CHANNELED SCABLANDS OF THE COLUMBIA PLATEAU 621 



plateau which descends in a general way from the northeast to the 

 southwest. 



The ice sheet approached and invaded the plateau from its 

 northern high margin. It barely crossed to the headwaters of the 

 consequent drainage. In places, it did not cross, but by blocking all 

 other escapeways, its waters were forced to cross. By about a 

 dozen different routes, at different altitudes and distributed along 

 more than 150 miles of the ice front, water entered the mature 

 drainage system. The capacity of the pre-existing valleys was 

 wholly inadequate for the volume of most of these streams. Fur- 

 thermore, gradients were high and the glacial waters eroded enor- 

 mously, sweeping away the overlying loessial material, crossing 

 low divides and isolating many groups of the maturely eroded 

 hills to produce the anastomosing pattern of the scablands, biting 

 deeply into the basalt to make the canyons and rock basins, and 

 spilling into the Snake and Columbia in three times as many places 

 as the pre-existing drainage had used. 



This procedure of glacial streams was unique, so far as the 

 writer is aware. It was unorthodox, at any rate, for no valley 

 trains and but two outwash plains' were built on the plateau 

 south of the basalt plain. The stream gravel of the scablands is 

 almost wholly in separate bars. 



The conception above outlined is amply sustained by every 

 feature and relationship of the scablands. All other hypotheses 

 meet fatal objections. Yet the reader of the following more 

 detailed descriptions, if now accepting the writer's interpretation, 

 is likely to pause repeatedly and question that interpretation. The 

 magnitude of the erosive changes wrought by these glacial streams 

 is nothing short of amazing. The writer confesses that during ten 

 weeks' study of the region, each newly examined scabland tract 

 reawakened a feeling of amazement that such huge streams could 

 take origin from such small marginal tracts of an ice sheet, or that 

 such an enormous amount of erosion, despite high gradients, 

 could have resulted in the very brief time these streams existed. 

 Not River Warren, nor the Chicago outlet, nor the Mohawk channel, 



^ The Hartline gravel flat and the Quincy basin fill, both in structural depressions 

 in the plateau. 



