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Touchet Formation
The Touchet Formation or Touchet beds consist of large quantities of gravel and fine sediment which overlay almost a thousand meters (several thousand feet) of volcanic basalt of the Columbia River Basalt Group in south-central Washington and north-central Oregon. The beds consist of between 6 to 40 distinct rhythmites - horizontal layers of sediment, each clearly demarcated from the layer below. These Touchet beds are often covered by windblown loess soils which were deposited later; the number of layers varies with location.〔〔 The beds vary in depth from 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 ().〔
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 16,450 to 13,750 years BCE.〔〔 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.〔
==Discovery and interpretation==
Although visible along the Walla Walla and Touchet rivers, the Touchet Beds were not subjected to study until the early 20th century. They remain at the center of scientific analyses to characterize the Missoula Floods.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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