242 RALPH ARNOLD 



folding and faulting were intense, the greatest disturbances accom- 

 panying the uplift of the mountain ranges to an altitude of thousands 

 of feet. In other regions low broad folds were formed during the 

 post-Monterey disturbance, and the strata were not upheaved to a 

 great altitude. Faulting on a most magnificent scale took place along 

 the earthquake rift and certain other fault-zones, especially that in the 

 Salinas Valley, and along these lines of displacement, masses of 

 granitic rocks, which during the preceding epoch had been subject 

 to little or no erosion, were suddenly thrust upward and left exposed 

 to the ravages of streams that assumed the proportions of torrents in 

 certain regions, as for instance adjacent to the Carrizo Plain in south- 

 central California. The post-Monterey disatrophic movements in 

 the Puget Sound province also produced sharp relief as is evidenced 

 by the coarse sediments deposited immediately following the disturb- 

 ance. The localization of movement during the period is exemplified 

 at numerous localities in the Coast Ranges. 



Throughout much of the coastal belt, and probably likewise in 

 the interior, great volcanic activity took place during the middle 

 Miocene, this being the last epoch of volcanism in the Coast Ranges 

 south of San Francisco. During this post-Monterey period of 

 diastrophism general subsidence took place over most of the areas 

 which were under water during the lower Miocene, and, in addition, 

 extended northward from San Francisco Bay into the Sacramento 

 Valley and along the coast to the California-Oregon line and south- 

 ward down the Willamette Valley of Oregon. A new channel was 

 apparently opened across the northwestern end of the Olympic 

 Peninsula, and the Colorado Desert country of southern California 

 and Arizona which for a very long time had presumably been free 

 from marine conditions was occupied by an arm of the sea. 



THE UPPER MIOCENE PERIOD 

 DISTRIBUTION AND CONDITIONS OF DEPOSITION 



With the possible exception of that in the Eocene the subsidence 

 immediately preceding and extending into the upper Miocene was 

 the most important in the Tertiary history of the Pacific Coast. As 

 a result, the formations of this epoch occupy a very considerable 

 percentage of the surface of the present land-area. The sediments 



