594 J. H. BRETZ GLACIAL DRAINAGE ON COLUMBIA PLATEAU 



utive or braided course of the Spokane glacial flood over a basalt surface 

 which possessed no adequate pre-Spokane valleys. Greater erosion in the 

 tilted basalt on the western margin of this tract finally drew these waters 

 off and down into what has become the lower canyon of Grand Coulee. 



There is evidence at the lower end of Grand Coulee that a short pre- 

 Spokane valley was entered a little north of Soap Lake. This lake lies 

 at the mouth of the coulee, where the canyon crosses a low anticline. 

 The syncline between it and the main monocline to the north is low and 

 wide open as a structural valley at the east to the Quincy basin. Some 

 drainage did go through here, but most of it crossed the anticline — a 

 thing which would not have been possible if an antecedent drainage line 

 had not existed at this place. 



The original slope on which the glacial waters flowed from the Colum- 

 bia Valley to the Quincy structural basin Avas steplike in a general way. 

 a steeper descent existing between the plateau summit to the north and 

 the- Hartline basin, and another such on the northern margin of the 

 Quincy basin. These steeper slopes were determined by structure, 

 though they did not conform exactly to it. The monoclinal fold north 

 of the Hartline basin has a maximum dip of 30 degrees. If the slope 

 which the Spokane waters found was only one-third of this, that glacial 

 river, in effectiveness over this stretch, must have been an enormous 

 ^))0untain torrent. The vertically and closely jointed basalt must have 

 been eroded with great rapidity, and, where favorable conditions for 

 sapping were discovered in the channels, falls developed and retreated 

 much more rapidly even than Niagara. 



While the talus in the higher abandoned channels reaches up three- 

 fourths to four-fifths of the total original height of the cliffs, that in 

 Grand Coulee itself is only halfway up the cliffs (where the flows are 

 laorizontal) and has an almost invariable slope of 35 degrees. It clearly 

 dates from the Wisconsin glaciation, not the Spokane. 



Quincy Valley or Basin. — The Quincy structural basin is bounded on 

 the south by the Frenchman Hills anticline, but its drainage escapes 

 southward around the east end of this fold. All glacial drainage routes 

 of the plateau west of Medical Lake, except Moses Coulee, have been 

 traced to it. Enormous quantities of basaltic debris have been swept out 

 of the hUtidreds of miles of such channels and into this catch-basin. The 

 till covers 600 square miles and the maximum knoA\TL thickness is about 

 400 feet. The loAver three-fourths of this fill is clay, silt, and sand, 

 doubtless lacustrine in origin. In this have been found shells of fresh- 

 water boreal mollusks.^^ Only the upper 100 feet, approximately, arc 



" ScUwenneson and Meinzer : Ground Avater in Quincy Valley, Washington. U. S. 

 Geol. Survey, Water Supply I'aper 425 E, 1918, p. 143. 



