Vol. 5] 



Weaver. — San Pablo Formation. 



251 



name, occurs in Contra Costa County on the east side of San 

 Pablo Bay near the town of Rodeo. It extends as a belt about 

 one mile in width from El Cierbo southeasterly to the head of 

 Franklin Canon and from there it swings around to the south- 

 west and reaches San Pablo Bay just north of the powder works 

 at Hercules. Here the San Pablo, together with the Chico, Mon- 

 terey, and Pinole Tuff, has been folded into a synclinal trough. 

 Just south of here the Monterey is very completely developed 

 and is composed of nine well defined divisions of sandstone and 

 shale. The uppermost division is known as the Quercan sand- 

 stone and it very closely resembles the characteristic San Pablo 

 sandstone. In this region the Monterey apparently lies \mcon- 

 formably beneath the San Pablo. Areal mapping shows the 

 different divisions of the Monterey to extend diagonally beneath 

 the strike of the San Pablo beds. On the north flank of this 

 syncline all of the formations dip at very high angles toward 

 the axis. Here the Monterey is represented by the two upper 

 divisions only, the Herculean shale member and the Quercan 

 sandstone. They rest unconformable upon the Chico, and al- 

 though no actual exposures reveal them unconformable beneath 

 the San Pablo, yet from observations taken on the south flank it 

 would seem most probable that an unconformity exists here also. 



The section made across the strike of these beds is well de- 

 fined and was measured in detail by following the cliffs along 

 the bay shore. On the south side of the syncline the dip is much 

 lower than on the north and only the Monterey, San Pablo, ami 

 Pinole Tuff are exposed in the fold. Near the base of the San 

 Pablo the average clip is about 30 degrees north, and this grad- 

 ually decreases towards the top until at the contact with the tuff 

 it is only 20 degrees. The general character and appearance of 

 the strata is the same on both flanks of the syncline. The total 

 thickness of the strata in this section is about seventeen hundred 

 feet. The formation is made up of conglomerates, thick-bedded 

 sandstones which are in places conglomeratic and cross-bedded, 

 sandy shales and clay shales. The sandstones generally have a 

 bright blue or gray-blue color, but very often weather to a yellow- 

 ish or reddish tinge. The conglomerates appear to be most 

 abundant in the lower half of the formation. 



