626 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



The foregoing table, though somewhat abridged, is fuller than it need be 

 to show Willis's conception of the events which are of importance in the present 

 connection. It will be observed that the relatively short period of time repre- 

 sented in the Pliocene and Pleistocene combined, is crowded with events to a 

 degree seldom if ever matched in a modern geological study of a complex 

 mountain system. No ■ definite statement is given as to the strength of the 

 initial relief which was brought low in the Methow peneplanation, but the 

 implication from the paper is that the relief at the middle or later part of the 

 Miocene was considerable. Further, we may believe that Willis shares the view 

 stated by Smith in the companion paper of the same volume, which reads as 

 follows : — 



' The evidence of the reduction of a large area of folded Tertiary 

 rocks to form the Cascade (Methow stage) lowland appears conclusive. 

 The date of the development of this lowland is fairly well determined, 

 since folds involving late Miocene strata are truncated, while on the other 

 hand the subsequent history of a large part of the region has been so event- 

 ful that the production of the lowland surface could not reasonably have 

 been later than Pliocene. Previous to this Pliocene reduction, erosion does 

 not appear to have ever produced anything like a peneplain in the northern 

 Cascades, as far as its history has been determined. In view of the eventful 

 character of the whole of the Tertiary, it is plain that the period of reduc- 

 tion to base-level can not be considered as including any large part of 

 Tertiary time, as has been suggested by Russell. Uplifts or subsidences of 

 the extent that are known to have occurred during Eocene and Miocene 

 time in this area must be considered as inaugurating new topographic 

 cycles. Furthermore, the land surface that was flooded by the basalt flows 

 at the beginning of the Miocene possessed considerable relief. This pre- 

 Miocene topography has been preserved in a large measure from later 

 erosion by the basalt, and where the capping is partially eroded away and 

 stream canyons are cut down into the underlying formations the contact 

 shows very conclusively the character of the old surface. Such a locality 

 is the valley of Taneum creek, where it is at once seen that the prebasalt 

 surface was such as to deserve to be termed rugged topography. t It seems 

 necessary, therefore, to restrict the period of the development of the Cascade 

 lowland to the Pliocene.'* 



Both Willis and Smith agree that the deformation of the Pliocene pene- 

 plain was not a simple uparching but a more complex uplift of the Cascade 

 range through the association of local upwarps and downwarps. In the down- 

 warps new river-courses were established, which are typified by those of the 

 Methow, Wenatchee, upper Skagit, and the Pasayten rivers. These and other 

 streams are thus supposed to be consequent on the late Pliocene warping of the 

 peneplain developed in earlier Pliocene time. The lower course of the Skagit 

 where it crosses the Skagit range, and the lower course of the Fraser where it 



* G. 0. Smith, Prof. Paoer, No. 19. U.S. Geol.. Survey. 1903. p. 28. 



