THE SPOKANE GLACIATION 



591 



drainage in the two areas. The cliffs of the Palonse scabland, though 

 less prominent on the whole than those of the Crab Creek canyons, ever}^- 

 where possess the same average ratios of talus height to depth of channel 

 and the same degree of slope. Further use of this criterion will be made 

 when the Wisconsin glacial drainage channels are examined. 



Grand Coulee. — Grand Coulee heads in the south wall of Columbia 

 Yalley, about 550 feet above the river. At its head it is 1,000 feet deep 

 and about 3 miles wide. In the middle of this valley, 10 miles from the 

 Columbia, stands Steamboat Eock, a basalt mesa with its square mile of 

 summit area at the general altitude of the plateau on either side of the 

 coulee. There are also numerous pre-basalt hills of granite on the floor 

 between Steamboat Eock and the Columbia, formerly buried in the basalt 



Figure 7. — Post-Spokane Talus in "The Potholes" south of Trinidad 



and later exhumed in the erosion of the coulee. About 13 miles from 

 the head the canyon narrows to about 2 miles and maintains this width 

 and a depth of about 800 feet as far south as Coulee City. Here the 

 canyon form is lost for 1: or 5 miles, the eastern wall descending, because 

 of a monoclinal flexure, until it is not more than 200 feet above the floor 

 of the coulee. In this broadened portion is a great abandoned cataract 

 with a fall of 400 feet. The width of the falls is nearly 3 miles, the full 

 width of the bottom of the coulee ; but it was broken into two different 

 parts in its later history through erosion of the floor above the falls. A 

 central portion of the earlier floor, which escaped ijiuch of this erosion, 

 became an island on the brink. The western half. Dry Falls or Grand 

 Falls, is the more definite and in itself was a double fall at the close of 

 the history of this great cataract, with a "Goat Island" in the middle. 

 N"o water now flows over these falls except in times of heavy rain. 



