608 J. H. BRETZ GLACIAL DRAINAGE OX COLUMBIA PLATEAU 



scablauds, much more than a thousand square miles of the plateau, and 

 more than a tenth of the total area of the plateau (as the term is used 

 in this 23aper) have been denuded of overlying sedimentary deposits b} 

 running water. But, despite the enormous amount of erosion by escaping' 

 glacial waters in both Spokane and Wisconsin epochs, no permanent 

 derangement of drainage lines, save that of the Palouse from Hooper to 

 Snake Eiver, resulted. The Palouse formerly flowed to Esquatzel Coulee 

 by way of Washtucna Coulee. Its course to Snake Eiver was shortened 

 50 miles by this diversion, probably during the Spokane epoch. 



The Wisconsin history of Drumheller channels and Moses Coulee has 

 been outlined in the description of these features under "Spokane Glacia- 

 tion." It remains to note that the small amount of erosion in the syn- 

 clinal plexus of Moses Coulee and the small valley train built by drainage 

 from the tip of the Okanogan lobe suggest that this lobe may have evap- 

 orated in considerable part. This suggestion of large evaporation may 

 also explain the relatively small amount of Wisconsin water which came 

 from the Cordilleran ico east of the Okanogan lobe. 



Discussion 



Dr. ]\I. M. Leightox : The speaker is to be congratulated on the char- 

 acter of the work he has done in eastern Washing-ton. A few years ago 

 I liad the opportunity to see some of the features which he describes, and 

 the case for at least two glaciations is perfectly clear and, as he suggests, 

 there may have been three. In support of this, I may say that in a gravel 

 pit in the southwestern part of Spokane I found balls of apparently old 

 glacial till in what seemed to be gravel of Spokane age. 



Mr. Oscar E. Meixzer : I was especially interested in this excellent 

 paper, because of my own brief field work a few years ago in this region. 

 The o'lacial features of the region are on a o-rand scale and verv strikinii'. 

 I understand that in the Spokane Stage the lake in Quincy Valley dis- 

 charged directly into the gorge of the Columbia and also through the 

 outlet east of Frenchman Hills, and that the latter was over lava rock. 

 How is the deep trenching of the plain in Quincy Valley during the 

 Wisconsin Stage accounted for? 



Author's reply to ]\Ir. Meinzer: Only the upper or gravelly part of 

 the Quincy Valley fill is surely of Spokane age. Wisconsin waters added 

 but little to this deposit. They eroded it instead, because of downcutting 

 in the basalt at the Drumheller Channels at this time. All the remark- 

 able features of that tract are in basalt and were produced largely by 

 Wisconsin waters. 



Brief remarks were also made by Mr. Leverett, with reply by the author. 



