CHANNELED SCABLANDS OF THE COLUMBIA PLATEAU 623 



raphy have been eroded to varying depths into the basalt. The 

 bottoms of such as lead out across the basalt plain to the north are 

 lower among the hills than the general surface of the plain. The 

 profile of the plain, extended back among the hills, cuts their slopes 

 somewhere between hill summits and valley bottoms. And this 

 , transection is at the contact of loess on basalt. The ice sheet which 

 covered the plain, therefore, simply removed the upper, weaker 

 formation, and only to a minor extent altered the surface of the 

 basalt formation. 



THE MATURE TOPOGRAPHY 



This is the dominant type of topography of the Columbia 

 Plateau. The major drainage lines are structurally controlled, 

 but the minor ones constitute a dendritic network. The pattern 

 is eroded largely in a weak sedimentary deposit, chiefly loessial, 

 which overlies the basalt. Maturity is expressed in the complete 

 development of the drainage system, in the reduction of the original 

 surface to valley slopes^ and in the concavity of the lower part 

 of many of these slopes. This maturity has been developed with 

 reference to the underlying basalt as a base level, for progress of 

 the cycle of erosion in the loess has been very much more rapid. 

 Neglecting the loessial cover, the basalt plateau is in early youth, 

 and will still be when the loess has been entirely removed. Never- 

 theless, the absence of deep trenches in basalt in the interior of the 

 plateau, similar to Spokane, Columbia, and Snake River valleys 

 about its margin, and the cutting through of the loess has allowed 

 the development of shallow mature valleys in the upper part of the 

 basalt. 



The loessial deposit varies in thickness, in some places being only 

 a soil, and in others being 200 feet or more in thickness. It is not 

 all loess. There are places where it is chiefly a residual soil from the 

 basalt, and others where it is a waterlaid sediment.^ But many 

 widely distributed sections show a succession of loessial deposits, 



' A few broad, undissected divides are still left. Michigan Prairie, south of Lind, 

 is a good example. 



^ Probably the EUensburg formation. The Pleistocene Ringold formation, in 

 Franklin County, is younger than the mature topography. 



