This paper deals with the heavy Miocene lava flows, known as the Columbia River basalt, which cover a large part of Oregon and Washington ; but more particularly with that part of their area along the boundary between those two States, where the basalt was in Pliocene time folded in strong anticlines, trending and pitching southwest in the Cascade Mountains, and faulted in a long, east-west trough just east of the mountains. The trough, here called Maryhill trough, was formed by a gentle down-bending of the lava so that its surface, called the Shaniko surface, slants gently northward to a strong, south-facing fault-scarp, from 3,000 to 350 feet high and about 80 miles long (Figure 1).
The Pliocene Columbia River took a consequent course along the trough; thus guided, it ran beyond the entrance of the deep canyon which it has since then trenched across the anticlinal folds of . . .