620 / HARLEN BRETZ 



part of the plateau/ This basalt plain bears many glaciated erratic 

 bowlders and some patches of till, but no channeled scabland, no 

 mature topography, and no loessial soil. 



17. Only where the minor valleys of the mature topography 

 adjacent to this basalt plain open northward on to the plain are 

 any glacial erratics found in them.^ 



18. There are but ten scabland openings to this basalt plain to 

 the north. 



19. Scabland tracts are invariably traceable down-gradient to 

 Snake River on the south or to Columbia River on the west. There 

 are nine places where scabland tracts enter these two streams Only 

 three of them were drainage ways before the scablands were formed. 



20. There is no channeled scabland on the plateau in western 

 Idaho or south of Snake River or west of Columbia River. 



2 1 . Nowhere in the scablands or the maturely dissected country, 

 during ten weeks of field study, has a till been found, or any deposit 

 of doubtful genesis which could be interpreted more satisfactorily 

 as till than as non-glacial in origin.^ 



GENERALIZED STATEMENT OF THE ORIGIN OF THE 

 CHANNELED SCABLANDS 



This unique combination of topographic features of the Colum- 

 bia Plateau in Washington has only one interpretation consistent 

 with all the foregoing items. The channeled scablands are the 

 erosive record of large, high-gradient, glacier-born streams. The 

 basalt plain records the southern limit reached by the ice sheet 

 from which these streams took origin. Before this glaciation 

 occurred, the entire plateau of Washington was covered with a 

 loessial soil, varying in depth from a few feet to 200 feet. This 

 and the underlying basalt had been eroded to maturity and a net- 

 work of drainage lines covered it. The major water courses of this 

 mature topography were consequent on the warped surface of the 



' Grand Coulee and Moses Coulee are exceptions. Grand Coulee opens up- 

 gradient into Columbia River VaUey and the upper end of Moses Coulee is obliterated 

 by the drift of a later glaciation. 



^ Exceptions due to a later episode in the history of the region are noted later. 



3 One exception, noted later. 



