300 HISTORICAL GEOLOGY 



seen, but probably not more than a chain of islands was then 

 developed. Another time of moderate elevation occurred during the 

 Eocene. The Oligocene was a time of considerable elevation and 

 erosion in the Coast Range district, though the relief could not 

 have been strong, as shown by the fine-grained sediment which was 

 deposited during the Oligocene in the limited areas of southern 

 California and western Oregon and Washington. 



What may really be called the " Coast Range Revolution" 

 took place in mid-Miocene time. According to Arnold, " one of the 

 most widespread and important periods of diastrophism in the 

 Tertiary history of the Pacific Coast was that immediately follow- 

 ing the deposition of the Monterey or lower middle Miocene. Its 

 effects are visible from Puget Sound to southern California. It is 

 marked as much by readjustment, by local faulting and folding, 

 as by general movements of elevation and subsidence. In some 

 regions the folding and faulting were intense, the greatest dis- 

 turbance accompanying the uplift of the mountain ranges to an 

 altitude of thousands of feet (Fig. 183). In other regions low 

 broad folds were formed during the post-Miocene disturbance, and 

 the strata were not upheaved to a great altitude. Faulting on a 

 most magnificent scale took place along the (San Francisco) earth- 

 quake rift (fault) and certain other fault-zones. . . . The post- 

 Monterey diastrophic movements in the Puget Sound province 

 also produced sharp relief, as is evidenced by the coarse sediments 

 deposited immediately following the disturbance." 1 



Other important mountain-making movements took place 

 during the Quaternary, with minor activity continuing to the 

 present time. The combination of the various diastrophic move- 

 ments and erosion in the Coast Range belt since early Tertiary 

 time has given rise to the Pacific border mountains as we see them 

 today. 



Sierra Nevada and Cascade Ranges. — We have already seen 

 that the Sierras were produced by crustal disturbance toward the 

 close of the Jurassic period. From that time till late in the Miocene 

 epoch the mountain mass had undergone profound erosion, so that 

 it was reduced to a range of hills or low mountains with no great 

 relief features. In other words, it approached the condition of a 

 peneplain. Then, late in the Miocene or early in the Pliocene, 



1 Ralph Arnold: Outlines of Geologic History, by Willis and Salisbury, 

 p. 242. 



