Bulletin of the Geological Society of America . ^-where possess the same average ratios of talus height to depth of channeland the same degree of slope. Further use of this criterion will be madewhen the Wisconsin glacial drainage channels are examined. Grand Coulee.—Grand Coulee heads in the south wall of ColumbiaYalley, about 550 feet above the river. At its head it is 1,000 feet deepand about 3 miles wide. In the middle of this valley, 10 miles from theColumbia, stands Steamboat Eock, a basalt mesa with its square mile ofsummit area at the general altitude of the plateau on either side of t
Bulletin of the Geological Society of America . ^-where possess the same average ratios of talus height to depth of channeland the same degree of slope. Further use of this criterion will be madewhen the Wisconsin glacial drainage channels are examined. Grand Coulee.—Grand Coulee heads in the south wall of ColumbiaYalley, about 550 feet above the river. At its head it is 1,000 feet deepand about 3 miles wide. In the middle of this valley, 10 miles from theColumbia, stands Steamboat Eock, a basalt mesa with its square mile ofsummit area at the general altitude of the plateau on either side of thecoulee. There are also numerous pre-basalt hills of granite on the floorbetween 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 fromthe head the canyon narrows to about 2 miles and maintains this widthand a depth of about 800 feet as far south as Coulee City. Here thecanyon form is lost for 1: or 5 miles, the eastern wall descending, becauseof a monoclinal flexure, until it is not more than 200 feet above the floorof the coulee. In this broadened portion is a great abandoned cataractwith a fall of 400 feet. The width of the falls is nearly 3 miles, the fullwidth of the bottom of the coulee; but it was broken into two differentparts in its later history through erosion of the floor above the falls. Acentral portion of the earlier floor, which escaped ijiuch of this erosion,became an island on the brink. The western half. Dry Falls or GrandFalls, is the more definite and in itself was a double fall at the close ofthe history of this great cataract, with a Goat Island in the wa
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1890
