. Vegetation of Oregon and Washington. Botany Oregon Ecology; Botany Washington (State) Ecology. 30 meters thick. Bottom portions of individ- ual flows are dense, dark-gray basalt, but near upper margins the basalt becomes scoriaceous. In some areas, deformation of the Columbia River Basalt produced ridges and hills during the Pleistocene epoch. In the central portion of eastern Washing- ton's Columbia Basin province is a unique geologic feature — the Channeled Scablands. This is a gigantic series of dry, deeply cut channels in Columbia River Basalt (fig. 15) which form an extensive and comple
. Vegetation of Oregon and Washington. Botany Oregon Ecology; Botany Washington (State) Ecology. 30 meters thick. Bottom portions of individ- ual flows are dense, dark-gray basalt, but near upper margins the basalt becomes scoriaceous. In some areas, deformation of the Columbia River Basalt produced ridges and hills during the Pleistocene epoch. In the central portion of eastern Washing- ton's Columbia Basin province is a unique geologic feature — the Channeled Scablands. This is a gigantic series of dry, deeply cut channels in Columbia River Basalt (fig. 15) which form an extensive and complex drain- age network. Many of the deeply entrenched drainageways diverge upstream only to con- verge again further downstream. Perhaps the best known- feature in the Channeled Scab- lands is Grand Coulee with its spectacular Dry Falls. Although the origin of these puzzling features is still debated, Bretz (1959) prob- ably offered the most satisfactory theory. He suggests that flood waters, pouring from gla- cial Lake Missoula (western Montana) as a re- sult of dam failure during the Pleistocene epoch, were responsible for cutting the chan- nels. Plio-Pleistocene deposits cover the Colum- bia River Basalt over extensive areas. The most widespread deposit is the Palouse loess which mantles an elliptical area 160 kilome-. Figure 15. — General view of channeled scabland in the Columbia Basin of central Washing- ton; this area is characterized by numer- ous dry channels cut in Columbia River basalt and generally shallow, stony soils (photo courtesy of H. W. Smith). ters long in southeastern Washington. This material, deposited during the Pleistocene epoch, is made up of massive, tan-colored silt which may be over 45 meters thick. The Palouse area is characterized by smoothly roll- ing hills (fig. 16) and soils of high fertility which are generally used for wheat and pea Figure 16. — Rolling Palouse Hills, composed of deep loess deposits, near Pullman, Washington, Columbia Bas
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