Touchet Formation

[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.

In 1923, J Harlen Bretz published a paper arguing that the channeled scablands in Eastern Washington were caused by massive flooding in the distant past.

Shaw's team proposed that the rhythmic Touchet beds are the result of multiple pulses, or surges, within a single larger flood.

[14] The Evidence for periods of nondeposition and subaerial exposure between the deposition of individual beds have been further documented and the occurrence of multiple Missoula Floods confirmed by later research.

[3][15] Recent scientific investigations support Waitt's proposed separation of layers into records of sequential flood events.

Some of the suspended materials settled out, creating thick Touchet Formation layers, or rhythmites, which are found throughout these valleys.

He postulated that these floods could occur only when glacial Lake Missoula existed,[10] which estimates place at 18,400 to 15,700 calendar years ago[5] More recent studies have used radiocarbon dating to establish the approximate ages of deposition for the various layers.

The number of layers observed decreases as one progresses up-valley, indicating that the floodwater lakes varied in depth and distance upstream from the various flood dams.

Clastic dike intrusions are observed in some locations penetrating the layers vertically; the dikes are infilled with uniform-size fine-grained materials and are indications that the period between floods was long enough that the deposits dried, forming vertical cracks and allowing wind-blown eroded materials to infill the cracks.

This area was filled to varying depths by the successive Missoula Floods, and sediments settled out to form the Touchet beds over much of the basin, creating what is today a fertile farmland region.

[7][22] The Tucannon River and its tributary, the Pataha Creek also have valleys partially filled with alluvial deposits from the Missoula and Bonneville floods.

One notable location is the White Bluffs, which are made up of more thickly bedded deposits from the eddy that formed in the floodwaters behind the Saddle Mountains.

[8] As one example, silt-sand rhythmites associated with the Missoula Floods are present along Willow Creek, a valley tributary to the Columbia River.

Excellent exposures of the unit (which contains abundant clastic dikes) can be found in roadcuts along Hwy 74 (Heppner Highway) near the ranching community of Cecil, Oregon.

This photo shows a series of horizontal layers of soil in a road cut. In the center of the photo – barely visible – there is a geologist's hammer and a leather glove to establish perspective – they suggest that the layers are about 1 meter thick. In geology the type locale is a location where the first example of a newly described object was found – this is the type locale. This set of layered beds, although nowhere near as clearly pronounced in layering as in several of the other photos, shows the first location where the Touchet Formation was identified in the published literature.
Touchet beds in the Touchet River valley.
This photo shows several layers of the horizontal Touchet beds – the layers are not as clearly demarked as they were in some of the prior photos. There is a geologist's hammer and a glove in the photo to establish perspective – it suggests the layers are about 1 meter thick. Most significantly, there is a thin, white horizontal line running through the top of one of the layers. This layer consists of two layers of ash deposited by an eruption of Mount St. Helens separated by a thin layer. The ash allows an alternative confirmation of the dating the layer.
Touchet beds in the Touchet River valley. Note distinct white line, which consists of two layers of Mount St. Helens Set S ash that fell in eastern Washington about 16,000 calendar years ago. [ 15 ]
This photo shows four profiles of Lake Lewis at various flood levels. It illustrates that the lake back flooded several valleys in which the Touchet Beds were found.
Lake Lewis was formed when the restricted flow of waters [ 18 ] from periodic cataclysmic floods from Glacial Lake Missoula, pluvial Lake Bonneville, and perhaps from subglacial outbursts, backed up through the constriction formed by the Wallula Gap in the Horse Heaven Hills (southern Washington). [ 19 ]
This photo shows several layers of the Touchet Formation, as seen close-up. There is a geologist's hammer and a leather glove in the photo to provide perspective – they suggest the layers are about 1 meter deep. There is a vertical seam of a different material running through the horizontal layers – this is the clastic dike which was introduced by later geologic processes.
Carson and Pogue identified these as clastic dikes penetrating Touchet layers. [ 8 ] Photo taken in a road cut several miles up the Touchet Valley.
This photo shows another canyon cut into the surrounding flat soil with about 30 distinct horizontal layers of sediment, each clearly demarked from the layer below. Above the canyon a telephone pole can be seen in the distance – the pole provides the perspective that helps the viewer establish that the cut is 30–40 ft (9.1–12.2 m) deep. In the foreground one observes the near edge of the canyon, which help one establish that the canyon is quite narrow and steep walled.
Touchet beds near Lowden in the Walla Walla valley. Note distinct layers.
This photo shows a canyon cut into the surrounding flat soil with over 20 distinct horizontal layers of sediment, each clearly demarked from the layer below. Above the canyon lip a number of boxes used for transporting harvested apples are stacked; the size of these boxes indicates that the layers each are 0.5–1 meter in depth.
Touchet beds on the Yakima River in Zillah, Washington . Note distinct layers. Apple crates at top provide scale (each is 1 meter high) for layer thickness.