630 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



The present writer believes that the same conclusion must be drawn after 

 an attentive comparison of the Cascade topography with that described in the 

 Klamath mountains of Oregon and northern California. Diller describes a 

 peneplain there locally developed on the relatively weak rocks of the upturned 

 Shasta-Chico series in the immediate vicinity of the sea or of the Sacramento 

 river which has long been near sealevel. It is possible, as Diller holds, that 

 this proved peneplain once extended over the harder rocks of the range as well 

 as over the Sierra Nevada, although Lindgren shows that the Sierra was not a 

 peneplain at the opening of the Auriferous Gravel period. We may quote 

 Diller's summary : — 



' The erosion necessary to develop the baselevel [peneplain in the 

 sense meant in the present report] out of the topography resulting' from 

 the uplift at the close of the Shasta-Chico period must have occupied a long 

 interval of time, possibly beginning in the later part of the Cretaceous and 

 continuing through the Eocene and earlier portion of the Miocene, but as 

 the plain appears to have attained its maximum extent during the Miocene, 

 it may be referred to as the Miocene baselevel.'* 



Thus, in a Cordilleran region which probably underwent erosion at about 

 as fast a rate as that characterizing Tertiary erosion at the Forty-ninth 

 Parallel, we have a Miocene peneplain still preserved on rocks (Cretaceous) 

 which are much weaker than the staple rocks of the Cascades. All the more 

 readily can we exclude the possibility of a well perfected Pliocene peneplain 

 ' in the northern range. 



The foregoing argument applies also, with nearly all its force, against the 

 hypothesis of Russell that the Cascades were peneplained in post-Eocene time 

 from a condition of strong, mountainous relief in the late Eocene period. It 

 does not seem necessary to restate the argument for this case. 



As an alternative hypothesis, therefore, the present writer offers the view 

 that all post-Laramie time has been occupied in the production of mature 

 mountain topography in the Cascades. The initial stage is taken to be that of 

 the new relief left as a result of the Laramide orogenic revolution. Local, 

 often severe deformations have, at a few intervals since (especially in the late 

 Miocene), complicated the history of the range which was hoisted up in that 

 revolution. There is, further, good reason to think that near the beginning of 

 the Pliocene there was some, rather general uplift of the system, still further 

 adding to the task of producing the deep canyons and wider valleys of these 

 mountains. Such crustal movements have formed episodes in a single period of 

 erosion in a district which has always been mountainous since the Laramide 

 revolution. Before the later, probably Pliocene, massive uplift to which many 

 of the deep, narrow canyons are due, the relief may have approximated late 

 maturity of form or locally even old age — a mountain-torso landscape — but 

 true peneplanation on a large scale within Tertiary time is expressly excluded 

 by this alternative hypothesis. Large-scale peneplanation of large parts of the 



* J. S. Diller, 14tn Ann. Rep. U.S. Geol. Survey, Part 2, 1894, p. 420. 



