156 Adventures in Scenery 



when the sea covered the region at different times, and these 

 have been carried away by erosion during the long time during 

 which the upheaval of the mountain was going on. During 

 the time from late Jurassic to the end of the Cretaceous there 

 were five epochs of submergence and sedimentation through- 

 out the Mt. Diablo area, and during Tertiary time the region 

 was submerged beneath the sea at two different times. It is 

 thought that 20,000 feet of Cretaceous and Tertiary rocks have 

 been removed from the top of Mt. Diablo, and how much of 

 the upthrust Franciscan rocks have been worn away it is im- 

 possible to tell. Mt. Diablo as a topographic feature has devel- 

 oped during the long time during which 20,000 feet, more or 

 less, of Tertiary and Cretaceous rocks were being eroded from 

 the conical mass of Franciscan rocks which had been thrust up 

 through the axial zone of the great Diablo anticline. 



Mass Forced Upward as Vast fault Plug 



It requires some exercise of the imagination to picture to 

 the mind such a gigantic thing as the Mt. Diablo overthrust. 

 When the region of Mt. Diablo was submerged beneath five 

 miles of sediments, as it was at the end of Tertiary time, and 

 was subjected to mountain-building stresses deep seated in the 

 earth, the rocks moved as a plastic mass in the direction of least 

 resistance. A mass of Franciscan rocks four miles in diameter 

 was forced upward as a fault plug into the younger rocks above, 

 and was thrust forward a distance of 10 miles over younger 

 rocks below and under still younger rocks above. The moun- 

 tain as it is today is a conical mass of eroded and weathered 

 igneous, metamorphic, and sedimentary rocks of Franciscan age 

 standing high above the San Joaquin Valley on the east and San 

 Ramon Valley on the west. Cretaceous and Tertiary rocks 

 flank the anticline in eroded parallel ridges northeast and south- 

 west of the mountain. 



The Franciscan rocks of Mt. Diablo are intensely folded, 

 crumpled and fractured. They may be grouped roughly into 



