5g8 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



probable. That Merced strata once covered the entire val- 

 ley of the Pleistocene Walker Creek is shown by the small 

 remnant of Merced on the northwest end of Burdell Moun- 

 tain mass on the south side of San Antonio Creek and the 

 Merced of Spring Hill on the north side of the same stream 

 near its present headwaters. Most of the area south of the 

 Walker-San Antonio Creek valley is now composed of Fran- 

 ciscan rocks and the only suggestion of Merced obtained in 

 this region is the presence of an old erosion surface which 

 appears to rise as one travels from Petaluma southward. 

 This old erosion surface is also seen as one looks eastward 

 from Bolinas Ridge, a mile west from Tocaloma. The physi- 

 ography of the southern half of the Petaluma Quadrangle 

 was not given detailed study by the writer, but he thinks that 

 this old surface across these ridges indicates that the San 

 Francisco-Marin Block was uplifted higher and given even 

 a greater eastern tilt than the area further north. 



A reconnaissance trip southwest on the Point Reyes-Peta- 

 luma road which passes through Hick's Valley in the 

 Petaluma Quadrangle and Pomponio Creek yielded some 

 interesting observations. Salmon creek and its tributary, 

 Arroyo Sausal, are cut in solid Franciscan rock, and the 

 tortuous course of these streams when studied upon the 

 combined charts, particularly in the vicinity of Hick's Valley, 

 clearly indicates that they are superimposed streams. Hick's 

 Valley when its bottom is examined is found composed of 

 alluvium which completely surrounds some hills of Franciscan 

 rocks, and this recent material appears peaty in places, thus 

 evidencing a ponding of the upper tributaries of Arroyo 

 Sausal. A low divide separates Hick's Valley drainage from 

 that of Arroyo Nicasio. In the vicinity of the Pacheco 

 School and around Nicasio a similar ponding was observed. 

 At Pacheco School, on a small tributary the ponded condition 

 is still present and marshy ground in a relatively broad val- 

 ley which is evidently due to recent filling was seen. This 

 tributary and its neighboring tributary as one goes south- 

 east from the schoolhouse are separated by a very low di- 

 vide and a ponded condition is evident here as well as on 

 the main stream around Nicasio village. As one ascends 

 Nicasio Creek, following up Lucas Valley this ponding dis- 



