Smith.] 



Islands of Southern California. 



229 



of the Pliocene valley deposits was largely accomplished during 

 the first post-Pliocene elevation, while the coastal terraces were 

 formed during the second. If the deposits along the Tres Pinos 

 Creek are of Pliocene age,* then, since a well-defined series of 

 stream-cut terraces has been developed in these gravels by the 

 stream which has trenched the present valley, and from the upper 

 limit of the deposits down, a single general elevation since these 

 deposits were laid down would seem to be indicated. For it is 

 reasonable to suppose that these stream-terraces were formed at 

 the same time as the coastal wave-cut terraces; and since such a 

 series of stream-cut terraces, on the sides of a narrow valley, could 

 not well be formed after the valley as a whole (or even in large 

 part) had been carved, the present form of this valley, at least, 

 must have been developed during the elevation of the coast which 

 gave rise to the series of wave-cut terraces.. The same will hold 

 true for other valleys of the coast where Pliocene deposits have 

 been terraced by stream cutting. 



It is generally assumed that, to produce terraces along a sea 

 coast, periods of movement must alternate with periods of com- 

 parative quiescence. If variation in movement must be postulated, 

 in order that terraces may be formed during a general elevation, 

 we can not limit these variations to periods of temporary cessation 

 of movement or irregularity in the rate of upward movement, but 

 we must also include the possibility of reverse movements. For, 

 the same causes which give rise to cessation or variation in rate of 

 motion may also be capable of producing reverse movements, — for 

 example, oscillations or movements of depression during a general 

 elevation, — and the possibility of such oscillations must be taken 

 account of in any working hypothesis. 



Such oscillations would explain the presence of Elephas remains 

 on Santa Rosa Island. For, irrespective of variations in the height 

 of the submarine ridge at the eastern end of the Santa Barbara 

 Channel (see page 204), an elevation somewhat above the present 

 level would be required to connect Santa Rosa with Santa Cruz 

 Island, and thus with the mainland; while a depression on the 



*See, Lawson, The Post-Pliocene Diastrophism of the Coast of Southern 

 California, Bull. Dept. Geol., Univ. Calif., Vol. I, No. 4, 1893, p. 152. 



