Only the northern part of the Columbia Plateau is concerned in this study and only waters from the Cordilleran ice-sheet. Two glacial epochs are involved—one the Wisconsin, the other as yet undated. Besides contributing to the geological history of the region, this article endeavors to show that glacier-born streams, under proper conditions, are erosive agents of great vigor over large tracts far from the front of the melting ice.
The Columbia basalt plateau is separated from several mountain ranges on the north and from the Cascade Range on the west by the trench in which flows Columbia River. On the east, the plateau abuts against and interfingers with the Coeur d’Alene Mountains of Idaho. When the Miocene basalt flows of this region ceased, the lava plain abutted in a similar fashion against the mountains on the north and west. The cutting of the Columbia Valley . . .