The Touchet Formation is a widespread sequence of layered flood sediments in south-central Washington and north-central Oregon, deposited during repeated late Pleistocene floods, chiefly the Missoula Floods between about 18,400 and 15,700 years ago, with some contribution from the Bonneville Flood. As floodwaters were constricted at Wallula Gap and ponded into temporary Lake Lewis, sediment settled in quiet side valleys to form dozens of distinct graded layers. These beds became crucial evidence that the channeled scablands were shaped by many immense floods rather than one event, and they remain a key record of flood history while also underpinning fertile soils in places like the Walla Walla basin.