GEOLOGY OF WEST COAST REGION OF UNITED STATES 



The Coast Ranges of California received their 

 first and greatest folding at this time. The Fran- 

 ciscan series is pressed into a regular system of folds 

 which often consist of open synclines and broken, 

 compressed anticlines, but locally of isoclinal struc- 

 tures. Throughout the region affected the trend of 

 the folding is northwestward. The folds of the 

 Coast Ranges of California are cut diagonally by 

 the present coast line, the system disappearing 

 under the ocean in the neighborhood of the boun- 

 dary line between California and Oregon. The 

 Klamath Mountains, which may be considered as 

 a mountain knot connecting the Coast Ranges with 

 the Sierra Nevada-Cascade system, belong structur- 

 ally with the folded complex of the latter range. 

 The Oregon and Olympic mountains west of the 

 Puget Sound trough have been considered as the 

 continuation of the Coast Ranges. They are, how- 

 ever, gentlv folded, and possibly were affected only 

 by the middle Miocene revolution, which was much 

 less violent than the one under consideration. 



In the southern Cascades, the folded structures 

 produced by the Cordilleran revolution are buried 

 under the thick Tertiary lavas, but in a few places 

 a closely folded complex of the older rocks has 

 been uncovered by erosion of the lavas, indicating 

 that the folded zone persists under the later cover. 

 In the northern Cascades, the rocks are even more 

 closely crushed and folded than in the Sierra Ne- 

 vadas. Here we have the super-imposition of the 

 movements and metamorphism accompanying the 

 intrusion of batholiths of Cretaceous and later age, 

 upon the effects of the Cordilleran revolution. 



The Cordilleran revolution impressed upon the 

 west coast region its present structural pattern, 

 and outlined the positive (rising) and negative 

 (sinking) elements which persist today (fig. 5). 

 The main negative element is the great central 

 trough extending from Puget Sound to the Gulf of 

 Mexico, via the Willamette Valley, the Valley of 

 California, and the Salton Sink. This trough is 

 separated into three main basins by the mountains 

 north and south of the Valley of California. The 

 positive elements are the highlands and mountain 

 ranges on either side of the trough. 



In California, especially, the folded zones initi- 

 ated during late Jurassic times have governed sub- 

 sequent deformation. The great faults, for the most 

 part of subsequent origin, show a general parallel- 

 ism to the trend of the folds. 



The Cordilleran revolution also resulted in a 

 complete re-arrangement of the distribution of the 



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