CHAPTER XIII 

 MOUNT DIABLO 



An Up-Thrust Folded Mountain 



An outstanding geologic feature of central California is 

 Mount Diablo, about 20 miles east of San Francisco Bay. It 

 is located at the northern end of the Mt. Diablo Range, the 

 central group of the Coast Ranges, between the great Central 

 Valley of California and the Pacific Coast adjacent to San 

 Francisco. Rising from near sea level to an elevation of nearly 

 4,000 feet (3,849 feet), Mt. Diablo is a conspicuous landscape 

 feature. It is in view from the eastern slopes of the Sierra 

 Nevada Mountains to the Golden Gate. From a geologic 

 standpoint the mountain and its immediate vicinity constitute 

 what is pronounced the most conspicuous structural feature in 

 middle California. 



Following the end of Tertiary time Coast Range mountain 

 building became more active, with increased uplifting, folding, 

 and faulting. At this time the Mt. Diablo uplift began, and 

 during its progress, which continued probably into late Quater- 

 nary time, the Franciscan complex was thrust up through the 

 Cretaceous in the apex of the structure. The Franciscan mass 

 of Mt. Diablo is the remnant of this upthrust complex remain- 

 ing after the erosion of the Cretaceous from above and about it. 



Two distinct structural features stand out in Mt. Diablo. 

 First, there is the great anticlinal fold, the Mt. Diablo uplift, 

 which may be termed the "great Mt. Diablo anticlinal fold," 

 developed at the close of Tertiary time. Second, there is the 

 Mt. Diablo upthrust of an igneous and metamorphic complex 

 through the upper Jurassic and Cretaceous strata in the center 

 of the great anticlinal fold. The structure of the Mt. Diablo 

 anticline involves the entire sedimentary series below the top of 



