53? CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



These canyons have effected the dissection of a plateau which is 

 a marine wave-cut terrace, representing one of the later stages 

 of the epeirogenic uplift of the coast." Concerning the pos- 

 sibility of a Pliocene peneplain Professor Lawson says : "It 

 may, perhaps, be well to state here that in those portions of 

 the coast which once served as areas of Pliocene sedimenta- 

 tion, as in the vicinity of the Bay of San Francisco, much of 

 the geomorphic character was evolved in pre-Pliocene time, 

 and had simply been revealed and modified by the stripping 

 off of the Pliocene accumulations." The accuracy of this 

 conclusion will be better appreciated after the evidence for 

 this surface has been described around Petaluma and Free- 

 stone. At these places the conditions are essentially in ac- 

 cord with Lawson's statement. Lawson suggested that Peta- 

 luma Valley was once occupied by the Russian River. His 

 statement is as follows : "Associated with the subsidence 

 which flooded the Bay of San Francisco, there were prob- 

 ably other deformations of the crust which seem to have had 

 an important influence on the drainage. The most notable 

 instance of this kind is the shifting of the divides of the 

 hydrographic basin of the Russian River. This stream once 

 clearly flowed through Petaluma Valley to the main drain- 

 age outlet at the Golden Gate. A low divide in the middle 

 of the old valley now causes the drainage to flow westward 

 at right angles to its former southerly course, and seek the 

 coast by the present transverse route. The change in the 

 drainage may be due to stream capture or to crustal warping. 

 The latter is most probably the cause; but the problem has 

 not yet been studied sufficiently." The writer is not in agree- 

 ment with this statement in its entirety, but warping, as 

 Lawson points out, is one of the controlling influences in the 

 development of the physiography of this country. Lawson 

 also discusses this region in the San Francisco Folio. Mr. 

 F. M. Anderson 7 mapped and described the Point Reyes 

 Quadrangle and his excellent mapping is used in this publi- 

 tion with but slight alteration. These changes are the ad- 

 dition of the Pleistocene formations in Tomales Bay region 

 and reconnaissance mapping of Merced strata around To- 



7 Anderson, F. M., Geology of Point Reyes Peninsula, Univ. Calif. Publ., Bull. Dept. 

 Geol., vol. 2, No. S, 1903. 



