GEOLOGY OF WEST COAST REGION OF UNITED STATES 

 which may be considered broadly as a great fault 

 block, with an important zone of faulting on the 

 east side. The movements consisted in a tilting 

 of the block towards the west, the eastern portion 

 being elevated above the desert basin region of 

 Nevada. In early Tertiary times erosion gained 

 the upper hand and the Sierras were reduced in 

 part to a peneplain. In late Tertiary and Quater- 

 nary times uplifts culminating in great elevations 

 during the Quaternary period gave birth to the 

 present Sierra Nevada, and subsequent erosion is 

 as yet in its youthful stage. Except for this tilting, 

 this great mountain block has acted as a resistant 

 buttress and has not suffered the intense deforma- 

 tion that affected the weaker regions to the east 

 and west in Tertiary and Quaternary times. 



The early Tertiary history of the Cascade ranges 

 is largely hidden under the great lava mantle. The 

 southern Cascades owe their present height in part 

 to constructional processes, majestic volcanic cones 

 dominating the range. Since Miocene times these 

 ranges seem to have suffered a gentle upwarping 

 but no important folding. The history of the north- 

 ern Cascades and adjacent regions is much more 

 complicated, folding probably occurring in the 

 Cretaceous, at the close of the Eocene and in Mio- 

 cene times. The present elevation, as in the other 

 ranges, is largly the result of relatively recent up- 



The Klamath Mountains received the full force 

 of the later compressive movements, intensified pos- 

 sibly by the resistance offered by the unyielding 

 Sierra Nevada block. The boundary between the 

 Coast Ranges and the Klamath Mountains is marked 

 by a great thrust, along which crystalline schists 

 have overridden the Jurassic and Cretaceous rocks 

 to the west, and similar thrust faults have been 

 described in Oregon. 



Along the Coast Ranges compressive stresses ac- 

 cumulated throughout the Tertiary. Locally the 

 relief and crumpling took place at various times, 

 the most pronounced folding occurring in the mid- 

 dle Miocene. In this same region great faults de- 

 veloped in general parallel to the trend of major 

 folds. Some of the faults are old and date back 

 to early Tertiary or possibly Cretaceous times. The 

 fault movements seem to have reached a maximum 

 during the elevation of the entire west coast region 

 in Quaternary and Recent times, and are continuing 

 with perhaps undiminished intensity at present. 



The elevated beach lines show that recent eleva- 

 tions of the coast have been important. Local 



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