488 POPULAR SCIENCE MONTHLY. 



than the encircling basalt, they still remain in bold relief, as is illus- 

 trated by Steptoe and Kamiack bnttes, 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 bound- 

 ary 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 characteris- 

 tics of flat-bottomed canons with vertical walls. 



The remarkable circular valley surrounded by an almost continu- 

 ous 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 w^hat is known as the Great 

 Plain of the Columbia, or more familiarly as the " Big Bend coun- 

 try," in central Washington.. The Grande Coulee is a flat-bottomed 

 canon 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 Coulee, as can be seen from Coulee City, 

 the lava sheets on its eastern side dip gently eastward, while the 

 beds comprising its western Avail are apparently horizontal. This fact 

 led me to infer that the Grande Coulee, like several other similar but 

 smaller canons in the lava, is due to stream erosion along a line of 



* A Reconnoissance in Southeastern Washington. Water Supply and Irrigation Papers 

 of the United States Geological Survey, No. 4, 1897, pp. 37-40. 



