66 



University of California Publications. [Geology 



The thickness of this member varies from 150 feet to 1000 feet. 

 The predominant conglomeratic constituent of this member is 

 fragmentary Monterey shale which shows very little water action. 

 Over small areas, especially in the northern part of the region, 

 Franciscan sandstones and cherts enter largely into Jie make-up 

 of these conglomerates. While marine fossils ar~ entirely absent 

 from these basal beds, there is an abundant scattering of soft 

 carbonaceous, evidently plant, material. These beds rest uncon- 

 formably upon the Monterey shale (see pi. 15), and upon 

 the Franciscan rocks in the northern part of the area. 



The best section for detailed study is just north of Peseadero 

 Creek, beginning at the actual contact, here plainly visible, 

 between the conglomerates of the San Pablo and the Monterey 

 shales shown in plate 15. 



The shale beds here strike N. 48° B. and dip 57° to the 

 northwest. Upon the fairly even erosion surface of the shale 

 rest the San Pablo conglomerates, with a strike almost due east 

 and west and with a northerly dip of 10°. The unconformable 

 relations between the formations are therefore clear and decisive. 

 The detailed section of Member A is as follows : 



Some of the beds of this section are worthy of special 

 description. Numbers 1, 4, and 6 are very friable and soft and 

 do not give rise to prominent outcrops. The sandy and pebbly 

 layers are of irregular thickness. Number 2 is a fairly hard, 

 dark-brown sandstone of even texture. It forms a prominent 

 outcrop standing out as a vertical wall on the ridge where the 

 section was studied. Numbers 3, 5, and 7 are the prominent 

 beds of the section. They are exceedingly coarse in texture, the 

 conglomeratic material being mostly irregular shale fragments 

 of all sizes. These three beds resist erosion exceptionally well 



feet. 



7. 

 6. 

 5. 

 4. 

 3. 

 2. 

 1. 



Coarse conglomerates 



Conglomerates and sandstones 



Coarse conglomerates 



Conglomerates and sandstones 



Coarse conglomerates 



Hard sandstone 



Conglomerates and sandstones 



Unconformity 



Monterey Shale 



50 

 25 

 30 

 20 

 40 

 6 

 20 



191 



