The clay deposits of eastern Washington and northern Idaho are along the eastern edge of the Columbia River Plateau physiographic province. Three types of clay occur in the area: (a) residual clay derived from the Columbia River basalt of Tertiary age; (b) residual clay derived from the Cretaceous granodiorite and related intrusive rocks of the Idaho batholith; and (c) transported clay. The two types of residual clay were formed during a protracted lull in the outpouring of the Columbia River basalt, when the relief of land surface was low to moderate and the climate was warm and humid. The topography and climate during this interval were probably ideal for leaching and oxidation. The transported clay is chiefly a product of the weathered granodiorite and related intrusive rocks. Most of it accumulated in basins formed by damming of streams by basalt flows.

Kaolinite and halloysite are the principal clay minerals found in the area. The residual clay derived from basalt is chiefly composed of halloysite. Most of this clay is colored blue, gray or grayish black by finely disseminated ilmenite and locally it is stained blown by limonite; most of it has a relict basaltic texture.

The residual clay derived from granodiorite and related intrusive rocks contains both kaolinite and halloysite, and the transported clay is predominantly kaolinite. Both of these clays are white and contain abundant quartz grains and mica flakes. The residual clay has a relict granitic texture, and the transported clay has a well-developed bedding and is interbedded with elastic material.