576 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



At the beginning of the Pleistocene, the San Francisco- 

 Marin Block was thickly covered by nearly horizontal Mer- 

 ced strata, and at least in the northern portion of this block, 

 excellent evidence that the Pleistocene sea truncated Merced 

 strata, may be seen. As was pointed out above, fine ter- 

 races occur around Freestone and to the northwest of this 

 village. After this marine plain (or plains) was formed, the 

 San Francisco-Marin Block was lifted above sea level, conse- 

 quent Pleistocene streams began their downward and side- 

 wise cutting, and quickly worked through the soft Merced 

 strata to the old erosion surface cut across Franciscan rocks. 

 At this time the drainage was across the block from east to 

 west. Pleistocene Russian River 26 was probably the most 

 vigorous of these streams, as it cut the longest and deepest 

 canyon in the Franciscan. Later in the Pleistocene, the San 

 Francisco-Marin Block was uptilted on its western edge and 

 the lower portions of the consequent streams, with an in- 

 creased gradient, were enabled to cut canyons in the under- 

 lying Franciscan rocks. All the streams on this block which 

 now flow into the Pacific Ocean or Tomales Bay from the 

 Russian River to Elk Creek, have canyons of greater or less 

 length cut into the Franciscan. The tilting variously affected 

 the middle and upper courses of the streams, producing in 

 some cases ponding and in others spilling them out by way 

 of their headwater tributaries, reversing the stream direction. 



Topography and Physiography 



The principal topographic units of this region, as well as 

 orogenic divisions, are the Point Reyes Peninsula, the Rift 

 Valley of Tomales Bay, the San Francisco-Marin Block which 

 extends from Tomales Bay and the Pacific Ocean, to Peta- 

 luma Valley and the northwestern extension of the Berkeley 

 Hills Block, Point Reyes Triangle. 



The Point Reyes Triangle, as will be seen by consulting 

 the topographic map of the Geological Survey, has its north- 

 eastern base sharply determined by the San Andreas Rift 

 Zone. Point Reyes is the apex opposite this base, and 

 Tomales Point and Bolinas headland are the other apices of 



28 Holway, R. S., The Russian River, a Characteristic Stream of the California 

 Coast Ranges, Univ. Calif. Publ., Geog. Dept., Vol. 1. No. 1. 1913. 



