2017

September - The Palouse, Washington State

The peculiar and picturesque loess hills which characterize the Palouse Prairie are underlain by wind-blown sediments of the Palouse Loess that covers the surface of over 50,000 km2 (19,000 sq mi) on the Columbia Plateau in southeastern Washington, western Idaho, and northeastern Oregon. The Palouse Loess forms a fine-grained mantle of variable thickness that lies upon either the Miocene Columbia River Basalt Group, non-glacial Pliocene fluvial sediments of the Ringold Formation, or Pleistocene glacial outburst flood sediments that are known informally as the Hanford formation. At its thickest, the Palouse Loess is up to 75 meters (246 ft) thick. It consists of multiple layers of loess separated by multiple well-defined calcrete paleosols and erosional unconformities.

Although superficially resembling sand or other types of dunes, the loess hills of the Palouse are of far different origin. Internally, they lack any evidence of cross-bedding or erosion of interbedded layers of loess and calcrete that characterize dunes formed by moving currents. Instead, these hills consist of alternating layers of loess and calcrete that are more or less concordant with the modern surface of these hills. This layering demonstrates that the Palouse hills loess accumulated from the airfall of wind-silt from suspension. In addition, the ubiquitous homogenization of the loess by innumerable plant roots and insect burrows as it accumulated further supports the conclusion drawn from numerous thermoluminescence and optically stimulated luminescence dates that individual layers of loess accumulated over an extended period of time in terms of thousands of years. Finally, the calcrete horzons are paleosols that represent the periodic cessation of loess accumulation for periods of thousands of years during which they formed within the surface of a loess layer.

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