Age of the "Satsop" and the Dalles Formations of Oregon and Washington

Abstract

Geologists have differed regarding the ages of the "Satsop" and the Dalles formations of the Columbia River Gorge region. Because of their bearing on the history of the Gorge and for other reasons their ages are important. During a brief investigation of these beds under the auspices of the Carnegie Institution of Washington, the writers secured fragmentary mammalian fossil remains from the Dalles formation representing not a Quaternary, but approximately an upper Miocene or lower Pliocene stage. This age determination is corroborated by the lithologic resemblance of the Dalles beds to the middle Neocene Ellensburg formation of central Washington, by the apparently similar relations of these two formations to the Columbia lavas, and by the induration of the Dalles beds, which is equal to that of lower or middle Neocene deposits of the west, but much greater than that of Quaternary formations.