Abstract
Results obtained in mapping the geology of the Wenatchee-Chelan and of the Okanogan district suggest modifications of the earlier interpretations regarding the erosional history:
1) The existence of a “Methow Plain” or “Cascade Peneplain” is questioned. Most of the areas formerly considered to be remnants of this peneplain are surfaces controlled by the structure of the Columbia River basalt flows without regard to a widespread base level of erosion. One remnant, which bevels the Columbia River basalt and the Ellensburg formation on the flanks of an anticline in the Yakima district, is believed to be a pediment of very local extent.
2) The mature surface of the “Entiat stage” is well developed only on resistant granodiorite and gneiss and is exceedingly fragmentary or absent in areas underlain by greenstone, phyllite, schist, and Tertiary sedimentary rocks. It has not been recognized in areas underlain by the Columbia River basalt. The writer interprets nearly all the residuals of the “Entiat surface” described in the literature as resurrected fragments of a widespread late mature erosion surface buried beneath the Miocene Columbia River lavas and associated sediments and then partially exhumed in post-Miocene time. Certain fragmentary remnants of this surface on massive crystalline rocks beyond the limit reached by the basalt flows are believed to have been similarly preserved by burial under andesitic pyroclastics erupted from volcanoes in the Cascade Range.
