This paper describes the general geology of the Lower Columbia River and aims to show how the river came to have its present course through the Cascade Mountains. Because the events in the geologic history of the Columbia River are complex, a summary is provided as a guide in following the detailed evidence which is presented.
The region provides some evidence of an early Oligocene and Eocene terrestrial formation that was eroded to a mature surface in lower Oligocene time. Through this surface erupted the Eagle Creek volcanoes, some of the products of which were carried by the wind into central and eastern Oregon, where they formed the John Day formation of middle Oligocene age. An equally large quantity of these volcanic products remained in situ as agglomerates, ashes, and breccias. A small disconformity separates this mass of material from the reworked material forming the lower Miocene upper part . . .