Mount Diablo 159 



jero flexure is on the east side of Riggs Canyon and on the north 

 side of the main Riggs Canyon fault a little farther to the east. 

 It is on the surface an overturned syncline, the rocks of which 

 are composed of upper Cretaceous shales and Eocene (Tertiary) 

 deposits. The Eocene rocks in the flexure have been doubled 

 back upon themselves. The Eocene deposits west of the flex- 

 ture stand vertically, and in the area south of it they are over- 

 turned. The curved fault around the flexure is not easily seen 

 in the field because it is in shale and exposures are poor. 



The Mt. Diablo upthrust cuts through Cretaceous strata on 

 all sides. The contact of the uplifted Franciscan Complex is 

 that of a fault plane with Cretaceous rocks on one side and the 

 heterogeneous mass of the intruded complex on the other. The 

 contact of the Franciscan rocks and the surrounding Cretaceous 

 deposits wherever exposed is that of faulting. On all sides of 

 the mountain rocks have broken and moved in landslides, these 

 extending in many places over the edges of the adjacent Cre- 

 taceous formations. On the southwest side of Mt. Diablo and 

 on the north side of North Peak particularly, gravity landslides 

 of very large proportions have extended to the base of the 

 mountain. The road that approaches the mountain from the 

 west crosses such landslides for several miles, as does the road 

 approaching the mountain from the south for a lesser distance. 



