592 J. H. BRETZ GLACIAL DRAIXAGE OX COLUMBL\ PLATEAU 



Below the falls the coulee again becomes a canyon, both because of the 

 oorge left by retreat of the falls and because the basalt surface rises in 

 that direction. For 15 miles or so this lower coulee is a wild, spectacular 

 feature, its western wall fully 1,000 feet high and its bottom less than 

 a mile in width. For most of this distance the ancient river flowed on 

 the strike of a monoclinal fold the dip of which averages 45 degrees and 

 is to the southeast. The falls took origin where the glacial stream passed 

 from horizontal flows to the tilted structure, about 3 miles below their 

 present location. Grand Coulee debouches in a broad, shallow structural 

 depression, the Quincy basin. Into this same basin discharges Crab 

 Creek, which carried the combined flow of all of the glacial rivers 

 previously listed. 



As above outlined. Grand Coulee is a relatively simple affair, with two 

 canyoned portions separated by a shallower part, due to local structural 

 conditions, Avith a remarkable waterfall in midlengih and wdth a part of 

 its lower course along tilted flows; but the preglacial conditions and the 

 glacial history are not as simple as in the region farther east. 



There is no evidence of a preglacial drainage line along the upper 

 canyon between Coulee City and the Columbia. There are but two 

 tributary gorges in the 30 miles of this canyon and these are short and 

 youthful. A mature topography, untouched by glacial ice or water, lies 

 on the plateau to the east, with no drainage lines leading to the coulee. 

 To the west the plateau was glaciated to the edge of the coulee during 

 the Wisconsin epoch, but preglacial topograjDhic features still control and 

 give no suggestion of drainage toward the coulee. Moreover, the altitude 

 of the basalt on the jDrecipitous edge of the canyon is about that of the 

 Columbia River-Crab Creek divide ; and on both sides of the coulee, near 

 the head, es23ecially well shown on the unglaciated east side, is a scabland 

 tract (see figure 3), with the mature hills gone and the basalt etched 

 and roughened by a maze of anastomosing minor cliannels and rock- 

 basins, separated by low buttes and knobs. Elongated meadows, and even 

 lakes, mark some channel courses. Xo channels are more than 75 feet 

 deep. Glacial erratics of diorite, argillite, slate, schist, and quart^^ite 

 occur liere and there. Especially significant is the talus, which has 

 climbed three-fourths to four-fifths of the total original lieights of the 

 channel walls and nowhere is steeper than 20 degrees. On the east side 

 this summit scabland is 3 miles wide, as wide as the floor of the coulee 

 itself, but 1,000 feet higher. The altitude here is aljout 2,500 feet 

 above tide. 



In order that glacial waters should have spilled across this place, the 

 ice-sheet must have blocked the Columbia, both to the west and the east, 



