Mount Diablo 155 



the Tertiary, aggregating not less than 43,000 feet. The ex- 

 tent of the protrusion of the Franciscan upthrust mass through 

 later Cretaceous and Tertiary strata it is not possible to state 

 because these strata, totaling upward of 20,000 feet, have been 

 removed from the central part of the anticline around the 

 mountain, and an unknown amount of the Franciscan rocks 

 has been worn away. 



What is called the Mt. Diablo anticlinal fold extends from 

 the edge of the San Joaquin Valley south of Tracy more than 

 50 miles in a northwesterly direction through Mt. Diablo, 

 across Carquinez Strait, and includes Sulphur Springs Moun- 

 tains near its north end. The upheaval and overthrust of Mt. 

 Diablo occurs about midway of this great fold or anticline. 

 The mountain proper is an ovate mass of metamorphic and 

 igneous rocks 1 5 square miles in area thrust up and protruding 

 through the center of the great anticlinal fold. Structurally 

 the mountain is an overturned and overthrust anticline, that 

 is, an uplifted bent and folded segment of the earth's crust, 

 thrust upward and forward by the force of a molten mass in- 

 jected from below, and the fold overturned. The injected 

 mass of igneous and metamorphic rocks was thrust upward 

 through the upper Jurassic and Cretaceous strata in the center 

 of the anticlinal. The mountain mass proper is what is geo- 

 logically termed a "horst," a segment of the crust of the earth 

 uplifted and bounded on all sides by faults. 



Great Thickness of Rocks Eroded 

 D^tring Uplift 



The central mass of Mt. Diablo is a "plug" of rocks four 

 miles in diameter of the old underlying Franciscan formation. 

 Rocks of Cretaceous age, which were thrust into and uplifted, 

 have been eroded away, so that the old Franciscan rocks are 

 now exposed and form the outstanding mass of Mt. Diablo. 

 Cretaceous and Tertiary rocks having a total thickness of as 

 much as five miles were laid down over what is now Mt. Diablo, 



