CHANNELED SCABLANDS OF THE COLUMBIA PLATEAU 633 



basalt. The minor valleys, in general, are not eroded through the 

 loessial mantle. 



Except near the bounding canyons of Spokane, Columbia, and 

 Snake Rivers, or in exceptionally upwarped parts of the plateau, 

 the preglacial valleys in basalt which were unviolated by the glacial 

 flood, have the same mature slopes as those in the loess. Where 

 glacial streams found routes eroded but slightly in basalt, they 

 commonly spread widely at first^ and only later eroded canyons, 

 if they did so at all. Such wide scablands commonly are bounded 

 by steep bluffs of loess. Many of the mature valleys in basalt were 

 sufficiently capacious to contain the glacial waters which entered 

 them. In such eases^ the scabland of the route lies on the sides and 

 bottoms only, and unsteepened slopes of the bounding older topog- 

 raphy may come to the edge of the scabland. Such a tract is narrow 

 and instead of having a multitude of lateral shallow channels anas- 

 tomosing with the main canyon, it consists of exceedingly roughened 

 ledges of basalt outcropping on the slopes of the main valley. 

 Shoulders in the curves of these valleys were treated with especial 

 vigor, in some cases being wholly cut away to leave prominent cliffs 

 on the valley walls. 



By smoothing out these ledges, something of the cross-section 

 of the preglacial valley may be obtained. Remnants of the old 

 floors are recorded in isolated buttes of basalt on the present floor 

 and in the lowest prominent rock terraces, below which is the 

 canyoned channel eroded in mid-current by the huge glacial stream. 



Instances. — In Cow Creek, southwest of Ralston, the remarkable 

 number of knobs and buttes in the lower part of the valley indicates 

 clearly that the preglacial floor must have been at least 75 feet higher 

 than the present. In Crab Creek Valley, between Krupp (Marlin) 

 and Stratford, there are prominent buttes, isolated or partially 

 isolated on the floor of the canyon. The tops of these are remnants 

 of the preglacial floor and their height (100 feet in places) is a 

 minimal measure of the depth of canyon-cutting by the glacial 

 waters. 



' As in the Telford and the Cheney-Hooper areas. 



^ Washtucna CovJee, Esquatzel Coidee, Lower Moses Coulee, Lind Coulee, Pine 

 Creek above Rock Lake,»Rock Creek (Lincoln County) and Coal Creek are examples. 



