Page:Popular Science Monthly Volume 53.djvu/506

488 than the encircling basalt, they still remain in bold relief, as is illustrated by Steptoe and Kamiack buttes, described elsewhere by the present writer.

When, however, the projecting portions of the nearly submerged peaks and ridges were of granite, volcanic tuff, limestone, etc., which weather more rapidly than the surrounding lava, they have wasted away so as to give origin to basins, valleys, and canons, with boundary walls of basalt. The sheets of basalt in these escarpments in many instances rest on less resistant rocks, and a recession of the cliffs due to the breaking away and falling of large masses of their capping layers takes place. The fallen blocks disintegrate and waste away in the manner described above, and the canons and valleys increase in size. The ground plans of the depressions originating and enlarging in this manner, vary according to the shapes of the islandlike rock masses which have been removed; some of the depressions are nearly circular, others are greatly elongated, and now have the characteristics of flat-bottomed cañons with vertical walls.

The remarkable circular valley surrounded by an almost continuous palisade in eastern Oregon, known as Grande Ronde Valley, from which a river of the same name flows northward to join the Snake, is an illustration of the class of topographic forms produced in the manner described above. I can not testify from personal observation as to the nature of the soft rocks beneath the lava in the walls of Grande Ronde Valley, but other similar valleys, less regular in outline, near at hand, have resulted from the removal of islandlike masses of soft volcanic tuff.

Another unique feature in the topography of the region drained by the Columbia is the Grande Coulee in what is known as the Great Plain of the Columbia, or more familiarly as the "Big Bend country," in central Washington. . The Grande Coulée is a flat-bottomed cañon some thirty miles long and varying in width from two to four miles. In its vertical walls, usually about three hundred and fifty feet high, the edges of several sheets of Columbia lava are exposed. This great trench through the but little disturbed plain of lava was in existence previous to the Glacial epoch, and furnished an avenue of escape for Columbia River which was dammed by a glacier. At the southern end of the Grande Coulée, as can be seen from Couleé City, the lava sheets on its eastern side dip gently eastward, while the beds comprising its western wall are apparently horizontal. This fact led me to infer that the Grande Coulée, like several other similar but smaller canons in the lava, is due to stream erosion along a line of