| Touchet Formation | |
|---|---|
| Stratigraphic range: Late Pleistocene (Rancholabrean) ~ | |
 Touchet beds in Burlingame Canyon (the "Little Grand Canyon") near Lowden in the Walla Walla valley. Note distinct layers. | |
| Type | Geological formation |
| Underlies | Loess |
| Overlies | Columbia River Basalt Group |
| Area | 7,000 square miles (18,000 km2) |
| Thickness | up to 330 feet (100 m)[1] |
| Lithology | |
| Primary | Sediment[specify] |
| Other | Basalt, loess |
| Location | |
| Coordinates | Walla Walla Basin 46°15′49″N 120°31′00″W / 46.26361°N 120.51667°W Touchet Basin 46°06′00″N 118°39′00″W / 46.10000°N 118.65000°W White Bluffs 46°42′25″N 119°27′23″W / 46.70694°N 119.45639°W Yakima Basin 46°20′30″N 119°27′23″W / 46.34167°N 119.45639°W |
| Region | Washington, Oregon |
| Country |  United States |
| Extent | Columbia Basin of Western North America |
| Type section | |
| Named for | Touchet, Washington |
| Named by | Foster Flint |
| Year defined | 1938[1] |
 Map showing the elevations of the Mid-Columbia region. The lowland areas in brown routinely were flooded during the Missoula Floods, creating lakes. | |
The Touchet Formation or Touchet beds consist of well-bedded, coarse to fine sand and silt which overlays local bedrock composed of Neogene basalt of the Columbia River Basalt Group in south-central Washington and north-central Oregon. The beds consist of more than 40 to 62 distinct rhythmites – horizontal layers of sediment, each clearly demarcated from the layer below. These Touchet beds are often covered by windblown loess which were deposited later; the number of layers varies with location.[2][3][4] The beds vary in thickness from 330 ft (100 m) at lower elevations where a number of layers can be found to a few extremely thin layers at the maximum elevation where they are observed (1,150 ft (350 m)).[1]
The Touchet beds are one element in a chain of evidence which helped identify and define the progression of the Missoula Floods, which occurred around 18,400 to 15,700 calendar years ago.[4][5] During the floods, flow through the Wallula Gap was slow enough such that water pooled in a temporary lake, Lake Lewis. Lake Lewis back-flooded up the Yakima, Walla Walla, Touchet and Tucannon River Valleys. In these relatively calm arms of the lake, the slack waters deposited the suspended materials eroded from the scabland regions north of Lake Lewis, and redeposited them in pronounced layers before receding.[2]
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