Vol. 4] Osmont. — Geological Section of Coast Ranges. 



81 



To the east is the wide, flat-bottomed Berry essa Valley, which 

 is also of the nature of a subsequent valley. It is not located in 

 a syncline, however, being along a contact between Knoxville 

 shales and sandstones dipping to the northeast, and large masses 

 of serpentine and Franciscan rocks. 



The Tertiary voleanics once arched over this district, as 

 shown by the sections, and the valley may have started in a syn- 

 cline of Tertiary rocks which have now all been removed by 

 erosion. Immediately east of this valley is a very high ridge, 

 the crest of which is a very level sharp line about 2,600 feet above 

 sea-level for many miles north and south. This ridge owes its 

 even crest line to differential erosion, the alternation of hard 

 sandstone and limestone with soft shale in the Knoxville series 

 being peculiarly adapted to such weathering. ■ 



East'of the above mentioned high ridge, but not visible from 

 Mt. St. Helena, are the subseipient valleys of Capay and Pleas- 

 ant. Here again the streams flow along a contact between com- 

 paratively hard rocks and later softer ones, the Tertiary gravels 

 and soft sandstones overlying the Knoxville along this line. 



Marine Terraces. — Anderson states that at Point Reyes Penin- 

 sula well defined wave-cut terraces occur up to 300 feet above 

 sea-level. In the portion of the coast with which the writer is 

 most familiar, namely, the vicinity of Bodega Bay, the only well 

 defined shelf is lower than this. On the eastern side of the bay 

 and northward toward the Russian River is a wide wave-cut 

 shelf of an average width of about one-quarter of a mile and an 

 elevation at its front edge of from twenty-five to thirty-five 

 feet above sea-level. This slopes gently upward toward the hills, 

 and at its back are frequently seen residual stacks and talus 

 from the old sea-cliff. A thin veneer of gravel occurs in places 

 up to eighty or a hundred feet above sea -level. On the west 

 shore of Bodega Bay, as already mentioned under Pleistocene 

 deposits, is a distinct shelf cut in the diorite just about at high- 

 water mark. It is about 300 yards wide, and covered with gravel 

 and sand to a depth of 113 feet. The difference in the height of 

 this terrace on the two sides of the bay may perhaps be due to 

 comparatively recent movement along the Tomales Fault. 



No well defined terraces were observed at Bodega Bay above 



