FORMATIONS AND PHYSICAL HISTORY 



587 



Ellensburg 

 Formation 

 1000-1500 ft. 



Non-marine deposits, largely of volcanic material, occur in 

 British Columbia between the Coast and Gold ranges. Miocene 

 (It-posits are known in Alaska, but erosion rather than deposition 

 was the dominant process there, so far as present data show. 



Igneous activity during the Miocene. The widespread igneous 

 activity which began with the close of the Cretaceous, perhaps 

 reached its climax during the Miocene. Igneous materials abound 

 in the sedimentary formations of the 

 system throughout the west, and igneous 

 activity affected nearly or quite every state 

 west of the Rocky Mountains, the erup- 

 tions being from fissures as well as 

 volcanoes. Among the conspicuous centers 

 of activity the basin of the Columbia 

 and the Yellowstone National Park may 

 be mentioned. Locally, forests were 

 buried by the volcanic ejecta, and in 

 favorable situations their trunks were 

 petrified (Fig. 495). The lavas of at least 

 a considerable part of 200,000 or 300,000 

 square miles of lava-covered country in 

 the western part of the United States 

 issued during this period, or during the 

 time of crustal deformation which brought 

 it to a close. Volcanoes were active in 

 the Antillean region of Central America 

 and the West Indies, and the Andean 

 system of South America, as well as in 

 North America. 



Close of the Miocene. Slow warpings 

 of the surface seem to have been in prog- 

 ress throughout the Cordilleran region 

 during the Miocene period, accompanied by faulting and vulcanism, 

 and locally, by pronounced erogenic movements; but toward the 

 close of the period movements were more general. Pronounced 

 deformation affected the coastal regions of Oregon and northern 

 California, tilting and folding the Miocene and older formations. 

 The principal folding of the existing Coast Ranges of both these 

 states has been assigned to this time, but it now appears that some 

 of the deformations formerly referred to the end of the Miocene took 



Fig. 493. Columnar sec- 

 tion showing the succession 

 of formations in central 

 Washington. (G. O. Smith, 

 U. S. Geol. Surv.) 



