158 Adventures in Scenery 



morphic sedimentary main mass of the mountain and the dia- 

 base basalt area of Mt. Zion, Eagle Point, and Black Point. 



Tilted Rocks Visible on Ride to Top 

 of Mountain 



A ride to the top of Mt. Diablo is one to be remembered. 

 The fine highway makes the ride enjoyable, and the view from 

 the top is in itself compensation for the climb. One does not 

 have to be a geologist to enjoy seeing what nature has done. 

 All the upturned rocks that are passed going up may not be 

 distinguished as Cretaceous or Tertiary, as metamorphic or 

 igneous, but tilted beds of chert, shale and sandstone will be 

 interesting. The complex metamorphic sedimentary and igne- 

 ous rocks that make the central mass of the mountain, though 

 weathered and mantled with soil at the top, where exposed in 

 eroded gullies or in highway cuts, will fascinate and hold the 

 interest of layman or geologist. 



Formations Vertical and Overturned 

 Exposed in Faults 



The Riggs Canyon fault cuts across the southwestern base 

 of the mountain. For several miles to the southeast of the base 

 of the mountain the fault is covered by the Cretaceous deposits 

 above the Diablo thrust. The formations along the south side 

 of the Riggs Canyon fault zone stand vertically or are over- 

 turned. In the immediate vicinity of Mt. Diablo beds of 

 Knoxville (Lower Cretaceous) age border the fault. An anti- 

 clinal fold, which strikes into the fault at right angles, occurs 

 in the Knoxville beds on the north side of the fault and exposed 

 in a cut on the road that leads up the mountain from the west 

 side. This is north of Pine Canyon and a little southeast of 

 Arroyo del Cerro. Just south of Windy Point, to the east, 

 lower Chico (Cretaceous) sandstones and shales are folded into 

 a syncline with steeply dipping beds on either side. In a broken 

 flexure in this syncline the beds stand vertically. The Tassa- 



