Loess

Loess and the Ice Age Floods

Very fine particles of sand or dust blown in on arid or glacial wind is called loess. This is a Scandinavian word pronounced like "look" with an "S" sound instead of a "K" as in loo-ss. The usual source of loess is old lakebeds, eroded hilsides and flood plains.

Palouse area loess is a tawny color with even texture and tends to stick together like a cake of face powder. This is evident in the steep roadside cuts and is why it resisted being swept away by the great floods. Some remaining deposits are 200 or so feet deep. The picture at right shows the tawny brown loess on top of layers of sand and gravel near Spokane on US 2.

The diagram at left shows a loess island over a basalt coulee with talus. Not only was the basalt stripped of soil but its columns were removed to form the coulee. This happened during the Ice Age Floods over 10,000 years ago. You can find out more about these floods on the Channeled Scablands and Glacial Lake Missoula pages.

"Bretz (1923) first recognized that the hundreds of isolated loessial hills of the eastern scablands possessed remarkably steep, ungullied marginal hillslopes. These slopes converge to form a definite prow that points up the local scabland gradient. Although Bretz and others (1956) interpreted these hills as fluvially-eroded (eroded by surface water action) loessial "islands" high-water mark reconstruction (Baker, 1973) has shown that many of the hills were eroded subfluvially (underwater erosion)...Water velocities averaged from 12-15 m/sec for depths of 30-60 m in these areas (Baker, 1974)." [Field Guide 2]

Volcanic ash from Mount St Helens and other Cascade Volcanoes is mixed in with the loess and makes for excellent soil for agriculture. Wheat, peas, lentils, lawn bluegrass and other crops are grown in the region. The picture at right shows a deep layer of loess in a road cut near Spokane on I-90.

Loess stripped from the Palouse by the floods was taken down the Columbia River and deposited as far away as the Willamette Valley in western Oregon, some 500 miles away. In addition to the loess, dirt, sand, gravel and silt deposited in the Willamette Valley, ice rafts brought very large boulders downstream and as the rafts melted left them in the valley.

There is evidence that a tremendous amount of loess, sand and gravel was washed out the mouth of the river into ocean trenches. See map at right. Scientists have identified Palouse loess in the trenches as far south as Northern California. Core samples from the Escanaba Trough just off the coast of California show that there were at least 12 major floods that were large enough to carry material down the Cascadia Channel and into the trough. Other deposits have been found in the Astoria Channel.

Geology Terms

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Here are some basic terms used on this page. Find more geology terms in the Glossary.

Ash

Fragments of less than 2 millimeters in diameter of lava or rock blasted into the air by volcanic explosions.

Basalt

Volcanic rock caused by partial melting of the Earth's crust.

Channel

The deepest part of a river or bay.

Channeled Scabland

Area in Washington state where huge floods made channels in a large, deep basalt flow. Named by J Harlan Bretz during the 1920's in various publications. See also Channeled Scabland and Glacial Lake Missoula.

Coulee

Long winding channel cut through lava formations. A term primarily used in the northwestern United States.

Erosion

Lifting and removal of rock, dirt, sand and the like caused by wind, water, or glacial ice.

Loess

Fine dirt deposited by wind usually from arid or glaciated areas.

Scabland

A flat region with exposed lava rock and a thin layer of soil and sparse vegetation. Usually cut through with channels.

Talus

Rock debris that has fallen from the sides of a cliff or steep slope.

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