Geologic mapping of the Yakima district has been neglected since publication of the Ellensburg and Mount Stuart folios (G. O. Smith, 1903a; 1904; see also Calkins, 1905). Reports on other parts of the Columbia River plateau, however, pose new problems and call for revision of some interpretations. This paper presents the results of extending Smith's mapping eastward into the adjacent Yakima East quadrangle, and of considerable reconnaissance work in adjoining areas. The new mapping shows that to the east and south the lower part of the Ellensburg formation interfingers with the Yakima basalt. It also indicates that local diastrophism and volcanism continuously modified and interrupted deposition of the Ellensburg formation.

Smith's conclusions that the great topographic ridges are growing anticlines, not fault blocks, and that the major rivers are antecedent to these folds, are confirmed. The anticlines appear to have grown in a single epoch of deformation instead of in two orogenic episodes separated by a period of peneplanation. The “Cascade lowland” is a local pediment formed on the flanks of a growing anticline, not a remnant of a widespread peneplain.

The new data do not support Warren's hypothesis of defeat and diversion of Columbia River by rise of the Horse Heaven uplift, nor Flint's hypothesis of the cutting of Wallula Gap by a river thus diverted.

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