THE JURASSIC PERIOD 223 



peneplain and the next sediments (Comanchean) were deposited 

 upon the eastern portion of that worn-down surface. For instance, 

 on Staten Island and in northern New Jersey, the Comanchean 

 beds may be seen resting directly upon the deeply eroded Triassic 

 rocks, and hence the proof is conclusive that during much, if not 

 all, of the Jurassic period active erosion was taking place, and this 

 in turn implies that the Triassic beds were well elevated in the 

 early Jurassic. 



Close of the Jurassic (Sierra Nevada Revolution). — The 

 close of the period witnessed profound geographic changes in the 

 western part of the continent. During both the Triassic and 

 Jurassic periods, as well as throughout much of Paleozoic time, 

 there had been more or less continuous deposition of sediments on 

 the Pacific slope over the sites of the present Sierra, Cascade, and 

 Coast Range Mountains. Toward the close of the Jurassic period 

 these thick sediments, particularly in the Sierra region, were 

 subjected to a tremendous force of lateral compression, the strata 

 being upheaved, folded, and crumpled (Fig. 132). Thus the Sierra 

 Nevada Mountains of California were borne out of the ocean and 

 the Pacific shore line was transferred to the western base of the 

 newly formed range. The Sierras, in this their youth, were most 

 likely a lofty range, but were later much worn down by erosion, 

 their present great altitude having been produced by later (Ter- 

 tiary) movements. Accompanying the orogenic movements, the 

 deeply buried sediments were metamorphosed and the vast quan- 

 tities of granite were probably intruded at the same time, this 

 granite being now exposed to view only because of profound sub- 

 sequent erosion. As a result of the metamorphism the thick 

 Mesozoic shales were converted into the hard (Mariposa) slates. 



The best evidence indicates that this orogenic disturbance also 

 affected the strata of the Klamath Mountains in northwestern 

 California and the Cascade Mountain region through Oregon, 

 Washington, and even British Columbia. 



The strata then occupying the site of the present Coast Ranges 

 were somewhat deformed, but probably only enough to form a 

 chain of islands or a very low mountain range. This is proved by 

 the fact that Lower Cretaceous strata are found resting uncon- 

 formably upon the deformed Jurassic rocks. The orogenic move- 

 ments which produced the Coast Range Mountains as we now see 

 them came later. Some other mountains of the west, such as the 



