial floods known as The Missoula Floods. First, the Clark Fork flooded the valleys of Idaho and western Montana as far north as Canada. Known as Lake Missoula,this body of water was 2,000 ft. deep in places and as large as Lake Ontario and Lake Erie combined. There are two theories regarding what happened next. The water either rose until it was deep enough to float the ice-damn or until it accumulated enough force to weaken and eventually breach it. Either way, the ice failed, and a 500-foot wall of water, ice and debris swept across eastern Oregon and Washington. When the water reached the Wallula Gap in the Columbia River Gorge, it backed-up and flooded the Walla Walla, Yakima, Snake and Columbia river valleys, reaching depths of 1000 feet in places. Eventually the flood made its way through the Columbia River Gorge, floodinig the Willamette Valley as far south as Eugene, before continuing west to the Pacific Ocean. Amazingly, as the glacial ice continued south, it sealed off the Clark Fork again. Experts believe this cycle was repeated 80 to 100 times and that it took 50 to 75 years to complete.
The Missoula Floods had a tremendous impact on the Walla Walla Valley. As each flood engulfed the valley, reaching elevations as high as 1,200 ft., the receding water left behind a layer of sediment and debris. These layers are known as the Touchet Beds. Sediment from at least thirty floods reaches depths of 100 feet in the Touchet Beds. The beds are rich in minerals such as quartz and mica that are not found in the underlying basalt. In addition to forming the Walla Walla Valley's basalt bedrock, Cascade volcanoes such as Mount Mazama and Mount St. Helens erupted repeatedly during the last ice age, covering the valley with fine layers of ash.
Wind also had a great impact on the Walla Walla Valley during this time. Strong southwest winds lifted the finest particles of silt and sand from the Touchet Beds and piled it into mounds. This fine, wind-blown soil is known as loess, and it covers much of the valley. The Walla Walla River, Mill Creek and other major streams in the Blue Mountains participated in the process, too. During the last ice age, these streams had much greater flows than today, and floods from this era left behind large deposits of basalt river-rock. These rocky floodplains can be found in and around Walla Walla and Milton-Freewater.
Sources: Forces of Nature, Wine Press Northwest. Fall 2005. Dr. Kevin R. Pogue, Professor and Chair, Department of Geology, Whitman College. Cole Danehauer, Oregon Wine Report.