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



41 



PACE 



Geomorphology 



The Elevated Coastal Peneplain 



Wide, Flat-Bottomed Valleys of Erosion . . 

 Synclinal Valleys and Subsequent Drainage 



Marine Terraces 



Fault Origin of Mt. St. Helena 



Eecent Submergence 



Geomorphic Cycle 



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Historical Resume 



INTRODUCTION. 



The work of which this paper is the outcome was undertaken 

 during the Fall of 1901, when the writer was a graduate student 

 in geology at the University of California. The subject was sug- 

 gested by Prof. Andrew C. Lawson, who rendered the writer 

 invaluable assistance by his advice and personal supervision of 

 the work, which has occupied the latter 's spare time for the past 

 three years. Acknowledgments are also due Dr. J. C. Merriam, 

 Dr. Gidley, Dr. W. J. Sinclair, and C. E. Weaver for palaeon- 

 tological aid. 



No topographical maps of most of the area being available, 

 county maps had to be used, and since these represent the topo- 

 graphy poorly or not at all, it was not found feasible to map in 

 the areal geology. 



Two continuous cross-sections were made. Section AB starts 

 at the south end of Bodega Peninsula, and runs in a northeast 

 direction across Bodega Bay, through the towns of Bodega and 

 Freestone, through the low hills east of Freestone, across the 

 Laguna de Santa Rosa, and enters the broad valley of Santa 

 Rosa a little north of Sebastopol ; crosses the hills west of this 

 valley about midway between Windsor and Mark West Creek: 

 passes through Mark West Springs, crosses the watershed of that 

 creek, passing about two miles north of the Petrified Forest ; 

 crosses Franz Valley and the high ridge east of it, and comes out 

 at the low divide on the southwest blank of Mt. St. Helena, which 

 separates Knights Valley from Napa Valley ; thence it* swings 

 somewhat more to the north through Mt. St. Helena, and thence 

 in an easterly direction across St. Helena Creek, passing a little 

 north of the Oat Hill Quicksilver Mine, following approximately 



