REPORT OF THE CHIEF ASTRONOMER 



625 



SESSIONAL PAPER No. 25a 



of the Entiat stage is mature. It occurs as a spur or divide below occasional 

 residuals of the Methow stage -and above features of later stages. 



1 "Within the relief of the Entiat stage there were cut deeper channels, 

 some of them canyons of impressive depth, many of them simply mountain 

 ravines. They constitute the most marked and everywhere the most char- 

 acteristic features of the topography of the region. Any large stream 

 might be chosen as exhibiting the type, but probably none shows it in 

 various degrees better than the Twisp, which from its junction with the 

 Methow to its source in the Cascades lies in a canyon that varies from a 

 few hundred to 4,000 feet in depth, as can be seen on the Methow topo- 

 graphic atlas sheet. This stage is accordingly named Twisp. The char- 

 acteristic of the Twisp stage is a canyon, the typical feature of topographic 

 youth, but the development progressed far toward maturity. 



' The Twisp stage closed with accumulations of glacial ice, which 

 occupied the canyons and in many instances greatly modified them. . . 



' It is somewhat difficult to place these several stages in geologic time. 

 On the evidence of fossil plants from the Ellensburg formation, the Methow 

 plain in the Yakima district is post-Miocene. The data are fully presented 

 by G. 0. Smith, and the unity of the feature throughout the Cascade 

 range is discussed by the writer under its proper head below. The very 

 long time required to accomplish such extensive and uniform leveling 

 appears reasonably to occupy most of the Pliocene and to bring the date 

 of the next stage near the close of that epoch. . . . 



' The following tabulation expresses the most reasonable estimates of 

 correlation for the several stages in geologic time :— 



PHYSIOGRAPHIC DEVELOPMENT OF THE CASCADE RANGE. 



Physio- 

 graphic 

 stage. 



m , r . Nature of char- 

 Type locality. < acteristic activity 



Date, if one 

 uninterrupted 

 Glacial epoch. 



Date, if two or 



more Glacial 



epochs. 



Period . 



Stehekin. 



Stehekin sources Glacial retreat and 



Post-Glacial to pre- 



Post- Glacial to 



Recent. 





and valley. 



re-excavation of 

 old valleys. 

 Glacial occupation 



sent. 



present. 





Chelan. 



Gorge of Lake Che- 



Glacial epoch. 



Latest Glacial 



Pleistocene. 





lan and terraces 



of canyons. 





epoch. 







of the Columbia. 











Twisp . 



Canyon of the 

 TwiJ-p, Methow 

 quadrangle. 



General acceler- 

 ation of corrasion. 



Pre- Glacial. 



Inter-Glacial. 





Entiat. 



Basin of the Enti<*t. 



Development o f 



Pre-Glacial. 



Earlier Glacial 



Pliocene or 







mature topogra- 





epoch. 



Plei s t o - 







phy generally 







cene (Sier- 







throughout the 







ran Age). 







Cascade plateau. 









Methow . 



Generally through- 

 out the broad 

 mountain dis- 

 trict. 



Planation by ero- 

 sion to a low 

 plain with mon- 

 adnocks. 



Pro-Glacial. 



Pre-Glacial. 



Pliocene. 



