THE SATSOP FORMATION 455 



burg formation, largely a sandstone of volcanic debris. Both the 

 Yakima and Ellensburg formations have been folded into a number 

 of east- west anticlinal ridges. Ahtanum Ridge is one of these 

 folds in the southern part of the Ellensburg quadrangle. On this 

 ridge the quartzite pebbles lie on the eroded edges of both the 

 Yakima basalt and the Ellensburg sandstone. Toppenish Ridge, 

 between the Ahtanum and Simcoe anticlines, is similarly oriented, 

 of similar origin, and bears abundant quartzite gravel on the edges 

 of both formations. 



Relation of the Satsop formation to the Cascade^ Range. — -It is 

 obvious from data already presented that the outpouring of gray 

 basalt immediately succeeded, and in part was contemporaneous 

 with, the deposition of the Satsop formation in the Cascade Range. 

 From the position of the Satsop formation in these mountains, it is 

 also clear that it and the overlying lava-flows were deposited before 

 the Cascade Range was formed. 



Relation of the Satsop formation to the Methow peneplain. — ■ 

 Russell first advanced the hypothesis that the accordant summit 

 levels of the Cascade Mountains in central and northern Washington 

 record a warped and dissected peneplain. Willis and Smith 1 have 

 named this the Methow peneplain. They have identified it on the 

 eastern slopes of the Cascades from Lake Chelan on the north to 

 the Yakima valley on the south. In the Ellensburg quadrangle the 

 Methow pleneplain is thought to truncate the Ellensburg sandstone 

 and the underlying Yakima basalt. As interpreted by Smith, these 

 formations were gently folded before the peneplanation. Develop- 

 ment of the peneplain brought the surface of these folds down to 

 base-level. Renewed folding along the same axial lines is thought 

 to have followed the truncation so that the Methow peneplain is 

 now a warped surface lying on the tops and flanks of the anti- 

 clinal ridges. 



The significant item here contributed is that the mantle of 

 Satsop quartzite pebbles lies unconformably on the tops and flanks 

 of at least some of these anticlinal ridges. If they constituted a 

 stratified deposit across the eroded edges of the underlying forma- 

 tions, the case for planation between two epochs of folding would 



1 Bailey Willis and George Otis Smith, U.S. Geol. Surv., Prof. Paper 19; 1903. 



