Vol. XI] DICKERSON—PT. REYES AND SANTA ROSA QUADRANGLES 537 



drainage of its eastern headwaters to the east, ponded them 

 somewhat, and they fell an easy prey to the more powerful 

 stream, the Russian River, which succeeded in maintaining 

 a strong course completely across the Berkeley Hills and San 

 Francisco-Marin blocks. 



Estero Americano apparently had a similar history. Like 

 Salmon Creek it cut a gorge at its mouth in Franciscan rock. 

 The wide valley stage is seen in the region just east of Valley 

 Ford and in and around Valley Ford the Merced strata rest 

 upon Franciscan rocks exposed in the bottom of the valley. 

 The stream further south, Estero San Antonio and its main 

 tributary, Tomales Creek, exhibit the same characters. 



Estero San Antonio and Tomales Creek probably had a 

 drainage that originated in Sonoma Mountain and it is pos- 

 sible that the two old stream channels described under the 

 heading "Undifferentiated Pleistocene" near Waugh School 

 are remnants of this East-West drainage. 



When the gorge of Walker Creek is compared to that of 

 Estero San Antonio, one finds that it is far better developed, 

 but its present drainage basin is far less than that of Estero 

 San Antonio. Professor Holway has described the relation- 

 ship between Walker Creek and San Antonio Creek and has 

 pointed out the ponded area at their present headwaters 

 (See Plate XXIV, Figure 2), and that the present tributaries 

 of San Antonio Creek enter the main stream in certain cases 

 at acute angles whose vertices point toward the source of the 

 stream and not toward its mouth. These anomalous features 

 indicated to Professor Holway that the former course of 

 this drainage was entirely toward the Pacific and that San 

 Antonio Creek of today was in reality the headwaters of a 

 more powerful, ancient Walker Creek. When the Point 

 Reyes and Petaluma quadrangle sheets are placed together 

 Professor Holway's evidence is clearly shown, and if you 

 place a stream line across Chileno Valley and ink the tribu- 

 taries of San Antonio Creek, the evidence will be even clearer. 

 The writer thinks that the broken backed condition of Pleis- 

 tocene Walker Creek was, like the changes in other streams 

 previously discussed, due to tilting in late Pleistocene time. 

 That Pleistocene Walker Creek was developed upon a Pleisto- 

 cene marine plain which graded into a low upland appears 



